Variations and Temperature Dependence of the Excited State

Craig J. Medforth,‡ Kevin M. Smith,‡ Jack Fajer,§ and Dewey Holten*,† ...... (24) (a) Connolly, J. S.; Samuel, E. B.; Janzen, A. F. Photochem. ...
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J. Phys. Chem. B 1997, 101, 1247-1254

1247

Variations and Temperature Dependence of the Excited State Properties of Conformationally and Electronically Perturbed Zinc and Free Base Porphyrins Steve Gentemann,† Nora Y. Nelson,‡ Laurent Jaquinod,‡ Daniel J. Nurco,‡ Sam H. Leung,‡ Craig J. Medforth,‡ Kevin M. Smith,‡ Jack Fajer,§ and Dewey Holten*,† Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130; Department of Chemistry, UniVersity of California, DaVis, California 95616; and Department of Applied Science, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: October 9, 1996; In Final Form: December 9, 1996X

The photophysical properties and their temperature dependence are reported for the sterically encumbered nonplanar zinc and free base 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrins (ZnDPP and H2DPP), and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrins (ZnOETPP and H2OETPP), and the zinc complex of 5,10,15,20-tetra-tert-butylporphyrin (ZnT(t-Bu)P). Compared to planar 5,10,15,20-tetraphenylporphyrins (ZnTPP and H2TPP), the above compounds exhibit reduced lifetimes of the lowest excited singlet state, reduced fluorescence yields, and large shifts between their absorption and emission maxima at room temperature. ZnT(t-Bu)P, which is known to adopt a ruffled conformation, displays dramatically altered photophysical properties including a 7 ps 1(π,π*) lifetime compared to one of ∼2 ns for ZnTPP at 296 K. Equally noteworthy is the return of the ZnT(t-Bu)P singlet lifetime to a “normal” value of 2.5 ns at 78 K. An analogous temperature dependence has been observed previously for the free base analog H2T(t-Bu)P. The other porphyrins investigated, with different modes of nonplanarity, display smaller temperature variations but also tend toward more normal properties at low temperatures. A more extreme case of perturbation to the tetrapyrrole electronic structure is found in zinc 2,3,5,5′,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3oxopropyl)isoporphyrin perchlorate, a porphyrin tautomer with an interrupted π system. This zinc isoporphyrin also exhibits a short excited state lifetime of 130 ps at 296 K, which again increases to 0.8 ns at 78 K. The results for the various nonplanar porphyrins and for the isoporphyrin in several solvents indicate that the principal cause of the altered excited state lifetimes is the ability of the molecules to traverse multiple conformational surfaces in the excited state. These surfaces appear to be separated by only small energy barriers that vary with the types of conformational distortions and their concomitant perturbations of the electronic structures of the chromophores.

Introduction Nonplanar conformations of porphyrinic protein complexes are being increasingly recognized in crystallographic characterizations of photosynthetic antenna and reaction centers as well as of heme proteins.1-3 As a result, synthetic efforts have targeted macrocyclic deformations believed to occur in ViVo as a result of steric and covalent interactions within the proteins in order to determine the factors that modulate the optical, vibrational, redox, and excited state properties of the tetrapyrrole cofactors in the natural systems and thus control their function. A number of metalloporphyrins and free base porphyrins have been synthesized recently that exhibit significant deviations from planarity as a result of steric crowding by the peripheral substituents.4-7 Crystallographic structures reveal that these steric interactions are manifested in several classes of distortion modes in both free base porphyrins and metalloporphyrins.4-9 Tilting of the pyrrolic units is generally associated with a “saddle-type” structure while twisting of these units is often reflected in a “ruffled” structure.8 Mixtures of the these two general types of deformations, as well as “waved” and “domed” structures have been observed. Electronic and vibrational spectroscopic studies have revealed dramatic effects of nonplanar distortions on emission yields and * To whom correspondence should be addressed. † Washington University. ‡ University of California. § Brookhaven National Laboratory. X Abstract published in AdVance ACS Abstracts, February 1, 1997.

