CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201400078
Does Excited-State Proton-Transfer Reaction Contribute to the Emission Behaviour of 4-Aminophthalimide in Aqueous Media? Dinesh Chandra Khara, Sanghamitra Banerjee, and Anunay Samanta*[a] 4-Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent-sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time-dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited-state solvent-mediated proton-transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide-H protected derivative, N-BuAP, to prove that a solvent-assisted excited-
state keto–enol transformation does not contribute to the steady-state and time-resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogenbonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited-state proton-transfer reaction in the system, is explained by considering the difference in the influence of H2O and D2O on the nonradiative rates and ground-state exchange of the proton with the solvent.
1. Introduction The photophysics of 4-aminophthalimide (AP) has been studied extensively due to its attractive fluorescence properties, in particular, high fluorescence quantum yield, long fluorescence lifetime and sensitivity of these properties to the surrounding environment. The molecule is used routinely as a fluorescence probe for studies of solvation dynamics, to estimate the polarity of unknown media and probe the organised environments of complex chemical and biological systems.[1–36] It is generally believed that photoexcitation of AP leads to enhanced separation of charge and the molecule emits from an intramolecular charge-transfer (ICT) state, whose energy is determined by the polarity and hydrogen-bonding ability of the solvent.[3, 8, 10] The fluorescence quantum yield and lifetime of AP are also highly sensitive to the environment.[3, 8, 10] In contrast to its behaviour in aprotic media, where AP is strongly fluorescent and possesses a long fluorescence lifetime, in protic solvents the molecule emits weakly and has a short fluorescence lifetime. The solute– solvent hydrogen-bonding interaction was considered to be responsible for a large Stokes shift of the fluorescence maximum, a low fluorescence yield and a short fluorescence lifetime of the molecule in protic media. However, an alternative [a] Dr. D. C. Khara,+ S. Banerjee, Prof. A. Samanta School of Chemistry University of Hyderabad Hyderabad 500 046 (India) E-mail: [email protected] [+] Current address: Institute of Atomic and Molecular Sciences Academia Sinica, Taipei 106 (Taiwan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201400078.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
explanation in terms of solvent-assisted intramolecular proton transfer (Scheme 1) was also proposed, according to which the emission of AP originates from its enol form in protic media.[12]
Scheme 1. Solvent-assisted intramolecular proton transfer in AP.
Even though no kinetic evidence in support of this transformation was available, a higher fluorescence lifetime and quantum yield of AP in D2O compared to H2O was considered to be consistent with this mechanism.[13] However, even though subsequent studies revealed drawbacks of this mechanism,[14] the concept of a solvent-mediated excited-state keto–enol transformation of the system was reintroduced recently to account for observed excitation-wavelength-dependence of the emission spectrum and a progressive blue shift of the emission maximum of AP with time in aqueous media.[22] Biexponential fluorescence-decay behaviour and time-dependent shift of the fluorescence spectrum are argued to be direct kinetic evidence of the excited-state reaction of AP in aqueous media.[22] As we could not reproduce the excitation wavelength dependence of steady-state fluorescence spectrum of AP, we did not take the mechanism seriously.[23] However, the kinetic data presented in ref. [22] in support of the excited-state reaction required careful consideration as ChemPhysChem 0000, 00, 1 – 7
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a biexponential decay behaviour and time-dependent shift of the emission spectrum were not expected in the absence of an excited-state reaction. Moreover, even though dynamic Stokes shift of the fluorescence spectrum due to excited-state reaction is common, a progressive shift of the spectrum towards higher energy is very unusual, if not unprecedented. It is also puzzling that the spectral shift is not continuous, but mostly occurs between 2–4 ns.[22] To address these points and establish that excited-state proton-transfer reaction does not influence the photophysics of AP in aqueous media, we have undertaken this work on AP and its derivative (N-BuAP, Figure 1), in which the imide hydrogen atom, which is shown to be involved in water-assisted excited-state proton-transfer process, is replaced by an n-butyl group.
Figure 1. Molecular structures of AP and its derivative (used in this work).
2. Results and Discussion
Figure 2. Steady-state emission spectra of a) AP and b) N-BuAP (lex = 370 nm) in acetonitrile and water.
Figure 2 compares the steady-state fluorescence spectra (lex = 370 nm) of AP and N-BuAP in aprotic (acetonitrile, ACN) and protic (water) solvents. N-BuAP also exhibits a large difference in the lem max values in the two solvents ( 100 nm, Table 1), which led to the postulation that the emission of AP originates
Table 1. Comparison of some of the fluorescence parameters of AP and N-BuAP. Solvent
lem max
ACN H2O
AP 460 565
N-BuAP 472 577
(IAP/IN-BuAP)[a]
ff (AP)
1.2 1.5
0.63[18] 0.022[18]
[a] IAP and IN-BuAP are integrated areas under the fluorescence spectra of AP and N-BuAP, respectively, recorded under identical experimental conditions.
