LETTER pubs.acs.org/JPCL
Origin of Ultrafast Radiationless Deactivation Dynamics of Free-Base Subpyriporphyrins Kil Suk Kim,† Jong Min Lim,† Radomir Mysliborski,‡ Mizosz Pawlicki,‡ Lechoszaw Latos-Gra_zynski,*,‡ and Dongho Kim*,† † ‡
Department of Chemistry, Yonsei University, Seoul 120-749, Korea Department of Chemistry, University of Wroczaw, 14 F. Joliot-Curie Street, Wroczaw 50 383, Poland
bS Supporting Information ABSTRACT: In this study, we have investigated the photophysical properties of free-base subpyriporphyrins by changing the peripheral substituents from phenyl to nitrophenyl groups. While the electron withdrawing nature of nitrophenyl substituent gives rise to a noticeable perturbation in the absorption spectrum arising from the charge-transfer (CT) transition, both molecules show similar excited state dynamics at ambient temperature. This feature is mainly due to the fact that the CT transitions caused by the nitrophenyl substituent in free-base subpyriporphyrin are located higher in energy than the lowest excited S1 state. Moreover, through the temperature dependence and protonation experiments, we have demonstrated that the NH-tautomerization is a key factor in determining the excited state dynamics of free-base subpyriporphyrins. In this sense, we can suggest that the control of substituent and solvents in free-base subporphyrinoids could be a new strategy for the fine-tuning of photophysical properties. Furthermore, to the best of our knowledge, this is the first study to illustrate the photophysical properties of free-base subporphyrinoids. SECTION: Kinetics, Spectroscopy
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ubporphyrinoid is a contracted porphyrin congener containing three pyrroles or pyrrole-related rings linked through meso-sp2 carbon atoms.1 In recent years, there has been a considerable interest in the macrocycles of this type owing to their potential applications as green dyes, nonlinear optical (NLO) materials, and photonic devices.2-4 In this regard, extensive studies on the novel synthetic protocols and characterizations of subporphyrinoids have been conducted. Since subphtahalocyanine was first synthesized by Meller and Ossko in 1972,5 subporphyrazines,6 tribenzosubporphyrins,7 and mesoaryl subporphyrins8,9 have also been newly synthesized by pioneer research groups. Recently, numerous studies have focused on spectroscopic investigations of subporphyrinoids such as steady- and excitedstate dynamics,10 NLO properties,11,12 magnetic circular dichroism (MCD) analysis,13 electron transfer phenomena,14 and so forth, depending on modifications of molecular structures and substituents. Nevertheless, most of the studied subporphyrinoids have boron centers (i.e., 1, Scheme 1), revealing strong diatropic ring-current effects in NMR spectra associated with H€uckel aromaticity, suggesting the existence of a 14π-electron aromatic core.15 Furthermore, these subporphyrinoids generally reveal the blue-shifted absorption spectra related to their corresponding 18π tetrapyrrolic molecules and intense fluorescence behaviors.15 However, photophysical characterizations of freebase subporphyrinoids have never been studied hitherto because of a lack of synthetic methodologies. r 2011 American Chemical Society
Scheme 1. Schematic Molecular Structures of Subporphyrin Boron Complex (1) and Subpyriporphyrins (2)
In 2006, Latos-Gra_zynski and co-workers reported the synthesis and structural characterization of the first free-base type subporphyrinoid named as subpyriporphyrin (2a) containing two pyrroles and one pyridine in its macrocyclic ring (Scheme 1).16 Potentially, subpyriporphyrin can be included into multielement porphyrin arrays using perimeter adjustments, developed for tetraarylporphyrins. It should be noted that, while most 14π aromatic boron-centered subporphyrinoids Received: December 27, 2010 Accepted: February 2, 2011 Published: February 14, 2011 477
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Figure 2. Temperature-dependent absorption changes at 77-293 K and fluorescence spectrum at 77 K of 2a in 2-Me-THF. Inset shows a fluorescence decay profile (black line) and fitting line (red line) at 77 K.
Figure 1. Steady state absorption and calculated vertical transition energies of 2a (a) and 2b (b).