S1089-5647(96)03113-6 CCC: $14.00

SCHEME 1: Zinc and Free Base Porphyrins (Left) and Zinc Isoporphyrin (Right)

excited state lifetimes.7,10,11-13 For example, the 1(π,π*) lifetime of saddle-shaped dodecaphenylporphyrin, H2DPP (Scheme 1), is ∼800 ps compared to 10-15 ns for the essentially planar H2TPP and H2OEP.11 The meso-substituted tetra-tert-butylporphyrin, H2T(t-Bu)P, has an even shorter 1(π,π*) lifetime of 2550 ps at 295 K.12 Suprisingly, this lifetime increases to 14 ns at 78 K. Similar behavior (8 ps at 296 K vs 7 ns at 78 K) is found for the tetraadamantyl analog, H2T(Ad)P. Both H2T(tBu)P and H2T(t-Bu)P are assumed to be ruffled in analogy to ZnT(t-Bu)P.5b We have proposed12 that conformational dynamics in the 1(π,π*) state involving thermal population of low© 1997 American Chemical Society

1248 J. Phys. Chem. B, Vol. 101, No. 7, 1997 frequency out-of-plane vibrational modes of the macrocycle underlie this temperature-dependent excited state relaxation. Many questions remain unanswered regarding the perturbed photophysical properties of these and other nonplanar free base porphyrins. Not the least of these is whether the novel temperature-dependent 1(π,π*) lifetimes and fluorescence yields exhibited by H2T(t-Bu)P and H2T(Ad)P reflect the properties of (i) ruffled metal-free porphyrins, (ii) the ruffled class of porphyrins in general, or (iii) nonplanar porphyrins as a whole. To address these issues, we have extended our investigations of the temperature dependence of the photophysics to ZnT(tBu)P as well as to the nonplanar zinc and free base DPP and OETPP complexes (Scheme 1). In contrast to the ruffled H2T(t-Bu)P and ZnT(t-Bu)P,5b ZnOETPP4c and H2OETPP4b are saddle-shaped complexes. The crystal structures of H2DPP show that this molecule has a saddle conformation,6 although recently it has been found that dodecaarylporphyrins are conformationally flexible and can assume saddle, ruffle, and wave distortions.6 In addition, we report on the temperature dependence of the excited state relaxation properties of a zinc isoporphyrin. The molecule is a porphyrin tautomer with an interrupted π system (Scheme 1).14 Unlike the above nonplanar porphyrins, this isoporphyrin has a planar ground state structure.15 However, like the nonplanar free base porphyrins, the planar zinc isoporphyrin exhibits a substantially reduced 1(π,π*) lifetime compared to the canonical zinc porphyrins and chlorins (130 ps vs 1-3 ns) at room temperature.16 Furthermore, like the nonplanar porphyrins, the perturbed excited state properties of the isoporphyrin appear to derive, at least in part, from a photoinduced change in structure. Structural differences between the excited singlet and ground electronic states are evidenced by substantial shifts between the absorption and fluorescence maxima for both the isoporphyrin and the nonplanar porphyrins.11-13,16 Parallel studies of the temperature dependence of the photophysical properties of saddled and ruffled porphyrins and of the planar isoporphyrin tautomer were thus undertaken to elucidate the connections between ground and excited state conformations, the electronic structures, and the substantial variations in the excited state properties of these chromophores. Experimental Section ZnTPP, H2TPP, and H2OEP were purchased from Porphyrin Products (Logan, UT) and purified by column chromatography. All other free base and zinc porphyrins were prepared as reported elsewhere.4c,5a,6b,14 2-Methyltetrahydrofuran (MeTHF) was dried by refluxing over sodium in the presence of benzophenone and vacuum transferred into the spectroscopic cell containing the porphyrin. HPLC grade toluene, 3-methylpentane, isopentane, and acetonitrile were also dried by refluxing with sodium. Although data for most of the compounds were acquired in a number of solvents, all the data for ZnOETPP were obtained in toluene in the presence of the axial ligand 4-(N,N-dimethylamino)pyridine (DMAP) in order to stabilize the complex with respect to loss of the metal ion. An Oxford Instruments liquid nitrogen cryostat was employed for low-temperature experiments. Ground state absorption spectra were acquired on a Cary Bio-1 spectrophotometer. Emission spectra were obtained on a Spex Fluorolog II equipped with a cooled Hamamatsu R928 photomultiplier tube. Spectra are corrected for detection wavelength sensitivity. Excitation spectra confirm that the emission detected originates from the compound under investigation. Time-resolved absorption data were obtained as previously described.17 Fluorescence lifetimes

Gentemann et al.