from the enol form of the molecule that is produced as a result of excited-state water-assisted transfer of the imide proton (Scheme 1). The strikingly similar fluorescence spectra of the two systems thus clearly rule out the possibility of the enol form of AP as the emitting species in protic solvents such as water. One can argue against the excited-state solvent-mediated proton-transfer mechanism from another angle. As in aprotic solvent acetonitrile AP emits (lmax = 460 nm) from its normal (keto) form, if the emission of AP in aqueous media (lmax = 565 nm) indeed originates from its enol form, then on gradual 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
addition of water to an acetonitrile solution of AP, a progressive increase of the fluorescence intensity of the 560 nm band at the expense of the 460 nm emission is expected. By considering the low fluorescence quantum yield of AP in water (compared to acetonitrile), one expects the decrease in intensity at 460 nm to be more pronounced than the increase in intensity at 565 nm. However, the spectral behaviour of AP and N-BuAP presented in Figure 3 shows a gradual shift of the emission spectrum towards longer wavelength instead of the expected trend, which thus disproves the excited-state proton-transfer reaction of AP. Moreover, a very similar fluorescence response (including the time-resolved behaviour, see Supporting Information) of AP and N-BuAP, the latter lacking the imide hydrogen atom essential for the formation of the enol form of the molecule, further reinforces our conclusion. 2.1. Isotope Effect As stated in the Introduction, a significantly higher fluorescence quantum yield (ff) and lifetime (tf) of AP in D2O when compared with that in H2O was thought to be consistent with excited-state solvent-mediated transfer of the imide hydrogen atom to the carbonyl oxygen atom.[13, 16, 22] However, as the results presented above rule out this possibility, one needs to reexamine the isotope effect carefully and then determine its origin. For this purpose, we first find out whether N-BuAP, which lacks the imide hydrogen atom, also exhibits a deuteriChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org systems, this does not necessarily mean that no proton of the fluorescent systems is exchanged with the solvent (H2O or D2O) in the ground state. Both AP and N-BuAP have amino protons, which are sufficiently acidic due to the inductive influence of the carbonyl group and can be exchanged with solvent H or D atoms in the ground state. Evidence of this kind of exchange process is often obtained from 1H NMR measurements, where the signals due to the acidic protons are seldom observed. We emphasise, however, that as these rates are lower than the radiative and nonradiative rates of the systems by several orders of magnitude, they do not play a noticeable role in the fluorescence behaviour of the systems. Under these conditions, in deuterated solvents, one is dealing not only with the original molecule, but also the deuterium-exchanged molecule. If the fluorescence efficiencies of a system and its deuterated form are different due to different rates of the nonradiative relaxation process (which is quite likely),[42] the deuterium isotope effect can also be observed. Hence, in our opinion the isotope effect exhibited by AP and N-BuAP arises from one or both the factors described above. 2.2. Wavelength-Dependent Fluorescence Decay and TimeResolved Emission Spectrum
Figure 3. Steady-state emission spectra of a) AP and b) N-BuAP in acetonitrile–water mixture (lex = 370 nm).
um isotope effect. Like AP, the ff and tf values of N-BuAP are also higher in D2O (than in H2O) by factors of 2.0 and 4.5, respectively. The values of ff and tf of both AP and N-BuAP are about threefold higher in CD3OD than in CH3OH. Thus, as the isotope effect is real, but not due to the excited-state protontransfer reaction, its origin must lie elsewhere. Solvent deuterium isotope effect on ff and tf suggests a change in the nonradiative rate of the emitting state of the system on isotopic substitution.[37] This change is expected to be most pronounced if the solvent proton participates in the reaction that acts as one of the nonradiative decay channels.[38] However, this reaction need not be the only pathway for the radiationless process. Solvent often plays a crucial role in influencing the nonradiative relaxation (intersystem crossing and internal conversion) rates by directly interacting with the solute.[39] A strong interaction of the electronic motion of the fluorescent system with the nuclear motion of the solvent can provide a mechanism for the conversion of electronic energy of the system into the vibrational energy of the solvent, which thus results in radiationless interconversion that affects a change of the ff and tf values. In such a situation, isotopic substitution in the solvent can influence the coupling between the electronic motions of the solute and vibrational motion of the solvent and thus can contribute to the isotope effect.[40, 41] A second factor can contribute to the isotope effect. Even though we have ruled out the influence of excited-state proton-transfer reaction on the emission behaviour of the two 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
We now reinvestigate carefully the time-resolved fluorescence behaviour of the two systems and find out whether it is due to an excited-state reaction. Considering that the emission peak of AP and N-BuAP is observed at around 565 nm in aqueous media, we have measured the fluorescence decay profiles at this wavelength. These fluorescence intensity versus time profiles of the two systems (Figure 4) can be fitted satisfactorily to a single exponential decay function that yields lifetimes of around 1.10 ns, which is consistent with literature values.[13, 18] Considering the recent report[22] of time-dependent shift of the fluorescence spectrum we studied the time-resolved fluorescence response of the systems in more detail. As the timeresolved emission spectra (TRES) of a system can be constructed from its fluorescence decay profiles at several wavelengths covering the entire emission spectra, we also measured the fluorescence decay profiles of the two systems at various other wavelengths, and the results (Figure 5) are quite revealing. Contrary to the decay profiles measured at the peak maxima or at higher wavelengths, which are mainly characterised by a single exponential decay function, those measured at shorter wavelengths are clearly biexponential and consist of a longlifetime (minor) component of lifetime about 10.0 ns, in addition to the short-lifetime component of around 1.1 ns (major). With an increase in the monitoring wavelength, the contribution of the long component diminishes gradually and as the wavelength corresponding to the emission peak position is reached, the long component vanishes almost completely to give a single exponential decay with a lifetime of around 1.1 ns, which is commonly taken as the fluorescence lifetime of AP in aqueous media. We shall discuss the origin of the two specific lifetime components later. However, having observed two lifetime components and the nature of variation of its amplitude with monitoring wavelength, it became clear how ChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org a blue shift of the TRES, as reported recently,[22] is observed. The TRES recorded at early times consisted primarily of the short component and those recorded beyond 3 ns had no contribution from the short component, which is why the blue shift was observed in the 3–4 ns time range.
