Photophysical Properties of Phenylethyne-Linked Porphyrin and

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8190

J. Phys. Chem. B 2004, 108, 8190-8200

Photophysical Properties of Phenylethyne-Linked Porphyrin and Oxochlorin Dyads Eve Hindin,† Christine Kirmaier,† James R. Diers,‡ Kin-ya Tomizaki,§ Masahiko Taniguchi,§ Jonathan S. Lindsey,*,§ David F. Bocian,*,‡ and Dewey Holten*,† Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889, Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: NoVember 26, 2003; In Final Form: March 19, 2004

A set of porphyrin-porphyrin and oxochlorin-oxochlorin dyads has been prepared in which the constituent pigments are joined at the meso-positions by a phenylethyne linker. Attachment of an ethynyl substituent to the meso-position of a tetrapyrrolic macrocycle strongly perturbs the electronic properties of the ring. The inherent asymmetry of the phenylethyne linker affords the possibility of perturbing either end of the dyad. The porphyrin dyads include bis-Zn, Zn-free base (Fb), and bis-Zn species wherein 0, 1, or 2 of the three nonlinking aryl rings of one of the Zn porphyrins are perfluorinated. The two oxochlorin dyads are both ZnFb species in which the meso-ethyne substituent is located on either the Zn complex or the Fb. The dyads have been studied using static and time-resolved absorption and emission spectroscopy and electrochemical techniques. The optical and electrochemical properties of a series of monomeric reference compounds were also examined. The time-resolved optical studies reveal that energy transfer in the phenylethyne-linked dyads is faster by ∼10-fold or more than in analogous dyads joined by a diphenylethyne linker. In particular, energy transfer occurs with a rate constant of >(20 ps)-1 between the phenylethyne-linked oxochlorins and an efficiency of >98%, to be compared with a rate of (140 ps)-1 and an efficiency of 83% found previously for diphenylethyne-linked oxochlorins. Taken together, these results should be useful in the design of multipigment architectures that absorb in the red and undergo fast and efficient energy transfer, as required for lightharvesting applications.

Introduction Photosynthetic antenna complexes employ large numbers of porphyrinic pigments (e.g., chlorophylls, bacteriochlorophylls) to absorb sunlight and funnel energy to the reaction centers. A major objective of artificial photosynthesis is to design and construct synthetic architectures that rival the natural systems in light absorption and excited-state energy-transfer properties. The synthetic systems created to date are quite primitive by comparison with the natural systems and largely incorporate porphyrins rather than the more biologically relevant chlorin or bacteriochlorin analogues.1 A general feature of many synthetic approaches is the use of covalent linkers to join porphyrinic pigments in an array, thereby fixing the distance of separation between the pigments. In addition to its mechanical role, the linker can provide a conduit that enables through-bond electronic communication between chromophores for energy transfer.2 In this respect, the linker provides a feature that has no counterpart in the natural systems, where the pigments are assembled via noncovalent interactions. Toward the goal of preparing artificial light-harvesting systems, we have previously characterized the photophysical properties of a variety of dyads comprised of a Zn porphyrin and a free base (Fb) porphyrin that are joined via different types of linkers.2 Energy transfer in all of the ZnFb dyads was found to be dominated by a through-bond (TB) process with a weaker through-space (TS) contribution. Furthermore, the energy* Corresponding authors. E-mail: [email protected]; David. [email protected]; [email protected]. † Washington University. ‡ University of California. § North Carolina State University.

transfer rates from the Zn-porphyrin to the Fb-porphyrin depend on the distance between adjacent porphyrins and the electron density of the porphyrin frontier molecular orbitals at the position of linker attachment. The rates were found to be (38 ps)-1 for 4,4′-diphenylbutadiyne,3 (24 ps)-1 for 4,4′-diphenylethyne,4 and (3.5 ps)-1 for p-phenylene linkers5 in meso-linked tetraarylporphyrins. More recently, we examined the lightharvesting and energy-transfer properties of several diphenylethyne-linked chlorin dyads and oxochlorin dyads (Chart 1).6 Such hydroporphyrins are more similar in their structure and red-region absorption to chromophores employed in the photosynthetic systems than are porphyrin derivatives. Although the hydroporphyrin dyads exhibit enhanced TS coupling, these dyads exhibited energy-transfer rates of (110-140 ps)-1 that are 5-fold slower than the porphyrin analogues. The rates are slower because the TB coupling is greatly reduced in the hydroporphyrin dyads and the enhanced TS coupling is insufficient to compensate for this attenuation. In this paper, we describe the electronic features and energytransfer properties of porphyrin dyads and oxochlorin dyads linked with a phenylethyne group. We selected the phenylethyne linker owing to the anticipated increase in electronic coupling and consequent faster energy transfer. Studies of meso-alkynylsubstituted porphyrins originated 25 years ago with the work of Arnold, who prepared porphyrin monomers bearing a single meso-ethyne group as well as meso-butadiynyl-linked dimers.7-9 Since then, a wide variety of molecular constructs containing porphyrins bearing up to four alkynyl groups have been prepared and examined in detail.10,11 The general features of alkynyl porphyrins are as follows:10,12-16 (i) significantly red-shifted absorption spectra, (ii) intensified Q-bands relative to the Soret