Figure 1. Ground state absorption spectrum of ZnT(t-Bu)P in MeTHF at 78 K. The Q bands have been multiplied by the indicated factor for clarity.

were acquired using 30 ps excitation flashes at 532 or 416 nm at 7 Hz. Two schemes were employed. In one, emission was isolated with a monochromator (10 nm band-pass) and detected with a Hamamatsu R928 photomultiplier tube whose output was fed into a Stanford Instruments SR445 amplifier followed by a Tektronix 7912AD transient digitizer; the instrument response (fwhm) of this setup is ∼2 ns. In the second setup, emission was isolated with colored glass filters and detected with a Newport 818-BB-21 PIN photodiode whose output was connected directly to the digitizer, giving an instrument profile of ∼500 ps. Results Ground State Absorption Spectra. The ruffled conformation of ZnT(t-Bu)P has been established crystallographically.5c Figure 1 shows the ground state absorption spectrum of the molecule in MeTHF at 78 K. Virtually identical spectra are observed at room temperature in MeTHF and at both temperatures in 3-methylpentane. The most prominent distinction between the spectra for ZnT(t-Bu)P and planar analogs such as ZnTPP is the ∼50 nm red shift of the Soret and visible bands in the nonplanar complex. Figure 2 shows ground state absorption spectra of ZnDPP, H2DPP, and H2OETPP, in MeTHF at room and low temperature. X-ray data show that H2DPP,6b-d ZnDPP,6d H2OETPP,4b and ZnOETPP4c adopt saddle conformations. H2OETPP (Figure 2C) exhibits absorption bands similar to those of the planar free base porphyrins H2TPP and H2OEP, but all the bands are red-shifted and broadened. At 78 K, the four Q bands of H2OETPP sharpen and remain near the same positions as at 296 K. The broad Soret band of H2OETPP is insensitive to temperature. At 296 K, the same spectra are obtained in toluene, acetonitrile, or a 50/50 mixture of 3-methylpentane and isopentane. The absorption bands of ZnOETPP (not shown) are also red-shifted from the planar analogs.4 The Soret band of this molecule in toluene plus DMAP does not change position when the temperature is lowered from 296 to 200 K, although modest changes in the positions and relative intensities of the visible bands are observed. The absorption bands of H2DPP (Figure 2B) at room temperature in MeTHF are also broadened and red-shifted relative to those of the planar H2TPP. Virtually the same spectrum is found in toluene and hydrocarbon solvents. Unlike H2OETPP, the spectrum of H2DPP changes when the temperature is lowered. The Soret band sharpens and red shifts, and the positions and relative intensities of the Q bands change at 78 K compared to 296 K (Figure 2B). These spectral changes are not due to formation of the dication, based on comparison with authentic spectra of H4DPP2+.18 The temperature effects on the electronic spectrum are reversible. Warming the sample

Excited State Properties of Zn and Free Base Porphyrins

J. Phys. Chem. B, Vol. 101, No. 7, 1997 1249

Figure 3. Ground state absorption spectra and emission spectra of zinc isoporphyrin in 2-MeTHF at 296 K (dashed lines) and at 78 K (solid lines). Spectra are normalized for comparison.

Figure 2. Ground state absorption spectra in MeTHF of ZnDPP (A), H2DPP (B), and H2OETPP (C) at 296 K (dashed lines) and at 78 K (solid lines). Q bands have been multiplied by the indicated factors relative to the Soret band. Spectra at different temperatures are scaled for comparison.

Figure 4. Emission spectra of ZnT(t-Bu)P in MeTHF at 296 and at 78 K. Spectra are scaled to similar intensity for comparison.

regenerates the original 296 K spectrum. The Q bands in the room temperature absorption spectrum of H2DPP in acetonitrile (not shown) are very similar to those of H2DPP in MeTHF at 78 K, although the Soret band is at 470 and 485 nm, respectively. The absorption bands of ZnDPP are also broadened and redshifted relative to those of the planar ZnTPP at room temperature in MeTHF, toluene, and 3-methylpentane, although the spectrum shows notable differences among the three solvents. For example, compared to the spectrum in MeTHF at 296 K (Figure 2A), all the absorption bands in toluene are blue-shifted (by 10-25 nm), and the Q(1,0) band is about twice as intense as the Q(0,0) band. The bands are further blue shifted (by 5-10 nm) when the solvent is changed to 3-methylpentane. When the temperature is reduced to 78 K, the Soret and Q bands of ZnDPP in either MeTHF or 3-methylpentane red shift and sharpen, and the Q(0,0) band becomes about twice as intense as the Q(1,0) band. Qualitatively similar effects are observed at room temperature when the potential axial ligand DMAP is added to ZnDPP in toluene. Likewise, a similar spectrum is obtained in neat acetonitrile, a potentially ligating solvent. This comparison suggests that ZnDPP becomes ligated as the temperature is reduced. However, a similar temperature dependence of the spectrum in 3-methylpentane and MeTHF (both rigorously dried) suggests that the temperature effects are not due simply to a change in ligation state. This view is corroborated by the finding of spectral changes between 296 and 200 K when ZnDPP in toluene is ligated with DMAP. As with H2DPP, the spectral changes observed at lower temperatures are inconsistent with demetalation and subsequent formation of the dication. Furthermore, the changes are reversible with temperature. The Q-band region of the electronic absorption spectrum of the zinc isoporphyrin in MeTHF at 296 and 78 K is shown in Figure 3. As previously reported, the spectra are insensitive to