2.3. Possible Origin of The Two Lifetime Components While the dependence of the fluorescence decay profiles on the monitoring wavelength (as described above) explains the blue shift of TRES (as observed by Durantini et al),[22] one does not expect decay profiles of this kind in the case of an excitedstate reaction. Had the two species been related through an excited-state reaction, there would have been a rise component in the fluorescence time profiles monitored at longer wavelengths. That the time-resolved behaviour, depicted in Figure 6, is strikingly similar for AP and N-BuAP clearly suggests
Figure 4. Fluorescence decay profiles of a) AP and b) N-BuAP monitored at 565 nm in water. Solid lines represent a single exponential fit to the decay profiles (lex = 375 nm).
Figure 6. Time-resolved emission spectra of a) AP and b) N-BuAP in water (lex = 375 nm).
Figure 5. Wavelength-dependent fluorescence decay profiles of a) AP and b) N-BuAP in aqueous solution (lex = 375 nm).
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that the emission of AP in aqueous media has nothing to do with the enol form. The nature of the decay profiles observed here indicates the presence of two separate emitting species that are not related through an excited-state reaction. We next attempt to find out the possible origin of the two lifetime components in water. The fluorescence quantum yield and lifetime of AP in polar media depend on the hydrogenbond donating/accepting ability of the solvents.[18] These ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES values are low in polar protic solvents and the lowest in water. The low ff and tf values in polar protic solvents are believed to be due to stabilisation of the emitting state of the system as a result of strong hydrogen-bonding interaction of AP with the solvent and consequent increase in the nonradiative rate of the emitting state. Two different kinds of hydrogen-bonding interaction (Figure 7) are documented in the literature.[18] The
Figure 7. Types of hydrogen-bonding interactions possible between AP and water.
most common of these interactions is hydrogen-bond formation between the carbonyl oxygen atom of AP and hydrogenbond-donating protic solvents such as hexafluoroisopropanol or trifluoroethanol, which leads to a red shift of the emission maximum and a decrease in ff and tf values.[18] The other possible mode of hydrogen-bonding interaction involves the amino hydrogen atom of AP and hydrogen-bond-accepting solvents (in solvents such as in DMSO), which leads to a blue shift of the emission spectrum and an increase in ff and tf values.[18] Solvents such as alcohols and water, which can act both as H-bond donor and acceptor, can therefore interact with AP through both types of hydrogen-bond formation (Figure 7). As the hydrogen-bonding interaction with the amino moiety leads to a blue shift and that with the carbonyl moiety results in a red shift of the emission spectrum, when one monitors at wavelengths corresponding to the blue side of the spectrum, the molecules hydrogen-bonded at the amino end (and are characterised by long lifetime) contribute more to the fluorescence. On the other hand, as one moves towards longer wavelength, the contribution due to hydrogenbonded species with the carbonyl moiety dominates and one observes only the species with a short lifetime.
3. Conclusions We have convincingly established that solvent-mediated excited-state proton-transfer reaction does not contribute to the fluorescence behaviour of AP and its derivatives. We have also shown that the time-dependent blue shift of the fluorescence spectrum of AP in aqueous media, which was recently presented as evidence in support of the water-mediated excited state keto–enol transformation of the molecule, arises due to the presence of two different types of hydrogen-bonded species of AP with distinctly different fluorescence lifetimes. The solvent isotope effect on the fluorescence properties of AP and N-BuAP is explained by considering the difference in the influence of H2O and D2O on the nonradiative relaxation rates of the systems. It is suggested that ground-state exchange of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org amino hydrogen atom of the systems with the solvent can also contribute to the isotope effect.
Experimental Section AP was obtained from TCI (Japan) and was recrystallised from ethanol prior to use. N-BuAP was synthesised and purified according to a literature procedure.[14] The purity of the molecules was ascertained by NMR spectroscopy and also by the appearance of a single spot in thin-layer chromatography. Methanol and acetonitrile were obtained from Merck (India) and purified according to standard procedures prior to use.[43] Deuterated water and methanol were obtained from Merck (India) and used without any treatment. Double-distilled water was used throughout. The NMR spectra were recorded in Bruker AVANCE 400 MHz NMR spectrometer. The absorption and steady-state fluorescence spectra were recorded on a UV/Vis spectrophotometer (Cary100, Varian) and spectrofluorimeter (FluoroLog, Horiba Jobin Yvon), respectively. The fluorescence spectra were corrected for the instrumental response. Time-resolved fluorescence decay measurements were carried out using a time-correlated single-photon-counting (TCSPC) spectrometer (Horiba Jobin Yvon IBH). A diode laser (lex = 375 nm) was used as excitation source, and a microchannel plate (MCP) photomultiplier (Hamamatsu R3809U-50) was used as detector (response time 40 ps). The instrument response function (IRF) of the setup (50 ps) was limited by the full-width at half-maximum (FWHM) of the exciting laser pulse. The lamp profile was recorded by placing a scatterer (dilute solution of Ludox in water) in place of the sample. Decay curves were analysed by using a nonlinear least-squares iteration procedure using IBH DAS6 (Version 2.2) decay analysis software. The qualities of the fits were assessed by analysis of the c2 values and distribution of the residuals. Time-resolved emission spectra (TRES) were recorded directly in the TCSPC fluorescence spectrometer described above.