10.1021/jp037614l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004

Phenylethyne-Linked Porphyrin and Oxochlorin Dyads CHART 1

J. Phys. Chem. B, Vol. 108, No. 24, 2004 8191 CHART 2

CHART 3 bands, and (iii) less negative reduction potentials (but little perturbed oxidation potentials). While the early studies of alkynyl porphyrins generally employed ethynyl or butadiynyl substituents, more recent studies have turned to arylethynyl substituents.13,17-29 The latter enable incorporation of a variety of substituents on the aryl unit, and compared with ethyne or butadiyne substituents, impart greater solubility to the porphyrin. The presence of arylethynyl substituents causes a greater red shift and intensification of the Q-bands versus that of ethynyl groups alone. Indeed, the spectral perturbation is so pronounced for meso-tetrakis(phenylethynyl)porphyrin versus meso-tetraphenylporphyrin that the former afford green solutions, prompting Milgrom to term the former “chlorphyrins”.18 While the general physicochemical features of arylethynylporphyrins have been described,13,22,23 only one report concerns energy-transfer properties of phenylethynelinked porphyrins.27 Only limited studies have been reported for alkynyl-linked chlorins,30 and to our knowledge, no studies have been reported on alkynyl oxochlorins. The prior study of energy transfer focused on all-zinc porphyrin triads wherein the central porphyrin was substituted with two phenylethynyl groups.27 The studies reported herein focus on dyads composed of porphyrins or oxochlorins, with all-zinc, zinc-free base, or free base-zinc metalation states, and use of one or two pentafluorophenyl groups to tune the energy levels of the porphyrinic units. This work complements prior studies of phenylethyne-substituted porphyrinic compounds and illustrates the versatility of this motif for achieving very fast and efficient excited-state energy transfer.

Molecular Architectures. The compounds examined herein include porphyrin dyads (Chart 2) and their monomeric porphyrin reference compounds (Chart 3), oxochlorin dyads (Chart 4), and their monomeric reference compounds (Chart

8192 J. Phys. Chem. B, Vol. 108, No. 24, 2004 CHART 4

Hindin et al. CHART 5

5). Note that the linkers employed previously (diphenylethyne, diphenylbutadiyne, and p-phenylene) are symmetric with respect to their two attachment sites. The phenylethyne group differs in that the two ends are not identical. Attachment of the phenylethynyl unit to a porphyrinic macrocycle (X) via the phenyl unit is designated “X-pe” whereas attachment via the ethynyl unit is designated “pe-X”. Experimental Section The following compounds were prepared as described in the literature: Zn-pe,31 OxoZn-pe,32 OxoFb-pe,32 and OxoZn.32 The synthesis of the ethyne-substituted porphyrin reference compounds and dyads [pe-Zn, (pe)2-Zn, pe-ZnF5, pe-ZnF10, (pe)2-ZnF10, Zn-pe-Zn, Zn-pe-Fb, Zn-pe-ZnF5, Zn-pe-ZnF20] is described elsewhere.33 The synthesis of the ethyne-substituted oxochlorin reference compounds and dyads [pe-OxoZn, peOxoFb, OxoZn-pe-OxoFb, and OxoFb-pe-OxoZn] will be described in due course. The electrochemical measurements were performed using techniques and instrumentation previously described.34 The solvent was acetonitrile containing 0.1 M Bu4NPF6 as the supporting electrolyte. The potentials are versus Ag/Ag+ (E1/2 for FeCp2/FeCp2+ ) 0.19 V). The static absorption and fluorescence and time-resolved absorption measurements on the compounds in toluene or benzonitrile at room temperature were performed using techniques and instrumentation previously described.35 Results and Discussion Electrochemistry of Selected Reference Compounds. The redox characteristics were measured for selected phenylethynyl porphyrin and oxochlorin reference compounds, including peZn, (pe)2-Zn, pe-ZnF10, (pe)2-ZnF10, and pe-OxoZn. These values are included in Table 1. The redox potentials for two compounds that lack phenylethynyl substituents, zinc-tetraphenylporphyrin (ZnTPP) and Oxo-Zn, are also included in the table as reference points for evaluating the effects of the phenylethynyl substituents. The general features of the redox characteristics of the reference porphyrins are consistent with the results of earlier measurements on other types of mesoalkynyl-substituted porphyrins.10,12-16 Here, we have included the additional effects of pentafluorophenyl substituents and the use of the oxochlorin macrocycle to further explore the electronic origins of the effects of meso-alkynyl groups. The salient features of the electrochemical data are as follows.

(1) The introduction of phenylethynyl substituents has a negligible effect on the oxidation potentials of both the porphyrins and oxochlorins. For example, the Eox 1/2 values for pe-Zn and (pe)2-Zn are 0.55 and 0.56 V; the Eox 1/2 value is 0.56 V for ZnTPP. Similarly, the Eox 1/2 value is 0.59 V for peOxoZn, whereas the Eox 1/2 value is 0.59 V for OxoZn (no phenylethynyl substituents). (2) The introduction of phenylethynyl substituents shifts the reduction potentials to less negative values for both the porphyrins and oxochlorins. For example, the Ered 1/2 value is is -1.58 V for ZnTPP. -1.45 V for pe-Zn, whereas Ered 1/2 value is -1.40 V for pe-OxoZn, whereas Similarly, the Ered 1/2 is -1.50 V for OxoZn. The addition of a second Ered 1/2 phenylethynyl substituent results in a further positive shift in the reduction potential. In particular, the Ered 1/2 value is -1.23 V for (pe)2-Zn. (3) Similar trends as noted above are also observed upon introduction of phenylethynyl substituents into complexes that also contain pentafluorophenyl substituents. In particular, the

Phenylethyne-Linked Porphyrin and Oxochlorin Dyads

J. Phys. Chem. B, Vol. 108, No. 24, 2004 8193

TABLE 1: Photophysical and Redox Propertiesa CtCb

C6F5c

λabsd,e

ZnTPPl Zn-pel pe-Zn pe-ZnF5 pe-ZnF10 (pe)2-Zn (pe)2-ZnF10 Zn-pe-Zn Zn-pe-Fbm Zn-pe-ZnF5 Zn-pe-ZnF10