solvent.14-16 The substantial red shift of the Q(0,0) band from the 530-580 nm region for ZnOEP and ZnTPP to near 800 nm is characteristic of the isoporphyrin structure. The molecular structures of Zn isoporphyrin with one water15a or two THF15b axial ligands have been determined. The molecule is planar in both cases. Fluorescence Spectra and Yields. Figure 4 shows the fluorescence spectra of ZnT(t-Bu)P at 296 and 78 K in MeTHF. At room temperature, the spectrum has a single broad feature centered near 740 nm. The fluorescence quantum yield (φF) of 3 × 10-5 at room temperature is orders of magnitude smaller than that observed for planar zinc porphyrins such as ZnTPP (φF ) 0.0319). Similar results (within a factor of 2) are obtained in acetonitrile. In MeTHF at 78 K, the emission spectrum of ZnT(t-Bu)P is better resolved with two maxima near 670 and 725 nm, presumably the partially resolved Q(0,0) and Q(0,1) bands. However, at low temperature, the shift between the absorption and fluorescence maxima is ∼350 cm-1, a 3-fold reduction from that at 296 K (Figures 1 and 4). Additionally, the fluorescence yield increases dramatically to 0.04, an enhancement of 3 orders of magnitude from the value at 296 K (Table 1). In contrast, the emission yield for ZnTPP increases by only 30% over a similar temperature span.20 The emission spectra of ZnDPP, H2DPP, and H2OETPP in MeTHF and ZnOETPP in toluene plus DMAP are shown in Figure 5. The spectral features of all four compounds sharpen and blue shift with decreasing temperature. The shifts between the absorption and fluorescence maxima are also reduced at low temperature, but remain larger than those for the planar analogs (Table 1). As in the case of ZnT(t-Bu)P, the temperature variations of the Stokes shifts arise primarily from the temperature dependence of the fluorescence profile and not from changes of the low-energy absorption bands (Figures 1, 2, 4, and 5). The fluorescence quantum yields for H2DPP, H2OETPP, and ZnOETPP increase by no more than a factor of 2 at low

1250 J. Phys. Chem. B, Vol. 101, No. 7, 1997

Gentemann et al.

Figure 5. Emission spectra in MeTHF of ZnDPP (A), H2DPP (B), and H2OETPP (D) at 296 K (dashed lines) and at 78 K (solid lines). The spectra of ZnOETPP (C) were acquired in toluene + DMAP at 296 and 200 K. The spectra are scaled to similar intensity for comparison.

TABLE 1: Summary of Photophysical Data in MeTHFa fluorescence yieldb compound ruffled ZnT(t-Bu)P H2T(t-Bu)P saddled ZnDPPe H2DPP H2OETPP ZnOETPPa planar tautomer ZnisoP planar ZnTPPf H2TPPf

296 K

78 K

3 × 10-5 0.04 2 × 10-4 0.04 0.010 0.006 0.005 0.003

0.09 0.01 0.01 [0.003]a

0.004

0.01

0.03 0.11

0.04 0.14

1(π,π*) lifetimec

296 K 7 ps 51 psg

78 K

∆ (abs/fluor) (cm-1)d 296 K 78 K

2.5 ns 1300 14 ns 480

450 180

0.50 ns 4.4 ns 1360 500 0.83 ns 1.6 ns 900 360 0.66 ns 1.0 ns 770 300 0.15 ns [60 ps]a 1660 1220 0.13 ns 1.9 ns 14.7 ns

