Spectroscopic, Photophysical, and Redox Properties of Some Mes+

Spectroscopic and photophysical properties of unsubstituted free-base porphine and several symmetric meso- tetraalkyl- and meso-tetraarylporphyrins ha...
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J. Phys. Chem. 1993,97, 7024-7033

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Spectroscopic, Photophysical, and Redox Properties of Some Mes+Substituted Free-Base Porphyrins' Harold N. Fonda,l Julanna V. Gilbert,? Russell A. C o d e r , # Julian R. Sprague, Keiko Kamioka,g and John S. Connolly' Photoconversion Branch, Basic Sciences Division, National Renewable Energy Laboratory, Golden, Colorado 80401 Received: January 21, 1993; I n Final Form: March 31, I993

Spectroscopic and photophysical properties of unsubstituted free-base porphine and several symmetric mesotetraalkyl- and meso-tetraarylporphyrins have been studied. Detailed analyses of absorption and fluorescence spectra, fluorescence lifetimes and quantum yields, and 'HNMR spectra, as well as molar extinction coefficients and redox potentials, are reported. The experimental data are correlated with trends in the structural features of these molecules predicted by molecular mechanics calculations, including planarity of the macrocycles and the dihedral angles about the meso C-C bond. Conclusions concerning the role of excited-state lifetimes of some of the aryl-substituted porphyrins are presented. The significance of these results is discussed in terms of light-induced electron transfer in covalently linked porphyrin-containing assemblies.

Introduction Linked donor-acceptor molecules have assumed an increasingly important role in studies of photoinduced electron transfer (PET).2-5 Over the past 15 years or so, a widevarietyof covalently linked molecules with porphyrin donors and quinone acceptors (PQ) have been synthesized in order to gain a better understanding of the electron-transfer (ET) events in photosynthetic reaction centers. These systems range from relatively simple porphyrins with a single attached quinone (dyads) to more elaborate, multicomponent assemblies (triads, tetrads, pentads). Such molecules have provided the basis for systematic studies of the effects of driving force, distance and orientation dependence, the nature of the bridging group, and solvent on PET.Z4 The simplest PQ dyad is of the type first reported by Dalton and co-workers6a in which benzoquinone is attached directly at a meso-position on the porphyrin ring. Very rapid (C6ps) decay of the porphyrin excited singlet state (SI) of this molecule was observed by Bergkamp et The kinetics of the transient absorption profiles were attributed to formation of a chargeseparated radical-ion-pair (RIP) state by both singlet and triplet mechanisms. Some of this behavior may have been due to the relatively large (- -0.5 eV) driving force for forward ET, which places the energy of the RIP statevery close to that of the localized porphyrin triplet state. These complications were avoided in an analogous PAQ dyad reported by Cormier et ala,' in which anthraquinone (AQ) rather than benzoquinone was attached at the periphery of tritolylporphyrin (P). The energetics for electron transfer even from the excited singlet state (SI)of this molecule are not favorable since the sum of the redox potentials is essentially isoenergeticwith SI. Nevertheless, strongly solvent-dependent fluorescence quenching was observed, which indicates that the short distance between the P and AQ moieties ( 1.4 A edge-to-edge) can compensate for marginal energetics for ET. A further variation on this dyadic theme was reported recently by Kamioka et ale,*who observed charge-transfer emission from the Zn analogue of PAQ in low-

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* To whom correspondence should be addressed.

Associated Western Universities (AWU) Visiting Scholar, 1992. Permanent address: Department of Chemistry, University of Denver, Denver, CO 80208. t AWU Visiting Scholar. Permanent address: Department of Chemistry, Metropolitan State College of Denver, Denver, CO 80204. 8 AWU Visiting Scholar 1991-1993. I Permanent address: Department of Chemistry,Bloomsburg University, Bloomsburg, PA 17815. f

0022-365419312097-7024$04.00/0

polarity solvents. In this case, rovibrational motion about the bond connecting the two groups (Le., the dihedral angle between the planes) plays an important role in determining the kinetics and mechanism of intramolecular PET. A feature common to these dyads and a great many of the multicomponent molecules*4 is that they contain meso-arylporphyrins. One factor that has not received much attention in studies of linked PQ molecules is the role played by the aromatic mesosubstituentsin PET.z Specifically, it is important toknow whether these groups should be considered as components of the donor, the acceptor,or the link for purposes of calculatingdonor-acceptor distances and other geometry-dependent parameters such as the Gibbs free energy and reorganization energy. In the work presented here, we have investigated the spectroscopic, photophysical, and redox properties of several symmetric, free-base porphyrins with different meso-substituents (Scheme I). The most extensive and precise data are those obtained for P, TMesP, TPP, T l N P , and T2NP (defined in Scheme I); our understanding of the structural effects on the molecular properties of interest has been greatly enhanced by measurements on TMP, TPentP, and T2AP. We demonstrate that the properties of the prophyrin are influenced by the substituents through extension of the conjugated *-system, and we discuss the possible role of distortion of the ma~rocycle.~ These factors bear consideration in interpreting the photophysical and photochemical behavior of linked PQ systems and may have implications for the design of new molecules for studying PET. The properties of the Zn analogues of some of these molecules have been studied by Harriman and Davila.10 Throughout the text, we compare our results with theirs and note some similarities as well as some interesting dissimilarities that appear to be due to subtle differencesin molecular geometry. Sengel1 has recently reviewed the conformational flexibility of a wide variety of porphyrinic molecules, with emphasis on chlorophylls and relevance to photobiology.

