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Real-time observation of ultrafast electron injection at graphene–Zn porphyrin interfaces Dilshad Masih, Shawkat M. Aly,† Anwar Usman, Erkki Alarousu and Omar F. Mohammed* We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via
Received 24th December 2014, Accepted 24th February 2015
electrostatic and p–p stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and
DOI: 10.1039/c4cp06050d
which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC,
presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, forming ZnTMPyP + and GC , as indicated by a spectral feature at 650–750 nm, which is attributed to
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a ZnTMPyP radical cation resulting from the ET process.
Introduction Dye-sensitizers are attractive candidates for solar energy conversion technology, and they have attracted enormous interest due to easy fabrication, low cost of production, flexibility, and transparency, which are potential advantages for practical applications compared with the conventional crystalline silicon.1–4 Driven by such advantages, there have been many attempts to develop new efficient sensitizers for practical usage; one type of sensitizer of particular research interest is porphyrin because of the role of its related compounds in natural photosynthesis and the variety of derivatives with different functional groups and simple synthetic routes.5 Moreover, porphyrins have good light-harvesting properties because of high molar extinction coefficients of their Soret and Q-bands in the UV/Vis spectra, and they show unique photophysical and electrochemical properties that are tunable by varying the meso-functional groups and through interactions with metal atoms.4,5 On the other hand, the two-dimensional (2D) crystalline lattice of hybridized carbon atoms provided by graphene has been a new material of research interest, owing to its unique electronic, mechanical, and thermal properties, for future electronic and energy applications.6,7 There have been a number of systems comprising noncovalent association of porphyrins with small molecules,8,9 polymers,10 DNA,11 metallic nanoparticles,12 and semiconductor quantum dots.13,14 Recently, we reported that free positively
Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. E-mail: [email protected] † On leave from Chemistry Department, Assiut University, Egypt.
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charged porphyrins, 5,10,15,20-tetra(1-methyl-4-pyridino) porphyrin tetra( p-toluenesulfonate) (H2TMPyP) and 5,10,15,20-tetra(4trimethylammoniophenyl) porphyrin tetra( p-toluene-sulfonate) (TMAP), form complexes with negatively charged graphene carboxylate (GC) via electrostatic and p–p stacking interactions.15 It has been demonstrated that fluorescence of the porphyrins was quenched upon interaction with GC due to electron or/and energy transfer. We demonstrated that the rate of the photoinduced charge transfer and charge separation at the donor–acceptor interfaces crucially competes with the undesirable charge recombination. Possible effective strategies to modify the distance and electronic coupling are to introduce either different meso-functional groups or a metal atom in the porphyrin macrocycle. With these approaches, one can also expect to modify the redox properties of the porphyrin and, thus, the energy-level alignment between the porphyrins and the GC to optimize the charge separation. In the present work, we extended our study to non-covalent association between the metalated porphyrins and the GC. We focus on the association of meso tetrakis(4-N-methylpyridyl) zinc porphyrin, ZnTMPyP, with GC. We selected ZnTMPyP because of its well-defined optical properties and water solubility. Although the existence of the center Zn atom should increase the localization of the p-system compared with that in H2TMPyP, the presence of four pyridinium residues as meso-functional groups maintains the positive charges on the porphyrin, which can facilitate electrostatic binding with negatively charged GC. Steady-state absorption and emission spectroscopies indicate the affinity of ZnTMPyP on the GC surface, and efficient quenching of the porphyrin fluorescence is observed. Ultrafast time-resolved transient absorption (TA) spectroscopy clearly demonstrates that excited state absorption (ESA) decays rapidly in ps time scales attributable to ultrafast
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electron injection from the excited ZnTMPyP* to GC, as indicated by the formation of a porphyrin radical cation.
