Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic and DFT investigation on the photo-chemical properties of a push-pull chromophore: 4-Dimethylamino-4′-nitrostilbene Francesco Muniz-Miranda a,⁎,1, Alfonso Pedone a, Maurizio Muniz-Miranda b a b
Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia (UniMORE), Via Campi 103, 41125 Modena, Italy Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze (UniFI), Via Lastruccia 3, 50019 Sesto Fiorentino, Italy
a r t i c l e
i n f o
Article history: Received 18 April 2017 Received in revised form 15 August 2017 Accepted 31 August 2017 Available online 05 September 2017 Keywords: Push-pull molecule Raman spectra DFT Photoreaction
a b s t r a c t 4-Dimethylamino-4′-nitrostilbene (DANS), a π-conjugated push-pull molecule, has been investigated by means of a combined spectroscopic and computational approach. When the Raman excitation is close to the visible electronic transition of DANS, vibrational bands not belonging to DANS appear in the spectra, increasing with the laser power. These bands are observed at room temperature in the solid phase, but not at low temperature or in solution, and we interpret them as due to a thermally-activated photoreaction occurring under laser irradiation in the visible spectral region. Density-functional calculations correctly reproducing the electronic and vibrational spectra of DANS, describe the charge-transfer process, indicate that an azo-derivative is the product of the photoreaction of DANS and provide a reasonable interpretation of this process. © 2017 Elsevier B.V. All rights reserved.
1. Introduction “Push–pull” chromophores are characterized by an electron-donating group and an electron-accepting group linked by a conjugated π system [1–9]. They generally exhibit large first and second hyperpolarizabilities and can be usefully employed in non-linear optical (NLO) applications, for example as efficient second-harmonic generators and fast waveguide electro-optical modulators. In the case of push–pull molecules, two resonance electronic structures help to explain their molecular behavior, a neutral form and a zwitterionic form [10]. A two-state model is often proposed, where the neutral form is predominant in the ground state and the zwitterion, instead, in the lowest excited state [11]. As a consequence of the charge-transfer occurring in the excited state, the molecule undergoes a large change in dipole moment and a strong solvatochromism is observed, as for 4-nitroaniline [12–14], 4-dimethylamino-4′-nitrostilbene (DANS) [15], and 4nitroanisole [16]. Among these, DANS, whose resonance forms are shown in Fig. 1, presents peculiar fluorescence and photophysical behaviors, very sensitive to the polarity of the solvent [17–21]. These interesting properties are exploited in organic light-emitting diodes (OLEDs) for screen displays and solid-state lighting, where the organic molecule is used as a photoemitter and whose color is tuned by the environment (usually a polymeric matrix) where it is embedded.
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (F. Muniz-Miranda). 1 Current address: Center for Molecular Modeling (CMM), Universiteit Gent (UGent), Technologiepark 903, 9052 Zwijnaarde, Belgium.
http://dx.doi.org/10.1016/j.saa.2017.08.072 1386-1425/© 2017 Elsevier B.V. All rights reserved.
Furthermore, because of its high two-photon activity [22–24], DANS molecules are good probes for bio-imaging and radiation therapy applications [25,26]. For these reasons, in recent years DANS was the object of numerous studies based on different computational approaches [27–31]. However, like other push-pull molecules, DANS can undergo photochemical processes, which can modify their opto-electronic properties [32–34] and limits their potential applications. The synthesis of photoactive materials with good thermal, chemical and photochemical stabilities [35,36] requires a proper understanding of the relationship between structure and optical response, along with knowledge of the possibile photoproducts that may be generated once the molecule is excited with UV–vis electromagnetic waves. Therefore, in the present study, we have used experimental techniques (UV–vis absorption, FTIR spectroscopy and Raman scattering) and density functional theory (DFT) calculations to investigate the spectroscopic properties of DANS in both solution and solid state, along with its behavior under irradiation with different laser lines. 2. Experimental Section 2.1. Raman Spectra Raman spectra of DANS as powder sample and in CH2Cl2 solution (10− 3 M concentration) were recorded by excitation with the 457.9 nm, 488.0 nm and 514.5 nm lines of an Ar+ laser, by using a Jobin-Yvon HG2S monochromator equipped with a cooled RCAC31034A photomultiplier and a data acquisition facility. Power density measurements were performed with a power meter instrument (model 362; Scientech, Boulder, CO, USA) giving ~ 5% accuracy in the
34
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
excitation spectrum was obtained by considering twenty Sn ← S0 excitations. We computed the descriptors of charge-transfer with the spreadsheet provided in the Supplementary material of Ref. [45]. Molecular figures have been drawn with the Gaussview [46] and Jmol [47] softwares. 4. Results and Discussion 4.1. Structure and Electronic Spectra of DANS Fig. 1. Resonance structures of DANS (lower: benzenoid; upper: quinonoid).
300–1000 nm spectral range. Raman spectra of solid DANS were also recorded by using a micro-Raman spectrometer RM2000 Renishaw equipped with a diode laser emitting at 785 nm or with an Ar + laser emitting at 514.5 nm, a Leica optical microscope and a single grating monochromator. Sample irradiation was accomplished by using the 50 × microscope objective of a Leica Microscope DMLM. The beam power was ~ 3 mW, and the laser spot size was adjusted between 1 and 3 μm. The backscattered Raman signal was filtered by a double holographic Notch filter system and detected by an air-cooled CCD (2.5 cm−1per pixel). All spectra were calibrated with respect to a silicon wafer at 520 cm−1. In the low-temperature micro-Raman measurements we used the THMS 600 device (Linkam Scientific Instruments, U.K.), which allowed obtaining spectra of a microscopic sample under a cold gas flow coming from a liquid nitrogen dewar. By acting on the gaseous nitrogen flow it was possible to vary the sample temperature in the range from +20 °C to −180 °C. 2.2. Infrared Spectra
DANS is a push-pull molecule due to an electronic charge-transfer occurring from amino to nitro group through the π-conjugated system. Fig. 1 shows the resonance structures of DANS, one benzenoid and another quinonoid, where negative charges are localized on the oxygen atoms of the nitro group. The electronic spectrum (Fig. 2) shows essentially two bands: one in the UV region at about 305 nm, attributable to π → π* transition, and another one in the visible region with maximum at 433 nm, related to the excitation of the nitrogroup. The latter band, which moves depending on the solvent [15], is shifted to higher wavelengths in the solid state, with a maximum at 438 nm and a pronounced tail extending beyond 600 nm. Although not reported in the literature, the molecular structure of DANS can be considered planar on the basis of the DFT approach (but for H atoms of the methyl groups) that we have used here for the molecule in the trans form. The optimized structure, along with calculated energy and structural parameters, is shown in Fig. S1 (Supplementary material), whereas the ElectroStatic Potential (ESP) surface of DANS in the ground state is reported in Fig. S2 (Supplementary material). By observing the contour plots of the computed frontier orbitals (Fig. 3), we notice that the HOMO has a benzenoid character, while the LUMO has a quinonoid one. In particular, for the HOMO the central C_C bond has olefinic character, whereas for the LUMO a nodal plane