0.8 ns

530

450

2.6 ns 12.7 ns

170 70

50 1 ns) for ZnDPP, H2T(t-Bu)P, and ZnT(t-Bu)P obtained from transient absorption data at 78 K are corroborated by emission lifetimes. Excited state lifetimes were also measured for ZnTPP and H2TPP and agree with values reported previously.20 The data for all the compounds are summarized in Table 1. Discussion Previous investigations have shown that the photophysical properties of porphyrins are strongly modulated by distortions of the macrocycle.7,10-13 Our own studies have shown that saddle-shaped dodecasubstituted free base porphyrins such as H2DPP and H2OETPP display substantially shorter lifetimes of the lowest excited singlet state than nominally planar compounds such as H2TPP and H2OEP (400-800 ps vs 10-15 ns).11 Even shorter lifetimes (10-50 ps) are found for meso-substituted compounds such as H2T(t-Bu)P,12 which are assumed to be ruffled in analogy to ZnT(t-Bu)P.5b The reduced 1(π,π*) lifetimes (and concomitantly reduced fluorescence yields) of the metal-free nonplanar porphyrins derive from enhanced rates of both intersystem crossing and internal conversion. In the case of the ruffled complexes, the internal conversion route so dominates the excited state deactivation process as to lead to ground state recovery greater than 80% in tens of picoseconds. We have proposed that the enhanced rates of internal conversion in both classes of free base nonplanar porphyrins reflect increased values of the Franck-Condon factor for the nonradiative deactivation process. Although the 1(π,π*) state is lower in energy in the nonplanar complexes than in planar analogs, the enhanced vibrational overlap factors do not derive primarily from an energy gap effect. This conclusion follows from the observation that the 1(π,π*) lifetimes are shorter for

J. Phys. Chem. B, Vol. 101, No. 7, 1997 1251 the meso-substituted H2T(t-Bu)P than for dodecasubstituted H2DPP, even though the Q(0,0) absorption (and emission) of the latter is red-shifted relative to that of the former (Table 1 and Figures 1 and 2). We have suggested that the enhanced internal conversion derives from the propensity of the metal-free nonplanar porphyrins to undergo photoinduced changes in the macrocycle conformation, corresponding to a substantial coordinate displacement of the ground and excited state potential energy surfaces. Such a conformational change in the 1(π,π*) state is evidenced by the significant shifts between the absorption and fluorescence maxima and the correspondingly broad and featureless emission profiles. That the conformational changes are largely internal to the porphyrin and do not primarily reflect solvent reorganization follows from the observation of large absorption/fluorescence shifts and broad emission profiles in both polar and nonpolar solvents. In the DPP complexes, the Stokes shift is further enhanced in polar versus nonpolar solvents, indicating that excitation may cause changes in the interaction between the porphyrin and the medium, in addition to a photoinduced change in macrocycle structure. Although the photoinduced changes in porphyrin structure clearly involve adjustments in the conformations of the macrocycles, it is not clear whether excitation produces more or less planar molecules or involves additional modes of nonplanar distortion. The present work demonstrates that the substantially perturbed photophysical behavior exhibited at room temperature by the sterically crowded dodeca- and meso-substituted metal-free porphyrins is also exhibited by the analogous zinc porphyrins. The 1(π,π*) lifetime of ZnT(t-Bu)P at room temperature is reduced to ∼10 ps compared to ∼2 ns for ZnTPP, and the fluorescence yield is similarly lower. The 1(π,π*) decay occurs almost exclusively to the ground state, corresponding to an increase in the rate of internal conversion by a factor of 2000 compared to planar zinc porphyrins. The perturbations to 1(π,π*) deactivation behavior for ZnT(t-Bu)P relative to ZnTPP are thus comparable in magnitude to those found previously for H2(T(t-Bu)P relative to H2TPP. The 1(π,π*) lifetime and emission yield of ZnOETPP at room temperature are reduced by a factor of 10 or so compared to ZnTPP (or ZnOEP), which is one-half the change in the excited state properties of H2OETPP relative to H2TPP (or H2OEP). Structural data show that both H2OETPP4b and ZnOETPP4c adopt saddle-shaped conformations. The excited state lifetime and fluorescence yield for ZnDPP at 296 K are also reduced relative to ZnTPP, but the extent of the perturbations are significantly less than for the metal-free analog H2DPP (a factor of 3-4 vs 20). H2DPP is highly distorted from planarity, as is evidenced by the crystal structures of this molecule.6b-d The structure of ZnDPP has now been determined, and it is saddleshaped.6d The ground state absorption and the fluorescence spectra of H2DPP and ZnDPP show similar changes from those of the planar analogs. The smaller perturbations of the excited state lifetimes and emission yields for ZnDPP versus H2DPP do not appear to be an effect of axial ligation, since the overall results are not strongly dependent on the solvents employed or the addition of DMAP as an axial ligand. These observations suggest that the structure of ZnDPP in solution may be sufficiently different from that of H2DPP in the ground state, the excited state, or both to lead to a smaller enhancement in the rate of nonradiative decay compared to H2DPP. The presence of the central zinc ion may also stiffen the structure of ZnDPP sufficiently to reduce the extent of distortions possible in the excited electronic state. This possibility is supported by the finding that the perturbations to the excited