Acknowledgements This work was supported by the J.C. Bose National Fellowship (to A.S.) of the Department of Science and Technology (DST), Government of India. D.C.K. thanks the Council of Scientific and Industrial Research (CSIR) for a Senior Research Fellowship. S.B. acknowledges the Fellowship received from the University Grants Commission (UGC). Keywords: fluorescence spectroscopy · hydrogen bonding · isotope effects · keto–enol transformations · solvent effects [1] W. R. Ware, S. K. Lee, G. J. Brant, P. P. Chow, J. Chem. Phys. 1971, 54, 4729 – 4737. [2] P. Suppan, J. Chem. Soc. Faraday Trans. 1987, 83, 495 – 509. [3] D. Noukakis, P. Suppan, J. Lumin. 1991, 47, 285 – 295. [4] T. Soujanya, T. S. R. Krishna, A. Samanta, J. Photochem. Photobiol. A 1992, 66, 185 – 192. [5] T. Soujanya, T. S. R. Krishna, A. Samanta, J. Phys. Chem. 1992, 96, 8544 – 8548. [6] G. Saroja, A. Samanta, Chem. Phys. Lett. 1995, 246, 506 – 512. [7] T. O. Harju, A. H. Huizer, C. A. G. O. Varma, Chem. Phys. 1995, 200, 215 – 224. [8] T. Soujanya, R. W. Fessenden, A. Samanta, J. Phys. Chem. 1996, 100, 3507 – 3512. [9] D. Yuan, R. G. Brown, J. Phys. Chem. A 1997, 101, 3461 – 3466.
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CHEMPHYSCHEM ARTICLES [10] G. Saroja, T. Soujanya, B. Ramachandram, A. Samanta, J. Fluoresc. 1998, 8, 405 – 410. [11] G. Saroja, A. Samanta, J. Chem. Soc. Faraday Trans. 1996, 92, 2697 – 2701. [12] E. Laitinen, K. Salonen, T. Harju, J. Chem. Phys. 1996, 104, 6138 – 6148. [13] S. Das, A. Datta, K. Bhattacharyya, J. Phys. Chem. A 1997, 101, 3299 – 3304. [14] G. Saroja, A. Samanta, J. Chem. Soc. Faraday Trans. 1998, 94, 3141 – 3145. [15] G. Saroja, B. Ramachandram, S. Saha, A. Samanta, J. Phys. Chem. B 1999, 103, 2906 – 2911. [16] A. Datta, S. Das, D. Mandal, S. K. Pal, K. Bhattacharyya, Langmuir 1997, 13, 6922 – 6926. [17] D. E. Wetzler, C. Chesta, R. Fernndez-Prini, P. F. Aramenda, J. Phys. Chem. A 2002, 106, 2390 – 2400. [18] E. Krystkowiak, K. Dobek, A. Maciejewski, J. Photochem. Photobiol. A 2006, 184, 250 – 264. [19] K. Dobek, Photochem. Photobiol. Sci. 2008, 7, 361 – 370. [20] K. Dobek, J. Karolczak, D. Komar, J. Phys. Chem. A 2012, 116, 6655 – 6663. [21] A. Maciejewski, E. Krystkowiak, J. Koput, K. Dobek, ChemPhysChem 2011, 12, 322 – 332. [22] A. M. Durantini, R. D. Falcone, J. D. Anunziata, J. J. Silber, E. B. Abuin, E. A. Lissi, N. M. Correa, J. Phys. Chem. B 2013, 117, 2160 – 2168. [23] D. C. Khara, A. Samanta, J. Phys. Chem. B 2013, 117, 5387 – 5388. [24] A. M. Durantini, R. D. Falcone, J. D. Anunziata, J. J. Silber, E. B. Abuin, E. A. Lissi, N. M. Correa, J. Phys. Chem. B 2013, 117, 5389 – 5391. [25] A. M. Durantini, R. D. Falcone, J. D. Anunziata, J. J. Silber, E. B. Abuin, E. A. Lissi, N. M. Correa, J. Phys. Chem. B 2013, 117, 5392 – 5392. [26] R. Karmakar, A. Samanta, J. Am. Chem. Soc. 2001, 123, 3809 – 3817. [27] A. Maciejewski, J. Kubicki, K. Dobek, J. Phys. Chem. B 2003, 107, 13986 – 13999.