Results and discussion The absorption spectra of ZnTMPyP with successive additions of GC are shown in Fig. 1. The intense Soret band of ZnTMPyP shows a peak at 440 nm, and weaker Q-bands have two peaks at 567 and 611 nm. The intensity of the Soret band shows a dramatic decrease upon addition of GC. It should be noted that in the presence of high concentrations of GC B0.01 mg mL1, the decrease in the Soret band is almost 70%, which is accompanied by a red shift to 448 nm. In addition, the Q-bands are also red-shifted to 569 and 615 nm. The large red shifts in the ground-state absorption of porphyrins have been attributed to conformational changes, due to ground state charge transfer complexes.15–18 To further confirm the non-covalent association of ZnTMPyP with GC, the emission spectrum of a 5 mM aqueous solution of ZnTMPyP was measured after excitation at 563 nm in the presence of GC at different concentrations, as shown in Fig. 1 (right panel). The emission of ZnTMPyP is successively quenched upon GC addition, which can be attributed to photoinduced electron or/and energy transfer between ZnTMPyP and GC. The quenching efficiency was 50% at a GC concentration of 0.0057 mg mL1, and it abruptly reached 98% for GC of 0.011 mg mL1. This efficiency of ZnTMPyP is higher than that of H2TMPyP. This can be rationalized by an increase in localization of the p-system upon metalation of H2TMPyP as well as the decrease in oxidation potential from 1.30 for TMPyP to 1.18 for ZnTMPyP.13,19 The modified Stern–Volmer plot of the emission quenching of ZnTMPyP upon interaction with GC is presented in the inset of Fig. 1 (inset in right panel). The data show a linear correlation
Fig. 1 Steady-state absorption (left) and emission after excitation at 563 nm (right) of the ZnTMPyP at different GC concentrations (in mg mL1) as indicated. The inset displays the modified Stern–Volmer plot as a function of the GC concentration.
9016 | Phys. Chem. Chem. Phys., 2015, 17, 9015--9019
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with the reciprocal of GC concentration, which indicates that quenching proceeds according to a static mechanism.20,21 The efficient emission quenching could be due to electron or energy transfer from the excited ZnTMPyP to the GC, and this was further evaluated and deciphered by TA spectroscopy. Ultrafast TA spectroscopic measurements of the non-covalent association of ZnTMPyP with GC provide detailed information on the excited state dynamics. The TA spectra are shown in Fig. 2 for ZnTMPyP in the absence and presence of three different GC concentrations. For free ZnTMPyP (Fig. 2A), the excitation results in a strong negative absorption band corresponding to the ground state bleach (GSB) of the Soret band at 443 nm and two positive bands at 387 and 485 nm, which are related to excited state absorption (ESA). The features of the TA spectra are the characteristic signal of ZnTMPyP.13,22 The ESA band at 485 nm is overlapped with a newly emerged, long-lived, and slightly blue-shifted band, resulting in a band with a peak at 480 nm in long time delays. This latter band can be attributed to the triplet–triplet absorption.13,19 Within the 5.5 ns time observation window, the GSB is only recovered by 26% to the ground state, and the rest of the ZnTMPyP* decays into the long-lived triplet excited state and this long-lived band. Similar excited state relaxation dynamics of free ZnTMPyP to the triplet state have been reported after 400 nm excitation.13,19 Upon successive additions of GC concentrations, the GSB encounters a fast
Fig. 2 Transient absorption spectra observed at time delays from sub ps to a few ns for the ZnTMPyP (17 mM) at different GC concentrations; (A) 0 mg mL1, (B) 0.025 mg mL1, (C) 0.033 mg mL1, and (D) 0.041 mg mL1 after excitation at 630 nm.
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dynamic process developing and increasing with the increase of the GC concentration (Fig. 2B–D). Moreover, the GSB gets broader with the development of a red-shifted signal at B465 nm, which becomes more apparent, with added concentrations of GC, as shown in Fig. 2D. The broad and red-shifted GSB can be related to the ground state complex absorption, as indicated in Fig. 1. The ESA obtained for ZnTMPyP agrees with that reported in the literature;22 however, a new red-shifted broad transient spectrum in the presence of GC is observed. This new band detected for low concentrations becomes predominant with high GC concentration, as shown clearly in Fig. 2D. This TA feature in the spectral range of 650–750 nm as indicated in Fig. 3 can be attributed to the ZnTMPyP + radical cation.23,24 It is clear from this measurement that the new TA feature observed in the excited state absorption of ZnTMPyP in the presence of a high GC concentration (Fig. 2D) is a part of the ZnTMPyP + TA. Monitoring the radical cation unambiguously indicates ET from the excited ZnTMPyP* to GC (see eqn (1)). ðZnTMPyPGCÞ ground-state complex
hn
! ðZnTMPyPGCÞ
475nm
CT
CR
o 120fs
20ps
! ZnTMPyPþ þ GC !
ðZnTMPyPGCÞ ground-state complex (1)
Kinetic traces collected for ZnTMPyP in the absence and presence of GC are given in Fig. 4. Within the same time observation window, both GSB recovery and ESA decay are faster with the GC concentration in the solution (Fig. 4A and B). The kinetic trace for ESA decay collected at 392 nm for ZnTMPyP exhibits a time constant of 1.43 0.12 ns, which is in agreement with the literature.19,25 This excited-state absorption suffers from a very fast deactivation in the presence of GC as observed from the kinetic decay in Fig. 4B. The ESA decay of ZnTMPyP in the presence of 0.041 mg mL1 GC shows a time constant of 19.94 6.04 ps. Similarly, while the kinetic trace collected for ZnTMPyP alone at
Fig. 3 Transient absorption spectra observed at sub ps time delays for ZnTMPyP (17 mM) at different GC concentrations; (A) 0 mg mL1, and (B) 0.041 mg mL1 after excitation at 475 nm.