Experimental Section Synthesis and Materials. The aromatic meso-substituted porphyrins T l N P , T2NP, TMesP, and T2AP (Scheme I) were prepared by acid-catalyzed condensation of appropriate aromatic aldehydes with pyrrole. The procedure was based on the Lindsey equilibrium method of porphyrin synthesis.l2 The required aromatic aldehyde precursors were available commercially with the exception of 2-anthraldehyde, which was prepared according 0 1993 American Chemical Society

Meso-Substituted Free-Base Porphyrins

The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7025

SCHEME I: Meso-Substituted Free-Base Porphyrins Used in This Study

R

R

R

-B

Svmbol

H-

P

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TMP

CH3CH2CH2CH2CH2-

TPcntP

TlNP

RNP

\

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/

to a literature synthesis.13 T M P was synthesized in very low yield by condensation of 1,l-diethoxyethane with pyrr01e.l~ P and TPP were obtained from Porphyrin Products (Logan, Utah); additional quantities were gifts from Alan D. Adler (Western Connecticut State University). TPentP was donated by Michael R. Wasielewski (Argonne National Laboratory). Synthesized porphyrins were isolated by column chromatography (alumina, CH2Cl2) and purified by preparativeTLC (silica gel, CH2Clz). Samples of all porphyrins for spectroscopic and photophysical measurements were further purified by HPLC (silica, CHzCl2 containing a trace of pyridine) immediately before use. The structures were characterized principally by IH N M R and by comparing their spectroscopic and chromatographic properties with those in the literature.12J4Js Initial lH N M R spectra were taken with a JEOL FX90Q spectrometer operating at 89.56 MHz. High-resolution spectra were recorded at 300 MHz using a Varian Unity 300 system. In both cases, the solvent was CDCl3 with internal tetramethylsilane as reference. Solvents were of the highest purity commercially available (usually Burdick and Jackson) except for 2-methyltetrahydrofuran (MTHF), which was obtained from Aldrich, doubly distilled from calcium hydride, and stored at 5 O C under Nz until use. Spectroscopic Measurements. Electronic absorption spectra were recorded for dilute (-5 pM) solutions in 1.00-cm path length cuvettes on a Hewlett-Packard 8450A diode-array spectrophotometer, which was periodically checked for accuracy in wavelength (HozO3 filter) and absorbance (neutral density filters). Molar extinction coefficients were determined for some of the porphyrins with carefully weighed portions of the solutes (Sartorius Model 443 1 microbalance) diluted in volumetric flasks; results of duplicate runs were reproducible within 45%. Beer’s

law behavior was observed for all molecules for the concentrations used (620 pM). Fluorescence spectra were obtained on dilute solutions (absorbance < 0.2 at the excitation wavelength,

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Figure 4. Absorption (®ion) and emission spectra of all porphyrins in benzene solutions normalized to each other at their respective Qroo maxima: (a) P, (b) TINP, (c) TMP, (d) TPentP, (e) TMesP, (f) TPP, (8) T2NP, (h) TZAP. substituents interact with those of the cyclic tetrapyrrole. This Discussion trend does not hold for either the free-base or Zn form of T l N P , EffectsofStructureonSpectrcwcopy~ The progressiveredshifts observed in the absorption and emission spectra in the series TPP, which suggests that the extent of interaction of r-electrons of the T ~ N P and , TZAPgenerallyagreewith the trendseen by Hamiman l'-naphthyl group is not Only less than that in the 2'-iSOmerS but and DavilaIo for the Zn analogues of the first two porphyrins and also less than that in TPP. Based on the extinction coefficients for other Zn porphyrins with more extended *-conjugation in the of the aryl-substituted Zn porphyrins they studied, Harriman and Davilalosuggested that, as theextent of conjugation increases, meso-substituents. This indicates that the *-electrons of the

Meso-Substituted Free-Base Porphyrins

The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7029

TABLE V Oxidation and Reduction Potentials of TZNP, TPP, and TlNP this work literature porphyrina Eoab E d Eoa Ed T2NP 0.92 -1.21 TPP 0.98 -1.23 0.95c -1 .05d TlNP

1.03

-1.20

1.04'

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SCHEME 11: Ring Torsional Angles Used To Estimate Planarity of Porphyrin Ring

-1.03'

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a Listed in order of increasing E,. Measured in benzonitrileus Ag/ AgCl pseudoreference with TBAPF6 as supportingelectrolyte (see text); tabulated us SCE (estimated errors hO.01 V). Measured in methylene ch10ride.I~~Measured in dimethylsulfoxide.'* e Measured in dimethyl s~1foxide.l~~

the first and second excited states (i.e., Q and Soret bands, respectively) become more mixed. This interpretation was based on the observation that the intensities of the Q bands increase with the size of the meso-substituents, apparently a t the expense of the Soret transition. Such an intensity exchange is not observed in the free-base analogues for which absolute absorption data have been measured (Table 11). For the series TPP, T2NP, and TZAP, there is a progressive increase in the Qnm:Qmoabsorption-intensity ratio with increasing *-electron density of the substituent, indicating increased displacement between theSoandSI potentialcurves. TlNPdeviates from this trend, with an intensity ratio within the QXbands quite similar to that of unsubstituted porphine. This is another indication of weaker interaction between the porphyrin and naphthyl *-systems in the 1'- as compared to the 2'-substituted macrocycles. As discussed below, we ascribe this effect mainly to differences in dihedral angles. TMesP, in which rotation about the meso C-C bonds should be sterically restricted by the orthomethyl groups, strongly resembles TPP in its spectral properties with only slight shifts in the Qm and Q X ~absorption O bands (Figure Id and Table I) as well as in the Q*ml emission band (Figure 3 and Table I). Thus, the dihedral angle is not the only factor that influences the electronic spectra. In fact, the alkylsubstituted porphyrins, for which this paraameter is not relevant, display the largest red shifts in the Qx bands in both absorption and fluorescence (Table I). To investigate the extent to which the spectra can be correlated with the ground-state structures of these substitutued porphyrins, we carried out molecular-mechanics (MM+) calculations2' on all of the molecules shown in Scheme I. The parameters that appear to correlate with the differences in the spectroscopic data are the dihedral angles at the meso C-C bond and the planarity of the porphyrin ring, where the latter is defined in terms of torsional angles A, B, C, and D defined in Scheme 11. The results of the calculations are listed in Table VI. We have much more confidence in the treyds than in the absolute values of the calculated structural parameters. The MM+ program predicts similar structures for TPP, T2NP, andTZAP, all ~ithdihedralanglesof60~ andonlyslight deviations from the highly planar configuration of unsubstituted porphine. It issignificant that thecalculationsonTPPareingoodqualitative agreement with the X-ray structural data.22 The red shifts in the absorption and emission spectra of these three compounds appear to result mainly from increased *-conjugation with the mesosubstitutent, although progressive distortions from ring planarity may also be involved. In contrast, the MM+ calculations on T l N P predict a dihedral angle of 67O, no doubt the result of increased stericinteractions between the 8'-carbonon the naphthyl ring and the @-pyrrolic hydrogen atoms on the porphyrin. A larger dihedral angle should permit less interaction between the two *-systems, thereby diminishing the extent of conjugation in T l N P relative to T2NP. Concomitant with smaller dihedral angles, the molecular-mechanics calculations predict slightly larger distortions from planarity of the macrocyclic core. Thus, for TPP, TZNP, and T2AP, all with dihedral angles of 60°, the