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www.chemphyschem.org [28] S. Mukherjee, K. Sahu, D. Roy, S. K. Mondal, K. Bhattacharyya, Chem. Phys. Lett. 2004, 384, 128 – 133. [29] A. Maciejewski, J. Kubicki, K. Dobek, J. Phys. Chem. B 2005, 109, 9422 – 9431. [30] S. K. Mondal, D. Roy, K. Sahu, P. Sen, R. Karmakar, K. Bhattacharyya, J. Photochem. Photobiol. A 2005, 173, 334 – 339. [31] A. Samanta, J. Phys. Chem. B 2006, 110, 13704 – 13716. [32] A. Maciejewski, J. Kubicki, K. Dobek, J. Colloid Interface Sci. 2006, 295, 255 – 263. [33] R. Wang, C. Hao, P. Li, N.-N. Wel, J. Chen, J. Qiu, J. Comput. Chem. 2010, 31, 2157 – 2163. [34] A. Samanta, J. Phys. Chem. Lett. 2010, 1, 1557 – 1562. [35] D. C. Khara, J. P. Kumar, N. Mondal, A. Samanta, J. Phys. Chem. B 2013, 117, 5156 – 5164. [36] C. F. Chapman, R. S. Fee, M. Maroncelli, J. Phys. Chem. 1995, 99, 4811 – 4819. [37] J. Dresner, J. Prochorow, A. Sobolewski, Chem. Phys. Lett. 1978, 54, 292 – 294. [38] M. J. Mirbach, M. F. Mirbach, W. R. Cherry, N. J. Turro, P. Engel, Chem. Phys. Lett. 1978, 53, 266 – 269. [39] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science Business Media, New York, 2006. [40] T. Fçrster, K. Rokos, Chem. Phys. Lett. 1967, 1, 279 – 280. [41] G. W. Robinson, R. P. Frosch, J. Chem. Phys. 1962, 37, 1962 – 1973. [42] P.-A. Muller, C. Hogemann, X. Allonas, P. Jacques, E. Vauthey, Chem. Phys. Lett. 2000, 326, 321 – 327 and references therein. [43] D. D. Perrin, W. L. F. Armarego, D. D. Perrin, Purification of Laboratory Chemicals, Pergamon Press Ltd. Oxford, England, 1980.
Received: January 27, 2014 Published online on && &&, 2014
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ARTICLES Insight into photophysics: The time-resolved and time-integrated fluorescence response of 4-aminophthalimide and its derivative does not indicate photoinduced solvent-mediated intramolecular proton transfer in protic media. Two distinct types of hydrogen-bonded complex of the molecule with the solvent are identified and a new explanation for the solvent isotope effect is suggested.
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D. C. Khara, S. Banerjee, A. Samanta* && – && Does Excited-State Proton-Transfer Reaction Contribute to the Emission Behaviour of 4-Aminophthalimide in Aqueous Media?
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Does Excited-State Proton-Transfer Reaction Contribute to the Emission Behaviour of 4-Aminophthalimide in Aqueous Media? Dinesh Chandra Khara, Sanghamitra Banerjee, and Anunay Samanta*[a] 4-Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent-sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time-dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited-state solvent-mediated proton-transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide-H protected derivative, N-BuAP, to prove that a solvent-assisted excited-
state keto–enol transformation does not contribute to the steady-state and time-resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogenbonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited-state proton-transfer reaction in the system, is explained by considering the difference in the influence of H2O and D2O on the nonradiative rates and ground-state exchange of the proton with the solvent.
1. Introduction The photophysics of 4-aminophthalimide (AP) has been studied extensively due to its attractive fluorescence properties, in particular, high fluorescence quantum yield, long fluorescence lifetime and sensitivity of these properties to the surrounding environment. The molecule is used routinely as a fluorescence probe for studies of solvation dynamics, to estimate the polarity of unknown media and probe the organised environments of complex chemical and biological systems.[1–36] It is generally believed that photoexcitation of AP leads to enhanced separation of charge and the molecule emits from an intramolecular charge-transfer (ICT) state, whose energy is determined by the polarity and hydrogen-bonding ability of the solvent.[3, 8, 10] The fluorescence quantum yield and lifetime of AP are also highly sensitive to the environment.[3, 8, 10] In contrast to its behaviour in aprotic media, where AP is strongly fluorescent and possesses a long fluorescence lifetime, in protic solvents the molecule emits weakly and has a short fluorescence lifetime. The solute– solvent hydrogen-bonding interaction was considered to be responsible for a large Stokes shift of the fluorescence maximum, a low fluorescence yield and a short fluorescence lifetime of the molecule in protic media. However, an alternative [a] Dr. D. C. Khara,+ S. Banerjee, Prof. A. Samanta School of Chemistry University of Hyderabad Hyderabad 500 046 (India) E-mail: [email protected] [+] Current address: Institute of Atomic and Molecular Sciences Academia Sinica, Taipei 106 (Taiwan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201400078.
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explanation in terms of solvent-assisted intramolecular proton transfer (Scheme 1) was also proposed, according to which the emission of AP originates from its enol form in protic media.[12]
Scheme 1. Solvent-assisted intramolecular proton transfer in AP.