1252 J. Phys. Chem. B, Vol. 101, No. 7, 1997 state relaxation of ZnOETPP are also somewhat smaller than for H2OETPP relative to the planar analogs, although the differences are not as great as for the DPP complexes. Nonetheless, significant photoinduced changes in structure occur for ZnDPP and ZnOETPP. This is evidenced by the observation that the shifts between the absorption and emission maxima are comparable to those found for the free base analogs at both room and low temperature (Figures 4 and 5 and Table 1). Overall, our findings on the excited state relaxation of ZnT(t-Bu)P, ZnOETPP, and ZnDPP at room temperature reinforce those found previously for H2T(t-Bu)P, H2OETPP, and H2DPP. The combined results further demonstrate that (i) nonplanar saddle-shaped and ruffled porphyrins exhibit dramatic perturbations of the electronic spectra and excited state deactivation behavior, (ii) the perturbations of the excited state properties are larger for the ruffled class, and (iii) the modified photophysical properties occur whether the complex bears two central protons or a closed-shell metal ion. The robustness of the conclusions are further confirmed by recent results on NiDPP, NiOETPP, and NiT(t-Bu)P.22 One of the most interesting observations made previously on the metal-free nonplanar porphyrins was the novel temperature dependence of the 1(π,π*) lifetime and fluorescence yield for H2T(t-Bu)P.12 Specifically, both excited state properties increase by over 2 orders of magnitude as the temperature is reduced from 296 to 78 K, approaching the values for H2TPP (Table 1). The 1(π,π*) lifetime and emission yield for the ruffled H2T(Ad)P show a similarly strong dependence on temperature.12 Conceivably, this unusual temperature dependence might be associated with the two central protons, e.g., tautomerization, in the excited state. This possibility is clearly eliminated by the data obtained here on ZnT(t-Bu)P. This compound shows the same temperature dependence of the 1(π,π*) lifetime and fluorescence yield as do H2T(t-Bu)P and H2T(Ad)P (Table 1). Given the comparable results for the zinc and metal-free meso-substituted porphyrins, a common mechanism likely underlies their temperature-dependent photophysics. Deactivation of photoexcited ZnT(t-Bu)P via a zinc-to-porphyrin charge transfer excited state that has dropped closer to 1(π,π*) due to the nonplanar distortion, although plausible, would not provide an explanation for the metal-free analog. The previous observation that the 1(π,π*) relaxation behavior of H2T(t-Bu)P varies smoothly with temperature12 eliminates the possibility that a phase transition in the medium at its freezing point is involved. The observation that the ground state absorption spectrum of ZnT(t-Bu)P (Figure 1), like those of H2T(t-Bu)P and H2T(Ad)P,12 is basically independent of temperature further rules out the explanation that a reduction in temperature transforms these molecules from ruffled to planar in the electronic ground state. Rather, the origin of the temperature-dependent 1(π,π*) lifetimes and fluorescence yields for the metal-free and zinc ruffled meso-substituted porphyrins must rest with a property of the electronic excited state. The substantial temperature dependence of the emission spectrum of ZnT(t-Bu)P (Figure 3), H2T(t-Bu)P, and H2T(Ad)P12 is consistent with this point of view. We proposed previously for the metal-free complexes that the photoexcited ruffled porphyrin can easily access other nonplanar macrocycle configurations (either of similar or different types of distortion). Traversing these conformers in the 1(π,π*) excited state may involve thermal population of the appropriate out-of-plane deformation modes of the macrocycle. Such modes (e.g., ruffling, saddling, doming) appear to have rather low frequencies (30-60 cm-1).9b A reduction in temperature from 296 to 78 K would significantly reduce the