Infrared spectra of solid DANS in KBr pellet were obtained in the 4000–450 cm−1 region by a Perkin-Elmer FT-IR RX/I spectrometer. 2.3. UV–visible Absorption Spectra The absorption spectra of DANS dissolved in CH2Cl2 solution or deposited as thin solid film on quartz plate (Hellma, GmbH & Co KG, Germany) were observed in the 200–800 nm spectral region by means of a Cary 5 Varian spectrophotometer. The absorption band shape of DANS in solution was independent of concentration in these experiments. 3. Computational Details All the calculations were carried out using the GAUSSIAN 09 package [37]. Optimized geometries were obtained at the DFT level of theory with the Becke 3-parameter hybrid exchange functional combined with the Lee–Yang–Parr correlation functional (B3LYP) [38,39], along with the 6-311G(d,p) basis set. All the parameters were let free to relax and all the calculations converged toward optimized geometries corresponding to energy minima, as revealed by the lack of negative values in the subsequent frequency calculation. A scaling factor of 0.98 for the calculated harmonic wavenumbers was employed, as usually performed in calculations at this level of theory [40–43]. The DFT calculations of DANS were performed for the trans form with respect to the olefinic C_C bond, as well as for the cis form. The DFT calculations for the azo-derivative were performed by considering a trans arrangement with respect to the central N_N bond. Time-dependent density functional theory (TD-DFT) calculations [44] were performed for DANS with the same functional and basis set on the optimized geometry in order to obtain a correct assignment of the observed bands in the UV–visible absorption spectra. The simulated
Fig. 2. UV–vis absorption spectra of DANS in CH2Cl2 solution (A) and in the solid phase (B). The spectral positions of the visible laser lines used for the Raman excitation are shown. The simulated spectrum obtained by TD-DFT calculations is also reported in green for sake of comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
35
This effect can be best pictured in Fig. 4, where the difference between the S0 and S1 electron densities is showed. As can be seen, electronic charge is pushed toward the nitrogroup from the whole DANS molecule (even the\\N(CH3)2 group undergoes an electronic depletion). The transition dipole moment, represented as a superimposed red arrow, also yields a straightforward representation of this transfer of negative charge. For sake of completeness, details of the descriptors of charge-transfer are reported in Table S2 of the Supplementary material, whereas the energies of the calculated TD-DFT transitions, along with their oscillator strengths and attributions, are reported in Table S3 of the Supplementary material. 4.2. Infrared and Raman (785-nm Excitation) Spectra of Solid DANS
Fig. 3. Contour plots of the frontier orbitals of DANS: HOMO (lower), LUMO (upper). The positions of the localized double-bonds are shown in white.
in the middle of the central C_C bond occurs, along with an accumulation of electronic charge on the oxygen atoms of the nitro group, compared to the electronic structure of the HOMO orbital. In order to obtain information on the nature of the low-energy electronic transition occurring around 435 nm, as shown in Fig. 2, we have performed time-dependent density functional theory (TDDFT) calculations. Fig. 2 shows the simulated extinction spectrum obtained by considering twenty excited states, which satisfactory matches the experimental ones. In particular, the lowest-energy band can be totally attributed to the HOMO-LUMO transition, as well as obtained previously with another functional and more limited basis set [31], confirming that it corresponds to an excitation of the molecule with internal electronic charge-transfer toward the nitro group, from a benzenoid electronic structure to a quinonoid resonant form, as proposed in Fig. 1. This can also be assessed computing Mulliken [48], Hirshfeld [49] and Merz-Kollman [50] partial charges (reported in the Table S1 of the Supplementary material), all showing an increase of negative charge on the nitro group going from the ground (S0) to the first singlet electronic excited state (S1). This charge-transfer can be better quantified adopting suitable charge-transfer descriptors, such as those proposed in Ref. [51]. In the present paper, we exploit the Merz-Kollman (ESP) charges, as suggested in Ref. [45], to quantify the transferred charge (qCT) and the average distance between the barycenters of the increase and depletion of charge (dCT), finding values of 0.72 e and 4.44 Å, respectively, with e being the unsigned electron charge. Also, with this method the computed transition dipole moment is | μCT | = dCTqCT = 15.4 Debye, in almost perfect agreement with the computed difference between the dipole moments of S0 (10.7 Debye) and S1 (26.1 Debye) states as extracted by direct Gaussian 09 calculations.
The vibrational spectra of solid DANS are shown in Fig. 5: Raman in powder sample (upper panel), IR in KBr pellet (bottom panel). The excitation laser line at 785 nm is used to obtain the Raman spectrum of DANS without resonance effects. This spectrum is quite similar to that previously obtained [52] by laser excitation at 1064 nm. The symmetrical stretching mode of the nitro group observed at about 1330 cm− 1 is among the most intense bands in both infrared and Raman spectrum, together with that at about 1580 cm−1, due to a deformation mode of the aromatic rings. To correctly assign the observed bands and validate our computational protocol, we have carried out DFT calculations that allow satisfactorily attributing both IR and Raman bands (Table S4 of the Supplementary material) and predicting their intensities (Fig. S3 of the Supplementary material). Our assignment is in agreement with that already proposed by Vijayakumar et al. for DANS [52]: the nitro group bands are located at about 1526 cm−1 (asymmetric stretching mode), observed in the IR spectrum, 1330 cm−1 (symmetric stretching mode), observed in both spectra, and 852 cm−1 (bending mode), observed in the IR spectrum. The very strong Raman band around 1580 cm−1, which has a strong counterpart in the IR spectrum, corresponds to the in-phase quadrant deformation mode of the aromatic rings. The medium intensity band observed in both spectra at about 1180 cm−1 can be attributed to in-plane bending mode of the hydrogen atoms linked to the aromatic rings. The stretching mode of the olefinic C_C bond is matched with the highest-wavenumber Raman band around 1630 cm−1, which is, instead, almost undetectable in the infrared spectrum. 4.3. Raman Spectra of DANS by Excitation with 457.9-nm, 488.0-nm, 514.5nm Laser Lines We also examined the resonance Raman spectra of DANS in solution and in solid phase (see Fig. 6), by using laser lines (457.8, 488.0, 514.5 nm) near to the maximum of the visible excitation band shown in Fig. 2. The solution Raman spectra correspond to that recorded in
Fig. 4. Contour plots of the depletion (light blue) and increase (orange) of electron density occurring in the S1 ← S0 electronic transition. The red arrow represents the transition dipole moment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
36
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
Fig. 5. Infrared and Raman (excitation laser line: 785 nm) spectra of solid DANS (baseline correction).
the solid sample by laser excitation at 785 nm, as shown in Fig. 5. In particular, the band at 1336 cm− 1, corresponding to the symmetrical stretching mode of the nitrogroup, is the most intense one. In the solid phase, however, “spurious” bands are observed with all three exciting lines, at about 1138, 1404 and 1443 cm−1, as shown in Fig. 6. The deformation band of the aromatic rings at about 1580 cm− 1 is shifted to 1590 cm−1, increasing in intensity. The appearance of these bands
Fig. 6. Raman spectra of DANS in CH2Cl2 solution and in solid phase (excitation: 457.9 nm, 488.0 nm, 514.5 nm; baseline correction). Laser powers: 100 mW (514.5 nm); 100 mW (488.0 nm); 70 mW (457.9 nm).