Even though no kinetic evidence in support of this transformation was available, a higher fluorescence lifetime and quantum yield of AP in D2O compared to H2O was considered to be consistent with this mechanism.[13] However, even though subsequent studies revealed drawbacks of this mechanism,[14] the concept of a solvent-mediated excited-state keto–enol transformation of the system was reintroduced recently to account for observed excitation-wavelength-dependence of the emission spectrum and a progressive blue shift of the emission maximum of AP with time in aqueous media.[22] Biexponential fluorescence-decay behaviour and time-dependent shift of the fluorescence spectrum are argued to be direct kinetic evidence of the excited-state reaction of AP in aqueous media.[22] As we could not reproduce the excitation wavelength dependence of steady-state fluorescence spectrum of AP, we did not take the mechanism seriously.[23] However, the kinetic data presented in ref. [22] in support of the excited-state reaction required careful consideration as ChemPhysChem 0000, 00, 1 – 7
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a biexponential decay behaviour and time-dependent shift of the emission spectrum were not expected in the absence of an excited-state reaction. Moreover, even though dynamic Stokes shift of the fluorescence spectrum due to excited-state reaction is common, a progressive shift of the spectrum towards higher energy is very unusual, if not unprecedented. It is also puzzling that the spectral shift is not continuous, but mostly occurs between 2–4 ns.[22] To address these points and establish that excited-state proton-transfer reaction does not influence the photophysics of AP in aqueous media, we have undertaken this work on AP and its derivative (N-BuAP, Figure 1), in which the imide hydrogen atom, which is shown to be involved in water-assisted excited-state proton-transfer process, is replaced by an n-butyl group.
Figure 1. Molecular structures of AP and its derivative (used in this work).
2. Results and Discussion
Figure 2. Steady-state emission spectra of a) AP and b) N-BuAP (lex = 370 nm) in acetonitrile and water.
Figure 2 compares the steady-state fluorescence spectra (lex = 370 nm) of AP and N-BuAP in aprotic (acetonitrile, ACN) and protic (water) solvents. N-BuAP also exhibits a large difference in the lem max values in the two solvents ( 100 nm, Table 1), which led to the postulation that the emission of AP originates
Table 1. Comparison of some of the fluorescence parameters of AP and N-BuAP. Solvent
lem max
ACN H2O
AP 460 565
N-BuAP 472 577
(IAP/IN-BuAP)[a]
ff (AP)
1.2 1.5
0.63[18] 0.022[18]
[a] IAP and IN-BuAP are integrated areas under the fluorescence spectra of AP and N-BuAP, respectively, recorded under identical experimental conditions.
from the enol form of the molecule that is produced as a result of excited-state water-assisted transfer of the imide proton (Scheme 1). The strikingly similar fluorescence spectra of the two systems thus clearly rule out the possibility of the enol form of AP as the emitting species in protic solvents such as water. One can argue against the excited-state solvent-mediated proton-transfer mechanism from another angle. As in aprotic solvent acetonitrile AP emits (lmax = 460 nm) from its normal (keto) form, if the emission of AP in aqueous media (lmax = 565 nm) indeed originates from its enol form, then on gradual 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
addition of water to an acetonitrile solution of AP, a progressive increase of the fluorescence intensity of the 560 nm band at the expense of the 460 nm emission is expected. By considering the low fluorescence quantum yield of AP in water (compared to acetonitrile), one expects the decrease in intensity at 460 nm to be more pronounced than the increase in intensity at 565 nm. However, the spectral behaviour of AP and N-BuAP presented in Figure 3 shows a gradual shift of the emission spectrum towards longer wavelength instead of the expected trend, which thus disproves the excited-state proton-transfer reaction of AP. Moreover, a very similar fluorescence response (including the time-resolved behaviour, see Supporting Information) of AP and N-BuAP, the latter lacking the imide hydrogen atom essential for the formation of the enol form of the molecule, further reinforces our conclusion. 2.1. Isotope Effect As stated in the Introduction, a significantly higher fluorescence quantum yield (ff) and lifetime (tf) of AP in D2O when compared with that in H2O was thought to be consistent with excited-state solvent-mediated transfer of the imide hydrogen atom to the carbonyl oxygen atom.[13, 16, 22] However, as the results presented above rule out this possibility, one needs to reexamine the isotope effect carefully and then determine its origin. For this purpose, we first find out whether N-BuAP, which lacks the imide hydrogen atom, also exhibits a deuteriChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org systems, this does not necessarily mean that no proton of the fluorescent systems is exchanged with the solvent (H2O or D2O) in the ground state. Both AP and N-BuAP have amino protons, which are sufficiently acidic due to the inductive influence of the carbonyl group and can be exchanged with solvent H or D atoms in the ground state. Evidence of this kind of exchange process is often obtained from 1H NMR measurements, where the signals due to the acidic protons are seldom observed. We emphasise, however, that as these rates are lower than the radiative and nonradiative rates of the systems by several orders of magnitude, they do not play a noticeable role in the fluorescence behaviour of the systems. Under these conditions, in deuterated solvents, one is dealing not only with the original molecule, but also the deuterium-exchanged molecule. If the fluorescence efficiencies of a system and its deuterated form are different due to different rates of the nonradiative relaxation process (which is quite likely),[42] the deuterium isotope effect can also be observed. Hence, in our opinion the isotope effect exhibited by AP and N-BuAP arises from one or both the factors described above. 2.2. Wavelength-Dependent Fluorescence Decay and TimeResolved Emission Spectrum
Figure 3. Steady-state emission spectra of a) AP and b) N-BuAP in acetonitrile–water mixture (lex = 370 nm).