0 0 1 1 1 2 2 1 1 1 1

0 0 0 1 2 0 2 0 0 1 2

588 (601) 589 (603) 611 (627) 606 (621) 601 (615) 640 (660) 626 (641) 617 (636) 673 (674) 611 (628) 606 (621)

OxoZn-pep,q OxoZnp OxoFb-pep,q pe-OxoZn pe-OxoFb OxoZn-pe-OxoFbm OxoFb-pe-OxoZnm

0 0 0 1 1 1 1

0 0 0 0 0 0 0

610 (614) 609 643 (643) 626 (631) 658 (657) 664 (665) 629 (633)

compound

τ (ns)e,h

kf-1 i (ns)

Porphyrins 596 (610) 0.030 (0.050) 596 (611) 0.034 (0.050) 619 (635) 0.075 (0.061) 613 (626) 0.060 (0.072) 618 (627) 0.040 (0.029) 650 (670) 0.19 (0.080) 636 (649) 0.069 (0.065) 627 (650) 0.14 (0.087) 675 (670) 0.21 (0.038) 618 (637) 0.10 (0.058) 615 (633) 0.068 (0.015)

2.1 (1.9) 2.4 (2.0) 2.3 (2.0) 2.3 (2.1) 2.2 (2.2) 1.9 (1.7) 2.5 (2.1) 2.0 (1.5) 9.7 (2.6n) 2.1 (1.2) 2.2 (0.6o)

70 (38) 71 (40) 31 (31) 38 (29) 55 (76) 10 (21) 36 (32) 14 (17) 46 (68) 21 (21) 32 (40)

Oxochlorins 611 (616) 0.040 (0.041) 609 0.044 (0.039) 643 (643) 0.14 628 (632) 0.073 (0.058) 658 (659) 0.18 (0.15) 665 (667) 0.10 (0.037) 630 (639) 0.11 (0.049)

0.7 (0.6) 0.9 (0.9) 8.8 1.1 (1.2) 8.2 (8.9) 9.1s (8.1t) 4.4 (1.3u)

18 (15) 20 (23) 63 15 (21) 46 (59) 91 (219) 40 (27)

λeme,f

Φfe,g

j Eox 1/2

k Ered 1/2

+0.56 (+0.51)

-1.58

+0.55 (+0.52)

-1.45

+0.78 (+0.78) +0.56 (+0.51) +0.77 (+0.74) (+0.54)

-1.34 -1.23 -1.20

+0.59r

-1.50r

+0.59

-1.41

a The data were obtained in toluene at room temperature unless indicated otherwise. b Number of phenylethyne groups, with the ethyne moiety at the meso carbon of the tetrapyrrole macrocycle. c Number of pentafluorophenyl groups; for the dyads, this refers only to the macrocycle with the ethyne moiety at the meso carbon. d Position of the longest wavelength absorption band. e Values in parentheses were obtained in benzonitrile at room temperature. f Position of the shortest wavelength emission band. g Fluorescence quantum yield ((10%) using Soret-band excitation (406 nm for the porphyrins and 445 nm for the oxochlorins) using Φf ) 0.13 for free base tetraphenylporphyrin in toluene as the standard.38 The same values within 10% were obtained for the monomeric zinc porphyrin and Zn-pe-Zn using Q-band excitation (550-565 nm). h Lifetime of the lowest excited-singlet state ((5%) determined using fluorescence detection. For the dyads, the value is that for the energy acceptor; the value for the energy donor is ∼10 ps or less as determined using transient absorption spectroscopy (see text). i Inverse of the radiative (fluorescence) rate constant for decay of the excited singlet state calculated via the formula (kf)-1 ) τ ‚ Φf. j First oxidation potential in acetonitrile containing 0.1 M Bu4NPF6 versus Ag/Ag+ (E1/2ox for FeCp2/FeCp2+ is 0.19 V34). The values in parentheses were obtained in CH2Cl2 with other conditions being the same. k First reduction potential in acetonitrile containing 0.1 M Bu4NPF6 versus Ag/Ag+. l Data from refs 34, 39, and 40. m This dyad is a ZnFb construct. n Weighted average of two components with lifetimes of 3.8 ns (47%) and 1.3 ns (54%). o Weighted average of two components with lifetimes of 1.2 ns (36%) and 0.4 ns (64%). p From ref 41. q From ref 32. r Essentially identical value was obtained in butyronitrile41 or CH2Cl2.32 s Weighted average of two components with lifetimes of 10.1 ns (86%) and 2.6 ns (14%). t Weighted average of two components with lifetimes of 10.0 ns (78%) and 1.2 ns (22%). u Weighted average of two components with lifetimes of 5.2 ns (18%) and 1.6 ns (82%).

oxidation potentials are not affected by phenylethynyl substituents, whereas the reduction potentials are shifted to less negative values. For example, the Eox 1/2 values for pe-ZnF10 and (pe)2-ZnF10 are 0.78 and 0.77 V, whereas the Ered 1/2 values are -1.34 and -1.20 V. The fact that both the oxidation and reduction potentials of the fluorinated phenylethynyl-substituted complexes are intrinsically more positive than those of ZnTPP red (Eox 1/2 ) 0.56 V; E1/2 ) -1.58 V) is consistent with the known electron-withdrawing effects of the pentafluorophenyl substituents.34 In the case of reduction, the effects of fluorination augment the effects of phenylethynyl substitution. Accordingly, the Ered 1/2 values for pe-ZnF10 and pe-Zn are -1.34 V and -1.45 V. The fact that the meso-phenylethynyl substituents have relatively minor effects on the oxidation potentials of the various porphyrin complexes, and much larger effects on the reduction potentials, is qualitatively consistent with the results of molecular orbital calculations that have been previously conducted on porphyrins bearing various types of meso-alkynyl substituents.10,13,16 In particular, these calculations predict that mesoalkynyl substituents slightly destabilize the a2u-derived HOMO, but significantly stabilize the eg*-derived LUMO. In the case of the oxochlorins, energetic stabilization of the eg*-derived LUMO would also be consistent with a significant shift of the reduction potential of these complexes to less negative values. On the other hand, the observation that meso-phenylethynyl substituents have a negligible effect on the oxidation potentials of the oxochlorins is more difficult to understand. The molecular orbital calculations predict that meso-phenylethynyl substituents stabilize the a1u-derived orbital, which is the HOMO for the

