J. Phys. Chem. 1996, 100, 13867-13873
13867
Spectral and Photophysical Properties of Ethylene-Bridged Side-to-Side Porphyrin Dimers. 2. Femtosecond Transient Absorption and Picosecond Fluorescence Study of trans-1,2-Bis(meso-octaethylporphyrinyl)ethene Mirianas Chachisvilis,† Vladimir S. Chirvony,‡ Alexander M. Shulga,‡ Bruno Ka1 llebring,§ Sven Larsson,⊥ and Villy Sundstro1 m* Department of Chemical Physics, Chemical Center, Lund UniVersity, S-221 00 Lund, Sweden, Institute of Molecular and Atomic Physics, 70 F. Skaryna AVenue, 220072 Minsk, Belarus, Department of Biochemistry and Biophysics, Go¨ teborg UniVersity, S-413 90 Gothenburg, Sweden, and Department of Physical Chemistry, Chalmers UniVersity of Technology S-412 96 Gothenburg, Sweden ReceiVed: March 11, 1996X
Femtosecond transient absorption and picosecond fluorescence spectra and kinetics have been measured for an ethylene-bridged porphyrin dimer molecule, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbisdOEP). In part 1 of this work it was shown with the help of absorption and fluorescence spectroscopy that in solution the molecule exists in at least two different conformers, the P conformer with porphyrin-type spectral properties and the U conformer with spectral properties atypical for porphyrin molecules. As our quantum chemical calculations showed, the U conformer is characterized by a common π-conjugation through the double-bond bridge. In the present work we have found that the lifetimes of the lowest excited singlet states of the P and U conformers are extraordinary short, ∼6 ps for P and ∼7-9 ps for U in toluene at room temperature. Singlet-state lifetimes of both conformers were found to be strongly dependent on solvent viscosity with that of the U-form exhibiting the most pronounced dependence; a fluorescence lifetime of approximately 460 ps was measured for the U conformer in a frozen toluene solution at 77 K. Photoisomerization-like conformational relaxation in the S1 state, leading the system to the “point of funnel” characterized by decreased ∆E(S1-S0) energy gap, is proposed as a mechanism to explain the photophysics of the P and U conformers.
1. Introduction In this article we present the results of the time-resolved experiments on the ethylene-bridged porphyrin dimer molecule studied in the previous work.1 Until recently, sandwich dimers consisting of two tetrapyrrole macrocycles, held within a short distance by a common lanthanide or actinide metal ion, have been considered as the best spectral models of the photosynthetic reaction center special pair.2-3 The most prominent features of these dimers are: (1) the presence of additional absorption bands in the blue-green region between the Soret and Q bands (Q′′ absorption) and at longer wavelengths than the Qx band (Q′ absorption) and (2) the presence of a broad-band fluorescence in the near-IR region. Such a spectral behavior was explained as the result of strong through-space π-π interaction between the cofacial tetrapyrroles at van der Waals distance.3 Quantum mechanical calculations have confirmed that the metal orbitals are not involved in the low-energy absorption spectrum.4 We have observed analogous spectral features for a side-toside porphyrin dimer which is covalently bridged by ethylene in the meso positions [trans-1,2-bis(meso-octaethylporphyrinyl)ethene or tbisdOEP].1 Since the porphyrins are in a trans configuration,5 through-space overlap of the porphyrin π-electrons on different monomers is ruled out. Careful ground-state absorption and fluorescence measurements have shown1 that tbisdOEP exists in solution as an equilibrium mixture of two main conformers P and U with their relative contribution ratio of about 5:1. All non-porphyrin-like spectral features of tbisdOEP were found to be related to the conformer U, whereas †
Lund University. Institute of Molecular and Atomic Physics. § Go ¨ teborg University. ⊥ Chalmers University of Technology. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1996. ‡
S0022-3654(96)00738-1 CCC: $12.00
the P conformer exhibits spectral properties expected for excitoncoupled porphyrin dimers. In the present paper we describe the photophysical properties of both conformers. Our direct femtosecond transient absorption and picosecond fluorescence measurements show that the P and U conformers exhibit extraordinary short S1 state lifetimes, ∼6 and 7-9 ps, respectively, in toluene at room temperature. A strong dependence of these lifetimes on solvent viscosity is found. A qualitative model is proposed, including subpicosecond conformational relaxation in the S1 state, to explain the photophysics of the U and P conformers of tbisdOEP. 