© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 17, MAY 4, 2000
LETTERS Internal Conversion and Vibronic Relaxation from Higher Excited Electronic State of Porphyrins: Femtosecond Fluorescence Dynamics Studies Noboru Mataga,* Yutaka Shibata, and Haik Chosrowjan Institute for Laser Technology, Utsubo-Hommachi, Nishiku, Osaka 550-0004, Japan
Naoya Yoshida and Atsuhiro Osuka* Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan ReceiVed: NoVember 18, 1999; In Final Form: February 22, 2000
To elucidate the dynamics and mechanisms of radiationless transitions from higher excited electronic states as well as the ultrafast intramolecular vibronic relaxation in porphyrin derivatives, we have studied the fluorescence dynamics of Zn-tetraphenylporphyrin (ZnTPP) and Zn-diphenylporphyrin derivatives (ZnDPP) in fs-ps time regimes by means of fluorescence up-conversion technique. Detailed measurements on ZnTPP in ethanol have demonstrated fluorescence dynamics over the whole spectral range from 430 to 620 nm when excited to the S2 state. The time constant (∼2.3 ps) of the single-exponential decay of S2 fluorescence around 430 nm agreed with that of the single-exponential rise of S1 fluorescence around 600 nm (wavelength of 0-0 transition in the stationary spectrum), indicating that the relaxation by the ultrafast vibronic redistribution immediately after S2 f S1 internal conversion mainly gives lower vibronic states near the bottom of the S1 state. However, we have observed the dynamics of weak hot fluorescence probably from the nonrelaxed vibronic state immediately after internal conversion and also higher vibronic states in S1 formed in competition with the main product of the vibronic redistribution, all over the wavelength region between S2 and S1. Preliminary results of our studies on ZnDPP were very similar to those of ZnTPP.
Introduction Optical properties and photochemical reactions of porphyrin compounds have been studied for a long time from various viewpoints. They are key compounds to investigate mechanisms and dynamics of the energy and electron transfer in photosynthetic model systems. They are also of crucial importance for synthesizing various compounds aiming to construct ultrafast photoresponsive devices in general. For such purposes, thorough elucidation of their photophysical and photochemical properties, including ultrafast dynamics in excited electronic states, is supremely important. On the other hand, some Zn-porphyrin * To whom correspondence should be addressed.
compounds such as Zn-tetraphenylporphyrin (ZnTPP) and Zndiphenylporphyrin derivatives (ZnDPP) show rather easily detectable S2 fluorescence. These are interesting compounds in relation to the mechanisms of radiationless transitions from higher excited electronic states; namely, they are exceptions to the so-called Kasha’s rule. Since the advent of ultrafast laser spectroscopic methods, investigations on vibrational and vibronic relaxation of rather complex aromatic molecules including dyes in the ground state, S1 state, and higher excited electronic states have been made mainly by means of direct pump-probe measurements in psfs time regimes1,2 in solutions, and important information has been obtained.2 Nevertheless, there are still many problems to
10.1021/jp9941256 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/06/2000
4002 J. Phys. Chem. B, Vol. 104, No. 17, 2000 be elucidated concerning the mechanisms and dynamics of radiationless transitions and vibronic relaxation including those from the higher excited electronic states. From such viewpoint, various porphyrins and their derivatives seem to be the most favorable systems to clarify those rather complex problems. For example, while ZnTPP and ZnDPP show rather easily detectable S2 fluorescence as described above, their free bases show practically no S2 fluorescence. Also, there are alkylsubstituted porphyrins which are practically nonfluorescent from the S2 state. The mechanisms and dynamics of radiationless transitions from S2 and the ultrafast vibronic relaxation of these porphyrins are extremely interesting problems. From such viewpoint we are carrying out detailed and systematic fs fluorescence studies of those various porphyrins. Of course, the most fundamental information on the ultrafast intramolecular relaxation processes obtained by such studies is also very important for the studies of the dynamics and mechanisms of the ultrafast reaction such as the energy and electron transfer to combined acceptors competing with such intramolecular vibronic relaxation. We are also carrying out studies of such ultrafast electron transfer from S2 and S1 states of ZnDPP and H2 DPP to directly linked electron acceptors of various kinds with different reduction potentials. The results of such investigations will be reported shortly in a subsequent paper.3 In this letter, we report briefly the results of our studies on the ultrafast internal conversion and vibronic relaxation of ZnTPP and ZnDPP by means of a fs fluorescence up-conversion method.
Letters
Figure 1. The ground state absorption spectra (solid line) and the stationary state fluorescence spectra (dotted line) of ZnTPP excited at 405 nm in ethanol. Optical density of the S0 f S1 absorption band is 25 times magnified. [ZnTPP] ∼ 0.2 mM.