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Fig. 4 Normalized time profile of transient absorption kinetics of (A) GSB at 441 nm, (B) ESA at 392 nm, and (C) Radical cation TA at 740 nm of the ZnTMPyP in the absence and presence of GC. The solid lines represent the best fits to the obtained data.
740 nm shows an almost flat line over a 500 ps time window, fast decay is observed after addition of GC, as shown in Fig. 4C. The time constant collected for the ZnTMPyP/GC mixture at 740 nm, attributed to the ZnTMPyP + radical cation, is 20.18 6.49 ps.23,26 It is worth pointing out that the radical cation of ZnTMPyP was detected within our temporal resolution, indicating that the ET from Zn porphyrin to GC is ultrafast. It should be noted that the amplitude of unrecovered GSB is decreased with the increase in GC concentration. This finding indicates that (i) the porphyrin is in close contact with GC15 and (ii) photoinduced electron injection at the ZnTMPyP/GC interface takes place through the singlet state. This is in agreement with the electrostatic nature indicated by the ground-state measurements discussed above. Several reports in the literature pointed out the possibility of multichannel interaction in zinc porphyrin covalently attached to electron acceptors upon excitation in the Soret band.27–30 It is worth pointing out here that our excitation wavelengths are in the Q-band absorption at 630 nm and in the charge transfer absorption at 475 nm hence populating the S2 level is very unlikely. With only 0.35 eV separation between singlet and triplet energy levels,19 we might ask if the interaction is effective in the triplet state as well. To evaluate the dynamics of the long-lived species, Fig. 5 shows TA spectra up to ms time scales for ZnTMPyP in the absence and presence of GC. The intensity of the TA spectra
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porphyrin to the GC. Being in this regime, the strong electrostatic interaction between ZnTMPyP and GC might lead to very low reorganization energy for the ET, resulting in facile electron transfer at low driving forces as given by the fast ET kinetics observed in the femtosecond TA data.
Experimental section
Fig. 5 Transient absorption spectra observed at time delays from sub ns to a few ms for ZnTMPyP (17 mM) at different GC concentrations; (A) 0, (B) 0.025 mg mL1, and (C) 0.041 mg mL1, after excitation at 630 nm.
is drastically decreased with the addition of GC, and it is obviously not observed for high GC concentrations. For ZnTMPyP, the ESA band decays simultaneously with the GSB recovery, indicating direct relaxation of ZnTMPyP* from the triplet to ground state. The kinetic traces of the ESA and GSB bands also support this notion, as indicated by the almost same characteristic time constant of 2.83 0.02 ms. Such similar TA spectra and time constants indicate unambiguously that long-lived species can be attributed to the triplet excited state of free ZnTMPyP*. A similar TA spectrum of the long-lived species has been observed13 and attributed to the triplet state, with a strong absorption band with a peak at 470 nm.19 The decrease only in the amplitude of the TA signal with the successive additions of GC and no change in the lifetime of the triplet state indicate that electron transfer occurs only in the singlet excited state. Such a process is competing with the intersystem crossing and hence suppressing the triplet state population. This can be understood in terms of the fast photoinduced ET from ZnTMPyP* into the GC as discussed in the femtosecond TA measurements. Based on the lack of spectral overlap between the emission of ZnTMPyP and the absorption of GC, energy transfer between the donor–acceptor components can be excluded. Thus, the quenching process is very likely to be due to photo-induced ET involving ZnTMPyP* in the singlet state. In addition, a spectral feature at 740 nm, which is attributed to the ZnTMPyP + cation radical, provides crystal clear evidence for the ET from the Zn
9018 | Phys. Chem. Chem. Phys., 2015, 17, 9015--9019
ZnTMPyP and GC were purchased from Sigma-Aldrich and used without further purification. Steady-state absorption and emission measurements were performed using a rectangular quartz cell with a 1 cm optical path on a Cary5000 UV-visible spectrometer (Agilent Technologies) and a Fluoromax-4 spectrofluorometer (Horiba Scientific), respectively. In the measurements, the concentration of ZnTMPyP in distilled water was kept constant at 5 mM, and the GC from 0 to 0.011 mg mL1 was successively added into the solution. The Stern–Volmer analysis was performed based on the fluorescence spectral data without intensity correction. TA spectroscopy was carried out by employing a white-light continuum probe pulse generated by a super continuum source and spectrally tunable pump fs pulses (240–2600 nm; 25 mJ) generated from an optical parametric amplifier (Newport SpectraPhysics). The detailed experimental setup has been previously provided elsewhere.31 In these experiments, we use pump excitation pulses at 630 nm. The pump and probe pulses were overlapped within a 2 mm thick cuvette cell containing 17 mM ZnTMPyP solution in the absence and presence of 0.025–0.041 mg mL1 GC, and the transient absorbance change (DA) detected by the probe pulse was successfully monitored using a broadband UV-Vis detector. To ensure a fresh volume available for each laser shot, the sample solution was constantly stirred using a magnetic stirrer during the TA experiments. To verify that the sample solutions were not degraded upon photo-excitation, the absorption spectrum of each sample was measured before and after each pump–probe experiment. In order to cover the transient spectra from fs to ms time delays after photoexcitation, a Helios and an EOS detection system with time resolutions of 120 fs and 200 ps and detection limits of 5 ns and 1 ms, respectively, were employed. All TA experiments were performed at the magic angle and at room temperature.