oxochlorins.10,13,16 This energetic stabilization is apparent in the optical characteristics of the complex (vide infra), yet there is little effect on the oxidation potential. Static Absorption Spectra of Selected Reference Compounds. The electronic ground-state absorption spectra of the meso-phenylethynyl-substituted porphyrins and oxochlorins were studied in both toluene and benzonitrile. The spectra of three zinc porphyrins (ZnTPP, pe-Zn, and (pe)2-Zn) bearing 0, 1, or 2 meso-phenylethynyl substituents (and phenyl rings at the other meso-positions) in toluene are shown in Figure 1 (;;). Each spectrum contains a strong S0 f S2 Soret band (400470 nm) and weaker S0 f S1 Q-bands (500-700 nm). The positions of the Q(0,0) bands for these molecules and for the others studied herein are listed in Table 1. Each replacement of a phenyl ring by a phenylethynyl group results in a substantial (up to ∼30 nm) red shift in the peak positions as well as an increase in intensity of the Q(0,0) band with respect to the Q(1,0) and Soret bands. For example, the intensity of the Q(0,0) band of (pe)2-Zn is ∼35-fold larger than that for ZnTPP (when normalized to the Q(1,0) feature). Generally similar effects are observed for the molecules in the more polar solvent benzonitrile. The latter solvent is capable of metal coordination, which also leads to red shifts and Q(0,0)/Q(1,0) intensity changes. The relatively large spectral perturbations noted above derive from bonding of the ethyne moiety of the phenylethynyl group directly to the porphyrin. Attachment of the ethyne to a phenyl ring on the porphyrin results in only small perturbations, as can be seen by the similarity of the spectra of ZnTPP in Figure 1A with that for Zn-pe in Figure 2A). The effect of the mesoalkynyl groups to red shift the absorption bands of porphyrins

8194 J. Phys. Chem. B, Vol. 108, No. 24, 2004

Figure 1. Electronic absorption spectra (solid) and fluorescence spectra (dashed) for the porphyrins with 0 (ZnTPP), 1 (pe-Zn), and 2 [(pe)2Zn] phenylethynyl substituents, in toluene at room temperature. The absorption spectra in the 480-750-nm region have been multiplied by the factors shown. Spectra in the respective regions have been normalized to the most intense feature, and the maxima ((1 nm) have been indicated.

has been noted previously and is a reflection of a decreased HOMO f LUMO energy gap.10,13,16 For the ZnTPP reference porphyrin, the HOMO is the filled a2u(π) orbital, with the filled a1u(π) orbital at slightly lower energy (using D4h symmetry notation). According to earlier calculations, the eg(π*) LUMOs and the a1u(π) orbital are significantly stabilized by meso-alkyne substitution, consistent with the fact that the molecules are easier to reduce (Table 1).10,13,16 These calculations also suggest that the a2u HOMO is slightly destabilized by these substituents, although this effect must be small given the negligible change in porphyrin oxidation potentials. Thus, the red-shifted absorption bands are consistent with predominant LUMO stabilization and maintenance of the a2u(π) orbital as the HOMO upon mesophenylethynyl substitution. Furthermore, the increased energy gap between the a2u(π) HOMO and the lower-energy a1u is principally due to stabilization of the latter orbital. The energy gap grows upon addition of each meso-alkyne group, as can be seen by the increased oscillator strength of the Q(0,0) transition relative to the Q(1,0) transition (Figure 1). Note that the intensity of the Q(1,0) feature is relatively constant among metal porphyrins because it derives intensity primarily via vibronic borrowing from the Soret transition; however, the Q(0,0) intensity increases as the energy spacing between the a2u and a1u orbitals increases.36 That the a2u(π) orbital remains above a1u upon the addition phenylethynyl substituents can be seen from the counterbalancing effects of pentafluorophenyl rings on the Q(0,0)/Q(1,0) intensity ratio (compare spectra of peZn, pe-ZnF5, and pe-ZnF10 in Figures 2B, 3B, and 4B). This finding is consistent with the related effects on the redox potentials. Halogenated meso-aryl groups are known to preferentially stabilize the a2u orbital, and in the case of meso-tetrakis(pentafluorophenyl)porphyrin, a2u drops below a1u so that the

Hindin et al.