(HOMO = highest occupied molecular orbital) transitions, indicating that the hypsochromic shift of 2b could be explained by an increased HOMO-LUMO gap. Consequently, the absorption spectral changes of 2b can be understood in terms of an interplay between the increased HOMO-LUMO gap and CT characteristics of 2b. Remarkable differences in the absorption spectra did not affect the emission properties of subpyriporphyrins. We could not observe any emission from the visible to near-IR region at 293 K for both molecules. This spectroscopic feature remains in sharp contrast with the intense fluorescence behaviors of most 14π subporphyrins.15 Thus, to find the reason for this feature, we have performed various temperature-dependent spectroscopic experiments. Upon lowering the temperature, the absorption spectra of 2a reveal vibronic peaks at 710 and 773 nm. At the same time, a well-resolved fluorescence was observed at 77 K (Figure 2). Furthermore, 2b shows similar temperature-dependent spectral changes in the absorption and fluorescence spectra (see Figure S4 in the SI). Importantly, these fluorescence spectra of free-base subpyriporphyrins were gradually decreased by increasing the temperature from 77 to 133 K, which corresponds to the solid phase zone of 2-Me-THF. Interestingly, above the melting point of 2-Me-THF, the fluorescence almost disappears (see Figure S5 in the SI). On the basis of these results, it is clear that the fluorescence features of 2a and 2b are sensitive to the phase of 2-Me-THF. Furthermore, we investigated the singlet excited state dynamics of 2a and 2b at 293 K in 2-Me-THF by femtosecond transient absorption spectroscopy. The S1 state lifetime of 2a was estimated to be about 8.1 ps, which is consistent with the nonfluorescent feature at 293 K. Similar to 2a, the S1 state of 2b also exhibits ultrafast deactivation process with the lifetime of only 7.6 ps at 293 K (Figure 3). Contrary to the ultrafast S1 lifetime of 2a at 293 K, its S1 lifetime dramatically increased from 8.1 to 458 ps in 2-Me-THF glass matrix at 77 K (inset in Figure 2). The fluorescence lifetimes of 2a remained almost constant with about 400 ps in the temperature range of 77113 K and drastically dropped down to the complete disappearance as the temperature reached the melting point of 2-Me-THF (see Figure S6 in the SI). In addition, the S1 lifetime of 2b was estimated to be 580 ps at 77 K, and similar temperature dependence behaviors were observed in the range of 77-153 K
show blue-shifted absorption spectra and intense fluorescence features with a few nanosecond lifetimes, 2a shows quite unique absorption spectra along with a nonfluorescence behavior at room temperature. In addition, the 1H NMR spectrum reveals a nonaromatic character of 2a with a strong N-H-N hydrogen bonding interaction.16 To understand unique photophysical behaviors of free-base subpyriporphyrin, we have investigated the excited state dynamics of 2a focused on the origin of radiationless deactivation processes by various temperaturedependent spectroscopic measurements, protonation experiments, and theoretical calculations. In addition, we have recently synthesized and characterized an analogue of 2a, where the electronic properties of meso-substituent were changed by involving a nitro group (2b, see Experimental Details in the Supporting Information (SI)). To the best of our knowledge, these experiments performed on both macrocycles present the very first study illustrating the photophysical properties of free-basetype subporphyrinoids. The steady-state absorption spectra of 2a and 2b measured at 293 K in 2-methyltetrahydrofuran (2-Me-THF) present significant differences. 2a shows an intense peak at 345 nm and an extremely broad band in the 600-900 nm region. On the other hand, 2b reveals remarkable absorption spectral perturbations such as two new peaks at 432 and 524 nm and a hypsochromic shift of the lowest absorption band (Figure 1). To find out the origin for the absorption spectral differences between 2a and 2b, we have performed theoretical calculations. The optimized molecular structures of both molecules exhibit fairly planar geometries (see Figure S1 in the SI) and the calculated vertical transitions match well with the experimental absorption spectra with respect to both positions and relative intensities (Figure 1). Compared with the molecular orbital (MO) diagram of 2a, 2b reveals unique MOs with localized electron density on nitrophenyl substituent (LUMOþ1 and LUMOþ2; LUMO = lowest unoccupied molecular orbital) due to the electron withdrawing effect (see Figure S2 in the SI). According to the time-dependent density functional theory (TD-DFT) calculations, the new peaks at 432 and 524 nm correspond to the strong charge-transfer (CT) transitions, involving these two localized MOs (see Figure S3 in the SI). Furthermore, the origin of the lowest absorption bands for both molecules was assigned mainly as HOMO-LUMO 478
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Figure 4. Schematic diagram for the energy relaxation pathways of subpyriporphyrin at 77 and 293 K.