2. Experimental Section trans-1,2-(meso-Octaethylporphyrinyl)ethene (tbisdOEP) was prepared, purified, and characterized by 1H NMR and optical absorption spectroscopy as described in ref 6. Measurements of the near-IR fluorescence kinetics were performed with a timecorrelated single-photon-counting system equipped with a microchannel plate PMT R 2609 U-05 (Hamamatsu Photonics), an ORTEC 567 time-to-amplitude converter, and a Nucleus personal computer analyzer. Excitation pulses (10 ps, 800 kHz) in the range 716-840 nm (fundamental) and 358-420 nm (second harmonic) were produced by a synchronously pumped mode-locked styryl 8 and styryl 9 dye laser. Frequency doubling was performed in a 1 mm LiIO3 crystal. The average power at the sample was less than 0.5 mW, and the illuminated sample area was about 2 × 1 mm2. The spectral width of the monochromator was 8 nm. The samples were placed in 1 cm quartz or glass cells, and the fluorescence emission was collected at 90°. Additional color glass filters were also used to check for the absence of scattered excitation light. The instrument response function was recorded before and after the kinetic measurements using a nonfluorescing paper. It was stable during each series of measurements and had a full width at half© 1996 American Chemical Society
13868 J. Phys. Chem., Vol. 100, No. 32, 1996 maximum (fwhm) of 55-60 ps. Deconvolution of the kinetics at individual wavelengths was performed with our own program, and decay associated fluorescence spectra (DAS) were obtained using a global analysis program (Globals Unlimited). The time resolution was about 3-5 ps in both single-wavelength and global-analysis fits. Kinetic models with up to three-exponential kinetic components were used. Both one- and two-color transient absorption measurements were performed; in the one-color experiments the excitation and probing pulses (790 nm) were generated using a regeneratively mode-locked Ti:sapphire laser, pumped by an argon ion laser. The output from this laser was ∼1 W at a repetition rate of 82 MHz, and to prevent buildup of long-lived photoproducts the pulse repetition rate was reduced to 40 kHz by using an acoustooptical pulse selector. Compensation for wavelength chirp was performed in a two-prism compressor and resulted in transform-limited pulses with a typical duration of 50-75 fs. In a two-color setup transient absorption spectra and kinetics were obtained by using for excitation either ∼200 fs pulses at 580 nm from an amplified dye laser system or ∼450 fs pulses at 790 nm from an amplified Ti:sapphire laser system. In both cases a white-light continuum for probing was generated in a 1 cm path length water cell. The dual-beam detection system based on a three-photodiode arrangement had a sensitivity (in ∆A) of ∼0.0006 by averaging the data from ∼20 pulses. This was quite essential to obtain spectra and kinetics of the presently studied samples with very low absorption in the region of the U′ band. The group velocity dispersion of the white-light pulse in the collimating and focusing optics led to a ∼4 ps chirp in the 400-900 nm wavelength range. Therefore the transient absorption spectra were recorded with simultaneous adjustment of the delay between probe and pump pulses to compensate for this chirp. The samples were placed in a 1 or 2 mm rotating cell with the optical density ∼0.5 at the wavelength of excitation. However, the low solubility of the compounds did not allow an optical density higher than 0.1 in the infrared region in a 2 mm cell. All fluorescence and transient absorption measurements were carried out in the presence of air at room temperature. Paraffin oil solutions were prepared by mixing a toluene solution of tbisdOEP with an appropriate volume of paraffin oil and further vacuum evacuation of the toluene. Rigid poly(acrylic acid) polymer films containing tbisdOEP were prepared as follows: Concentrated toluene solution of tbisdOEP was carefully mixed with the toluene solution of polyacrylic acid, and then the mixture was put on a glass plate surface. The samples were kept on the surface under normal atmospheric pressure during a few days until the toluene had evaporated, and then used for kinetic fluorescence measurements. 3. Results and Discussion 3A. Fluorescence Kinetics and DAS Spectra. In the fluorescence kinetic measurements carried out in the 800-1000 nm region the samples were excited at two different wavelengths, 760 and 380 nm, corresponding to the first and second harmonic of our dye laser system. We found that the observed kinetics of the near-IR luminescence do not depend on the wavelength of excitation; therefore all further experiments were carried out using the 760 nm excitation only, i.e., in the wavelength region where only the U conformer absorbs. The kinetic measurements showed that in low-viscosity organic solvents such as toluene, CHCl3, and THF, the emission decay kinetics are nonexponential and exhibit three decay components: a dominant 7 ( 3 ps component (relative amplitude is ∼93%), a 1 order of magnitude less intense 30 ( 10 ps component (∼7%), and a 520 ( 100 ps component of very low
Chachisvilis et al.