Fig. 7. Raman spectra of solid DANS obtained with different laser powers, rotating device and defocalized laser beam. Excitation: 514.5 nm; laser power: 50 mW.
cannot be due to a mere effect of resonance Raman; in fact, as shown in Fig. 7, under defocused laser irradiation at 514.5 nm, these bands grow in intensity, becoming dominant by increasing the laser power. This demonstrates that a photo-induced reaction occurs in the solid phase, unlike what takes place in solution. This reaction is presumed involving the nitrogroup, as the symmetric stretching mode of the nitrogroup (at about 1330 cm−1) does not yield anymore the most intense band of the spectrum. Moreover, the spectral persistence of the ring bands (around 1180 and 1590 cm−1) and of that due to the olefinic bond stretching (around 1630 cm−1) indicates that these groups are not involved in the photoreaction. In fact, it is known [53] that the nitro group can undergo different processes of reduction, with the formation of products such as amino-, azo- and azoxy-derivatives, as shown in the Scheme 1. In particular, the formation of an azo-derivative, by dimerization and acquisition of four electrons, originates bands that well correspond to those observed here. The Raman spectrum of azobenzene [53], in fact, shows a very intense band around 1140 cm− 1 and two intense bands between 1400 and 1500 cm− 1, which do not appear in the spectrum of nitrobenzene. Both aniline and azoxybenzene, instead, do not have intense Raman bands that may correspond to
Scheme 1. Reduction processes of nitroarenes (Ar-NO2).
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
37
Fig. 8. Simulated Raman spectra of DANS in the cis form (left panel) and of the proposed azo-derivative (right panel).
those observed by laser irradiation of DANS in the solid phase. The photoreaction of DANS occurs when the visible radiation is absorbed by the molecules of the solid sample. In solution, instead, the energy associated with the absorbed radiation is dissipated through the surrounding solvent molecules. The photoreduction of the nitro group occurs in the excited state of the molecule by using laser lines that match the visible excitation band of DANS; actually, the LUMO orbital (see Fig. 3) shows a large accumulation of electronic charge on the nitro group, in comparison with the electronic distribution of the HOMO orbital. This charge-transfer in the excited S1 state can play a determinant role in promoting the reduction of the nitro group, as observed in the Raman spectrum. However, we have also considered a possible alternative reaction, which consists in the transformation of the molecule from the trans planar configuration to a cis configuration, as observed previously by using femtosecond transient absorption spectroscopy [54]. In fact, this is possible under laser irradiation in resonance with the electronic absorption: in the excited state the molecule is no longer rigid, as a nodal plane occurs where a C_C double bond is present in the ground state, as shown in Fig. 3.
4.4. DFT Calculations of the Possible Photoproducts We have carried out the same type of DFT calculations done for DANS in the trans planar form, by considering a cis type structure, whose calculated energy and structural parameters are shown in Fig. S1 of the Supplementary material, compared with the corresponding values of DANS in the trans form. However, its Raman spectrum does not show at all the “spurious” bands observed in the solid sample under irradiation, as shown in Fig. 8 (left panel).
The simulated Raman spectrum appears in fact similar to that of the trans isomer, even if the structure of the molecule is no longer planar. Then, in order to find a confirmation to the formation of an azo-derivative by reduction of the nitro group, we have also performed similar DFT calculations for this hypothetical reaction product, whose optimized structure is shown in Fig. 9. The simulated Raman spectrum of the azo-derivative (Fig. 8, right panel) shows very intense bands, which appear quite similar, in both positions and relative intensities, to the “spurious” ones observed in the solid phase of DANS: the intense bands calculated at 1124, 1410, 1436–1447 and 1590 cm− 1 correspond to those observed at 1138, 1402, 1444 and 1590 cm − 1 , respectively. These bands can be assigned to vibrations involving the N_N central bond, which is formed by condensation of two DANS molecules. In particular, the Raman band at 1138 cm− 1 corresponds to the stretching mode of the C\\N bonds (N being the nitrogen atom engaged in the N_N double bond), along with in-plane deformations of the vicinal aromatic rings, while the bands observed at 1402, 1444 and 1590 to stretching modes of the N_N bond, also in combination with deformations of the aromatic rings. The corresponding normal modes in terms of atomic Cartesian displacements are shown in Fig. S4 of the Supplementary material. The simulated IR spectrum is reported in Fig. S5 of the Supplementary material: the intense IR bands do not match those Raman, because the molecule is almost centrosymmetric. 4.5. Low-temperature Raman (Excitation: 514.5 nm) Spectra Finally, in order to understand the mechanism of the photoreaction, we have performed micro-Raman experiments on a solid sample of DANS at different temperatures, by excitation with the 514.5 nm laser
Fig. 9. DFT-optimized structure of the azo-derivative. Distances in Angstrom.
38
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
Acknowledgements FM-M's two post-doctoral terms at UniMORE (overall, from April 2013 to April 2017) were supported by the Italian “Ministero dell'Istruzione, dell'Università e della Ricerca” (MIUR) through the “Futuro in Ricerca” (FIRB) Grant RBFR1248UI_002 entitled “Novel Multiscale Theoretical/Computational Strategies for the Design of Photo and Thermo responsive Hybrid Organic-Inorganic Components for Nanoelectronic Circuits”. The authors are grateful to Dr. Barbara Pergolese for some preliminary calculations. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.08.072.
References
Fig. 10. Micro-Raman spectra of solid DANS at different temperatures (room temperature: ~20 °C, low temperature: ~−180 °C). Excitation: 514.5 nm.
line, as reported in Fig. 10. At room temperature, the photoreaction occurs, showing a Raman spectrum identical to that observed with the macro-Raman apparatus and the same exciting line. When, instead, the sample temperature is lowered to about − 180 °C under cold gas flow coming from a liquid nitrogen dewar, as described in the Experimental section, the “spurious” bands are completely absent and the Raman spectrum is identical to that observed by excitation with the 785-nm laser line (Fig. 5). 5. Concluding Remarks The present investigation, based on a combined spectroscopic and density functional theory approach, allows collecting information on the photoreaction of DANS, a π-conjugated push-pull molecule, occurring in the solid phase, which was not previously observed. Intense Raman bands not corresponding to DANS are detected, suggesting that they are due to the formation of a photoproduct as azo-derivative under visible laser irradiation. DFT calculations allow identifying the reaction product by comparison between the simulated Raman spectrum of the proposed azo-derivative and that observed by irradiation with laser lines in the blue-green light region, where DANS electronically absorbs. In solid phase, this absorption ensures a thermal activation for the photoreaction, as evidenced by micro-Raman experiments at different temperatures. In solution, instead, the photoreaction does not occur, because the energy associated with the absorbed radiation is dissipated through the surrounding solvent molecules. The photoreduction of the nitro group occurs in the excited state of the molecule, where a large accumulation of electronic charge on the nitro group is pinpointed by the electronic distribution of the LUMO orbital. Hence, this molecular charge-transfer toward the NO2 group in the excited state, typical of a push-pull molecule such as DANS, plays a determinant role in promoting the reduction of the nitro group, as observed in the Raman spectra and verified by computational approach. Moreover, as demonstrated by the low-temperature experiments, the reaction is thermally activated due to the absorption of visible radiation in the blue-green spectral region. It is reasonable to think that this reaction involves two DANS molecules by eliminating two oxygen molecules: 2 Ar-NO2 → Ar-N = N-Ar + 2 O2. This study paves the way to the investigation of the photochemical processes occurring in push-pull molecules, which have great importance in determining their physical and optical properties and consequently in the NLO applications, also providing a suitable approach to unravel their mechanism.