b'

R angle A between bonds a and a' angle B between bonds b and b' angle C between bonds c and c' angle D between bonds d and d

TABLE VI: Calculated Structural Parameters for Various Porphyrins*' ring torsional anglesc/deg porphyrin" av dihedral anglebldeg A B C D P 0.00 0.00 0.00 0.00 TlNP 61 0.h 0.10 0.26 0.13 TMesP 88 -O).lsd 0.18 -0.45 0.31 TPP 60 0.54 0.54 0.65 0.65 T2NP 60 0.55 0.55 0.63 0.63 T2AP 60 0.53 0.65 0.53 0.65 TMP -3.Sd 3.8 4.2 3.9 TPentP -5.4d 4.8 4.8 4.6 OMTPP 5 1 (43.8)' -19.0~ 18.6 -19.0 18.6 -21.7d OETPP 60 (45.5)c 21.2 -22.1 19.6 a Listed in order of increasing ring torsional angles (absolute sum). Average of the four angles about the respective meso C-C bonds in each structure; estimated precision &lo. Defined in Scheme 11. Estimated precision *0.lo; additional significant figures are listed only to show trends. Alternating signs depict "saddle" shape of the macrocyclic core. e Experimental average dihedral angle in the Zn deri~ative.~ porphyrin ring is predicted to be more or less equally distorted relative to P. Furthermore, the calculated dihedral angle in TMesP is the closest to 90°, and this porphyrin ring is estimated to be the most planar of the aryl-substituted compounds. Of all the calculated structures, the alkyl-substituted porphyrins show the largest deviations from planarity in the macrocycle, with TPentP being somewhat more distorted than TMP. We ascribe the strong red shifts in the spectra of these two molecules to a combination of hyperconjugation and large deviations from planarity. The latter influence has been demonstrated experimentally by Barkigia et ~ 1for. two ~ fully substituted porphyrins in which X-ray structural data show that steric repulsions at the periphery of the macrocycle induce severe distortions from planarity accompanied by significant red shifts in the absorption spectra. Accordingly, we included these two structures, OMTPP and OETPP,23 in the MM+ calculations as a test of the validity of the predicted trends. As shown by the results in Table VI, there is good qualitative agreement between the predictions and the X-ray structures; uiz., in both cases the porphyrin rings are distinctly nonplanar. Taken together with the absorption and emission data, the MM+ calculations indicate that the dihedral angles in the aryl-substituted porphyrins and ring deformations in the alkyl analogues are the strongest determinants of electronic spectra. Further elucidation of the details of the ground-state structures of these molecules is provided by 1H N M R spectroscopy (300

7030 The Journal of Physical Chemistry, Vol. 97,No. 27, 1993

MHz). Specifically, we would like to correlate structural inferences pertaining to dihedral angles and *-conjugation with the electronic spectra and photophysical behavior. Such information should also permit us to address directly the possibility of atropisomersZ5 in these compounds. In their detailed reviews of the N M R spectroscopy of porphyrins, Katz and co-worker# summarized the effects of meso-substituents on the resonant frequency of the &pyrrolic protons. In general, this signal is clearly distinct and readily assigned, and it appears to be particularly sensitive to the nature of adjacent meso-groups and the resultant influence on the ring current of the macrocycle. For example, the phenyl substituent in TPP induces a considerable shielding influence on the j3-pyrrolic proton signal (68.75) relative to the corresponding signal observed in the spectrumofunsubstituted porphine (69.74). This shielding effect is most likely a consequence of a meso C-C dihedral angle much greater than Oo, thereby placing the j3-protons in a more shielded environment produced by the anisotropic effect of the substituent ring current. Consistent with and supportive of this interpretation is our observation of a more upfield resonance for the &pyrrolic protons (68.61) in TMesP, suggesting even more shielding by the substituent. This is in agreement with the modeling studies, which predict a dihedral angle near 90° for this sterically hindered porphyrin. We have also studied the lH N M R spectra of TZAP, TZNP, TlNP, and the Zn analogues of the latter two, which are qualitatively similar to published spectra.10 Although both sets of spectra were taken on high-field instruments, the splitting patterns of the multiplets appear to be more sharply resolved in our data. The 8-pyrrolic proton signals for TZAP (-68.8, not resolved) and TZNP (68.86) are quite similar to that of TPP (68.75), which indicates that the dihedral angles are comparable in the three compounds, as also predicted by the MM+ calculations. All signals in the spectrum of T l N P are shifted considerably upfield relative to those in the spectrum of T2NP, typified by the signal for the &pyrrolic protons a t 68.47 for T1N P as compared to 68.86 for TZNP. These shifts indicate, as Harriman and Davila’o pointed out for the Znderivatives, a greater shielding effect by ring currents of a 1’-substituted naphthyl group than those of a 2’-naphthyl substituent. Our molecular-modeling calculations predict a significantly larger dihedral angle in T 1NP than in TZNP, which is in line with a greater substituent-shielding effect in the former, as discussed above for the comparison between TPP and TMesP. The naphthyl protons in T l N P are correspondingly positioned to be more shielded by the porphyrin ring current, The modeling calculations further suggest that steric interactions between the &pyrrolic hydrogens and H-8’of a 1’-naphthyl substituent should be particularly effective in restricting rotation around the meso C-C bond. Evidence for this appears in the N M R spectrum of T l N P , wherein the signal for the @-pyrrolic protons (68.47) consists of at least threeclosely spaced resonances. The corresponding signal for TZNP (68.86) is an unambiguous singlet. For the case of ZnTlNP, in which ligation of the metal cation is expected to restrict the flexibility of the porphyrin ring more than in the free base, the signal for the P-pyrrolic protons appears to consist of four closely spaced peaks centered a t 68.56. Thus, the apparent equivalence of the 8-protons in TZNP (both free-base and Zn forms) can be attributed to the smaller steric bulk of the 2’-naphthyl group, which could allow thermal equilibration of thevarious atropisomers on the N M R time scale. Alternatively, the differences in the N M R spectra of the TZNP atropisomers may be too subtle to detect at ambient temperature, even at 300 MHz. On the other hand, the apparent nonequivalence of the B-pyrrolic protons in both T l N P and its Zn analoguelo provides direct evidence that these compounds are either mixtures of conformationally stable isomers or, less likely, a single configuration in which the 1’-naphthyl groups are not