Figure 1. Decay-associated fluorescence spectra of tbisdOEP in toluene; wavelength of excitation is 760 nm.
intensity (∼0.1%). These lifetimes and amplitudes were obtained through global analysis of the luminescence kinetics measured at 10 different wavelengths in the range 800-1000 nm. Figure 1 presents decay-associated spectra (DAS) obtained for the three kinetic components in this region. The main 7 ps component of the emission has a wide structureless spectrum with a maximum at approximately 860 nm and a fwhm of approximately 1600 cm-1. Since this spectrum is not corrected for the spectral sensitivity of the photodetector, its actual maximum may lie at slightly longer wavelengths and the longwavelength tail of the spectrum may also extend further than 1000 nm, as it was experimentally found for the steady-state spectrum.1 The 30 ps component displays a DAS which is qualitatively similar to the 7 ps component spectrum with approximately the same bandwidth but with a somewhat redshifted (to 880 nm) maximum. Finally, the spectrum of the very weak 520 ps component is a structureless tail decreasing monotonously from 800 to 1000 nm. We suggest that both the main 7 ps and weaker 30 ps components belong to the U conformer because their DAS are similar to the steady-state fluorescence spectrum of the U conformer.1 The presence of the minor 30 ps component with a red-shifted fluorescence spectrum may be a result of some ground-state inhomogeneity of the U conformer. As emphasized in ref 1, absorption and fluorescence properties of the U conformer have an elevated sensitivity to its particular geometry, due to the presence of a common π-conjugation through the ethylene bridge. Most likely, some small part of the U-form has a slightly different geometry leading to increased excitedstate decay time. As for the very low intensity 520 ps component with quite different spectrum, most likely it is the red tail of the similar 350 ( 100 ps component of the red fluorescence observed in the 620-750 nm region (vide infra). The decay kinetics of the near-IR emission were found to depend strongly on solvent viscosity. Figure 2 presents the emission decay kinetics obtained at λ ) 860 nm in toluene, in a mixture of paraffin oil and toluene (10:1), and in a rigid poly(acrylic acid) polymer film. Lifetimes and amplitudes of these kinetics are summarized in Table 1. As we can see in the paraffin oil/toluene mixture, the luminescence decay kinetics is quite close to monoexponential with a dominating time constant of 75 ( 7 ps (A ∼ 97%). In the polymer film the kinetics is more nonexponential; however, three components were observed with time constants 70 ( 7 (A ) 62%), 230 (
Side-to-Side Porphyrin Dimers. 2
J. Phys. Chem., Vol. 100, No. 32, 1996 13869
Figure 2. Fluorescence decay kinetics observed in the near-IR region (λdet ) 860 nm) for tbisdOEP in different solvents.
Figure 3. Absorption difference spectrum obtained for tbisdOEP in toluene at a delay of 0.3 ps after excitation at 790 nm.
TABLE 1. Relative Intensities of Decay Components of tbisdOEP near-IR Fluorescence solvent toluenea paraffin oil/toluene (10:1)a poly(acrylic acid) filma tolueneb
decay components, ps
peak intensities, %
7(3 30 ( 6 520 ( 50 75 ( 7 290 ( 30 820 ( 80s 70 ( 7 230 ( 25 620 ( 50 460 ( 25
93 6.9 0.1 97.4 2.2 0.4 62 27 11 >99
a At room temperature, λdet ) 860 nm. b At 77 K, λdet > 800 nm (through cutoff filter).
30 (A ) 27%), and 620 ( 50 ps (A ) 11%). The kinetics of the near-IR emission of tbisdOEP was also measured at 77 K in a frozen toluene solution. In this measurement all emission above 800 nm was detected (a cutoff filter was used instead of a monochromator) and the observed kinetics are well described by a single monoexponential decay law with a time constant of 460 ( 25 ps (amplitude contribution of this component is higher than 99%). We suggest that the more complicated character of the fluorescence decay kinetics found in the polymer film most likely originates from some inhomogeneity of the polymer matrix, because we did not undertake measures to remove small remainders of toluene. Our purpose here was only to show that further increase of solvent viscosity, as compared to paraffin oil, leads to further increase of the S1 state lifetime of the U conformer even at room temperature. Indeed, despite of the complicated decay kinetics, our measurements showed that the “effective” fluorescence lifetime (the time necessary to decrease the fluorescence intensity to e-1 of the maximal value) is about 0.5 ns in this polymer film, which is about the same as in rigid toluene at 77 K. This viscosity dependence of the lifetime correlates well with the solvent viscosity dependence of the quantum yield and of the spectral position of steady-state fluorescence of the U conformer, found in part 1.1 All these data suggest that some conformational motion (relaxation) takes place in the S1 state, and that this relaxation can be retarded by increased solvent viscosity. In section 3C we will discuss a model which qualitatively explains this behavior. As for the decay kinetics of the weak fluorescence detected in the 620-750 nm region and essentially assigned to the P′ spectral form (see Figure 4 in ref 1), the component dominating
Figure 4. Transient absorption kinetics of tbisdOEP in toluene at different wavelengths of observation.