Compared with the neutral 2a molecule, protonated 2a reveals more distorted structure due to the repulsions between additional hydrogens.20 That is, if the NH tautomerization of 2a does not play a major role in the radiationless deactivation process at room temperature, it is expected that the nonradiative deactivation process becomes more effective for distorted protonated 2a than for neutral 2a. On the contrary, the protonated 2a reveals distinct fluorescence features, which is opposite to the above hypothesis (see Figure S10 in the SI). Furthermore, we also investigated the excited state dynamics of protonated 2a (see Figure S11 in the SI). As mentioned before, the excited-state lifetime of neutral 2a was estimated to be 8.1 ps. However, the protonated 2a shows distinct biexponential decay behavior with the time constants of about 8 and 95 ps. After adding TFA to the solution, the contribution by the short decay component gradually decreases from 100 to 11%, while the long decay component increases from 0 to 89%. The increasing portion of longer lifetime component at the expense of shorter one upon adding TFA seems to be correlated with the tautomerization locking for the protonated 2a, which is consistent with the detailed 1H NMR titration results (see Figure S8 in the SI). In other words, the short lifetime contribution observed for both protonation steps can be explained by the presence of free pyridine, allowing the suggested decay mechanism to occur. It leads to a conclusion that the nonradiative process takes place for neutral and monocationic forms where the NH tautomerism is possible, while the long lifetime is observed solely for the dication (see Figure S12 in the SI). On the basis of these experiments, we could suggest that the NH-tautomerization process in 2a involving intramolecular hydrogen bonding is responsible for ultrafast excited-state decay due to the acceleration of nonradiative decay processes at room temperature. To explain the longer excited-state dynamics of free-base subpyriporphyrins in a low-temperature glass matrix more clearly, we obtained the singlet and triplet energy level diagrams of accessible tautomers (2a and 2a0 ). According to the DFT calculations, the S0 state of 2a is energetically more stable than that of 2a0 by 4.8 kcal/mol (Figure 4). Additionally, the activation barrier between two tautomers was estimated to be 5.4 kcal/mol via transition energy calculations. In contrast with the S0 state, however, the S1 state of 2a is more unstable than that of 2a0 by 2.1 kcal/mol. In this regard, there is a considerable validity in effective excited state proton transfer from 2a to 2a0 . Furthermore, the S0-S1 energy gap of 2a0 (1.39 eV) is much smaller than that of 2a (1.69 eV) (see Figure S13 in the SI). In addition, the molecular structure of 2a0 reveals more twisted conformation
Figure 3. Femtosecond transient absorption spectra of 2a (a) and 2b (b) at 293 K in 2-Me-THF. Insets reveal representative kinetic profiles (black dot) and fitting lines (red line) of 2a and 2b.
(see Figure S6 in the SI). Interestingly, in contrast with considerable differences in the absorption spectra between 2a and 2b by electron withdrawing substituent effect, both molecules do not show substituent dependence on the excited S1 state dynamics, presumably because CT transitions in 2b are located higher in energy than the lowest excited S1 state. Instead, their excited state dynamics exhibit a dramatic change according to the phase of solvent matrix. As the organic solvent surrounding the molecular system becomes a glass matrix, the molecular motion becomes restricted. Thus, as conformational dynamics such as tautomerization,17 cis-trans isomerization,18 and so forth exist in molecular systems, these motions could be inhibited in rigid environment. In fact, Waluk et al.17,19 suggested that the NHtautomerization of free-base porphycene is an important factor to induce the radiationless deactivation processes through conical intersection from a planar trans tautomer to a twisted cis one, and these tautomerization processes accompanied by molecular distortion could be hindered in a rigid polymer matrix. In a similar manner, we have investigated the fluorescence spectrum and lifetime of 2a in a poly(methyl methacrylate) (PMMA) matrix at 293 K. Interestingly, we observed fluorescence and a much longer excited-state lifetime compared with that of 2-MeTHF (see Figure S7 in the SI). From these features, the temperature-dependent photophysical behaviors of free-base subpyriporphyrins seem to be associated with the locking of tautomerization caused by the rigidification of 2-Me-THF. To obtain further evidence on this conjecture, we have performed a protonation experiment with trifluoroacetic acid (TFA) to restrict the tautomerization process of free-base subpyriporphyrin. As reported previously, 2a reveals two-step protonation processes in the 1H NMR titration spectra, and the protonated 2a exhibits more distorted structure than that of a neutral one due to inner hydrogen repulsion inside the macrocycle.20 During spectrophotometric titration, the spectral changes revealed two-step processes clearly, which is in a good agreement with the 1H NMR titration results (see Figures S8 and S9 in the SI). In general, since the structural distortion can accelerate the nonradiative deactivation process, distorted molecules exhibit lower fluorescence quantum yield and shorter excited-state lifetime than their corresponding planar ones.21-24 479
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The Journal of Physical Chemistry Letters than that of 2a (see Figure S14 in the SI). From this point of view, the ultrafast relaxation dynamics of 2a0 can be explained by the structural distortion and reduced S0-S1 energy gap, which cause an acceleration of nonradiative decay processes. As 2-Me-THF becomes solid matrix upon lowering temperature below its melting point, the excited-state tautomerization is blocked through a combinative effect of temperature lowering and environmental rigidification. Thus, as the tautomerization was locked in the low-temperature solid matrix, we observed the unique optical properties of 2a without a perturbation by 2a0 . As a result, the phosphorescence spectrum of 2a could be observed at 77 K, and the energy level of the triplet state is quite consistent with the calculated T1 energy level of 2a (see Figure S15 in the SI). Furthermore, the excited triplet state dynamics of 2a shows a similar behavior to the excited singlet state dynamics (see Figure S16 in the SI). Consequently, based on our experimental and theoretical investigations, we can propose the energy relaxation pathways of free-base subpyriporphyrin at 293 and 77 K (Figure 4). At 293 K, the tautomerization of free-base subpyriporphyrin causes the effective radiationless deactivation processes. However, once the tautomerization is locked into the most stable tautomer at 77 K, we have solely observed the photophysical properties of the most stable form. In summary, we have investigated the photophysical properties of free-base subpyriporphyrins by changing the peripheral substituents from phenyl to nitrophenyl groups. While the electron-withdrawing nature of the nitrophenyl substituent gives rise to a noticeable perturbation in the absorption spectrum arising from the CT transitions, both of molecules show similar excited-state dynamics at ambient temperature. This feature is mainly due to the fact that the CT transitions caused by the nitrophenyl substituent in 2b are located higher in energy than the lowest excited S1 state. On the basis of temperature dependence and protonation experiments, we have demonstrated that the NH-tautomerization is a critical factor in determining the excited state dynamics of 2a and 2b. In this sense, we can suggest that the control of substituents and solvents in free-base subporphyrinoids could be a new strategy for the fine-tuning of photophysical properties.