[1] H.S. Nalwa, S. Miyata, Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, 1997. [2] J. Zyss, Molecular Nonlinear Optics: Materials, Physics, and Devices, Academic Press, Boston, 1993. [3] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [4] S.-S. Sun, L.R. Dalton, Introduction to Organic Electronic and Optoelectronic Materials and Devices, II ed. CRC Press, Boca Raton, 2016. [5] L.R. Dalton, A.W. Harper, B. Wu, R. Ghosn, J. Laquindanum, Z. Liang, A. Hubbel, C. Xu, Adv. Mater. 7 (1995) 519–540. [6] R.A. Rijkenberg, D. Bebelaar, W.J. Buma, J.W.J. Hofstraat, J. Phys. Chem. A 106 (2002) 2446–2456. [7] M. Cha, W.E. Torruellas, G.I. Stegeman, W.H.G. Horsthius, G. Möhlmann, R.J. Meth, Appl. Phys. Lett. 65 (1994) 2648–2650. [8] D. Beljonne, J.L. Bredas, M. Cha, W.E. Torruellas, G.I. Stegeman, J.W. Hofstraat, W.H.G. Horsthius, G.R. Möhlmann, J. Chem. Phys. 103 (1995) 7834–7843. [9] M. Tommasini, C. Castiglioni, M. Del Zoppo, G. Zerbi, J. Mol. Struct. 480–481 (1999) 179–188. [10] A.M. Moran, G.P. Bartholomew, G.C. Bazan, A. Myers Kelley, J. Phys. Chem. A 106 (2002) 4928–4937. [11] F. Bureš, RSC Adv. 4 (2014) 58826–58851. [12] C.-K. Wang, Y.-H. Wang, Y. Su, Y. Luo, J. Chem. Phys. 119 (2003) 4409–4412. [13] A.M. Moran, A.M. Kelley, J. Chem. Phys. 115 (2001) 912–924. [14] E.D. Schmid, M. Moschallski, W.L. Peticolas, J. Phys. Chem. 90 (1986) 2340–2346. [15] L.C.T. Shoute, H.Y. Woo, D. Vak, G.C. Bazan, A.M. Kelley, J. Chem. Phys. 125 (2006) 054506/1–054506/10. [16] M. Muniz-Miranda, J. Raman Spectrosc. 44 (2013) 1416–1421. [17] W. Rettig, W. Majnez, R. Lapouyade, F. Haucke, J. Photochem. Photobiol. A Chem. 62 (1992) 415–427. [18] R. Lapouyade, A. Kuhn, J.F. Letard, W. Rettig, Chem. Phys. Lett. 208 (1993) 48–58. [19] H. Gruen, H. Görner, J. Phys. Chem. 93 (1989) 7144–7152. [20] J. Oberlé, E. Abraham, G. Jonususkas, C. Rulliere, J. Raman Spectrosc. 31 (2000) 311–317. [21] H. Görner, J. Photochem. Photobiol. A Chem. 40 (1987) 325–339. [22] Md.M. Alam, M. Chattopadhyaya, S. Chakrabarti, J. Phys. Chem. A 116 (2012) 11034–11040. [23] L. Antonov, K. Kamada, K. Ohta, F.S. Kamounah, Phys. Chem. Chem. Phys. 5 (2003) 1193–1197. [24] P.N. Day, K.A. Nguyen, R. Pachter, J. Phys. Chem. B 109 (2005) 1803–1814. [25] B.B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X.-L. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51–54. [26] J. Arnbjerg, A. Jiménez-Banzo, M.J. Paterson, S. Nonell, J.I. Borrell, O. Christiansen, P.R. Ogilby, J. Am. Chem. Soc. 129 (2007) 5188–5199. [27] B. Moore, H. Sun, N. Govind, K. Kowalski, J. Autschbach, J. Chem. Theory Comput. 11 (2015) 3305–3320. [28] A.J. Garza, O.I. Osman, A.M. Asiri, G.E. Scuseria, J. Phys. Chem. B 119 (2015) 1202–1212. [29] C. Singh, R. Ghosh, J.A. Mondal, D.K. Palit, J. Photochem. Photobiol. A Chem. 263 (2013) 50–60. [30] N.A. Murugan, J. Kongsted, Z. Rinkevicius, K. Aidas, K.V. Mikkelsen, H. Ågren, Phys. Chem. Chem. Phys. 13 (2011) 12506–12516. [31] I.D. Petsalakis, D.G. Georgiadou, M. Vasilopoulou, G. Pistolis, D. Dimotikali, P. Argitis, G. Theodorakopoulos, J. Phys. Chem. A 114 (2010) 5580–5587. [32] S.J. Lord, H.-l.D. Lee, R. Samuel, R. Weber, N. Liu, N.R. Conley, M.A. Thompson, R.J. Twieg, W.E. Moerner, J. Phys. Chem. B 114 (2010) 14157–14167. [33] X.F. Xu, A. Kahan, S. Zilberg, Y. Haas, J. Phys. Chem. A 113 (2009) 9779–97791. [34] Q. Zhang, M. Canva, G. Stegeman, Appl. Phys. Lett. 73 (1998) 912–914. [35] C. Zhang, L.R. Dalton, M.-C. Oh, H. Zhang, W.H. Steier, Chem. Mater. 13 (2001) 3043–3050. [36] M. Cigán, A. Gáplovský, I. Sigmundová, P. Zahradník, R. Dĕdic, M. Hromadová, J. Phys. Org. Chem. 24 (2011) 450–459.
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39 [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT (USA, 2009. [38] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [39] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [40] M. Muniz-Miranda, B. Pergolese, A. Bigotto, Vib. Spectrosc. 43 (2007) 97–103. [41] M. Muniz-Miranda, M. Pagliai, F. Muniz-Miranda, V. Schettino, Chem. Commun. 47 (2011) 3138–3140. [42] M. Pagliai, F. Muniz-Miranda, V. Schettino, M. Muniz-Miranda, Progr. Colloid Polym. Sci. 139 (2012) 39–44. [43] M. Muniz-Miranda, F. Muniz-Miranda, S. Caporali, Beilstein J. Nanotechnol. 5 (2014) 2489–2497. [44] E. Runge, E.K.U. Gross, Phys. Rev. Lett. 52 (1984) 997–1000. [45] D. Jacquemin, T. Le Bahers, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 14 (2012) 5383–5388.