the integrated fluorescence spectrum has ∼5.3 ns time constant (A ) 26%, integral amplitude ∼93%). Simultaneously, two more short-lived decay components are detected with time constants of ∼350 ps (A ) 12%, 5% integral contribution) and ∼25 ( 10 ps (A ) 62%, 1.8% integral contribution). The 350 ps component most likely belongs to some P-type form having an intermediate lifetime between the lifetimes of the P′ form (5.3 ns) and the P form (∼6 ps, see section 3B). The red tail of this fluorescence component can be seen as a ∼520 ps component of very low intensity in the near-IR region. From the high amplitude (at t ) 0) of the ∼25 ps component in this spectral region, we can conclude that it most likely belongs to the short-lived P form since it is expected to fluoresce in this spectral region (see also ref 1 and section 3B). 3B. Femtosecond Transient Absorption Data. Difference absorption spectra and kinetics of tbisdOEP were obtained with excitation at two wavelengths, 790 (Figures 3-5) and 585 nm (Figures 6-8). According to our earlier results,1 the absorption at 790 nm is almost exclusively due to the U′ absorption of the U conformer, whereas 585 nm radiation is absorbed primarily by the P conformer and the P′ spectral form since the U conformer has an absorption minimum at ∼585 nm (see below and in ref 1). Figure 3 shows the difference absorption spectrum
13870 J. Phys. Chem., Vol. 100, No. 32, 1996
Chachisvilis et al.
Figure 5. Time evolution of the transient absorption difference of tbisdOEP in chloroform at 789 nm and T ) 50 °C.
Figure 7. Time evolution of the absorption difference of tbisdOEP in toluene at wavelengths 420, 545, and 661 nm after excitation at 585 nm.
Figure 6. Absorption difference spectra of tbisdOEP in toluene recorded at delays 1, 4, 9, 19, and 31 ps after excitation at λ ) 585 nm.
at 0.3 ps after excitation at λ ) 790 nm. One can see that the difference absorption spectrum (bleaching downward) closely resembles the fluorescence excitation spectrum of the Uconformer (see Figure 6 in ref 1). A main intense and narrow bleaching band is observed at approximately 490 nm which exactly corresponds to the U′′ absorption of the U conformer.1 A considerably less intense and structureless bleaching is observed in the 520-850 nm region, correlating well with the U′ absorption band of the U conformer. The only noticeable difference between the fluorescence excitation and bleaching spectra is a higher intensity of the bleaching signal in the 800850 nm region as compared to the relative intensity of the U′ absorption in the same region.1 We believe that stimulated emission contributes to the ∆A signal in this region because the U-conformer fluorescence is characterized by a broad band in the 750-1100 nm region.1 Figure 4 shows the normalized transient absorption kinetics measured at different probing wavelengths subsequent to excitation at 790 nm. The kinetics are clearly nonexponential
Figure 8. Time evolution of the absorption difference of tbisdOEP in toluene and paraffin oil/toluene mixtures at 421 nm after excitation at 585 nm.
with at least three exponential components, a fast component with a time constant comparable to or shorter than the pump pulse duration (∼450 fs) and at least two slower components with time constants of about 10 and 30-40 ps. The relative contributions of the three decay components are probewavelength dependent; at the center of the sharp bleaching band (495 nm) a maximum amplitude of the two longest lifetimes is observed while at longer wavelengths (750 and 650 nm) as well
Side-to-Side Porphyrin Dimers. 2 as at wavelengths on the slopes of the bleaching band a high amplitude of the shortest lifetime is observed. Because of the limited signal-to-noise ratio of the dual-color transient absorption measurements, more accurate kinetics were measured in a onecolor experiment using 75 fs pulses. Figure 5 shows results of such measurements at λ ) 790 nm and at T ) 50 °C. The decay of the bleaching is three exponential with the lifetimes τ1 ) 30 fs (14%), τ2 ) 260 ( 50 fs (31%) and τ3 ) 9 ( 1 ps (55%). The first lifetime is most likely a consequence of the coherence artifact, usually present in one-color pump-probe signals. The third lifetime is in good agreement with the 10 ps component of the decay kinetics at 750 nm, shown in Figure 4. The same measurement was also carried out at T ) 20 °C. In this case the first lifetime remained the same, while τ2 and τ3 inreased to τ2 ) 430 ( 50 fs and τ3 ) 11 ( 1 ps. The decay kinetics presented in Figures 4 and 5 may in principle reflect the decay of the initially populated singlet state of the U conformer, decay of the lower triplet states populated via the S1 state, or relaxation in the ground state after excited state(s) deactivation. Comparison with the fluorescence decay lifetimes of the U conformer (lifetimes 7 ( 3 ps (amplitude contribution 93%) and 30 ( 6 ps (amplitude contribution 7%)), shows that the ∼10 ps and 30-40 ps lifetimes observed in transient absorption and fluorescence probably have a common origin and represent deactivation of the U conformer’s excited state. It is difficult to directly compare the amplitudes of the decay components observed in the fluorescence and transient absorption measurements, because the two methods have very different temporal resolution and in the transient absorption kinetics an additional very fast subpicosecond component is observed. The transient absorption kinetics of the U conformer with 790 nm excitation, carries at many wavelengths a very fast 200400 fs kinetic component (Figures 4 and 5). One possible origin of this fast process is vibrational relaxation, which is expected to influence the shape and position of the excited-state spectrum. However, excitation at 790 nm is close to the electronic 0-0 