Fonda et al. TABLE Vn: Splittings within the QX Bands of the Absorption and Fluorescence Spectra and 0-0 Energies of the s, States. absorption/nm splittingb emission/nm porphyrin Qno Qroo Au/cm-l Q*m Q*ml Au/cm-l &levd A. Approximate Mirror-Image Symmetry P 562 616 1560 618.0 684.5 1570 2.01 TlNP 590 646 1470 651.0 718.5 1445 1.91 TMP 606 666 1490 668.5 741.5 1475 1.86 TPentP 602 662 1510 665.5 738.5 1485 1.87 B. Reverse Mirror-ImageRelationships

TMesP 592 650 1510 652.0 723.5 1515 1.90 TPP 590 648 1520 652.5 720.0 1435 1.91 TSNP 594 652 1500 659.0 724.0 1360 1.89 T2AP 596 656 1530 666.0 729.5 1305 1.88 Spectral data from Table I; molecules listed in same order as in Figure 4. Energy difference between Qno and Qroo bands, listed to nearest 10 cm-I (estimatedstandarderrorsf40 cm-I). e Energydifferences between Q*mand Q*ml bands, listed to nearest 5 cm-1 (estimated t Qroo and Q*mmaxima standard errors f20 cm-I). d M i d ~ i n of (estimated standard errors fO.01 eV). equivalent (specifically, three directed up and one down relative to the plane of the porphyrin ring). Either case constitutes a demonstration of atropisomerism.25 While the exact structure of TlNP must await X-ray crystallographic studies, the N M R evidence strongly suggests that the 1’-naphthyl substituents are conformationally stable with larger dihedral angles than in the case of TZNP, supporting the predictions of the MM+ calculations. Such a structure, moreover, is consistent with less extensive *-conjugation in T l N P , which agrees with our interpretation of the electronic spectra. To a large extent, the fluorescence spectra reflect the trends seen in absorption; viz., larger red shifts in emission accompany increased *-conjugation in the aryl-substituted porphyrins and increased distortion from planarity in the alkyl-substituted macrocycles. As noted in Table I and shown in Figure 4, the molecules fall into two broad categories: those with approximate mirror-image symmetry (Figure 4a-d) and those with what we have termed reverse mirror-image symmetry (Figure 4e-h). The first category can be further subdivided into two classes: those for which the &:QUlo ratio in absorption and the Q*m:Q*ml ratio in emission are less than one (Figure 4a,b) and those for which these ratios aregreater than one (Figure 4c,d). The former class consists of the two most planar porphyrins ( P and TlNP), while the nonplanar macrocycles carrying alkyl substituents (TMP and TPentP) constitute the latter class. With regard to the second broad category, as shown in Table I and Figure 4 e h , an increase in the Stokes shift coincides with increased red shifts and bandwidths and, perhaps, with less-pronounced deviations from mirror-image symmetry in the aromatic series TMesP, TPP, TZNP, and TZAP. A further distinction between these latter molecules and those with more distinct mirror-image symmetry is the magnitude of thesplitting between the t w o a b a n d s i n absorption and emission. The Qu maxima listed in Table I are shown again in Table VI1 together with the splittings (in cm-I) between the Qnw, and Q ~ o absorption bands and between the Q * m and Q*ml fluorescence bands. For the series P, TlNP, TMP, and TPentP, the splittings in the absorption spectra ( 1500 cm-l) are preserved in emission within about f l 5 cm-l. The splittings in the absorption spectra of TMesP, TPP, TZNP, and TZAP are nearly the same as for the other porphyrins, but there is a clear trend of narrower splittings in emisison than in absorption. With the exception of TMesP, the energy difference between the Q*mand Q*m, maxima is less than the corresponding splitting in absorption and progressively decreases with increasing size of the aryl substituent. All of the spectral data suggest that the extent of *-conjugation and N