um isotope effect. Like AP, the ff and tf values of N-BuAP are also higher in D2O (than in H2O) by factors of 2.0 and 4.5, respectively. The values of ff and tf of both AP and N-BuAP are about threefold higher in CD3OD than in CH3OH. Thus, as the isotope effect is real, but not due to the excited-state protontransfer reaction, its origin must lie elsewhere. Solvent deuterium isotope effect on ff and tf suggests a change in the nonradiative rate of the emitting state of the system on isotopic substitution.[37] This change is expected to be most pronounced if the solvent proton participates in the reaction that acts as one of the nonradiative decay channels.[38] However, this reaction need not be the only pathway for the radiationless process. Solvent often plays a crucial role in influencing the nonradiative relaxation (intersystem crossing and internal conversion) rates by directly interacting with the solute.[39] A strong interaction of the electronic motion of the fluorescent system with the nuclear motion of the solvent can provide a mechanism for the conversion of electronic energy of the system into the vibrational energy of the solvent, which thus results in radiationless interconversion that affects a change of the ff and tf values. In such a situation, isotopic substitution in the solvent can influence the coupling between the electronic motions of the solute and vibrational motion of the solvent and thus can contribute to the isotope effect.[40, 41] A second factor can contribute to the isotope effect. Even though we have ruled out the influence of excited-state proton-transfer reaction on the emission behaviour of the two 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
We now reinvestigate carefully the time-resolved fluorescence behaviour of the two systems and find out whether it is due to an excited-state reaction. Considering that the emission peak of AP and N-BuAP is observed at around 565 nm in aqueous media, we have measured the fluorescence decay profiles at this wavelength. These fluorescence intensity versus time profiles of the two systems (Figure 4) can be fitted satisfactorily to a single exponential decay function that yields lifetimes of around 1.10 ns, which is consistent with literature values.[13, 18] Considering the recent report[22] of time-dependent shift of the fluorescence spectrum we studied the time-resolved fluorescence response of the systems in more detail. As the timeresolved emission spectra (TRES) of a system can be constructed from its fluorescence decay profiles at several wavelengths covering the entire emission spectra, we also measured the fluorescence decay profiles of the two systems at various other wavelengths, and the results (Figure 5) are quite revealing. Contrary to the decay profiles measured at the peak maxima or at higher wavelengths, which are mainly characterised by a single exponential decay function, those measured at shorter wavelengths are clearly biexponential and consist of a longlifetime (minor) component of lifetime about 10.0 ns, in addition to the short-lifetime component of around 1.1 ns (major). With an increase in the monitoring wavelength, the contribution of the long component diminishes gradually and as the wavelength corresponding to the emission peak position is reached, the long component vanishes almost completely to give a single exponential decay with a lifetime of around 1.1 ns, which is commonly taken as the fluorescence lifetime of AP in aqueous media. We shall discuss the origin of the two specific lifetime components later. However, having observed two lifetime components and the nature of variation of its amplitude with monitoring wavelength, it became clear how ChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org a blue shift of the TRES, as reported recently,[22] is observed. The TRES recorded at early times consisted primarily of the short component and those recorded beyond 3 ns had no contribution from the short component, which is why the blue shift was observed in the 3–4 ns time range.
2.3. Possible Origin of The Two Lifetime Components While the dependence of the fluorescence decay profiles on the monitoring wavelength (as described above) explains the blue shift of TRES (as observed by Durantini et al),[22] one does not expect decay profiles of this kind in the case of an excitedstate reaction. Had the two species been related through an excited-state reaction, there would have been a rise component in the fluorescence time profiles monitored at longer wavelengths. That the time-resolved behaviour, depicted in Figure 6, is strikingly similar for AP and N-BuAP clearly suggests
Figure 4. Fluorescence decay profiles of a) AP and b) N-BuAP monitored at 565 nm in water. Solid lines represent a single exponential fit to the decay profiles (lex = 375 nm).
Figure 6. Time-resolved emission spectra of a) AP and b) N-BuAP in water (lex = 375 nm).
Figure 5. Wavelength-dependent fluorescence decay profiles of a) AP and b) N-BuAP in aqueous solution (lex = 375 nm).
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that the emission of AP in aqueous media has nothing to do with the enol form. The nature of the decay profiles observed here indicates the presence of two separate emitting species that are not related through an excited-state reaction. We next attempt to find out the possible origin of the two lifetime components in water. The fluorescence quantum yield and lifetime of AP in polar media depend on the hydrogenbond donating/accepting ability of the solvents.[18] These ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES values are low in polar protic solvents and the lowest in water. The low ff and tf values in polar protic solvents are believed to be due to stabilisation of the emitting state of the system as a result of strong hydrogen-bonding interaction of AP with the solvent and consequent increase in the nonradiative rate of the emitting state. Two different kinds of hydrogen-bonding interaction (Figure 7) are documented in the literature.[18] The
Figure 7. Types of hydrogen-bonding interactions possible between AP and water.
most common of these interactions is hydrogen-bond formation between the carbonyl oxygen atom of AP and hydrogenbond-donating protic solvents such as hexafluoroisopropanol or trifluoroethanol, which leads to a red shift of the emission maximum and a decrease in ff and tf values.[18] The other possible mode of hydrogen-bonding interaction involves the amino hydrogen atom of AP and hydrogen-bond-accepting solvents (in solvents such as in DMSO), which leads to a blue shift of the emission spectrum and an increase in ff and tf values.[18] Solvents such as alcohols and water, which can act both as H-bond donor and acceptor, can therefore interact with AP through both types of hydrogen-bond formation (Figure 7). As the hydrogen-bonding interaction with the amino moiety leads to a blue shift and that with the carbonyl moiety results in a red shift of the emission spectrum, when one monitors at wavelengths corresponding to the blue side of the spectrum, the molecules hydrogen-bonded at the amino end (and are characterised by long lifetime) contribute more to the fluorescence. On the other hand, as one moves towards longer wavelength, the contribution due to hydrogenbonded species with the carbonyl moiety dominates and one observes only the species with a short lifetime.