Figure 2. Electronic absorption spectra (solid and dotted) and fluorescence spectra (dashed) for Zn-pe-Zn and reference porphyrins with the phenyl (Zn-pe) or ethyne (pe-Zn) of the phenylethyne group bonded to a meso-carbon, in toluene at room temperature. In panel A, the Q-region absorption is shown for Zn-pe (solid) and ZnTMP (dotted). Other conditions are as those in Figure 1.

latter becomes the HOMO.34 These combined data are indicative of the significant electronic effects of meso-alkyne substituents. The electronic ground-state absorption spectra of phenylethynyl-linked Zn-pe-Zn and monomeric zinc porphyrin reference compounds in toluene are shown in Figure 2 (;;). The most notable observation is that the spectrum of the dyad is not well represented by the sum of the spectra of the reference compounds. Spectral shifts, splittings, and intensity-ratio changes are always observed in the near-UV region (400-470 nm) for diphenylethyne-linked and p-phenylene-linked dyads due to dipole-dipole coupling of the strong Soret transition dipoles of the two constituents. The same is found here for Zn-pe-Zn. However, the spectra that we have reported previously in the Q-band region (500-600 nm) for diphenylethyne- and pphenylene-linked dyads (e.g., Zn-pep-Zn and Zn-p-Zn in Chart 1) are altered only slightly, if at all, from the combined features of the constituents.4,5 In contrast, the Q(0,0) band at 617 nm for Zn-pe-Zn is red shifted by 6 nm and has gained intensity relative to the Q(1,0) band at 568 nm when compared to the spectrum of reference alkynylporphyrin pe-Zn (Figure 2B and 2C). Similar effects are found for Zn-pe-ZnF5 and Zn-peZnF10, in which one or two, respectively, of the mesityl groups at the nonlinking meso-positions of the ethynyl-substituted porphyrin is replaced with electron-withdrawing pentafluorophenyl rings (Figures 3 and 4). The same is true for Zn-peFb, in which the ethynyl-substituted chromophore is a free base porphyrin instead of a zinc chelate (Table 1). These observations indicate that substantial electronic interactions extend from the porphyrin via the directly attached ethyne to the phenyl ring and to the second porphyrin in the dyad. The same conclusion has been drawn previously concerning phenylethyne-linked porphyrin triads.27 The effects are also reminiscent of those that we observed previously for a perylene-porphyrin dyad employing a phenylethynyl linker in which the optical bands of perylene

Phenylethyne-Linked Porphyrin and Oxochlorin Dyads

Figure 3. Electronic absorption spectra (solid) and fluorescence spectra (dashed) for Zn-pe-ZnF5 and reference porphyrins Zn-pe and pe-ZnF5, in toluene at room temperature. Other conditions are as those in Figure 1.

Figure 4. Electronic absorption spectra (solid) and fluorescence spectra (dashed) for Zn-pe-ZnF10 and reference porphyrins Zn-pe and peZnF10, in toluene at room temperature. Other conditions are as those in Figure 1.

progressively red shifted upon direct attachment to the perylene of an ethyne, followed by the phenyl ring, followed by the porphyrin.37 The addition of a phenylethynyl group at the 5-position of an oxochlorin also has substantial effects on the absorption spectra of these molecules (Table 1). For example, the Qy(0,0) band of pe-OxoZn at 626 nm lies ∼15 nm to the red of that

J. Phys. Chem. B, Vol. 108, No. 24, 2004 8195

Figure 5. Electronic absorption spectra (solid) and fluorescence spectra (dashed) for OxoFb-pe-OxoZn and reference oxochlorins OxoFb-pe and pe-OxoZn, in toluene at room temperature. Other conditions are as those in Figure 1.

for the OxoZn-pe and has gained some intensity with respect to the other Q-bands and the Soret (Figure 5A versus 6A). A comparable red shift is observed for the free base analogue peOxoFb compared to OxoFb-pe, along with position shifts and a redistribution of intensity involving the x- and y-polarized Soret components (Figure 6B versus 5B). Thus, the symmetry lowering effects associated with phenylethynyl substitution previously noted for porphyrins10,13,16 are amplified in oxochlorins (which have intrinsically lower symmetry than porphyrins). It is curious that the red shifts caused by phenylethynyl substitution of the oxochlorin monomers are not paralleled by an altered oxidation potential. As noted above, the saturated pyrrole ring of oxochlorins raises the a1u-derived orbital energetically well above the a2u-derived orbital so that the former is the HOMO. Thus, to the extent that stabilization of the a1ulike orbital predicted by calculations10,13,16 on porphyrins (which have the a2u HOMO) is also appropriate for oxochlorins, one would have expected some effect on the oxidation potentials (and perhaps even a blue shift in the optical spectra). However, like the porphyrins, the meso-phenylethynyl substituents have negligible effects on the oxidation potentials of the oxochlorins (Table 1). Thus, the red shifts observed for oxochlorins upon addition of these substituents must derive from preferential stabilization of the LUMOs, which is consistent with the effects on the reduction potentials. The absorption spectra of the phenylethynyl-linked oxochlorin dyads also deviate from those expected from the combined spectra of the reference compound. This deviation appears less dramatic when the phenylethynyl-substituted macrocycle is a zinc chelate as opposed to a free base compound. For example, the spectrum of OxoFb-pe-OxoZn is reasonably well approximated by the sum of the spectra of the reference compounds (Figure 5). On the other hand, the spectral perturbations in OxoZn-pe-OxoFb are more apparent (Figure 6). These include an ∼5 nm red shift in the Qy(0,0) band (658 to 664 nm), which is more closely associated with the alkynyl-substituted

8196 J. Phys. Chem. B, Vol. 108, No. 24, 2004

Figure 6. Electronic absorption spectra (solid) and fluorescence spectra (dashed) for OxoZn-pe-OxoFb and reference oxochlorins OxoZn-pe and pe-OxoFb, in toluene at room temperature. Other conditions are as those in Figure 1.