transition of the U conformer and the absorption spectrum in this region is relatively flat, thus making the influence from shifts of spectral bands less likely. Therefore, we believe that the 200-400 fs kinetics at 790 nm (Figure 5) reflect structural changes (structural relaxation) of the U conformer during its motion to the point of funnel in the S1 state, i.e., before S1 f S0 deactivation (see section 3C). This is also in agreement with the experimental observation that it becomes faster at higher temperatures. Figure 6 shows a set of transient absorption spectra obtained at different delays (1, 4, 9, and 31 ps) after excitation of tbisdOEP in toluene solution with 200 fs pulses at λ ) 585 nm, where mainly the P conformer absorbs. These difference spectra exhibit a strong bleaching maximum at approximately 420 nm and a broad induced absorption band in the region 510850 nm. At ∼500 and ∼590 nm, there are narrow minima in the broad positive absorption band. A comparison with the absorption spectrum of tbisdOEP shows that there is a good correlation between the bleaching bands and the sharp Soret and Qy absorption bands. The dip in the absorption at ∼590 nm is partly also caused by scattered excitation light. The difference absorption spectra of Figure 6 are qualitatively different from the spectrum shown in Figure 3, obtained under excitation at 790 nm. In general, the ∆A spectra of Figure 6 at short delay times (1 ps) resemble the well-known difference absorption spectra of porphyrins, characterized by bleaching of the Soret band and a structureless absorption extending from the red side of the Soret band to the near IR region.8,9 However, at slightly longer delay times (∼4 ps) a transient absorption band appears in the region of 480 nm. One possible explanation
J. Phys. Chem., Vol. 100, No. 32, 1996 13871 could be that this band represents the absorption by the vibrationally “hot” molecules of the P conformer after they have relaxed to the ground electronic state. On the other hand the transient absorption at 480 nm could be due to the U′′ band of the U conformer (495 nm), which has formed as a result of the P* f U conformational transformation on the excited-state potential surface (see section 3C). We have also measured the transient absorption kinetics at several probe wavelengths, using excitation at 585 nm. Three representative traces are shown in Figure 7 for probe wavelengths 420, 545, and 661nm. In contrast to the strongly probewavelength dependent kinetics found for the U form, all these kinetic curves are characterized by similar time constants and amplitudes, a dominating 6 ( 1 ps (A ∼ 90%) component, and a weaker 10-50 ps component. The lifetime of this slower component is difficult to determine because of the low amplitude and the presence of a third weak and very long-lived nanosecond component (not shown). Since the excited-state decay of the spectral form P′ is characterized by a 5.3 ns lifetime, it seems likely that the low-amplitude very long-lived component in the kinetics of Figure 7 is due to the P′ spectral form. This is furthermore supported by the fact that the U conformer has almost no absorption at 585 nm (see Figure 6 in ref 1). These observations enable us to interpret the difference absorption spectrum with 585 nm excitation as related mainly to the conformer P. Thus, these results show that conformer P has a strongly quenched excited state with a dominating short lifetime of 6 ( 1 ps and a much weaker decay component of 10-50 ps (the presence of this low-intensity decay component is most likely related to some structural heterogeneity of the conformer P). This, together with the spectral similarity of P and P′ explains why it was not possible to measure the steady-state fluorescence spectrum of P. The fluorescence decay in the 620-750 nm region carries, in addition to the 5.3 ns component, two shorter ∼25 and ∼350 ps components. Comparing the fluorescence and the transient absorption results, it seems very likely that at least the ∼25 ps fluorescence component is due to the P conformer. We have also found that the effective S1 state lifetime (the decay of the signal to the e-1 level) of the P form increases at higher solvent viscosities, but the increase is less prominent than for the U conformer; a change of solvent from toluene to toluene/paraffin oil mixtures, increases the lifetime of the P form from 6.8 to 10.2 ps in a 1:5 mixture and to about 20 ps in a 1:10 mixture (Figure 8), as compared to an increase from 7 ps (toluene) to 75 ps (1:10 mixture) for the U conformer. Notice also that the decay becomes more nonexponential at higher viscosities. These results suggests that the mechanism of the ultrafast radiationless deactivation of the S1 state of the P conformer, also includes some conformational relaxation which can be retarded by increasing solvent viscosity (see section 3C). 3C. Mechanism for the Ultrafast Radiationless Deactivation of the S1 Excited State of the U and P Conformers. It is known from extensive time-resolved studies of trans-stilbene10-12 and similar molecules containing the double-bond group,13-14 that a torsional motion around the double bond in the first excited state is very effective in deactivating the excited state. For this type of molecules the excited state lifetimes in solution are determined (A) by the height of the excite-state-potential barrier for torsional motion (if a barrier is present) and the friction exerted by the solvent on this motion (basically the viscosity of the solvent); and (B) by the final (reduced) energy gap between S1 and S0 states in the vicinity of the so-called funnel point, where the double bond is twisted in the S1 state. Our results on tbisdOEP show that the excited-state lifetimes of the U- and P-conformers are strongly viscosity dependent.