Meso-Substituted Free-Base Porphyrins

The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7031

TABLE VIII: Analysis of Photophysical Data for AU Porphyrins in Deaerated Benzene Solutions at 293 K porphyrin" sf/nsb @fc Ck/107d k,d/107e k,/107f A. Approximate Mirror-ImageSymmetry in Spectra 0.51 5.94 P 15.5 0.079 6.45 0.79 6.35 TlNP 14.0 0.111 7.14 8.4 TMP 11.0 9 .og 0.73 0.08 0.76 8.50 TPentP 10.8 0.082 9.26 B. Reverse Mirror-Image Relationshipsin Spectra TMesP 14.1 0.121 7.09 0.86 6.23 TPP (0.130)s 1.06 7.07 12.3 8.13 1.36 7.11 T2NP 11.8 0.161 8.47 7.3 8.77 1.49 T2AP 11.4 0.17 a Listed in order of decreasing Tf within each category. Standard errors kO.1 ns except as noted in Table 111. Relative quantum yields (Table 111) applied to the absolute value for TPP;16@f values for T2AP and TMP are less precise than those for the other six molecules. 1/71 (s-I). e @f X Zk (s-I). f Zk - krad(s-I). 8 Reference 16. ring planarity affect the shapes and displacementsof the potential curves representing the SOand SI electronic states in different ways. Effects of Structure on Photophysical Properties. One of the most prominent aspects of the photophysics of these molecules (Table 111) is that, in contrast with the Zn analogues,lo higher fluorescence quantum yields are not necessarily accompanied by longer lifetimes. This a consequence of the relative contributions of radiative and nonradiative pathways to the overall decay, as shown by the analysis presented in Table VIII. As in the case of the spectroscopic properties (Table I), there is a correlation of the radiative rate constant (k,,d) with molecular structure. In general, molecules with approximate mirror-image symmetry have lower radiative rate constants than those that display reverse mirror-image relationships, the exception being TMesP. Moreover, the magnitudes of kradappear to correlate with the extent of *-conjugation more closely than with distortion of the porphyrin ring. Withineach series, thesumof thenonradiative rateconstants (k,, = internal conversion intersystem crossing) increases with decreasing q. For k,,, however, there is a good correlation with ring planarity: the values for P and T l N P are markedly lower than those for T M P and TPentP. The (other) aryl-substituted porphyrins, again with theexception of TMesP, have intermediate values. In other words, the greater the distortions from planarity (Table VI), the faster the nonradiative rates. Thus, there appears to be a correlation between structure and excited-state behavior, viz., the absolute magnitudes of kradand the relative contributions of radiative and nonradiative pathways. It is worth noting that values of kradcalculated using our integrated extinction coefficient data (Table 11) and the well-known Strickler-Berg equation2' yield the same trend shown in Table VI11 for krad: T l N P C TPP < T2NP. This correlation is consistent with what is known experimentally for the extreme structural case of the octaalkyl TPP molecules. Barkigia et ~ 1 . have noted that the large distortions from planarity in these macrocycles are accompanied by very low fluorescence yields, i t . , higher values of knr. Whether the faster nonraadiative rates are due mainly to intersystem crossing (isc) or to internal conversion (ic) is not known. It would be of interest, therefore, to investigate the fractions of the nonradiative rate constants listed in Table VI11 due to these two processes. For TPP in benzene, the most reliable value of aiscappears to be 0.67 (ref 28a). Combining this result with Of = 0.13 (ref 16) gives aiC = 0.20, which is the same value as that obtained by Kajii et a1.28b for TPP in deoxygenated toluene solution. For comparison, we note that aicfor ZnTPP is 0.14, which is obtained by combining ,iQ = 0.83 (ref 31) and Of= 0.03 (ref 16). Since the fluorescent state of ZnTPP lies -0.2 eV higher than that of the free-base analogue, it is logical to expect aicto be somewhat higher in the latter case. Hence, a t room temperature internal

+

TABLE I X Analysis of Photophysical Data for P, TlNP, TPP. and TZNP in Deaerated MTHF Zk/107 293

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krad/

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76

kO/

porphyrin" K K 76K K K A,G* 1078 P 6.37 5.26 0.096 0.50 5.86 4.76 15. 6.3 TlNP 7.19 5.65 0.141 0.80 6.39 4.85 20. 7.1 TPP 8.20 7.41 0.144 1.07 7.1, 6.34 8.4 7.4 T2NP 8.13 7.58 0.174 1.32 6.81 6.26 6.0 7.0 a Listed in the same order as in Table IV. 1/7f (d)from Table IV. Extrapolated from the data in Tables IV and VI11 assuming that the longer lifetimes at 76 K result only from decreases in k,, (see text).d Of X Ek (s-l); assumed to be essentially independent of solvent and temperature (see text and Table VIII). e Zk - k d (s-I). /Apparent activation energy (cm-I). g From eq 1. conversion accounts for -25% of the total nonradiative decay of the SIstate of free-base TPP. This relates to our interpretation of the temperature-dependent data listed in Table IV with MTHF as the solvent, to which we now turn. There is a pronounced difference between the behavior of the planar macrocycles ( P and T l N P ) on the one hand and TPP and T2NP on the other. Specifically, the increase in 7f upon cooling the first two molecules (320%) is significantly greater than that for the second two (- 10%). We assume in all cases that the longer lifetimes are due entirely to energy barriers in the nonradiative pathways. Since the relative fluorescence spectra at room temperature in deaerated M T H F are quite similar to those in benzene (Figure 2), we further assume that the radiative rate constants listed for these four molecules in Table VI11 are essentially the same for both solvents and are independent of temperature over this range. This may not be true quantitatively, but the trends are probably valid. The M T H F data thus analyzed are contained in Table IX. The remarkable pairwise agreement in thecalculatedvaluesof kmat76 Ksuggests that wecancombine intersystem crossing and internal conversion into a single, temperature-dependent term: where both k, and the apparent activation energy, AE,are assumed to depend on structure; the other symbols have their usual meanings. The most striking result is that the effective activation energies are quite similar for the P and T l N P pair and again for TPP and T2NP, whereas there is no apparent trend in k,. As the results in Table IX show, AE is considerably larger for planar porphyrins than for molecules with even slightly distorted macrocyclic cores. Asnotedabove, for TPPin benzeneat room temperature -75% of k,, is due to intersystem crossing. On the basis of the spectral and photophysical similarities among the aryl-substituted porphyrins, we assume that the same is approximately true for T2NP. However, because of the higher values of hE for P and T l N P , ~we infer that a much larger fraction of nonradiative decay in these molecules is due to intersystem crossing. Whether or not this is true for other planar porphyrins remains to be determined. Harriman and Davilalo proposed that triplet yields are affected by symmetry changes induced in the porphyrin ring by meso-aryl substituents. Our data not only tend to confirm this suggestion but lead to further hypotheses on the detailed influences of molecular structure on spectroscopic, photophysical, and electrochemical properties of these and other porphyrins. Effects of Structureon Redox Properties. Barkigia et ~ 1have . ~ demonstrated that distortion of porphyrinic macrocycles significantly affects redox potentials as well as spectroscopic and photophysical properties. Specifically, ZnOETPP (Eox= 0.47 V us SCE in CHzClz) is easier to oxidize than either ZnTPP (0.75 V) or ZnOEP (0.63 V). The reduction potentials (us SCE in THF) are also affected by structure: ZnOETPP, -1.54 V; ZnTPP, -1.35 V; and ZnOEP, -1.63 V. Although the magnitudes