Experimental Section The measurements of the fluorescence dynamics were made by a fluorescence up-conversion apparatus similar to that described elsewhere.4,5 The exciting wavelength was 405 nm, blue edge of the Soret band, and the average power of the exciting beam was ca. 15 mW. The fwhm of the instrumental response was ca. 210 fs. Sample solutions of ZnTPP (ca. 0.2 m mol/L) in ethanol (spectro grade) purged by N2 gas were made to flow through a 1 mm optical cell. For the preliminary measurements on ZnDPP (Zn-5,15-bis(3,5-di-tert-butylphenyl)porphyrin) in tetrahydrofuran (THF, spectro grade), because of the scarcity of sample, we contained the solution purged by N2 gas in an optical cell of 2 mm path length and stirred it by using micro magnetic stirring bar. Measurements were made at room temperature (23 °C). These samples were synthesized and purified by standard methods. ZnTPP is considerably more stable for irradiation in solutions compared with ZnDPP. Because of this property, in addition to the measurements under the flowing of the solution, very accurate and much detailed measurements have been possible in the case of ZnTPP compared with ZnDPP up to now. Therefore, we show and discuss mainly the results obtained for ZnTPP in the following and show briefly that the dynamics and mechanisms of the internal conversion and vibronic relaxation of ZnDPP are essentially the same as those of ZnTPP. Results and Discussions We show in Figure 1 the ground state absorption as well as the stationary state fluorescence spectra of ZnTPP (excited at 405 nm) in ethanol. The 0-0 fluorescence bands are observed at 425 nm for S2 and at 600 nm for S1, respectively. The vibronic structures are observed at ∼1000 cm-1 lower- and higher-energy side of S1(0-0) band, probably corresponding to 0-1 and 1-0 transition, respectively. The fluorescence intensity is negligibly
Figure 2. Fluorescence rise and decay curves of ZnTPP excited at 405 nm in ethanol and observed at 15 different wavelengths from left to right: 430, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, 610, and 620 nm. The peak intensity at 430 nm (S2 fluorescence) is reduced to 10% of the observed value.
small between 480 and 550 nm of the stationary spectra, i.e., in the valley region between the S2 and S1 fluorescence bands. Nevertheless, we can recognize clearly the rise and decay of weak fluorescence emission by the fs fluorescence up-conversion method even in this region as described below. It should be noted here that the stationary absorption and fluorescence spectra of ZnDPP are rather similar to those of ZnTPP indicated in Figure 1, except that the former spectra are a little blue shifted. We have made detailed measurements on the fluorescence dynamics of ZnTPP excited at 405 nm in ethanol including not only the S2 and S1 fluorescence region but also the “valley” region of 490-580 nm between S2 and S1 fluorescence. The results are indicated schematically in Figure 2 by the threedimensional picture of the observed fluorescence rise and decay curves at various wavelengths . Our analysis of these fluorescence rise and decay curves shows that the decay of S2 fluorescence at 430 nm and the rise of S1 fluorescence at 600 nm can be reproduced by single-exponential functions, respectively, with the same time constant of 2.3 ps. This result agrees well with the previous report6 and seems to imply the very rapid relaxation to the lower vibronic levels closer to the bottom of the S1 state immediately after the S2 f S1 internal conversion. However, in view of our new findings on the weak fluorescence in the 490-580 nm region which shows rather complex
Letters
J. Phys. Chem. B, Vol. 104, No. 17, 2000 4003 By solving a series of coupled rate equations for timedependent populations of S2, Sv1, and Sb1 states, the time dependencies of these states are given as follows:
Sb1(t) ∼
Figure 3. A. Fluorescence rise and decay curves of ZnTPP in ethanol at several wavelengths from 430 to 600 nm. Solid lines are simulations. λ1 and λ2 are fixed to 4.6 × 1011 s-1 (inverse of the S2 lifetime) and 2.5 × 1013 s-1, respectively. Relative weight of Sh1(t) (eq 7) decreased and that of Sb1(t) (eq 4) increased, and λ′3 value decreased with increase of the wavelength of observation, λ′3 ) 21 × 1012 s-1 at 490 nm, 15 × 1012 s-1 at 530 nm, 0.7 × 1012 s-1 at 560 nm, and 0.5 × 1012 s-1 at 580 nm. B. Fluorescence dynamics of ZnDPP in THF at three different wavelengths where typical rise and decay curves depending upon wavelength can be observed. Inset shows the stationary state fluorescence spectrum of ZnDPP indicating the monitoring wavelengths. Fluorescence intensity is normalized to unity at the maximum of rise and decay curves in both A and B.