39
[46] R. Dennington, T. Keith, J. Millam, GaussView, Version 5, Semichem Inc., Shawnee Mission, KS, 2009. [47] Jmol, version 12.2.2+dfsg-1, http://www.jmol.org. [48] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833–1840. [49] F.L. Hirshfeld, Theor. Chim. Acta 44 (1977) 129–138. [50] U.C. Singh, P.A. Kollman, J. Comput. Chem. 5 (1984) 129–145. [51] G. Garcìa, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 15 (2013) 20210–20219. [52] T. Vijayakumar, I. Hubert Joe, C.P. Reghunadhan Nair, V.S. Jayakumar, Chem. Phys. 343 (2008) 83–99. [53] P. Gao, D. Gosztola, M.J. Weaver, J. Phys. Chem. 92 (1988) 7122–7130. [54] C. Singh, R. Ghosh, J.A. Mondal, D.K. Palit, J. Photochem. Photobiol. A Chem. 263 (2013) 50–60.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic and DFT investigation on the photo-chemical properties of a push-pull chromophore: 4-Dimethylamino-4′-nitrostilbene Francesco Muniz-Miranda a,⁎,1, Alfonso Pedone a, Maurizio Muniz-Miranda b a b
Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia (UniMORE), Via Campi 103, 41125 Modena, Italy Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze (UniFI), Via Lastruccia 3, 50019 Sesto Fiorentino, Italy
a r t i c l e
i n f o
Article history: Received 18 April 2017 Received in revised form 15 August 2017 Accepted 31 August 2017 Available online 05 September 2017 Keywords: Push-pull molecule Raman spectra DFT Photoreaction
a b s t r a c t 4-Dimethylamino-4′-nitrostilbene (DANS), a π-conjugated push-pull molecule, has been investigated by means of a combined spectroscopic and computational approach. When the Raman excitation is close to the visible electronic transition of DANS, vibrational bands not belonging to DANS appear in the spectra, increasing with the laser power. These bands are observed at room temperature in the solid phase, but not at low temperature or in solution, and we interpret them as due to a thermally-activated photoreaction occurring under laser irradiation in the visible spectral region. Density-functional calculations correctly reproducing the electronic and vibrational spectra of DANS, describe the charge-transfer process, indicate that an azo-derivative is the product of the photoreaction of DANS and provide a reasonable interpretation of this process. © 2017 Elsevier B.V. All rights reserved.
1. Introduction “Push–pull” chromophores are characterized by an electron-donating group and an electron-accepting group linked by a conjugated π system [1–9]. They generally exhibit large first and second hyperpolarizabilities and can be usefully employed in non-linear optical (NLO) applications, for example as efficient second-harmonic generators and fast waveguide electro-optical modulators. In the case of push–pull molecules, two resonance electronic structures help to explain their molecular behavior, a neutral form and a zwitterionic form [10]. A two-state model is often proposed, where the neutral form is predominant in the ground state and the zwitterion, instead, in the lowest excited state [11]. As a consequence of the charge-transfer occurring in the excited state, the molecule undergoes a large change in dipole moment and a strong solvatochromism is observed, as for 4-nitroaniline [12–14], 4-dimethylamino-4′-nitrostilbene (DANS) [15], and 4nitroanisole [16]. Among these, DANS, whose resonance forms are shown in Fig. 1, presents peculiar fluorescence and photophysical behaviors, very sensitive to the polarity of the solvent [17–21]. These interesting properties are exploited in organic light-emitting diodes (OLEDs) for screen displays and solid-state lighting, where the organic molecule is used as a photoemitter and whose color is tuned by the environment (usually a polymeric matrix) where it is embedded.
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (F. Muniz-Miranda). 1 Current address: Center for Molecular Modeling (CMM), Universiteit Gent (UGent), Technologiepark 903, 9052 Zwijnaarde, Belgium.
http://dx.doi.org/10.1016/j.saa.2017.08.072 1386-1425/© 2017 Elsevier B.V. All rights reserved.
Furthermore, because of its high two-photon activity [22–24], DANS molecules are good probes for bio-imaging and radiation therapy applications [25,26]. For these reasons, in recent years DANS was the object of numerous studies based on different computational approaches [27–31]. However, like other push-pull molecules, DANS can undergo photochemical processes, which can modify their opto-electronic properties [32–34] and limits their potential applications. The synthesis of photoactive materials with good thermal, chemical and photochemical stabilities [35,36] requires a proper understanding of the relationship between structure and optical response, along with knowledge of the possibile photoproducts that may be generated once the molecule is excited with UV–vis electromagnetic waves. Therefore, in the present study, we have used experimental techniques (UV–vis absorption, FTIR spectroscopy and Raman scattering) and density functional theory (DFT) calculations to investigate the spectroscopic properties of DANS in both solution and solid state, along with its behavior under irradiation with different laser lines. 2. Experimental Section 2.1. Raman Spectra Raman spectra of DANS as powder sample and in CH2Cl2 solution (10− 3 M concentration) were recorded by excitation with the 457.9 nm, 488.0 nm and 514.5 nm lines of an Ar+ laser, by using a Jobin-Yvon HG2S monochromator equipped with a cooled RCAC31034A photomultiplier and a data acquisition facility. Power density measurements were performed with a power meter instrument (model 362; Scientech, Boulder, CO, USA) giving ~ 5% accuracy in the
34
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
excitation spectrum was obtained by considering twenty Sn ← S0 excitations. We computed the descriptors of charge-transfer with the spreadsheet provided in the Supplementary material of Ref. [45]. Molecular figures have been drawn with the Gaussview [46] and Jmol [47] softwares. 4. Results and Discussion 4.1. Structure and Electronic Spectra of DANS Fig. 1. Resonance structures of DANS (lower: benzenoid; upper: quinonoid).
300–1000 nm spectral range. Raman spectra of solid DANS were also recorded by using a micro-Raman spectrometer RM2000 Renishaw equipped with a diode laser emitting at 785 nm or with an Ar + laser emitting at 514.5 nm, a Leica optical microscope and a single grating monochromator. Sample irradiation was accomplished by using the 50 × microscope objective of a Leica Microscope DMLM. The beam power was ~ 3 mW, and the laser spot size was adjusted between 1 and 3 μm. The backscattered Raman signal was filtered by a double holographic Notch filter system and detected by an air-cooled CCD (2.5 cm−1per pixel). All spectra were calibrated with respect to a silicon wafer at 520 cm−1. In the low-temperature micro-Raman measurements we used the THMS 600 device (Linkam Scientific Instruments, U.K.), which allowed obtaining spectra of a microscopic sample under a cold gas flow coming from a liquid nitrogen dewar. By acting on the gaseous nitrogen flow it was possible to vary the sample temperature in the range from +20 °C to −180 °C. 2.2. Infrared Spectra
DANS is a push-pull molecule due to an electronic charge-transfer occurring from amino to nitro group through the π-conjugated system. Fig. 1 shows the resonance structures of DANS, one benzenoid and another quinonoid, where negative charges are localized on the oxygen atoms of the nitro group. The electronic spectrum (Fig. 2) shows essentially two bands: one in the UV region at about 305 nm, attributable to π → π* transition, and another one in the visible region with maximum at 433 nm, related to the excitation of the nitrogroup. The latter band, which moves depending on the solvent [15], is shifted to higher wavelengths in the solid state, with a maximum at 438 nm and a pronounced tail extending beyond 600 nm. Although not reported in the literature, the molecular structure of DANS can be considered planar on the basis of the DFT approach (but for H atoms of the methyl groups) that we have used here for the molecule in the trans form. The optimized structure, along with calculated energy and structural parameters, is shown in Fig. S1 (Supplementary material), whereas the ElectroStatic Potential (ESP) surface of DANS in the ground state is reported in Fig. S2 (Supplementary material). By observing the contour plots of the computed frontier orbitals (Fig. 3), we notice that the HOMO has a benzenoid character, while the LUMO has a quinonoid one. In particular, for the HOMO the central C_C bond has olefinic character, whereas for the LUMO a nodal plane