13872 J. Phys. Chem., Vol. 100, No. 32, 1996
Figure 9. Hypothetical potential energy surface diagram.
Because of similarity of tbisdOEP with stilbene-like molecules (it could be called a porphyrin-disubstituted ethylene) and because the double-bond bridge is involved in a common π-conjugation at least in the U conformer, it seems highly probable that the efficient excited-state deactivation characteristic for the U and P forms, is a result of some conformational motion which reduces the S1-S0 energy gap. (The long lifetime of the P′ spectral form can be either due to restricted conformational flexibility, if the P′ form is of dimer origin, or simply due to possible monomeric origin of the P′ form; see Part 1.1) However, in contrast to the large amplitude trans-to-cis isomerization occurring in the stilbene-like molecules,10-14 a full trans-cis photoisomerization at the ethylene double bond can hardly occur in the case of tbisdOEP. Several facts argue against this posibility. The same degree of weakening of the ethylene double bond as in trans-stilbene is not expected here since the HOMO and LUMO involved in the S1 transition have less of ethylene π character as compared to the corresponding MOs of trans-stilbene. It would also be difficult to explain the very fast relaxation of the P and U forms, whose substituents are much larger than the phenyl rings of stilbene. A very low potential barrier would speed up the isomerization, but even on a barrierless potential surface it would take substantially more time than the observed ∼10 ps to twist the big and bulky porphyrinic rings through a substantial angle. Moreover, we did not find any evidence of cis-form formation after tbisdOEP excitation in the transient and steady-state spectra (absorption spectrum of the full cis isomer is known7). On the other hand, a plausible candidate for such a conformational rearrangement could be a “bicycle” motion of the ethylene bond, as considered in ref 15. In Figure 9 we present hypothetical potential energy surfaces for the ground and excited states of tbisdOEP. We point out that the reaction coordinate in this picture represents both change of ethylene bridge distances and porphyrin plane rotation (vide infra). It is reasonable to assume that there is some particular geometry of the excited dimer which is accessible from both excited Frank-Condon states P* and U*, and corresponding to the minimum of the S1(φ) curve (Figure 9). Apparently, from the point of view of symmetry this minimum might correspond (or be close) to the full-in-plane arrangement of the two OEP planes and the ethylene bridge plane; in the excited state the system has an increased tendency to planarity because such a geometry facilitates a more extensive common π-conjugation and, therefore, should lead to further energy minimization; however, the details of the energy minimum on the excitedstate energy surface are still unknown. As can be seen in Figure 7 of part 1,1 where model structures for both conformers P and U are presented, both conformers need to undergo some conformational movements to achieve full-in-plane geometry.
Chachisvilis et al. This can explain the viscosity dependence of the decay of the excited U* and P* conformers in the same way as in the case of deactivation of excited states of trans- and cis-stilbene. It is worth noting that only conformational movement in the vicinity of the ethylene bridge is required for P to reach full-in-plane geometry because in this case the porphyrin planes are coplanar, whereas for U a substantial rotation of porphyrin planes is required. In our opinion, this can explain why solvent viscosity is much more effective for U than for P in increasing the lifetime of the excited state. At the point of the funnel (point of minimum of the S1(φ) surface) direct radiationless deactivation of P* and U* takes place leading to P or to U in the ground state. It is possible that the fast appearance of new transient absorption at ∼480 nm, approximately 4 ps after excitation of the P conformer (Figure 7), is due to such a partial P* f U “photoconformational” transition. However, no experimental evidence was found that the U* f P transition occurs. The model presented in Figure 9 also enables us to explain the blue-shift of the U-conformer’s steady-state fluorescence spectrum (Figure 5 in part 11) and the increase of the fluorescence quantum yield and lifetime, observed in more viscous solvents. A highly viscous solvent is capable of slowing down the conformational motion toward the point of funnel on the excited-state potential surface of U*. As a consequence nonrelaxed conformations will start to emit light from higher positions on the potential energy surface of S1 (see Figure 9) and as a result a blue-shifted fluorescence will be observed. An increase of the fluorescence lifetime and quantum yield is also a consequence of this effect. A similar effect is in principle expected to occur for the P form, but difficulties in resolving its fluorescence against the relatively intense background of the long-lived P′ fluorescence prevent the detection of the effect. Generally, any reliable explanation of the very fast S1 f S0 decay rate (i.e., internal conversion rate) would require a detailed knowledge about the potential energy surfaces and solution of a nontrivial quantum mechanical problem of the dynamics on these energy