rise and decay dynamics as indicated in Figure 2, we need more systematic and detailed interpretations for the mechanisms and dynamics of relaxation from the S2 state of this typical porphyrin molecule. For the convenience of discussing the mechanisms of the internal conversion and vibronic relaxation in S1 state of ZnTPP, we show in Figure 3A the fluorescence rise and decay curves at 430 nm (S2), 600 nm (S1), and several selected ones in the 490-580 nm region. We also show representative rise and decay curves of ZnDPP in THF for comparison in Figure 3B. According to the previous fs laser spectroscopic studies on complex molecules in solutions,2 the intramolecular vibronic redistribution in the excited electronic state seems to take place within some 10 to 100 fs, which may be the case also in the S1 state of ZnTPP and ZnDPP immediately after the S2 f S1 internal conversion. Regarding the internal conversion and subsequent vibronic relaxations of these porphyrin compounds in solutions, we should consider here the reaction scheme of eq 1 for the dynamics of the S2 state, the vibronic state Sv1 immediately after internal conversion from S2, and the almost relaxed state Sb1 near the bottom of the S1 state formed by ultrafast vibronic redistribution from the Sv1 state
where kIC is the transition probability of the S2 f S1 internal conversion; kr is the rate constant of the vibronic redistribution from the Sv1 state; kd, kvd, and kbd are radiationless transition probabilities other than kIC and kr; and kf, kvf , and kbf are fluorescence radiative transition probabilities from the S2, Sv1 and Sb1, states, respectively.
S2(t) ∼ exp(-λ1t)
(2)
Sv1(t) ∼ [- exp(-λ2t) + exp(-λ1t)]
(3)
[
1 {exp(-λ3t) - exp(-λ1t)} + λ1 - λ3 1 {exp(-λ2t) - exp(-λ3t)} (4) λ2 - λ3
]
where λ1 ) kf + kd + kIC, λ2 ) kvf + kvd + kr, and λ3 ) kbf + kbd, and they are inverse of the lifetimes of the S2, Sv1, and Sb1 states, respectively. The Sv1 state is very short-lived due to the ultrafast vibronic redistributions (10 ∼ 100 fs) compared with the S2 state (∼ ps), and both of these states are very short-lived compared with the Sb1 state (∼ ns), i.e., λ3 , λ1 , λ2. Therefore, the second term in eq 4 is negligible compared with the first term, and the time dependence of Sb1(t) can be written approximately as
Sb1(t) ∼ [1 - exp(-λ1t)]
(5)
Equations 2 and 5 agree with the observed results for ZnTPP that the decay time of the S2 fluorescence is approximately the same as the fluorescence rise time of S1 state around 600 nm, as indicated in Figure 3A, and also with the similar result for ZnDPP in Figure 3B. On the other hand, eq 3 means that, because the intermediate vibronic state Sv1 undergoes vibronic redistribution much faster than the S2 f Sv1 internal conversion, the fluorescence from Sv1 has the same decay time as that of S2. However, as noted in the beginning of this section, the rather complex behaviors of very weak fluorescence at the wavelength region between the stationary fluorescence spectra of the S2 and S1 states as demonstrated in Figures 2 and 3 do not seem to be easy to interpret satisfactorily by simplified treatment as indicated above. At the present stage of investigation, we interpret the observed fluorescence dynamics as follows. In Figures 2 and 3A, the sharp rise and fast decay of the fluorescence at 490 nm is very similar to those of S2 fluorescence observed at 430 nm. The very weak transient fluorescence in the 500-540 nm region also shows a sharp rise and fast initial decay. In addition to those fluorescence time profiles with the short initial decay, we have observed a long tail in this wavelength region, and the relative contribution of this tail to the fluorescence intensity increases a little with increase of the monitoring wavelength. In the spectral region of 550-580 nm, where also the fluorescence intensity is negligible or very weak in the stationary measurements, transient fluorescence rises in a few hundreds fs and then decays with several ps time constant, and also shows long-life tail, the contribution of which to the fluorescence intensity is considerably larger than that in the 500-540 nm region and increases with increase of the monitoring wavelength. As discussed already on the basis of eqs 2-5, since the rise time of the Sb1 fluorescence at 600 nm (the wavelength of the (0-0) band peak in the stationary fluorescence) is approximately the same as the decay time of the S2 fluorescence, presumably a large part of the Sv1 molecules may relax by the ultrafast redistribution to vibronic levels near the bottom of the S1 state.