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National Research Foundation of Korea funded by the Ministry of Education, Science and Technology and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea. The quantum calculations were performed by using the supercomputing resource of the Korea Institute of Science and Technology Information (KISTI). The work at the University of Wroczaw was supported by the Ministry of Higher Education (Grant No. N204 021939).
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’ ASSOCIATED CONTENT
bS
Supporting Information. (1) Experimental details; (2) optimized molecular structures and MO diagrams of 2a and 2b; (3) temperature dependence absorption and fluorescence spectra of 2a and 2b; (4) fluorescence spectra and lifetime of 2a in PMMA; (5) spectrophotometric titration results of 2a with TFA; (6) femtosecond transient absorption decay profiles and spectra of neutral and protonated forms of 2a; (7) phosphorescence spectra and triplet excited state lifetime of 2a. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (D.K.);
[email protected]. wroc.pl (L.L-G.).
’ ACKNOWLEDGMENT The work at Yonsei University was financially supported by the World Class University (R32-10217) program through the 480
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(17) Sobolewski, A. L.; Gil, M.; Dobkowski, J.; Waluk, J. On the Origin of Radiationless Transitions in Porphycenes. J. Phys. Chem. A 2009, 113, 7714–7716. (18) Dugave, C.; Demange, L. Cis-Trans Isomerization of Organic Molecules and Biomolecules: Implications and Applications. Chem. Rev. 2003, 103, 2475–2532. (19) Gil, M.; Dobkowski, J.; Wiosna-Sazyga, G.; Urbanska, N.; Fita, P.; Radzewicz, C.; Pietraszkiewicz, M.; Borowicz, P.; Marks, D.; Glasbeek, M.; et al. Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: meso-Alkylated Porphycenes. J. Am. Chem. Soc. 2010, 132, 13472–13485. (20) Lanzo, I.; Quartarolo, A. D.; Russo, N.; Sicilia, E. Can Subpyriporphyrin and Its Boron Complex Be Proposed as Photosensitizers in Photodynamic Therapy? A First Principle Time Dependent Study. Photochem. Photobiol. Sci. 2009, 8, 386–390. (21) Chirvony, V. S.; Hoek, A.; Galievsky, V. A.; Sazanovich, I. V.; Schaafsma, T. J.; Holten, D. Comparative Study of the Photophysical Properties of Nonplanar Tetraphenylporphyrin and Octaethylporphyrin Diacids. J. Phys. Chem. B 2000, 104, 9909–9917. (22) Gentemann, S.; Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith, K. M.; Fajer, J.; Holten, D. Photophysical Properties of Conformationally Distorted Metal-Free Porphyrins. Investigation into the Deactivation Mechanisms of the Lowest Excited Singlet State. J. Am. Chem. Soc. 1994, 116, 7363–7368. (23) Gentemann, S.; Nelson, N. Y.; Jaquinod, L.; Nurco, D. J.; Leung, S. H.; Medforth, C. J.; Smith, K. M.; Fajer, J.; Holten, D. Variations and Temperature Dependence of the Excited State Properties of Conformationally and Electronically Perturbed Zinc and Free Base Porphyrins. J. Phys. Chem. B 1997, 101, 1247–1254. (24) Song, H. -E.; Cissell, J. A.; Vaid, T. P.; Holten, D. Photophysics of Reduced Silicon Tetraphenylporphyrin. J. Phys. Chem. B 2007, 111, 2138–2142.
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