Conclusions We investigated in detail the photoinduced charge transfer and charge separation in a non-covalent donor–acceptor system comprising positively charged ZnTMPyP and negatively charged GC containing carboxylate groups. Ground-state complex formation was confirmed by steady-state absorption and emission measurements. Absorption spectra changes revealed the high binding affinity between ZnTMPyP and GC via electrostatic and pp stacking interactions and rotations of methylpyridinium moieties toward a coplanar conformation. The emission of ZnTMPyP is quenched by GC due to the ET from the excited ZnTMPyP* to GC, and the modified Stern–Volmer plot indicates that the quenching is linearly correlated with the reciprocal GC concentration, demonstrating
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the static nature of the quenching mechanism. TA absorption spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially (76%) relaxes into the triplet state and (ii) ET reactions in the non-covalent association of ZnTMPyP/GC from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP + and GC radical ion pairs, which recombined back to the initial state with a characteristic time constant of 20.18 ps.
Acknowledgements Shawkat M. Aly is grateful for the post-doctoral fellowship provided by Saudi Basic Industries Corporation (SABIC). The work reported herein was supported by the King Abdullah University of Science and Technology.
Notes and references 1 B. O’Regan and M. Graetzel, Nature, 1991, 353, 737–740. ¨tzel, S.-J. Moon, R. Humphry-Baker, 2 M. Wang, C. Gra ¨tzel, Adv. N. Rossier-Iten, S. M. Zakeeruddin and M. Gra Funct. Mater., 2009, 19, 2163–2172. 3 M. Wang, J. Liu, N.-L. Cevey-Ha, S.-J. Moon, P. Liska, ¨tzel, P. Wang, R. Humphry-Baker, J.-E. Moser, C. Gra ¨tzel, Nano Today, 2010, 5, S. M. Zakeeruddin and M. Gra 169–174. 4 S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, ¨tzel, Nat. Chem., 2014, 6, 242–247. M. K. Nazeeruddin and M. Gra 5 J. Otsuki, Coord. Chem. Rev., 2010, 254, 2311–2341. 6 J. G. Radich, P. J. McGinn and P. V. Kamat, Interface, 2011, (Spring), 63–66. 7 X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666–686. 8 J. I. Khan, A. S. Abbas, S. M. Aly, A. Usman, V. Melnikov, E. Alarousu and O. F. Mohammed, Chem. Phys. Lett., 2014, 616–617, 237–242. 9 R. Laudien, I. Yoshida and T. Nagamura, J. Chem. Soc., Perkin Trans. 2, 2002, 1772–1777. 10 A. V. Udal’tsov and L. A. Kazarin, J. Photochem. Photobiol., A, 1999, 121, 169–176.