Gentemann et al. population of higher vibrational levels of such modes, access to other conformations, and the rate of internal conversion. A change in the thermal population of the low-frequency modes would in and of itself perturb the mean structure of the excited state because these out-of-plane modes are no doubt anharmonic.23 In addition to modulating the 1(π,π*) deactivation rates, the observed changes in the emission spectra of the ruffled porphyrins could be rationalized since the anharmonic coupling of the out-of-plane modes with the in-plane Franck-Condonactive high-frequency vibrations will depend on temperature because of its effect on the populations of the low-frequency modes. A temperature dependence of motions of the solvent could also contribute to conformational dynamics if these motions are coupled to the out-of-plane movements of the macrocycle and perhaps promote mixing between them. The above working hypothesis involving out-of-plane excursions of the porphyrin ring provides a qualitative basis for understanding the similar spectral and kinetic behaviors exhibited by ZnT(t-Bu)P, H2T(t-Bu)P, and H2T(Ad)P. However, many details need to be elucidated. One is whether this model can account quantitatively for the results. The second issue concerns the relationship between the deviations in the electronic spectra and those in the nonradiative decay rates of the nonplanar porphyrins relative to the planar analogs. For example, although the broad unstructured emission and the large shifts from the Q(0,0) absorption maxima consistently observed for the nonplanar porphyrins almost certainly indicate significant coordinate displacements between the ground and excited electronic states, the spectral perturbations do not uniformly track the perturbations in the 1(π,π*) lifetime and internal conversion rate.11,12 This relationship becomes even more difficult to assess when temperature is included as a variable. Additional work on these and related compounds needs to consider in greater detail the similarities and differences in the spectral and kinetic data among H2OETPP, H2DPP, ZnOETPP, and ZnDPP. When the temperature is reduced from 296 to 78 K, the fluorescence spectra of all four molecules exhibit a blue shift, increased structure, and a reduced spacing between the emission and Q(0,0) absorption maxima. These temperaturedependent spectral changes are similar to those found for the ruffled ZnT(t-Bu)P and H2T(t-Bu)P (Figure 4 and Table 1). Unlike ZnT(t-Bu)P and H2T(t-Bu)P, the fluorescence yields and 1(π,π*) lifetimes for H DPP, H OETPP, and ZnOETPP do not 2 2 change appreciably as the temperature is reduced. Within the framework of our hypothesis, we speculate that at both room and low temperature photoexcited saddle-shaped H2DPP, H2OETPP, and ZnOETPP are able to undergo conformational excursions that facilitate nonradiative deactivation. In this regard, we note that crystalline dodecaphenylporphyrins can assume a range of ruffle, saddle, and wave distortions in the ground electronic state.6c Hence, in solution relatively small energy barriers in the excited electronic state may make structures involving varying combinations and degrees of different distortions modes more accessible. Such configurational “plasticity” could provide a relatively temperatureindependent means for fast rates of nonradiative deactivation in H2DPP, H2OETPP, and ZnOETPP. Although H2DPP, H2OETPP, and ZnOETPP have much in common in their spectral and kinetic behavior, a property that distinguishes the compounds is that the ground electronic state absorption spectrum (particularly the position of the Soret band) of H2DPP changes much more substantially as the temperature is reduced from 296 to 78 K than do the spectra of the two OETPP complexes (compare parts B and C of Figure 2). These results could indicate that the equilibrium structure of H2DPP in the ground state is modulated as the temperature is changed.

Excited State Properties of Zn and Free Base Porphyrins In this regard, H2DPP differs not only from H2OETPP and ZnOETPP but also from all the other complexes investigated except for ZnDPP. The Soret band of the latter also red shifts as the temperature is reduced and the ratio of the Q bands is reversed (Figure 2A). The strong possibility that ZnDPP and H2DPP exist as a distribution of conformers in the electronic ground state is supported by resonance Raman,6b crystallographic,6b-d and transient absorption data22 and molecular mechanics calculations.6b Indeed, two different saddle conformations of crystalline H2DPP have been observed.6b,d In one, the alternate up and down displacements of the pyrrole rings are nearly equivalent. In the other, the displacements differ by more than 0.5 Å. In addition, DPP complexes appear to possess the additional property that solvent interactions with the twelve phenyl substituents may further tune the accessible conformational space. For example, the electronic absorption spectrum for NiDPP is significantly different in aromatic vs aliphatic solvents whereas the spectra for NiTPP and NiOETPP are not.22 Likewise, the absorption and emission spectra of ZnDPP and H2DPP vary much more with solvent than those of ZnOETPP, H2OETPP, ZnT(t-Bu)P, and H2T(t-Bu)P. It would seem that the precise macrocycle conformation of the DPP complexes and the electron density distribution in the porphyrin ring could be modulated by the solvent interactions and orientations of the phenyl groups. The combination of multiple accessible conformations (either degrees of saddling or other types of distortion, or both) and variations in the solvent interactions of the phenyl substituents could give rise to heterogeneity in the spectral and kinetic properties of ZnDPP and H2DPP. If so, then lowering the temperature would reduce the accessible conformational forms, providing an explanation for the temperature dependence of the electronic absorption spectra of the two molecules. Additional investigations on the dodecasubstituted porphyrins are needed to distinguish between the effects of nonplanarity and the orientational and electronic effects of the β-phenyl substituents and to further clarify the relationships among the various photophysical properties of these molecules. In spite of the unanswered questions, the data in hand for ZnDPP, H2DPP, ZnOETPP, and H2OETPP demonstrate that (i) the generic class of saddle-shaped porphyrins show substantial perturbations in photophysical properties from the planar analogs at room temperature and (ii) the molecules in this class generally show a much weaker dependence of excited state lifetimes and fluorescence yields on temperature than the ruffled class of nonplanar porphryins. Overall, these comparisons reveal that the conformational landscapes of both the 1(π,π*) and ground states differ for the two classes of macrocycles and that the differences have a substantial impact on the excited state dynamics. Finally, we turn to the effect of temperature on the excited state properties of the zinc isoporphyrin tautomer depicted in Scheme 1. This molecule obviously represents a different class than the nonplanar porphyrins describe above, with both a planar ground state and an interrupted π system.14,15 Previously,16 we found that the properties of the zinc isoporphyrin do resemble those of the nonplanar porphyrins. At room temperature, the zinc isoporphyrin has a considerably reduced 1(π,π*) lifetime and fluorescence yield, as well as a substantially larger shift between the absorption and fluorescence maxima compared to canonical tetrapyrroles such as ZnOEP and ZnTPP.16 For example, the 130 ps 1(π,π*) lifetime is an order of magnitude shorter than the values of 1-3 ns for normal “planar” porphyrins, chlorins, and bacteriochlorins.24 This difference is com-