free base oxochlorin and an increase in intensity of a feature at 588 nm that appears to derive from the feature at 564 nm in the spectrum of the nonalkynyl-substituted constituent. Static Fluorescence Spectra of Selected Monomers. The red shifts in the peak positions in the fluorescence bands incurred upon attachment of the ethyne moiety of the phenylethynyl group to the meso position of the porphyrin or oxochlorin generally track those described above for the absorption spectra (Table 1 and Figures 1-6, - - -). The same is true of the intensity ratio of the origin and vibronic components. In this regard, it is interesting to note that the absorption-fluorescence mirror symmetry for (pe)2-Zn is almost perfect (Figure 1C). This high degree of mirror symmetry is rarely seen for porphyrins, including for pe-Zn (Figure 1B), Zn-pe (Figure 2A), or ZnTPP (Figure 1A). In the absence of a fortuitous circumstance, the very high absorption-fluorescence mirror symmetry for (pe)2Zn may arise because the two meso-alkynyl groups push the a2u orbital sufficiently far above a1u so that the lowest singlet excited state derives largely from the a2u f eg* (D4h symmetry notation) one-electron HOMO f LUMO promotion, with far less contribution of the a1u f eg* configuration than in most porphyrins. In this regard, (pe)2-Zn is spectrally similar to oxochlorins (Figures 5 and 6), in which the a1u orbital lies far above a2u (the reversed orbital order) as a result of the presence of the reduced ring. The effects on the emission spectra for the phenylethynylsubstituted porphyrins upon addition of the second porphyrin (or oxochlorin) also generally track those found in absorption (Figures 2-6). However, closer inspection indicates that the emission-band intensity ratios are affected somewhat differently than those in absorption when comparing the spectra of the dyads and reference monomers. These differences further illustrate how the optical properties are affected by the addition of the second porphyrin across this linker. One of the most notable examples is found for the compounds in which the alkynyl-substituted porphyrin has pentafluorophenyl groups at

Hindin et al. the nonlinking meso-positions. In particular, the intensity ratio of the Q(0,0) and Q(0,1) fluorescence bands of Zn-pe-ZnF10 are reversed from those for reference compound pe-ZnF10 (Figure 4). In fact, for this dyad the Q(0,0)/Q(0,1) emissionintensity ratio appears to be reversed from the Q(0,0)/Q(1,0) absorption-intensity ratio. These findings are another indicator of (1) the electronic interactions between the two porphyrins mediated by the phenylethynyl linker and (2) more extensive conjugation in the excited than ground states. These conclusions in turn imply a photoinduced conformational change involving the spatial relationship between the ethyne and porphyrin. The fluorescence of the porphyrin dyads and oxochlorin dyads most closely resembles the spectra of the isolated lower-energy chromophore, although these spectra are altered from the reference compounds as noted above. This behavior is observed even with excitation into absorption features that one might normally associate more closely with the higher-energy chromophore based on the monomer spectra, although caution is warranted here because there are enhanced electronic interactions coupling the states of the two macrocycles in the phenylethyne-linked arrays that mix the states to some extent. If for the sake of argument we refer to the emission as occurring mainly from the lower-energy chromophore, then this is the component with the meso-ethyne attachment in all the dyads that we have studied except OxoFb-pe-OxoZn (Figures 2-6). For the latter case, ethyne attachment to the zinc constituent lowers the excited singlet state energy; however, this excitedstate remains ∼1.7 kT above that of the meso-phenyl-attached free base constituent. The energetic proximity of these excited states permits thermal equilibration to occur. Fluorescence Quantum Yields and Lifetimes. The fluorescence quantum yield (Φf) and lifetime (τ) of the lowest excited singlet states for each compound in toluene or benzonitrile are given in Table 1 (the values in parentheses are for the compound in the more polar solvent). Insight into the trends is aided by the formulas Φf ) kf‚τ and τ ) (kf + kic + kisc)-1, where the rate constants are for the fluorescence, internal conversion, and intersystem crossing excited-state decay pathways. The radiative (fluorescence) rate constant for each compound obtained from the Φf equation and the experimental data is listed in Table 1 as the time constant kf-1. It is often found for porphyrins (where normally kisc dominates) that substituents alter Φf much more substantially than τ via the effect on kf (and on the nonradiative rates to some extent). This is what we observe for the effect of a meso-phenylethynyl group, namely a modest increase in fluorescence yield with a relatively constant excited-state lifetime (Table 1). For the monomeric porphyrins in toluene, the replacement of a phenylethynyl for a phenyl group in ZnTPP to first give pe-Zn and then (pe)2-Zn affords a progressive increase in the S1 f S0 radiative emission rate kf (a shorter radiative lifetime) by a factor of 2 to 3. This trend parallels the increased S0 f S1 transition probability reflected in the integrated (500-700 nm) Q-region oscillator strength in the ground-state absorption spectrum (Figure 1). Similarly, kf increases when a phenylethynyl group is substituted for a phenyl ring in the presence of two pentafluorophenyl substituents (pe-ZnF5 and pe-ZnF10). Compound (pe)2-Zn exhibits the largest lifetime shortening (an ∼25% effect) of all the monomeric porphyrins, as expected for a primary effect on kf because this molecule has the greatest increase in fluorescence yield compared to ZnTPP. Generally similar effects of phenylethynyl substituents are observed for the monomeric porphyrins in the more polar solvent benzonitrile, although the fluorescence lifetimes and yields are modified to