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11 A. J. Bhattacharjee, K. Ahluwalia, S. Taylor, O. Jin, J. M. Nicoludis, R. Buscaglia, J. B. Chaires, D. J. P. Kornfilt, D. G. S. Marquardt and L. A. Yatsunyk, Biochimie, 2011, 93, 1297–1309. 12 S. Murphy, L. Huang and P. V. Kamat, J. Phys. Chem. C, 2011, 115, 22761–22769. 13 P. M. Keane, S. A. Gallagher, L. M. Magno, M. J. Leising, I. P. Clark, G. M. Greetham, M. Towrie, Y. K. Gunko, J. M. Kelly and S. J. Quinn, Dalton Trans., 2012, 41, 13159–13166. 14 S. Jagadeeswari, G. Paramaguru and R. Renganathan, J. Photochem. Photobiol., A, 2013, 276, 104–112. 15 S. M. Aly, M. R. Parida, E. Alarousu and O. F. Mohammed, Chem. Commun., 2014, 50, 10452–10455. 16 Z. Chernia and D. Gill, Langmuir, 1999, 15, 1625–1633. 17 Y. Xu, L. Zhao, H. Bai, W. Hong, C. Li and G. Shi, J. Am. Chem. Soc., 2009, 131, 13490–13497. 18 Y. Ishida, D. Masui, T. Shimada, H. Tachibana, H. Inoue and S. Takagi, J. Phys. Chem. C, 2012, 116, 7879–7885. 19 K. Kalyanasundaram and M. Neumann-Spallart, J. Phys. Chem., 1982, 86, 5163–5169. 20 J. Keizer, J. Am. Chem. Soc., 1983, 105, 1494–1498. 21 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science + Business Media, LLC, Singapore, 3rd edn, 2006. 22 M. Enescu, K. Steenkeste, F. Tfibel and M.-P. Fontaine-Aupart, Phys. Chem. Chem. Phys., 2002, 4, 6092–6099. 23 A. Harriman, G. Porter and P. Walters, J. Photochem., 1982, 19, 183–187. 24 P. Neta and A. Harriman, J. Chem. Soc., Faraday Trans. 2, 1985, 81, 123–138. 25 N. Eugster, D. J. Fermı´n and H. H. Girault, J. Am. Chem. Soc., 2003, 125, 4862–4869. 26 O. V. Gerasimov, S. V. Lymar and V. N. Parmon, J. Photochem. Photobiol., A, 1991, 56, 275–285. 27 M. V. Rogozina, V. N. Ionkin and A. I. Ivanov, J. Photochem. Photobiol., A, 2015, 301, 55–61. 28 B. Robotham, K. A. Lastman, S. J. Langford and K. P. Ghiggino, J. Photochem. Photobiol., A, 2013, 251, 167–174. 29 E. M. Russell, E. Cudjoe, M. E. Mastromatteo, J. P. Kercher, ´ray and A. Bodi, J. Phys. Chem. A, 2013, 117, 4556–4563. B. Szta 30 S. Wallin, C. Monnereau, E. Blart, J.-R. Gankou, F. Odobel ¨m, J. Phys. Chem. A, 2010, 114, 1709–1721. and L. Hammarstro 31 O. F. Mohammed, D. Xiao, V. S. Batista and E. T. J. Nibbering, J. Phys. Chem. A, 2014, 118, 3090–3099.
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Real-time observation of ultrafast electron injection at graphene–Zn porphyrin interfaces Dilshad Masih, Shawkat M. Aly,† Anwar Usman, Erkki Alarousu and Omar F. Mohammed* We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via
Received 24th December 2014, Accepted 24th February 2015
electrostatic and p–p stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and
DOI: 10.1039/c4cp06050d
which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC,
presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, forming ZnTMPyP + and GC , as indicated by a spectral feature at 650–750 nm, which is attributed to
www.rsc.org/pccp
a ZnTMPyP radical cation resulting from the ET process.
Introduction Dye-sensitizers are attractive candidates for solar energy conversion technology, and they have attracted enormous interest due to easy fabrication, low cost of production, flexibility, and transparency, which are potential advantages for practical applications compared with the conventional crystalline silicon.1–4 Driven by such advantages, there have been many attempts to develop new efficient sensitizers for practical usage; one type of sensitizer of particular research interest is porphyrin because of the role of its related compounds in natural photosynthesis and the variety of derivatives with different functional groups and simple synthetic routes.5 Moreover, porphyrins have good light-harvesting properties because of high molar extinction coefficients of their Soret and Q-bands in the UV/Vis spectra, and they show unique photophysical and electrochemical properties that are tunable by varying the meso-functional groups and through interactions with metal atoms.4,5 On the other hand, the two-dimensional (2D) crystalline lattice of hybridized carbon atoms provided by graphene has been a new material of research interest, owing to its unique electronic, mechanical, and thermal properties, for future electronic and energy applications.6,7 There have been a number of systems comprising noncovalent association of porphyrins with small molecules,8,9 polymers,10 DNA,11 metallic nanoparticles,12 and semiconductor quantum dots.13,14 Recently, we reported that free positively
Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. E-mail: [email protected] † On leave from Chemistry Department, Assiut University, Egypt.