Infrared spectra of solid DANS in KBr pellet were obtained in the 4000–450 cm−1 region by a Perkin-Elmer FT-IR RX/I spectrometer. 2.3. UV–visible Absorption Spectra The absorption spectra of DANS dissolved in CH2Cl2 solution or deposited as thin solid film on quartz plate (Hellma, GmbH & Co KG, Germany) were observed in the 200–800 nm spectral region by means of a Cary 5 Varian spectrophotometer. The absorption band shape of DANS in solution was independent of concentration in these experiments. 3. Computational Details All the calculations were carried out using the GAUSSIAN 09 package [37]. Optimized geometries were obtained at the DFT level of theory with the Becke 3-parameter hybrid exchange functional combined with the Lee–Yang–Parr correlation functional (B3LYP) [38,39], along with the 6-311G(d,p) basis set. All the parameters were let free to relax and all the calculations converged toward optimized geometries corresponding to energy minima, as revealed by the lack of negative values in the subsequent frequency calculation. A scaling factor of 0.98 for the calculated harmonic wavenumbers was employed, as usually performed in calculations at this level of theory [40–43]. The DFT calculations of DANS were performed for the trans form with respect to the olefinic C_C bond, as well as for the cis form. The DFT calculations for the azo-derivative were performed by considering a trans arrangement with respect to the central N_N bond. Time-dependent density functional theory (TD-DFT) calculations [44] were performed for DANS with the same functional and basis set on the optimized geometry in order to obtain a correct assignment of the observed bands in the UV–visible absorption spectra. The simulated
Fig. 2. UV–vis absorption spectra of DANS in CH2Cl2 solution (A) and in the solid phase (B). The spectral positions of the visible laser lines used for the Raman excitation are shown. The simulated spectrum obtained by TD-DFT calculations is also reported in green for sake of comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
35
This effect can be best pictured in Fig. 4, where the difference between the S0 and S1 electron densities is showed. As can be seen, electronic charge is pushed toward the nitrogroup from the whole DANS molecule (even the\\N(CH3)2 group undergoes an electronic depletion). The transition dipole moment, represented as a superimposed red arrow, also yields a straightforward representation of this transfer of negative charge. For sake of completeness, details of the descriptors of charge-transfer are reported in Table S2 of the Supplementary material, whereas the energies of the calculated TD-DFT transitions, along with their oscillator strengths and attributions, are reported in Table S3 of the Supplementary material. 4.2. Infrared and Raman (785-nm Excitation) Spectra of Solid DANS
Fig. 3. Contour plots of the frontier orbitals of DANS: HOMO (lower), LUMO (upper). The positions of the localized double-bonds are shown in white.
in the middle of the central C_C bond occurs, along with an accumulation of electronic charge on the oxygen atoms of the nitro group, compared to the electronic structure of the HOMO orbital. In order to obtain information on the nature of the low-energy electronic transition occurring around 435 nm, as shown in Fig. 2, we have performed time-dependent density functional theory (TDDFT) calculations. Fig. 2 shows the simulated extinction spectrum obtained by considering twenty excited states, which satisfactory matches the experimental ones. In particular, the lowest-energy band can be totally attributed to the HOMO-LUMO transition, as well as obtained previously with another functional and more limited basis set [31], confirming that it corresponds to an excitation of the molecule with internal electronic charge-transfer toward the nitro group, from a benzenoid electronic structure to a quinonoid resonant form, as proposed in Fig. 1. This can also be assessed computing Mulliken [48], Hirshfeld [49] and Merz-Kollman [50] partial charges (reported in the Table S1 of the Supplementary material), all showing an increase of negative charge on the nitro group going from the ground (S0) to the first singlet electronic excited state (S1). This charge-transfer can be better quantified adopting suitable charge-transfer descriptors, such as those proposed in Ref. [51]. In the present paper, we exploit the Merz-Kollman (ESP) charges, as suggested in Ref. [45], to quantify the transferred charge (qCT) and the average distance between the barycenters of the increase and depletion of charge (dCT), finding values of 0.72 e and 4.44 Å, respectively, with e being the unsigned electron charge. Also, with this method the computed transition dipole moment is | μCT | = dCTqCT = 15.4 Debye, in almost perfect agreement with the computed difference between the dipole moments of S0 (10.7 Debye) and S1 (26.1 Debye) states as extracted by direct Gaussian 09 calculations.
The vibrational spectra of solid DANS are shown in Fig. 5: Raman in powder sample (upper panel), IR in KBr pellet (bottom panel). The excitation laser line at 785 nm is used to obtain the Raman spectrum of DANS without resonance effects. This spectrum is quite similar to that previously obtained [52] by laser excitation at 1064 nm. The symmetrical stretching mode of the nitro group observed at about 1330 cm− 1 is among the most intense bands in both infrared and Raman spectrum, together with that at about 1580 cm−1, due to a deformation mode of the aromatic rings. To correctly assign the observed bands and validate our computational protocol, we have carried out DFT calculations that allow satisfactorily attributing both IR and Raman bands (Table S4 of the Supplementary material) and predicting their intensities (Fig. S3 of the Supplementary material). Our assignment is in agreement with that already proposed by Vijayakumar et al. for DANS [52]: the nitro group bands are located at about 1526 cm−1 (asymmetric stretching mode), observed in the IR spectrum, 1330 cm−1 (symmetric stretching mode), observed in both spectra, and 852 cm−1 (bending mode), observed in the IR spectrum. The very strong Raman band around 1580 cm−1, which has a strong counterpart in the IR spectrum, corresponds to the in-phase quadrant deformation mode of the aromatic rings. The medium intensity band observed in both spectra at about 1180 cm−1 can be attributed to in-plane bending mode of the hydrogen atoms linked to the aromatic rings. The stretching mode of the olefinic C_C bond is matched with the highest-wavenumber Raman band around 1630 cm−1, which is, instead, almost undetectable in the infrared spectrum. 4.3. Raman Spectra of DANS by Excitation with 457.9-nm, 488.0-nm, 514.5nm Laser Lines We also examined the resonance Raman spectra of DANS in solution and in solid phase (see Fig. 6), by using laser lines (457.8, 488.0, 514.5 nm) near to the maximum of the visible excitation band shown in Fig. 2. The solution Raman spectra correspond to that recorded in
Fig. 4. Contour plots of the depletion (light blue) and increase (orange) of electron density occurring in the S1 ← S0 electronic transition. The red arrow represents the transition dipole moment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
36
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
Fig. 5. Infrared and Raman (excitation laser line: 785 nm) spectra of solid DANS (baseline correction).
the solid sample by laser excitation at 785 nm, as shown in Fig. 5. In particular, the band at 1336 cm− 1, corresponding to the symmetrical stretching mode of the nitrogroup, is the most intense one. In the solid phase, however, “spurious” bands are observed with all three exciting lines, at about 1138, 1404 and 1443 cm−1, as shown in Fig. 6. The deformation band of the aromatic rings at about 1580 cm− 1 is shifted to 1590 cm−1, increasing in intensity. The appearance of these bands
Fig. 6. Raman spectra of DANS in CH2Cl2 solution and in solid phase (excitation: 457.9 nm, 488.0 nm, 514.5 nm; baseline correction). Laser powers: 100 mW (514.5 nm); 100 mW (488.0 nm); 70 mW (457.9 nm).