surfaces, which is out of scope of this article. However, in the discussion below we offer a tentative explanation based on the “energy gap law”.17 A phenomenological expression for the fluorescence lifetime is τf ) 1/(kd + kf), where kd is the radiationless and kf is radiative S1 f S0 decay rate (no discernible population in the triplet state of tbisdOEP was detected). Both these rates depend on the S1-S0 energy gap, but in our case kd . kf and therefore τf is mainly determined by kd. In accord with the ref 17, kd ) A exp(-R∆E), where A is a constant and R is a coefficient approximately constant for a series of related compounds with different ∆E values. This expression was found to be satisfied sufficiently well for chlorophyll-like molecules with R ) 1.35 × 10-3 cm.18 This results in about 100-fold increase of kd for ∆E ≈ 3500 cm-1 (the ∆E is obtained from the shift of the fluorescence maximum of tbisdOEP as compared to that of bis-(OEP)), which is close to the experimentally observed increase of the deactivation rate. Maybe a more appropriate comparison would be to calculate the change of the deactivation rate of the U conformer from the change of the steady-state fluorescence maximum position in toluene and paraffin oil (∆E ≈ 980 cm-1, see Figure 5 of part 11). This value leads to an approximately 4-fold increase of kd, while experimentally measured lifetime decreases from ∼75 to ∼7 ps (see Table 1), i.e., only 2 times more than predicted. It has to be stressed that the energy gap law implies that the internal conversion rate is entirely determined by the Franck-Condon factors and mainly those for high-frequency modes. Thus a simple explanation might be that the slow motion on the S1 potential is responsible only for structural reorganization which leads to the reduced energy gap and
Side-to-Side Porphyrin Dimers. 2 consequently to an enhanced Franck-Condon factor for internal conversion (mainly determined by high-frequency modes). However, the above-simplified analysis gives only a crude estimate of the internal conversion rate, since the dynamic effects of the surface crossing (involving low-frequency modes) in the funnel region were neglected here; yet this simple mechanism qualitatively well describes the trend in the experimental data. We conclude this paper with a short discussion of some alternative interpretations of our results. One possibility, still quite consistent with the CNDO/S calculated spectrum, is that the U form is entirely planar. The calculations show that the P and U conformers have very similar ground-state energies and that U* is the lowest excited state of the system (as is also evidenced by the experiments1) due to its favorable orientation of the ethyl groups. It is therefore difficult to imagine an excited-state conformation that could be lower in energy and thus participate in the relaxation process of the excited state and make it efficient. Another possibility would be to assume the presence of a silent charge-transfer (CT) state at a lower energy than the locally excited state of the U form. Such a charge-transfer state, suggested in ref 15 for a similar molecule, would probably require that the coupling to the locally excited state is reduced, which could occur if the porphyrin planes in the CT state are turned with respect to the ethylene bridge plane. However, this implies that the fast deactivation of the P conformer most likely would be through the same CT state, which then would have the same geometry as the P conformer. It would then be difficult to explain the viscosity dependence of the excited-state deactivation of the P conformer. Furthermore, in the case of ref 15 the charge-transfer state appears only in high-polarity solvents, whereas our experiments were carried out in toluene. We therefore do not expect the chargetransfer state to be energetically more stable as compared to the S1 state of the U form. The tbisdOEP dimer studied here has photophysical properties which in many respects parallel those of ethylene bridged anthracenes, extensively studied by Becker et al.19, 20 The crystal structure of the anthracene dimers shows that the ethylene bridge plane is close to perpendicular to the two anthracene planes. The ground-state conformation in solution is most likely very similar. However, substituents may change the conformation and make the 45°-45° geometry lower in energy. The anthracene dimers are observed to have a large fluorescence Stokes shift,19,20 suggesting a more planar geometry and more extensive conjugation in the excited state. Contrary to tbisdOEP, there is no evidence of an extended conjugation in the absorption spectra of the dimeric anthracenes,20 but in one monomeric case where the ethylene group is forced to be coplanar with the anthracene there is a considerable red-shift of the S1 state. This is consistent with our conclusion that conjugation leads to a red-shift of the S1 state. Summary We have established that the two main conformers of tbisdOEP exhibit the following photophysical properties: (1) Conformer P has an exceptionally short S1 state lifetime (∼6 ps in toluene) which makes the steady-state fluorescence too weak to be measured in the 620-750 nm region, in the presence of a strong long-lived fluorescence from the monomerlike P′ spectral form. A strong dependence of the S1 state lifetime on solvent viscosity was found.