Phenylethyne-Linked Porphyrin and Oxochlorin Dyads some degree from the values for the compounds in toluene via the solvent coordination to the central zinc ion in the metal chelates (and primarily the effect on kf). Generally similar effects are observed for the fluorescence quantum yields and excited-state lifetimes for the porphyrin dyads in toluene, because, in each case, the energy flows to the lower-energy chromophore upon excitation. In particular, τ remains at ∼2 ns for Zn-pe-Zn, Zn-pe-ZnF5, and Zn-pe-ZnF10. For Zn-pe-Fb, the lifetime of ∼10 ns (Table 1) is modestly shorter than τ ≈ 13 ns for free base tetraphenylporphyrin,34 again consistent with a slight increase in kf. For the porphyrin dyads in benzonitrile, the lifetimes are generally shorter and the fluorescence yields are lower than those in toluene, whereas this was generally not the case for the reference monomers as noted above. These observations imply that an additional process such as charge transfer involving the other chromophore in the dyad becomes competitive with the intrinsic excited-state decay channels of the lower-energy porphyrin in the more polar solvent. The excited-state quenching becomes progressively more important upon each addition of a pentafluorophenyl ring to the ethynyl-substituted porphyrin along the series Zn-pe-Zn < Zn-pe-ZnF5 < Zn-pe-ZnF10 (Table 1). When it is considered that the quenching in the more polar solvent derives significantly from stabilization of charge-separated states Por1+-Por2-, this finding is most consistent with the ethynyl-substituted porphyrin (which also contains the electron-withdrawing pentafluorophenyl substituents) acting as the electron acceptor. Additionally, there is evidence that the excited-state decays for Zn-pe-Fb and Znpe-ZnF10 are not single exponential in benzonitrile, suggesting that more than one form contributes. This may involve differences in the interactions between the two porphyrins involving photoinduced reorientations about the phenylethynyl linker, a possibility noted above from the fluorescence spectra. The fluorescence yields are also increased slightly, and the excited-state lifetimes are little affected for the oxochlorins upon addition of a phenylethynyl substituent (Table 1). For example, the Φf ) 0.073 and τ ) 1.1 ns for pe-OxoZn in toluene, in comparison with Φf ) 0.040 and τ ) 0.7 ns for OxoZn-pe. Similarly, for the free base analogues in toluene, Φf ) 0.18 and τ ) 8.2 ns for pe-OxoFb in comparison with Φf ) 0.14 and τ ) 8.8 ns for the reference compound OxoFb-pe. Similar excited-state properties to pe-OxoFb are found for OxoZn-peOxoFb (Φf ) 0.10 and τ ) 9.1 ns). Some excited-state quenching is found for this dyad in benzonitrile, although not to the extent found for the analogous porphyrin dyad Zn-peFb. The situation is more complex for OxoFb-pe-OxoZn: although the phenyl-substituted oxochlorin is the lower-energy chromophore, the fluorescence yield and excited-state lifetime are shorter than those for the OxoFb-pe reference oxochlorin even in toluene, with a further reduction in benzonitrile. This observation implies contributions from (1) some excited-state quenching of the free base subunit even in nonpolar media, (2) a contribution from an equilibrium population of the excited alkynyl-substituted zinc oxochlorin at ∼1.7 kT higher energy, or (3) significant interactions implying mixed excited-state parentage. The latter two possibilities are consistent with the static fluorescence data described above. These results indicate that, for ZnFb oxochlorin dyads, it is preferable to utilize ethyne substituents on the free base constituent to afford arrays that are excellent candidates for energy-transfer cascades even in polar media. Time-Resolved Absorption Measurements and EnergyTransfer Rates. A. Porphyrin Dyads. Figure 7 shows timeresolved absorption difference spectra for porphyrin dyad Zn-

J. Phys. Chem. B, Vol. 108, No. 24, 2004 8197

Figure 7. Representative time-resolved absorption spectra and kinetic profiles for Zn-pe-Zn and Zn-pe-Fb in toluene at room temperature obtained using excitation with a 545-nm 130-fs flash. (Similar data for Zn-pe-Fb were obtained using 507-nm excitation.) For each compound, the 0.5 ps spectrum (solid lines) was constructed from spectra at closely spaced time intervals to account for the time dispersion of wavelengths in the white-light probe pulse. The fit to the data in the insets is the convolution of the instrument response with a dual exponential plus a constant, giving time constants of 0.6 and 8 ps at 635 nm (A) and 0.5 and 13 ps at 675 nm (B). The average time constants deduced from measurements at a number of probe wavelengths are given in the text.

pe-Zn in toluene at selected times following excitation with a 130 fs excitation flash at 545 nm. As a starting point for discussion of these data, we will refer to optical features as being due mainly to one or the other constituent of the dyad, although (as discussed above) the features may be better described as arising from mixed-parentage states of strongly coupled chromophores. Based on the ground-state absorption spectra for this dyad and the reference porphyrins given in Figure 2, excitation at 545 nm should primarily pump into the Q(1,0) ground-state absorption band of the zinc porphyrin to which the phenyl ring of the phenylethynyl linker is attached (Porp). Nonetheless, the difference spectrum at 0.5 ps after excitation (Figure 7A, - - -) is dominated by the red-shifted features that can be ascribed in the simplest picture primarily to the porphyrin that has the directly attached ethyne of the phenylethynyl linker (Pore). The most prominent of these features is a trough at 621 nm, which is intermediate in wavelength between the positions of the Pore Q(0,0) ground-state absorption bleaching (617 nm) and Q(0,0) Pore* excited-state stimulated emission (627 nm) expected from the static absorption and fluorescence spectra in Figure 2C. The presence of Pore* at 0.5 ps is also reflected in the trough at ∼570 nm that is expected for bleaching of the Q(1,0) groundstate absorption band and a dip at ∼680 nm expected for Q(0,1) stimulated emission. Thus, to the extent that excitation initially produces the excited-state Porp* (rather than a “delocalized” excited state of a strongly coupled system, which may be a better description), energy flows to Pore in 98%). This fast and efficient transfer compares with the rates of ∼(110-140 ps)-1 and efficiencies of 83-93% that we found previously for diphenylethyne-linked oxochlorins or chlorins.6 In that earlier work, we noted that energy transfer between these chromophores has a substantially greater contribution from a Fo¨rster TS mechanism than in the porphyrin arrays due to greater dipoledipole coupling derived from the increases oscillator strength of the origin transitions. The TS process in the phenylethynelinked oxochorins is expected to be even more significant owing to the shorter distance between the chromophores relative to the diphenylethyne-linked arrays. Indeed, the calculated Fo¨rster rates for the phenylethyne-linked oxochorins are comparable to the measured values, indicating that the TS process dominates, even though optical data suggest that the TB coupling is also increased compared to the diphenylethyne-linked oxochlorins. Thus, it is clear that the use of the phenylethyne linker in oxochlorin systems has overcome the limitation posed by the modest TB contribution to energy transfer that was manifest in diphenylethyne-linked oxochlorins. The modest TB contribution of oxochlorins was in distinct contrast to diphenylethyne-linked porphyrins, where the observed energy-transfer rate is dominated by a TB contribution.6 Thus, energy-transfer rates on the order of ∼(20 ps)-1 or faster for the phenylethyne-linked oxochlorins are comparable to those of (24 ps)-1 for diphenylethyne-linked porphyrins. Conclusions We have examined the spectroscopic properties of phenylethyne-substituted porphyrins and oxochlorins, as well as the energy-transfer properties of phenylethyne-linked porphyrins dyads and oxochlorin dyads. The phenylethynyl group results in strong interactions with an adjacent porphyrinic macrocycle. From a light-harvesting standpoint, a porphyrin bearing a single meso-phenylethynyl group exhibits strong red absorption, resembling that of an aryl-substituted oxochlorin. Indeed, the red-region oscillator strength (610-620 nm) is slightly stronger for pe-Zn than for OxoZn. A key distinction between porphyrins and oxochlorins is that the latter exhibit shorter excitedstate lifetimes. The benchmark aryl-substituted oxochlorin (OxoZn) has a shorter excited-state lifetime than the benchmark porphyrin ZnTPP (0.7 versus 2.1 ns), and the phenylethyne-