Fig. 7. Raman spectra of solid DANS obtained with different laser powers, rotating device and defocalized laser beam. Excitation: 514.5 nm; laser power: 50 mW.
cannot be due to a mere effect of resonance Raman; in fact, as shown in Fig. 7, under defocused laser irradiation at 514.5 nm, these bands grow in intensity, becoming dominant by increasing the laser power. This demonstrates that a photo-induced reaction occurs in the solid phase, unlike what takes place in solution. This reaction is presumed involving the nitrogroup, as the symmetric stretching mode of the nitrogroup (at about 1330 cm−1) does not yield anymore the most intense band of the spectrum. Moreover, the spectral persistence of the ring bands (around 1180 and 1590 cm−1) and of that due to the olefinic bond stretching (around 1630 cm−1) indicates that these groups are not involved in the photoreaction. In fact, it is known [53] that the nitro group can undergo different processes of reduction, with the formation of products such as amino-, azo- and azoxy-derivatives, as shown in the Scheme 1. In particular, the formation of an azo-derivative, by dimerization and acquisition of four electrons, originates bands that well correspond to those observed here. The Raman spectrum of azobenzene [53], in fact, shows a very intense band around 1140 cm− 1 and two intense bands between 1400 and 1500 cm− 1, which do not appear in the spectrum of nitrobenzene. Both aniline and azoxybenzene, instead, do not have intense Raman bands that may correspond to
Scheme 1. Reduction processes of nitroarenes (Ar-NO2).
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
37
Fig. 8. Simulated Raman spectra of DANS in the cis form (left panel) and of the proposed azo-derivative (right panel).
those observed by laser irradiation of DANS in the solid phase. The photoreaction of DANS occurs when the visible radiation is absorbed by the molecules of the solid sample. In solution, instead, the energy associated with the absorbed radiation is dissipated through the surrounding solvent molecules. The photoreduction of the nitro group occurs in the excited state of the molecule by using laser lines that match the visible excitation band of DANS; actually, the LUMO orbital (see Fig. 3) shows a large accumulation of electronic charge on the nitro group, in comparison with the electronic distribution of the HOMO orbital. This charge-transfer in the excited S1 state can play a determinant role in promoting the reduction of the nitro group, as observed in the Raman spectrum. However, we have also considered a possible alternative reaction, which consists in the transformation of the molecule from the trans planar configuration to a cis configuration, as observed previously by using femtosecond transient absorption spectroscopy [54]. In fact, this is possible under laser irradiation in resonance with the electronic absorption: in the excited state the molecule is no longer rigid, as a nodal plane occurs where a C_C double bond is present in the ground state, as shown in Fig. 3.
4.4. DFT Calculations of the Possible Photoproducts We have carried out the same type of DFT calculations done for DANS in the trans planar form, by considering a cis type structure, whose calculated energy and structural parameters are shown in Fig. S1 of the Supplementary material, compared with the corresponding values of DANS in the trans form. However, its Raman spectrum does not show at all the “spurious” bands observed in the solid sample under irradiation, as shown in Fig. 8 (left panel).
The simulated Raman spectrum appears in fact similar to that of the trans isomer, even if the structure of the molecule is no longer planar. Then, in order to find a confirmation to the formation of an azo-derivative by reduction of the nitro group, we have also performed similar DFT calculations for this hypothetical reaction product, whose optimized structure is shown in Fig. 9. The simulated Raman spectrum of the azo-derivative (Fig. 8, right panel) shows very intense bands, which appear quite similar, in both positions and relative intensities, to the “spurious” ones observed in the solid phase of DANS: the intense bands calculated at 1124, 1410, 1436–1447 and 1590 cm− 1 correspond to those observed at 1138, 1402, 1444 and 1590 cm − 1 , respectively. These bands can be assigned to vibrations involving the N_N central bond, which is formed by condensation of two DANS molecules. In particular, the Raman band at 1138 cm− 1 corresponds to the stretching mode of the C\\N bonds (N being the nitrogen atom engaged in the N_N double bond), along with in-plane deformations of the vicinal aromatic rings, while the bands observed at 1402, 1444 and 1590 to stretching modes of the N_N bond, also in combination with deformations of the aromatic rings. The corresponding normal modes in terms of atomic Cartesian displacements are shown in Fig. S4 of the Supplementary material. The simulated IR spectrum is reported in Fig. S5 of the Supplementary material: the intense IR bands do not match those Raman, because the molecule is almost centrosymmetric. 4.5. Low-temperature Raman (Excitation: 514.5 nm) Spectra Finally, in order to understand the mechanism of the photoreaction, we have performed micro-Raman experiments on a solid sample of DANS at different temperatures, by excitation with the 514.5 nm laser
Fig. 9. DFT-optimized structure of the azo-derivative. Distances in Angstrom.
38
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39
Acknowledgements FM-M's two post-doctoral terms at UniMORE (overall, from April 2013 to April 2017) were supported by the Italian “Ministero dell'Istruzione, dell'Università e della Ricerca” (MIUR) through the “Futuro in Ricerca” (FIRB) Grant RBFR1248UI_002 entitled “Novel Multiscale Theoretical/Computational Strategies for the Design of Photo and Thermo responsive Hybrid Organic-Inorganic Components for Nanoelectronic Circuits”. The authors are grateful to Dr. Barbara Pergolese for some preliminary calculations. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.08.072.
References
Fig. 10. Micro-Raman spectra of solid DANS at different temperatures (room temperature: ~20 °C, low temperature: ~−180 °C). Excitation: 514.5 nm.
line, as reported in Fig. 10. At room temperature, the photoreaction occurs, showing a Raman spectrum identical to that observed with the macro-Raman apparatus and the same exciting line. When, instead, the sample temperature is lowered to about − 180 °C under cold gas flow coming from a liquid nitrogen dewar, as described in the Experimental section, the “spurious” bands are completely absent and the Raman spectrum is identical to that observed by excitation with the 785-nm laser line (Fig. 5). 5. Concluding Remarks The present investigation, based on a combined spectroscopic and density functional theory approach, allows collecting information on the photoreaction of DANS, a π-conjugated push-pull molecule, occurring in the solid phase, which was not previously observed. Intense Raman bands not corresponding to DANS are detected, suggesting that they are due to the formation of a photoproduct as azo-derivative under visible laser irradiation. DFT calculations allow identifying the reaction product by comparison between the simulated Raman spectrum of the proposed azo-derivative and that observed by irradiation with laser lines in the blue-green light region, where DANS electronically absorbs. In solid phase, this absorption ensures a thermal activation for the photoreaction, as evidenced by micro-Raman experiments at different temperatures. In solution, instead, the photoreaction does not occur, because the energy associated with the absorbed radiation is dissipated through the surrounding solvent molecules. The photoreduction of the nitro group occurs in the excited state of the molecule, where a large accumulation of electronic charge on the nitro group is pinpointed by the electronic distribution of the LUMO orbital. Hence, this molecular charge-transfer toward the NO2 group in the excited state, typical of a push-pull molecule such as DANS, plays a determinant role in promoting the reduction of the nitro group, as observed in the Raman spectra and verified by computational approach. Moreover, as demonstrated by the low-temperature experiments, the reaction is thermally activated due to the absorption of visible radiation in the blue-green spectral region. It is reasonable to think that this reaction involves two DANS molecules by eliminating two oxygen molecules: 2 Ar-NO2 → Ar-N = N-Ar + 2 O2. This study paves the way to the investigation of the photochemical processes occurring in push-pull molecules, which have great importance in determining their physical and optical properties and consequently in the NLO applications, also providing a suitable approach to unravel their mechanism.