J. Phys. Chem., Vol. 100, No. 32, 1996 13873 (2) Conformer U exhibits a broad-band, structureless fluorescence in the 750-1100 nm region. The lifetime of this fluorescence depends on solvent viscosity and the dependence is stronger than that found for the P conformer, an increase is observed from ∼7 ps in toluene at room temperature to ∼0.5 ns in a rigid polymer film and in the frozen toluene at 77 K. (3) The kinetic measurements show that some structural inhomogeneity is present in each of the two main conformers, giving rise to low-amplitude (∼10%) more long-lived components in the S1 state decay kinetics. As suggested by the calculations this inhomogeneity, however, is a natural property of the ethylene-bridged porphyrin dimers. (4) A fast photoconversion P* f U is suggested on the basis of femtosecond transient absorption measurements and a model is suggested for the reaction. Acknowledgment. We are grateful to the Swedish Natural Science Research Council and the Swedish Institute for financial support. One of us (V.S.C.) thanks the International Science Foundation for financial support (Grants MWR000 and MWR300). References and Notes (1) Chachisvilis, M.; Chirvony, V. S.; Shulga, A. M.; Ka¨llebring, B.; Larsson, S.; Sundstro¨m, V. J. Phys. Chem. 1996, 100, 13857. (2) Buchler, J. W.; De Cian, A.; Fischer, J.; Kihn-Botulinski, M.; Paulus, H.; Weiss, R. J. Am. Chem. Soc. 1986, 108, 3652. (3) (a) Girolami, G. S.; Milam, S. N.; Suslick, K. S. J. Am. Chem. Soc. 1988, 118, 2011. (b) Yan, X.; Holten, D. J. Phys. Chem. 1988, 92, 409. (c) Bilsel, O.; Rodriguez, J.; Holten, D.; Girolami, G. S.; Milam, S. N.; Suslick, K. S. J. Am. Chem. Soc. 1990, 112, 4075. (d) Bilsel, O.; Rodriguez, J.; Holten, D. J. Phys. Chem. 1990, 94, 3508. (e) Bilsel, O.; Rodriguez, J.; Milam, S. N.; Gorlin, P. A.; Girolami, G. S.; Suslick, K. S.; Holten, D. J. Am. Chem. Soc. 1992, 114, 6528. (f) Bilsel, O. Milam, S. N.; Girolami, G. S.; Suslick, K. S.; Holten, D. J. Phys. Chem. 1993, 97, 7216. (4) Estiu, G. L.; Rosch, N.; Zerner, M. C.; to be published. (5) All conformers of tbisdOEP are in the trans configuration of the ethylene bridge as evidenced by NMR data.6 Moreover, the recently synthesized cis isomer7 exhibits an absorption spectrum quite different from absorption spectra of all three spectral forms of tbisdOEP. (6) Shulga, A. M.; Ponomarev, G. V. Chem. Heterocycl. Compounds, 1988, 24, 276-280. (7) Ponomarev, G. V.; Borovkov, V. V.; Sugiura, K.; Sakata, Y.; Shulga, A. M. Tetrahedron Lett. 1993, 34, 2153-2156. (8) Dzhagarov, B. M.; Chirvony, V. S.; Gurinovich, G. P. In Laser Picosecond Spectroscopy and Photochemistry of Biomolecules, Letokhov, V. S., Ed.; A. Hilger: Bristol, UK, 1987; Chapter 3. (9) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111, 6500. (10) Lee, M.; Bain, A. J.; McCarthy, P. J.; Haseltine, J. N.; Smith, A. B.; Hochstrasser, R. M. J. Chem. Phys. 1986, 85, 4341. (11) Schroeder, J.; Schwarzer, D.; Troe, J.; Vos, F. J. Chem. Phys. 1990, 93, 2393. (12) Schroeder, J.; Schwarzer, D.; Troe, J.; Vo¨hringer, P. Chem. Phys. Lett. 1994, 218. (13) (a) Courtney, S. H.; Fleming, G. R. J. Chem. Phys. 1985, 83, 215. (b) Velsko, S. P.; Fleming, G. L. Ibid. 1982, 76, 3553. (14) A° kesson, E.; Hakkarinen, A.; Laitinen, E.; Helenius, V., Gillbro, T.; Korppi-Tommola, J.; Sundstro¨m, V. J. Chem. Phys. 1991, 95, 6508. (15) (a) Johnson, S. G.; Small, G. J.; Johnson, D. G., Svec, W. A.; Wasielewski, M. R. J. Phys. Chem. 1989, 93, 5437. (b) Johnson, D. G.; Svec, W. A.; Wasielewski, M. R. Isr. J. Chem. 1988, 28, 193. (16) Gilbert, A.; Bagott, J. Essentials of Molecular Photochemistry; Blackwell Scientific Publications: Oxford, 1991; pp.230-236. (17) Siebrand, W. J. Chem. Phys. 1967, 46, 440-447. (18) Sagun, E. I.; Losev, A. P.; Nichiporovich, I. N. Dokl. Akad. Nauk USSR. 1987, 31, 416 (in Russian). (19) Becker, H. D. AdV. Photochem. 1990, 15, 139 and references therein. (20) (a) Becker, H. D.; Engelhardt, L. M.; Hansen, L.; Patrick, V. A.; White, A. H. Aust. J. Chem. 1984, 37, 1329. Becker, H. D.; Hansen, L.; Skelto, B. W.; White, A. H. Aust. J. Chem. 1988, 41, 1557.
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