J. Phys. Chem. B, Vol. 108, No. 24, 2004 8199 substituted compounds pe-OxoZn and pe-Zn exhibit a similar disparity (1.1 versus 2.3 ns). While the shorter lifetime ultimately limits the yield of excited-state energy transfer to an adjacent chromophore in a dyad, use of the phenylethyne linker with oxochlorins affords an energy-transfer rate of ∼(20 ps)-1 that is sufficiently fast to afford a high quantum efficiency (>98%). The rate is substantially faster than that in an oxochlorin dyad with a diphenylethyne linker of ∼(140 ps)-1, which affords an efficiency of ∼83%. The strong interaction of the phenylethyne group with an adjacent porphyrinic macrocycle renders this motif attractive for use as a linker in energy-transfer systems. On the other hand, the resulting modulation of the properties of the chromophores presents challenges for the rational design of progressively larger arrays based on knowledge of the properties of the component parts. Thus, a favorable approach would be to use (1) phenylethynyl substituents at nonlinking positions for tuning the properties (red-shifted absorption with increased oscillator strength) and (2) p-phenylene linkers to achieve fast energytransfer rates while maintaining sufficiently weak coupling to preserve the characteristics designed into the chromophores. Acknowledgment. This research was supported by the NSF (CHE-9988142). References and Notes (1) (a) Harvey, P. D. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vol. 18, pp 63-250. (b) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. ReV. 2001, 101, 2751-2796. (2) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. (3) Youngblood, W. J.; Gryko, D. T.; Lammi, R. K.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 2111-2117. (4) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181-11193. (5) Yang, S. I.; Lammi, R. K.; Seth, J.; Riggs, J. A.; Arai, T.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 1998, 102, 9426-9436. (6) Taniguchi, M.; Ra, D.; Kirmaier, C.; Hindin, E. K.; Schwartz, J. K.; Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Knox, R. S.; Holten, D. J. Am. Chem. Soc. 2003, 125, 13461-13470. (7) Arnold, D. P.; Johnson, A. W.; Mahendran, M. J. Chem. Soc., Perkin Trans. 1 1978, 366-370. (8) Arnold, D. P.; Nitschinsk, L. J. Tetrahedron 1992, 48, 8781-8792. (9) Arnold, D. P.; Heath, G. A. J. Am. Chem. Soc. 1993, 115, 1219712198. (10) Anderson, H. L. Chem. Commun. 1999, 2323-2330. (11) Arnold, D. P. Synlett 2000, 296-305. (12) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-1111. (13) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, 11854-11864. (14) Arnold, D. P.; Heath, G. A.; James, D. A. New J. Chem. 1998, 12, 1377-1387. (15) Arnold, D. P.; Heath, G. A.; James, D. A. J. Porphyrins Phthalocyanines 1999, 3, 5-31. (16) Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 70177033. (17) Proess, G.; Pankert, D.; Hevesi, L. Tetrahedron Lett. 1992, 33, 269272. (18) Milgrom, L. R.; Yahioglu, G. Tetrahedron Lett. 1995, 36, 90619064. (19) Milgrom, L. R.; Yahioglu, G. Tetrahedron Lett. 1996, 37, 40694072. (20) Milgrom, L. R.; Rees, R. D.; Yahioglu, G. Tetrahedron Lett. 1997, 38, 4905-4908. (21) Milgrom, L. R.; Yahioglu, G.; Bruce, D. W.; Morrone, S.; Henari, F. Z.; Blau, W. J. AdV. Mater. 1997, 9, 313-316. (22) Henari, F. Z.; Blau, W. J.; Milgrom, L. R.; Yahioglu, G.; Phillips, D.; Lacey, J. A. Chem. Phys. Lett. 1997, 267, 229-233. (23) Lacey, J. A.; Phillips, D.; Milgrom, L. R.; Yahioglu, G.; Rees, R. D. Photochem. Photobiol. 1998, 67, 97-100.

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