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charged porphyrins, 5,10,15,20-tetra(1-methyl-4-pyridino) porphyrin tetra( p-toluenesulfonate) (H2TMPyP) and 5,10,15,20-tetra(4trimethylammoniophenyl) porphyrin tetra( p-toluene-sulfonate) (TMAP), form complexes with negatively charged graphene carboxylate (GC) via electrostatic and p–p stacking interactions.15 It has been demonstrated that fluorescence of the porphyrins was quenched upon interaction with GC due to electron or/and energy transfer. We demonstrated that the rate of the photoinduced charge transfer and charge separation at the donor–acceptor interfaces crucially competes with the undesirable charge recombination. Possible effective strategies to modify the distance and electronic coupling are to introduce either different meso-functional groups or a metal atom in the porphyrin macrocycle. With these approaches, one can also expect to modify the redox properties of the porphyrin and, thus, the energy-level alignment between the porphyrins and the GC to optimize the charge separation. In the present work, we extended our study to non-covalent association between the metalated porphyrins and the GC. We focus on the association of meso tetrakis(4-N-methylpyridyl) zinc porphyrin, ZnTMPyP, with GC. We selected ZnTMPyP because of its well-defined optical properties and water solubility. Although the existence of the center Zn atom should increase the localization of the p-system compared with that in H2TMPyP, the presence of four pyridinium residues as meso-functional groups maintains the positive charges on the porphyrin, which can facilitate electrostatic binding with negatively charged GC. Steady-state absorption and emission spectroscopies indicate the affinity of ZnTMPyP on the GC surface, and efficient quenching of the porphyrin fluorescence is observed. Ultrafast time-resolved transient absorption (TA) spectroscopy clearly demonstrates that excited state absorption (ESA) decays rapidly in ps time scales attributable to ultrafast
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electron injection from the excited ZnTMPyP* to GC, as indicated by the formation of a porphyrin radical cation.
Results and discussion The absorption spectra of ZnTMPyP with successive additions of GC are shown in Fig. 1. The intense Soret band of ZnTMPyP shows a peak at 440 nm, and weaker Q-bands have two peaks at 567 and 611 nm. The intensity of the Soret band shows a dramatic decrease upon addition of GC. It should be noted that in the presence of high concentrations of GC B0.01 mg mL1, the decrease in the Soret band is almost 70%, which is accompanied by a red shift to 448 nm. In addition, the Q-bands are also red-shifted to 569 and 615 nm. The large red shifts in the ground-state absorption of porphyrins have been attributed to conformational changes, due to ground state charge transfer complexes.15–18 To further confirm the non-covalent association of ZnTMPyP with GC, the emission spectrum of a 5 mM aqueous solution of ZnTMPyP was measured after excitation at 563 nm in the presence of GC at different concentrations, as shown in Fig. 1 (right panel). The emission of ZnTMPyP is successively quenched upon GC addition, which can be attributed to photoinduced electron or/and energy transfer between ZnTMPyP and GC. The quenching efficiency was 50% at a GC concentration of 0.0057 mg mL1, and it abruptly reached 98% for GC of 0.011 mg mL1. This efficiency of ZnTMPyP is higher than that of H2TMPyP. This can be rationalized by an increase in localization of the p-system upon metalation of H2TMPyP as well as the decrease in oxidation potential from 1.30 for TMPyP to 1.18 for ZnTMPyP.13,19 The modified Stern–Volmer plot of the emission quenching of ZnTMPyP upon interaction with GC is presented in the inset of Fig. 1 (inset in right panel). The data show a linear correlation
Fig. 1 Steady-state absorption (left) and emission after excitation at 563 nm (right) of the ZnTMPyP at different GC concentrations (in mg mL1) as indicated. The inset displays the modified Stern–Volmer plot as a function of the GC concentration.
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with the reciprocal of GC concentration, which indicates that quenching proceeds according to a static mechanism.20,21 The efficient emission quenching could be due to electron or energy transfer from the excited ZnTMPyP to the GC, and this was further evaluated and deciphered by TA spectroscopy. Ultrafast TA spectroscopic measurements of the non-covalent association of ZnTMPyP with GC provide detailed information on the excited state dynamics. The TA spectra are shown in Fig. 2 for ZnTMPyP in the absence and presence of three different GC concentrations. For free ZnTMPyP (Fig. 2A), the excitation results in a strong negative absorption band corresponding to the ground state bleach (GSB) of the Soret band at 443 nm and two positive bands at 387 and 485 nm, which are related to excited state absorption (ESA). The features of the TA spectra are the characteristic signal of ZnTMPyP.13,22 The ESA band at 485 nm is overlapped with a newly emerged, long-lived, and slightly blue-shifted band, resulting in a band with a peak at 480 nm in long time delays. This latter band can be attributed to the triplet–triplet absorption.13,19 Within the 5.5 ns time observation window, the GSB is only recovered by 26% to the ground state, and the rest of the ZnTMPyP* decays into the long-lived triplet excited state and this long-lived band. Similar excited state relaxation dynamics of free ZnTMPyP to the triplet state have been reported after 400 nm excitation.13,19 Upon successive additions of GC concentrations, the GSB encounters a fast
Fig. 2 Transient absorption spectra observed at time delays from sub ps to a few ns for the ZnTMPyP (17 mM) at different GC concentrations; (A) 0 mg mL1, (B) 0.025 mg mL1, (C) 0.033 mg mL1, and (D) 0.041 mg mL1 after excitation at 630 nm.