[1] H.S. Nalwa, S. Miyata, Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, 1997. [2] J. Zyss, Molecular Nonlinear Optics: Materials, Physics, and Devices, Academic Press, Boston, 1993. [3] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [4] S.-S. Sun, L.R. Dalton, Introduction to Organic Electronic and Optoelectronic Materials and Devices, II ed. CRC Press, Boca Raton, 2016. [5] L.R. Dalton, A.W. Harper, B. Wu, R. Ghosn, J. Laquindanum, Z. Liang, A. Hubbel, C. Xu, Adv. Mater. 7 (1995) 519–540. [6] R.A. Rijkenberg, D. Bebelaar, W.J. Buma, J.W.J. Hofstraat, J. Phys. Chem. A 106 (2002) 2446–2456. [7] M. Cha, W.E. Torruellas, G.I. Stegeman, W.H.G. Horsthius, G. Möhlmann, R.J. Meth, Appl. Phys. Lett. 65 (1994) 2648–2650. [8] D. Beljonne, J.L. Bredas, M. Cha, W.E. Torruellas, G.I. Stegeman, J.W. Hofstraat, W.H.G. Horsthius, G.R. Möhlmann, J. Chem. Phys. 103 (1995) 7834–7843. [9] M. Tommasini, C. Castiglioni, M. Del Zoppo, G. Zerbi, J. Mol. Struct. 480–481 (1999) 179–188. [10] A.M. Moran, G.P. Bartholomew, G.C. Bazan, A. Myers Kelley, J. Phys. Chem. A 106 (2002) 4928–4937. [11] F. Bureš, RSC Adv. 4 (2014) 58826–58851. [12] C.-K. Wang, Y.-H. Wang, Y. Su, Y. Luo, J. Chem. Phys. 119 (2003) 4409–4412. [13] A.M. Moran, A.M. Kelley, J. Chem. Phys. 115 (2001) 912–924. [14] E.D. Schmid, M. Moschallski, W.L. Peticolas, J. Phys. Chem. 90 (1986) 2340–2346. [15] L.C.T. Shoute, H.Y. Woo, D. Vak, G.C. Bazan, A.M. Kelley, J. Chem. Phys. 125 (2006) 054506/1–054506/10. [16] M. Muniz-Miranda, J. Raman Spectrosc. 44 (2013) 1416–1421. [17] W. Rettig, W. Majnez, R. Lapouyade, F. Haucke, J. Photochem. Photobiol. A Chem. 62 (1992) 415–427. [18] R. Lapouyade, A. Kuhn, J.F. Letard, W. Rettig, Chem. Phys. Lett. 208 (1993) 48–58. [19] H. Gruen, H. Görner, J. Phys. Chem. 93 (1989) 7144–7152. [20] J. Oberlé, E. Abraham, G. Jonususkas, C. Rulliere, J. Raman Spectrosc. 31 (2000) 311–317. [21] H. Görner, J. Photochem. Photobiol. A Chem. 40 (1987) 325–339. [22] Md.M. Alam, M. Chattopadhyaya, S. Chakrabarti, J. Phys. Chem. A 116 (2012) 11034–11040. [23] L. Antonov, K. Kamada, K. Ohta, F.S. Kamounah, Phys. Chem. Chem. Phys. 5 (2003) 1193–1197. [24] P.N. Day, K.A. Nguyen, R. Pachter, J. Phys. Chem. B 109 (2005) 1803–1814. [25] B.B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X.-L. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51–54. [26] J. Arnbjerg, A. Jiménez-Banzo, M.J. Paterson, S. Nonell, J.I. Borrell, O. Christiansen, P.R. Ogilby, J. Am. Chem. Soc. 129 (2007) 5188–5199. [27] B. Moore, H. Sun, N. Govind, K. Kowalski, J. Autschbach, J. Chem. Theory Comput. 11 (2015) 3305–3320. [28] A.J. Garza, O.I. Osman, A.M. Asiri, G.E. Scuseria, J. Phys. Chem. B 119 (2015) 1202–1212. [29] C. Singh, R. Ghosh, J.A. Mondal, D.K. Palit, J. Photochem. Photobiol. A Chem. 263 (2013) 50–60. [30] N.A. Murugan, J. Kongsted, Z. Rinkevicius, K. Aidas, K.V. Mikkelsen, H. Ågren, Phys. Chem. Chem. Phys. 13 (2011) 12506–12516. [31] I.D. Petsalakis, D.G. Georgiadou, M. Vasilopoulou, G. Pistolis, D. Dimotikali, P. Argitis, G. Theodorakopoulos, J. Phys. Chem. A 114 (2010) 5580–5587. [32] S.J. Lord, H.-l.D. Lee, R. Samuel, R. Weber, N. Liu, N.R. Conley, M.A. Thompson, R.J. Twieg, W.E. Moerner, J. Phys. Chem. B 114 (2010) 14157–14167. [33] X.F. Xu, A. Kahan, S. Zilberg, Y. Haas, J. Phys. Chem. A 113 (2009) 9779–97791. [34] Q. Zhang, M. Canva, G. Stegeman, Appl. Phys. Lett. 73 (1998) 912–914. [35] C. Zhang, L.R. Dalton, M.-C. Oh, H. Zhang, W.H. Steier, Chem. Mater. 13 (2001) 3043–3050. [36] M. Cigán, A. Gáplovský, I. Sigmundová, P. Zahradník, R. Dĕdic, M. Hromadová, J. Phys. Org. Chem. 24 (2011) 450–459.
F. Muniz-Miranda et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 33–39 [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT (USA, 2009. [38] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [39] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [40] M. Muniz-Miranda, B. Pergolese, A. Bigotto, Vib. Spectrosc. 43 (2007) 97–103. [41] M. Muniz-Miranda, M. Pagliai, F. Muniz-Miranda, V. Schettino, Chem. Commun. 47 (2011) 3138–3140. [42] M. Pagliai, F. Muniz-Miranda, V. Schettino, M. Muniz-Miranda, Progr. Colloid Polym. Sci. 139 (2012) 39–44. [43] M. Muniz-Miranda, F. Muniz-Miranda, S. Caporali, Beilstein J. Nanotechnol. 5 (2014) 2489–2497. [44] E. Runge, E.K.U. Gross, Phys. Rev. Lett. 52 (1984) 997–1000. [45] D. Jacquemin, T. Le Bahers, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 14 (2012) 5383–5388.
39
[46] R. Dennington, T. Keith, J. Millam, GaussView, Version 5, Semichem Inc., Shawnee Mission, KS, 2009. [47] Jmol, version 12.2.2+dfsg-1, http://www.jmol.org. [48] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833–1840. [49] F.L. Hirshfeld, Theor. Chim. Acta 44 (1977) 129–138. [50] U.C. Singh, P.A. Kollman, J. Comput. Chem. 5 (1984) 129–145. [51] G. Garcìa, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 15 (2013) 20210–20219. [52] T. Vijayakumar, I. Hubert Joe, C.P. Reghunadhan Nair, V.S. Jayakumar, Chem. Phys. 343 (2008) 83–99. [53] P. Gao, D. Gosztola, M.J. Weaver, J. Phys. Chem. 92 (1988) 7122–7130. [54] C. Singh, R. Ghosh, J.A. Mondal, D.K. Palit, J. Photochem. Photobiol. A Chem. 263 (2013) 50–60.