3. Conclusions We have convincingly established that solvent-mediated excited-state proton-transfer reaction does not contribute to the fluorescence behaviour of AP and its derivatives. We have also shown that the time-dependent blue shift of the fluorescence spectrum of AP in aqueous media, which was recently presented as evidence in support of the water-mediated excited state keto–enol transformation of the molecule, arises due to the presence of two different types of hydrogen-bonded species of AP with distinctly different fluorescence lifetimes. The solvent isotope effect on the fluorescence properties of AP and N-BuAP is explained by considering the difference in the influence of H2O and D2O on the nonradiative relaxation rates of the systems. It is suggested that ground-state exchange of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org amino hydrogen atom of the systems with the solvent can also contribute to the isotope effect.
Experimental Section AP was obtained from TCI (Japan) and was recrystallised from ethanol prior to use. N-BuAP was synthesised and purified according to a literature procedure.[14] The purity of the molecules was ascertained by NMR spectroscopy and also by the appearance of a single spot in thin-layer chromatography. Methanol and acetonitrile were obtained from Merck (India) and purified according to standard procedures prior to use.[43] Deuterated water and methanol were obtained from Merck (India) and used without any treatment. Double-distilled water was used throughout. The NMR spectra were recorded in Bruker AVANCE 400 MHz NMR spectrometer. The absorption and steady-state fluorescence spectra were recorded on a UV/Vis spectrophotometer (Cary100, Varian) and spectrofluorimeter (FluoroLog, Horiba Jobin Yvon), respectively. The fluorescence spectra were corrected for the instrumental response. Time-resolved fluorescence decay measurements were carried out using a time-correlated single-photon-counting (TCSPC) spectrometer (Horiba Jobin Yvon IBH). A diode laser (lex = 375 nm) was used as excitation source, and a microchannel plate (MCP) photomultiplier (Hamamatsu R3809U-50) was used as detector (response time 40 ps). The instrument response function (IRF) of the setup (50 ps) was limited by the full-width at half-maximum (FWHM) of the exciting laser pulse. The lamp profile was recorded by placing a scatterer (dilute solution of Ludox in water) in place of the sample. Decay curves were analysed by using a nonlinear least-squares iteration procedure using IBH DAS6 (Version 2.2) decay analysis software. The qualities of the fits were assessed by analysis of the c2 values and distribution of the residuals. Time-resolved emission spectra (TRES) were recorded directly in the TCSPC fluorescence spectrometer described above.
Acknowledgements This work was supported by the J.C. Bose National Fellowship (to A.S.) of the Department of Science and Technology (DST), Government of India. D.C.K. thanks the Council of Scientific and Industrial Research (CSIR) for a Senior Research Fellowship. S.B. acknowledges the Fellowship received from the University Grants Commission (UGC). Keywords: fluorescence spectroscopy · hydrogen bonding · isotope effects · keto–enol transformations · solvent effects [1] W. R. Ware, S. K. Lee, G. J. Brant, P. P. Chow, J. Chem. Phys. 1971, 54, 4729 – 4737. [2] P. Suppan, J. Chem. Soc. Faraday Trans. 1987, 83, 495 – 509. [3] D. Noukakis, P. Suppan, J. Lumin. 1991, 47, 285 – 295. [4] T. Soujanya, T. S. R. Krishna, A. Samanta, J. Photochem. Photobiol. A 1992, 66, 185 – 192. [5] T. Soujanya, T. S. R. Krishna, A. Samanta, J. Phys. Chem. 1992, 96, 8544 – 8548. [6] G. Saroja, A. Samanta, Chem. Phys. Lett. 1995, 246, 506 – 512. [7] T. O. Harju, A. H. Huizer, C. A. G. O. Varma, Chem. Phys. 1995, 200, 215 – 224. [8] T. Soujanya, R. W. Fessenden, A. Samanta, J. Phys. Chem. 1996, 100, 3507 – 3512. [9] D. Yuan, R. G. Brown, J. Phys. Chem. A 1997, 101, 3461 – 3466.
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Received: January 27, 2014 Published online on && &&, 2014
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ARTICLES Insight into photophysics: The time-resolved and time-integrated fluorescence response of 4-aminophthalimide and its derivative does not indicate photoinduced solvent-mediated intramolecular proton transfer in protic media. Two distinct types of hydrogen-bonded complex of the molecule with the solvent are identified and a new explanation for the solvent isotope effect is suggested.
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D. C. Khara, S. Banerjee, A. Samanta* && – && Does Excited-State Proton-Transfer Reaction Contribute to the Emission Behaviour of 4-Aminophthalimide in Aqueous Media?
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