1032

The Journal of Physical Chemistry, Vol. 97, No. 27, IF'93

are surely influenced by the electron density of the substituents, the trend appears to be due mainly to the degree of ring distortion. Our molecular-mechanics calculations (Table VI) predict that the macrocyclic cores of the free-base forms of OMTPP and OETPP are distorted from ring planarity roughly 4 times more than those of either T M P or TPentP, using the ring torsional angles (Scheme 11) as the criterion. We have not measured the redox potentials of the meso-alkyl porphyrins. However, for the tripentylmonophenyl analogue of a substituted ZnTPP, Wasielewski and co-workers32 found the former to be 0.19 V easier to oxidize than the latter; this is about half the difference between the potentials for ZnOETPP and ZnTPP. Our data for theoxidation potentials of TPP, T l N P , and T2NP (TableV) areconsistent with this trend. T l N P , which the MM+ calculations predict to be about half as distorted as the other two, is the hardest to oxidize while T2NP is the easiest, by +0.05 and -0.06 V, respectively, us TPP. The latter observation further suggests that the ring distortion in the ground state of T2NP is underestimated by the MM+ predictions. Force-field calculations do not, of course, take into account strictly electronic factors such as ?r-conjugation. As noted previously, we have ascribed the trends in the electronic spectra of TPP, T2NP, and TZAP (uiz., progressively larger red shifts, Stokes shifts, and band broadening) mostly to increased *-conjugation. However, slight increases in distortion of the porphyrin ring could also be involved. Thus, perturbation from ring planarity is needed to accommodate smaller dihedral angles, which, in turn, should lead to more extensive interactions between the 7-systems of the two aromatic moieties. Presumably, this is more important in the excited states (both singlet and triplet) due to the vacancy in the highest-filled molecular orbital, analogous to electron transfer from the appended aryl group to the excited porphyrin.

Summary

P and TlNP. Despite the apparently large structural differences between these two macrocycles, the MM+ calculations predict that both rings are planar or very nearly so. This projection seems to be borne out by the similarities in their spectral and photophysical properties. The approximate mirror-image symmetry between the absorption and fluorescence spectra indicates that the nuclear configurations of the respective SIstates are the same in absorption and emission. However, the fact that the Qm:Qxlo and Q*m:Q*ml ratios are less than unity means that the potential curves depicting the SOand SIstates are displaced from one another. If we take the ground states to be planar, we infer from the spectral data that the SIstates are distorted. The similarity between the apparent AE values (Table IX) suggests that the major fraction of nonradiative decay of the excited singlet states in both cases is due to intersystem crossing, since we do not expect there to be a high energy barrier for internal conversion. Accordingly, we predict that ai, should be quite high (0.8-0.9) for both molecules. The triplet yields for ZnTPP3l and other Zn porphyrins10 are in this range and are probably associated with the high symmetry of these molecules, including planarity.18b If the SIstate of a free-base porphyrin is indeed less planar than its SOstate, it should be somewhat easier to oxidize than is predicted from combining its ground-state oxidation potential with the excited-state (Em) energy.33 This is an important consideration in terms of fine-tuning the rates and yields of ET processes. TMP and TPentP. The MM+ calculations predict these molecules to be the most distorted of the meso-substituted porphyrins we have studied and show very littledifference between their predicted structures. Consistent with this projection, the observed spectroscopic and photophysical properties are very similar, and we therefore conclude that most tetraalkylporphyrins will show essentially the same behavior. As in the case of P and T l N P , the approximate mirror-image symmetry in the spectra