4004 J. Phys. Chem. B, Vol. 104, No. 17, 2000
Letters
Nevertheless, a small part of them may take a different pathway of redistribution to give higher vibrational states Sh1 which undergo subsequent relaxations due to couplings with residual modes as indicated by the reaction scheme in eq 6. From coupled
rate equations, the time dependency of the Sh1 state is given by the same type of equation as eq 4 where, however, λ3 is replaced by the sum of the radiative transition probability (khf ) and radiationless relaxation rate constant (khd) of the Sh1 state, i.e., λ′3 ) (khf + khd).
Sh1(t) ∼
[
1 {exp(-λ′3t) - exp(-λ1t)} + λ1 - λ′3 1 {exp(-λ2t) - exp(-λ′3t)} (7) λ2 - λ′3
]
Namely, the ultrafast vibronic redistribution process kr taking place from the Sv1 state may give Sh1 with some range of distribution of vibrational energy and its relaxation rate khd presumably depends on the vibrational energy level. At the low vibrational energy limit of the distribution, khd may become very small because the available residual modes will become very poor and, at the limit of the very small khd, Sh1(t) will approach to Sb1(t). However, at higher vibrational levels, λ′3 will take a large value due to an extensive coupling of khd with residual modes, and at the high vibrational energy limit, khd may approach kvd. If λ′3 is larger than λ1 though it is still smaller than λ2, the first term of eq 7 (together with minor contribution from the second term) will correspond to the rapid fluorescence rise of hundreds fs and a few ps decay observed in the 500-540 nm region. At lower vibrational levels, λ′3 may become considerably smaller as discussed above and the time profile of Sh1(t) will more resemble that of Sb1(t) as observed in the 550-580 nm region. In addition, it should be noted here that the very weak long-life tail observed in the fluorescence time profiles in the 500-540 nm region and a little stronger one observed in 550-580 nm region can be ascribed to the bottom part of the
fluorescence band of the Sb1 state. To reproduce the observed fluorescence dynamics quantitatively on the basis of the proposed rate equations, we have examined the fluorescence rise and decay curves at several wavelengths from 490 to 580 nm by adjusting the parameter values for λ1, λ2, and λ′3. In this simulation, λ1 is the decay rate constant of S2, λ2 is the decay rate constant of Sv1, and λ′3 will change depending upon the fluorescence wavelength as discussed above. The results of the fitting by taking convolution with exciting pulse and using eqs 4 and 7 for Sb1(t) and Sh1(t), respectively, are indicated in Figure 3A. We show also some preliminary results for ZnDPP in THF solution in Figure 3B. Although we have not yet accomplished detailed measurements on ZnDPP, its S2 fluorescence decay time at 430 nm agrees with the S1 fluorescence rise time at 583 nm (0-0 band of S1 fluorescence), in agreement with eq 3 and 5. Moreover, the fluorescence rise and decay at the intermediate wavelength 554 nm are quite similar to those at 560 nm for the ZnTPP, which suggests strongly that the fluorescence dynamics of ZnDPP at 554 nm can be interpreted by eq 7 in the same way as ZnTPP since both of these hot fluorescence states have nearly the same vibrational energy above the bottom of the S1 state. Thus, we have observed the dynamics of hot fluorescence emission probably from nonrelaxed vibronic states (Sv1) immediately after S2 f S1 internal conversion, and higher vibronic states (Sh1) in addition to the almost relaxed one near the bottom of S1 (Sb1) formed by ultrafast vibronic redistribution, all-over the wavelength region between S2 and S1. The exact mechanisms of the vibronic redistribution relaxation and hot fluorescence state formations are not very clear at the present stage of investigations, and we need more detailed ultrafast laser spectroscopic studies on various porphyrin derivatives, including those with a practically nonfluorescent S2 state, etc., which are going on in our laboratory. References and Notes (1) See, for example: Chemical Applications of Ultrafast Spectroscopy; Fleming, G. R.; Oxford University Press: New York and Clerendon Press: Oxford, 1986. (2) Elsaesser, T.; Kaiser, W. Annu. ReV. Phys. Chem. 1991, 42, 83. (3) Shibata, Y.; Chosrowjan, H.; Mataga, N.; Yoshida, N.; Osuka, A., in preparation. (4) Chosrowjan, H.; Mataga, N.; Nakashima, N.; Imamoto, Y.; Tokunaga, F. Chem. Phys. Lett. 1997, 270, 267. (5) Mataga, N.; Chosrowjan, H.; Shibata, Y.; Tanaka, F. J. Phys. Chem. B 1998, 102, 7081. (6) Gurzadyan, G. G.; Tran-Thi, T.-H.; Gustavsson, T. J. Chem. Phys. 1998, 108, 385.