J. Phys. Chem. B, Vol. 101, No. 7, 1997 1253 parable to those found for the zinc and metal-free saddle-shaped porphyrins relative to their planar analogs (Table 1). We find here that the 1(π,π*) lifetime of the zinc isoporphyrin increases by a factor of six as the temperature is reduced from 296 to 78 K. The emission yield increases as well (Table 1). Although this temperature effect is much smaller than the factor of 300 observed for the ruffled H2T(t-Bu)P and Zn(T-t-Bu)P, it is larger than that for the saddle-shaped porphyrins (H2OETPP, ZnOETPP, and H2DPP) except for ZnDPP. The narrowed and blue-shifted fluorescence spectrum (Figure 3) and the reduced shift between the absorption and emission maxima (Table 1) parallel the effects seen in the spectra of the nonplanar porphyrins. The zinc isoporphyrin is also similar to ZnT(t-Bu)P, H2T(t-Bu)P, ZnOETPP, and H2OETPP in that the electronic ground state absorption spectrum is essentially the same at 296 and 78 K, indicating that the ground state conformation remains unchanged with temperature. In our previous room-temperature study of the zinc isoporphyrin, we suggested that its reduced 1(π,π*) lifetime and fluorescence yield, and substantial Stokes shift, reflect a photoinduced change in molecular conformation. A possible configurational change in the excited state would be for the molecule to fold along an axis containing the tetrahedral carbon and the opposite meso carbon (see Scheme 1) to generate a roof shape. Such structures are exhibited by porphodimethenes5a,25 (two opposing meso carbons saturated) and by 5,15-di-tertbutylporphyrins26 (opposing meso carbons each bearing one bulky group). A potentially interesting connection between the isoporphyrin and the tetra-tert-butyl porphyrins arises when one considers that the latter molecules undergo facile addition to opposing meso carbon to form a porphodimethene with a roof structure.5a One could speculate on the basis of these considerations that both the ruffled ZnT(t-Bu)P and planar zinc isoporphyrin tend to undergo conformational changes toward a roof structure. Hence, in planar or ruffled structures in which electronic or steric effects poise the macrocycles to distort further, photoexcitation may provide the energy required to initiate motions leading to a more roofed or domed configuration. Clearly, many questions remain regarding the fundamental mechanisms underlying the similarities and differences in photophysical behavior among the classes of structurally and electronically perturbed porphyrins. The planar isoporphyrins and the nonplanar porphyrins studied to date have already demonstrated how significantly the photophysical properties of these tetrapyrroles can be modulated. The results clearly show that the properties of tetrapyrrole cofactors in biological systems may be strongly influenced by steric and electronic constraints imposed by a protein matrix. Our results further establish that once a tetrapyrrole is distorted from planarity, additional conformational regions can be readily accessed as a result of transient electronic perturbations such as photoexcitation. The ability of nonplanar porphyrins to undergo such triggerable conformational excursions may be of functional significance in the protein complexes and may be advantageous in applications such as molecular electronic devices. Acknowledgment. This work was supported by NIH Grants GM34685 (D.H.), HL22252 (K.M.S.), by NSF Grant CHE-9305577 (K.M.S.), and by the Division of Chemical Sciences, U.S. Department of Energy, under Contract DE-AC02-76CH00016 (J.F.). C.J.M. thanks Professor John A. Shelnutt (Sandia National Laboratories) for financial support from the U.S. Department of Energy Contract DE-AC04-94AL85000.

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