Fonda et al. indicates that absorption and emission connect the same two states. For these molecules, both the Qm:Qxlo and Q*m:Q*m, ratios are greater than one, which indicates that the nuclear configurations do not change appreciably during the excited-state lifetimes. In other words, the SIpotential curves are displaced very little from those of their respective SOstates. Accordingly, we infer that both the excited and ground states are nonplanar, and we account for the lower fluorescence yields (us TPP) in terms of faster rates for nonradiative relaxation, as shown in Table VIII. The rate constants for internal conversion and intersystem crossing are not known, but we predict that the aic/ aiscratios will prove to be the highest of the eight porphyrins in this study. From the trend of increasing values for k,, with macrocycle distortion, we propose that, for the extreme structural case of the octaalkyl TPPs? the low values of (Pf and Tf result primarily from internal conversion and, accordingly, that triplet formation is negligible in highly distorted porphyrins. The implications for fine-tuning PET reactions using porphyrins of this type are somewhat less interesting than those in the case of the planar molecules. If the geometries of these porphyrins in their SIstates are essentially the same as those in the SOstates, the excited-state oxidation potentials should be very close to the values calculated from their respective ground-state properties.33 TMesP, TPP, TZNP, and TZAP. The molecular-mechanics calculations indicate that the meso C-C dihedral angles in TPP, TZNP, and TZAP are all 60' with only slight distortions of the macrocyclic cores, in good agreement with the X-ray structural data.22 The calculations for TMesPpredict a much larger dihedral angle and, concomitantly, a nearly planar ring. The validity of these calculations would be greatly substantiated if the predictions were to be borne out by X-ray crystallographic measurements. Raman and resonance Raman spectroscopies34might also be used to probe the ground- and excited-state structures of these and other tetraarylporphyrins. An alternative description of the reverse mirror-image symmetry seen in the spectra of this series of molecules is that they resemble the planar porphyrins in absorption but imitate the nonplanar, tetraalkyl molecules in fluorescence. Thus, a Qm: Qxlo ratio less than one implies that, in absorption, the potential curve for SIis displaced from that of SO.On the other hand, a Q*m:Q*ml ratio greater than one means that, in emission, the two potential curves represent very similar nuclear configurations. The progressively larger red shifts and Stokes shifts, together with the increasing bandwidths and progressive narrowing of the Q*x splitting, are all consistent with this interpretation. In other words, the geometry of either the SIstate or the ground state to which it emits (or both) is altered during the excited-state lifetime. On the basis of earlier fluorescence-anisotropy data,3s Kajii et ~ 1ruled. out~N-H~ tautomerization ~ as a contributing factor to the photophysics of TPP on the nanosecond time scale. We suggest that this aspect of porphyrin excited-state dynamics should be re-examined using picosecond laser techniques. Assuming the SOstates in these molecules to be nearly planar, we propose that the relaxed SI state is less so. A plausible consequence of this hypothesis is that the dihedral angles could be smaller, which would permit greater *-conjugation in the excited states than in the ground states. This notion is reinforced by the Q*x splitting data listed in Table VI1 and is consistent with the effects of rotation about the meso C-C bond observed in intramolecular charge-transfer emission in a series of related porphyrin-anthraquinone molecules.*~36 To the extent that the tetraarylporphyrins become more distorted upon photoexcitation, the potential for fine-tuning PET reactions is similar to the case discussed above for P and TlNP. That is, the excited states should be somewhat easier to oxidize than estimated from the ground-state properties, in which case the forward ET rate would be faster than expected because of a

Meso-Substituted Free-Base Porphyrins slightly larger driving force. This assumes, of course, that the rate is not adversely affected by a change in donor-acceptor orientation, an effect that could go either way.2 Even if the forward rate were diminished by changes in orientation prior to the E T event, the lifetime of charge separation would be enhanced if there were also a structural barrier for direct conversion back to the ground state.