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dynamic process developing and increasing with the increase of the GC concentration (Fig. 2B–D). Moreover, the GSB gets broader with the development of a red-shifted signal at B465 nm, which becomes more apparent, with added concentrations of GC, as shown in Fig. 2D. The broad and red-shifted GSB can be related to the ground state complex absorption, as indicated in Fig. 1. The ESA obtained for ZnTMPyP agrees with that reported in the literature;22 however, a new red-shifted broad transient spectrum in the presence of GC is observed. This new band detected for low concentrations becomes predominant with high GC concentration, as shown clearly in Fig. 2D. This TA feature in the spectral range of 650–750 nm as indicated in Fig. 3 can be attributed to the ZnTMPyP + radical cation.23,24 It is clear from this measurement that the new TA feature observed in the excited state absorption of ZnTMPyP in the presence of a high GC concentration (Fig. 2D) is a part of the ZnTMPyP + TA. Monitoring the radical cation unambiguously indicates ET from the excited ZnTMPyP* to GC (see eqn (1)). ðZnTMPyPGCÞ ground-state complex
hn
! ðZnTMPyPGCÞ
475nm
CT
CR
o 120fs
20ps
! ZnTMPyPþ þ GC !
ðZnTMPyPGCÞ ground-state complex (1)
Kinetic traces collected for ZnTMPyP in the absence and presence of GC are given in Fig. 4. Within the same time observation window, both GSB recovery and ESA decay are faster with the GC concentration in the solution (Fig. 4A and B). The kinetic trace for ESA decay collected at 392 nm for ZnTMPyP exhibits a time constant of 1.43 0.12 ns, which is in agreement with the literature.19,25 This excited-state absorption suffers from a very fast deactivation in the presence of GC as observed from the kinetic decay in Fig. 4B. The ESA decay of ZnTMPyP in the presence of 0.041 mg mL1 GC shows a time constant of 19.94 6.04 ps. Similarly, while the kinetic trace collected for ZnTMPyP alone at
Fig. 3 Transient absorption spectra observed at sub ps time delays for ZnTMPyP (17 mM) at different GC concentrations; (A) 0 mg mL1, and (B) 0.041 mg mL1 after excitation at 475 nm.
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Fig. 4 Normalized time profile of transient absorption kinetics of (A) GSB at 441 nm, (B) ESA at 392 nm, and (C) Radical cation TA at 740 nm of the ZnTMPyP in the absence and presence of GC. The solid lines represent the best fits to the obtained data.
740 nm shows an almost flat line over a 500 ps time window, fast decay is observed after addition of GC, as shown in Fig. 4C. The time constant collected for the ZnTMPyP/GC mixture at 740 nm, attributed to the ZnTMPyP + radical cation, is 20.18 6.49 ps.23,26 It is worth pointing out that the radical cation of ZnTMPyP was detected within our temporal resolution, indicating that the ET from Zn porphyrin to GC is ultrafast. It should be noted that the amplitude of unrecovered GSB is decreased with the increase in GC concentration. This finding indicates that (i) the porphyrin is in close contact with GC15 and (ii) photoinduced electron injection at the ZnTMPyP/GC interface takes place through the singlet state. This is in agreement with the electrostatic nature indicated by the ground-state measurements discussed above. Several reports in the literature pointed out the possibility of multichannel interaction in zinc porphyrin covalently attached to electron acceptors upon excitation in the Soret band.27–30 It is worth pointing out here that our excitation wavelengths are in the Q-band absorption at 630 nm and in the charge transfer absorption at 475 nm hence populating the S2 level is very unlikely. With only 0.35 eV separation between singlet and triplet energy levels,19 we might ask if the interaction is effective in the triplet state as well. To evaluate the dynamics of the long-lived species, Fig. 5 shows TA spectra up to ms time scales for ZnTMPyP in the absence and presence of GC. The intensity of the TA spectra
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porphyrin to the GC. Being in this regime, the strong electrostatic interaction between ZnTMPyP and GC might lead to very low reorganization energy for the ET, resulting in facile electron transfer at low driving forces as given by the fast ET kinetics observed in the femtosecond TA data.
Experimental section
Fig. 5 Transient absorption spectra observed at time delays from sub ns to a few ms for ZnTMPyP (17 mM) at different GC concentrations; (A) 0, (B) 0.025 mg mL1, and (C) 0.041 mg mL1, after excitation at 630 nm.
is drastically decreased with the addition of GC, and it is obviously not observed for high GC concentrations. For ZnTMPyP, the ESA band decays simultaneously with the GSB recovery, indicating direct relaxation of ZnTMPyP* from the triplet to ground state. The kinetic traces of the ESA and GSB bands also support this notion, as indicated by the almost same characteristic time constant of 2.83 0.02 ms. Such similar TA spectra and time constants indicate unambiguously that long-lived species can be attributed to the triplet excited state of free ZnTMPyP*. A similar TA spectrum of the long-lived species has been observed13 and attributed to the triplet state, with a strong absorption band with a peak at 470 nm.19 The decrease only in the amplitude of the TA signal with the successive additions of GC and no change in the lifetime of the triplet state indicate that electron transfer occurs only in the singlet excited state. Such a process is competing with the intersystem crossing and hence suppressing the triplet state population. This can be understood in terms of the fast photoinduced ET from ZnTMPyP* into the GC as discussed in the femtosecond TA measurements. Based on the lack of spectral overlap between the emission of ZnTMPyP and the absorption of GC, energy transfer between the donor–acceptor components can be excluded. Thus, the quenching process is very likely to be due to photo-induced ET involving ZnTMPyP* in the singlet state. In addition, a spectral feature at 740 nm, which is attributed to the ZnTMPyP + cation radical, provides crystal clear evidence for the ET from the Zn
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ZnTMPyP and GC were purchased from Sigma-Aldrich and used without further purification. Steady-state absorption and emission measurements were performed using a rectangular quartz cell with a 1 cm optical path on a Cary5000 UV-visible spectrometer (Agilent Technologies) and a Fluoromax-4 spectrofluorometer (Horiba Scientific), respectively. In the measurements, the concentration of ZnTMPyP in distilled water was kept constant at 5 mM, and the GC from 0 to 0.011 mg mL1 was successively added into the solution. The Stern–Volmer analysis was performed based on the fluorescence spectral data without intensity correction. TA spectroscopy was carried out by employing a white-light continuum probe pulse generated by a super continuum source and spectrally tunable pump fs pulses (240–2600 nm; 25 mJ) generated from an optical parametric amplifier (Newport SpectraPhysics). The detailed experimental setup has been previously provided elsewhere.31 In these experiments, we use pump excitation pulses at 630 nm. The pump and probe pulses were overlapped within a 2 mm thick cuvette cell containing 17 mM ZnTMPyP solution in the absence and presence of 0.025–0.041 mg mL1 GC, and the transient absorbance change (DA) detected by the probe pulse was successfully monitored using a broadband UV-Vis detector. To ensure a fresh volume available for each laser shot, the sample solution was constantly stirred using a magnetic stirrer during the TA experiments. To verify that the sample solutions were not degraded upon photo-excitation, the absorption spectrum of each sample was measured before and after each pump–probe experiment. In order to cover the transient spectra from fs to ms time delays after photoexcitation, a Helios and an EOS detection system with time resolutions of 120 fs and 200 ps and detection limits of 5 ns and 1 ms, respectively, were employed. All TA experiments were performed at the magic angle and at room temperature.
Conclusions We investigated in detail the photoinduced charge transfer and charge separation in a non-covalent donor–acceptor system comprising positively charged ZnTMPyP and negatively charged GC containing carboxylate groups. Ground-state complex formation was confirmed by steady-state absorption and emission measurements. Absorption spectra changes revealed the high binding affinity between ZnTMPyP and GC via electrostatic and pp stacking interactions and rotations of methylpyridinium moieties toward a coplanar conformation. The emission of ZnTMPyP is quenched by GC due to the ET from the excited ZnTMPyP* to GC, and the modified Stern–Volmer plot indicates that the quenching is linearly correlated with the reciprocal GC concentration, demonstrating
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the static nature of the quenching mechanism. TA absorption spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially (76%) relaxes into the triplet state and (ii) ET reactions in the non-covalent association of ZnTMPyP/GC from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP + and GC radical ion pairs, which recombined back to the initial state with a characteristic time constant of 20.18 ps.
Acknowledgements Shawkat M. Aly is grateful for the post-doctoral fellowship provided by Saudi Basic Industries Corporation (SABIC). The work reported herein was supported by the King Abdullah University of Science and Technology.
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