The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7033

(8) Kamioka, K.; Cormier, R. A.; Lutton, T. W.; Connolly, J. S.J . Am. Chem. SOC.1992, 114,44144415. (9) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M. W.; Smith, K. M. J. Am. Chem. SOC.1990,112, 8851-8857. (10) Harriman, A.; Davila, J. Tetrahedron 1989, 45, 4737-4750. (11) Senge, M. 0. J . Photochem. Photobiol. B: Biol. 1992, 16, 3-36. (12) (a) Lindsey, J. S.;Hsu, H. C.; Schreiman, I. C. Tetrahedron Lett. 1986,27,49694970. (b) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Keamey, P. C.; Marguerettaz, A. M. J . Org. Chem. 1987,52, 827-836. (c) Lindsey, J. S.;Wagner, R. W. J . Org. Chem. 1989, 54, 828-836. (13) Arunjan, P.; Berlin, K. D. Org. Prep. Proc. Int. 1981,13, 368-371. Conclusions (14) Gonsalves, A. M.; Pereira, M. M. J . Heterocycl. Chem. 1985, 22, 931-933. The objective of this work has been to understand the effects (15) (a) Callot, H. J.; Schaeffer, E. Now. J. Chem. 1980,4,311-314. (b) of meso-substituents on the structural, spectroscopic, photophysAbraham, R. J.; Hawkes, G. E.; Hudson, M. F.;Smith, K. M. J . Chem. SOC., ical, and redox properties of porphyrins. In particular, we have Perkin Trans. 111975, 204-211. (c) Treibs, A.; Hiberle, N. Leibigs Ann. addressed the flexibility of the macrocycle with respect to ring Chem. 1968, 718, 183-207. (d) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1-13. planarity and dihedral angles, factors that have been largely (16) Quimby, D. J.; Longo, F. R. J . Am. Chem. SOC.1975,97,5111-5 117. overlooked in the area of PET in porphyrin-based systems. Until (17) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum recently, it had generally been assumed that both free-base and Press: New York, 1983; pp 4 2 4 3 . metalated porphyrins are planar in their ground states and remain (18) (a) Connolly, J. S.;Janzen, A. F.;Samuel, E. B. Photochem.Photobiol. 1982, 36, 559-563. (b) Connofly, J. S.;Samuel, E. E.;Janzen, A. F. Ibid. so in their excited states. Our experimental results are consistent 1982,36,565-574. (c) Additional fluorescencelifetimes were measured with with predictions of molecular modeling that substituents on the a picosecond laser source. periphery of the macrocycle can rotate more freely if the ring is (19) (a) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic: nonplanar. Our data also confirm the findings of Barkigia et ~ 1 . ~New York, 1977;Vol. V, pp 53-125. (b) Brown, G. M.; Hopf, F.R.; Ferguson, J. A.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. SOC.1973,95,5939-5942. that ‘porphinoid skeletons are flexible and that distortions can (c) Felton, R. H.; Linschitz, H. J . Am. Chem. SOC.1966,88, 1113-1 116. (d) be imposed by steric interactions.” Moreover, even relatively Ransdell, R. A.; Wamser, C. C. J. Phys. Chem. 1992, 96, 10572-10575. (20) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: subtle changes in structure can have profound effects on the New York, 1977; Vol. 111, pp 1-165 and references cited therein. physical properties of these molecules.’ We therefore conclude (21) Molecular-mechanics calculations were performed on HyperChemm that distortion of porphyrins in their excited states plays an using the MM+ routine with default force-fieldparameters and a convergence important role in the behavior of covalently linked systems. Such criterion of 0.001 kcal/.&. (22) Silvers, S.J.; Tulinsky, A. J . Am. Chem. SOC.1967,89, 3331-3337. considerations will become increasingly important in future (23) OMTPP and OETPP denote, respectively, 2,3,7,8,12,13,17,18assembly of supermolecules that incorporate those porphyrins octamethyl-5,10,15,20-tetraphenylporphyrin and the correspondingoctaethyl possessing the most useful combinations of excited-state and redox derivative. Barkigia et aL9 noted that the crystal structures of the Zn and free-base forms of the latter are quite similar, as are the structures of ZnTPP properties. 2-5 and H2TPP.*2.24 (24) Scheidt, W. R.; Mondal, J. U.; Eigenbrot, C. W.; Adler, A.; Acknowledgments. We thank Mary R. Posey, Libbie S. Pelter, Radonovi_ch, L. J.; Hoard, J. L. Inorg. Chem. 1986, 22, 795-799. and Thomas W. Lutton for technical assistance and helpful (25) Oki,M. Top. Stereochem. 1983, 14, 1-81. (26) Janson, T. R.; Katz, J. J. In The Porphyrins; Dolphin, D., Ed.; discussions; Alan D. Adler for the samples of P and T P P and Academic: New York, 1979; Vol. IV, pp 1-59. Scheer, H.; Katz, J. J. In Michael R. Wasielewski for the gift of TPentP. We are also Porphyrins andMetalloporphyrins;Smith, K. M., Ed.; Elsevier: Amsterdam, grateful to Daryl Myers for calibrating the E G t G diffuser plate 1975; pp 399-524. and to Alex Miedaner for assisting with the electrochemical (27) Strickler, S.J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814-822. (28) (a) Bonnett, R.;McGarvey, D. J.; Harriman, A.;Land, E. J.;Truscott, measurements. Special thanks go to Daniel L. DuBois, who made T. G.; Winfield, U.-J. Photochem. Photobiol. 1988,48,271-276. (b) Kajii, it possible for us to carry out the molecular-mechanics calculations. Y.; Obi, K.;Tanaka, I.;Tobita,S. Chem. Phys. Lett. 1984,111,347-349. (c) This work was supported by the Division of Chemical Sciences, Moore and co-workers29 have shown by optoacoustic measurements on the closely related free-base macrocycle TTP (5,10,15,20-tetratolylporphyrin) Office of Energy Research, US.Department of Energy. embedded in a polymer film that the product of the triplet energy (ET)and = 0.68, in excellent @&is0.98 eV. For ET = 1.44 eV,Sothis relation gives References and Notes agreement with the direct measurements cited above. (29) Moore, T. A.; Benin, D.; Tom, R. J. Am. Chem. SOC.1982, 104, (1) Apreliminaryaccountofthisworkwaspresentedatthe 196thNational 7356-7357. Meeting of the American Chemical Society, Los Angeles, CA, Sept. 26-30, (30) Gouterman, M.; Khalil, G.-E. J. Mol. Spectrosc. 1974, 53, 88-100. 1988 (Inorganic Chemistry Division, Abstract 309). Harriman,A. J . Chem.Soc.,Faraday Trans.2 1981,77,1281-1291. Magde, (2) Connolly, J. S.;Bolton, J. R. In Photoinduced Electron Transfer; D.; Connolly, J. S.,unpublished results (1982). Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part D, pp 303(31) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983, 392. 38, 9-14. (3) Wasielewski,M. R. In Photoinduced Electron Transfer; Fox, M. A., (32) Wasielewski, M. R.; Johnson, D. G.; Svec, W. A.; Kersey, K. M.; Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, pp 161-206. Minsek, D. W. J . Am. Chem. SOC.1988,110, 7219-7221. Wasielewski, M. R. Chem. Rev. 1992, 92, 435461. (33) Weller, A. 2.Phys. Chem. (Neue Folge) 1982, 133, 93-98. (4) Gust, D.; Moore, T. A. Top. Curr. Chem. 1991,159,103-151. Gust, (34) Abe, M.; Kitagawa, T.; Kyogoko, Y. J . Chem. Phys. 1978,69,4526D.; Moore, T. A. Adv. Photochem. 1991, 16, 1-65. 4534. Stein, P.; Ulman, A,; Spiro, T. G. J. Phys. Chem. 1984,88, 369-374. (5) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Fonda, H. N. Ph.D. Dissertation, Michigan State University, 1989. Horwood: London, 1991. (35) Sevchenko, A. N.; Gurinovich, G. P.; Solev’ev, K. N. Soviet Phys. (6) (a) Chan, A. C.; Dalton, J.; Milgrom, L. R. J . Chem. SOC.,Perkin Doklady 1961,5,808-811. Gouterman, M.; Stryer, L. J . Chem. Phys. 1962, Trans. II1982.707-709. (b) Bergkamp, M. A.; Dalton, J.; Netzel, T. L. J . 37,2260-2266. Am. Chem. SOC.1982,104, 253-259. (7) Cormier, R. A.; Bell, W. L.; Fonda, H. N.; Posey, M. R.; Connolly, (36) Kamioka,K.;Lutton,T. W.;Sprague,J. R.;Cormier,R.A.;Connolly, J. S., unpublished results. J. S. Tetrahedron 1989,45,48314843.