J. Phys. Chem. B 2001, 105, 8249-8258
8249
Synthesis and Excited-State Photodynamics in Perylene-Porphyrin Dyads 2. Effects of Porphyrin Metalation State on the Energy-Transfer, Charge-Transfer, and Deactivation Channels Sung Ik Yang,† Sreedharan Prathapan,‡,§ Mark A. Miller,‡ Jyoti Seth,| David F. Bocian,*,| Jonathan S. Lindsey,*,‡ and Dewey Holten*,† Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4899, and Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 ReceiVed: January 25, 2001; In Final Form: June 6, 2001
The photophysical properties of two perylene-porphyrin dyads have been examined in detail with the aim of expanding the functional utility of these constructs for molecular optoelectronics applications. The dyads consist of a perylene-bis(imide) dye (PDI) connected to either a magnesium porphyrin (Mg) or a free base porphyrin (Fb) via a diphenylethyne (pep) linker. The photophysical behavior of these two dyads show both similarities and differences to one another and to the dyad containing a zinc porphyrin (Zn) that was examined in the previous paper in this series. In the case of both PDI-pep-Fb and PDI-pep-Mg in toluene, the excited perylene unit (PDI*) decays rapidly (Fb ) 2.9 ps; Mg ) 2.5 ps) by energy transfer to the porphyrin forming PDI-pep-Por* in relatively high yield (Fb ∼ 85%; Mg ∼ 50%) and hole transfer to the porphyrin forming PDI--pep-Por+ (Fb ∼ 15%; Mg ∼ 50%). This behavior parallels that observed for PDI-pep-Zn, for which rapid (2.5 ps) decay of PDI* affords PDI-pep-Zn* and PDI--pep-Por+ with yields of 80% and 20%, respectively. The subsequent behavior of the Fb-containing dyad is distinctly different in two ways from that of the Zn or Mg porphyrin-containing dyads. (1) Charge recombination within PDI--pep-Fb+ primarily forms PDI-pep-Fb*, thereby complementing the formation of the latter species from PDI*pep-Fb. (2) PDI-pep-Fb* subsequently decays to the ground state via fluorescence emission with a rate and yield that are nearly identical to those of an isolated Fb porphyrin. In contrast, for both PDI-pep-Mg and PDI-pep-Zn, the predominant decay process for PDI-pep-Por* is electron-transfer yielding PDI-pep-Por+ (Zn ∼ 80%; Mg >99%). The rapid electron-transfer quenching of PDI-pep-Por* and the nonemissive character of PDI--pep-Por+ leads to negligible fluorescence from the two metalloporphyrincontaining dyads after photoexcitation. The PDI--pep-Por+ charge-separated product with Por ) Mg or Zn is very long-lived (>10 ns) in toluene but decays much more rapidly (10 10 5 ns were determined by fluorescence decay and have errors of (5%. Values 0.01 have errors of (0.005, and values 98% for PDI-pep-Mg and >75% for PDI-pep-Fb. Fluorescence Spectra, Quantum Yields, and Lifetimes for PDI-pep-Fb. The emission properties of PDI-pep-Fb and PDI-pep-Mg show both similarities and differences from one another and from PDI-pep-Zn. One similarity among all three dyads is that the peak positions and band-intensity ratios for the (predominant) porphyrin fluorescence and the (minor) perylene fluorescence are essentially the same as in the spectra of the isolated chromophores (perylene,1 Fb porphyrin,7,8 and Mg porphyrin7,8). This finding parallels the absorption results and supports the conclusion of relatively weak interchromophore electronic interactions. The emission spectrum for PDI-pep-Fb in toluene using excitation at 490 nm is shown in Figure 2A (dashed line). The emission occurs essentially exclusively from the Fb porphyrin (655 and 710 nm) even though the excitation light is absorbed primarily by the perylene component. This observation is indicative of efficient energy transfer from the excited perylene (PDI*) to the ground-state porphyrin (forming Fb*). A very small amount of perylene emission can be seen at 535 and 580 nm (Figure 2A). However, the amplitude of these features is reduced ∼1000-fold compared to the PDI-m monomer (as deduced from experiments on optically matched samples). This reduction corresponds to a PDI* fluorescence yield of Φf ) 0.001 in the dyad compared to 0.97 in the PDI-m reference compound (Table 1). Although the emission of PDI* in the dyad is dramatically reduced from that in the isolated pigment, the Fb* emission is basically the same in PDI-pep-Fb as in the FbU′ monomer (Φf ) 0.12 versus 0.13). This assessment is made from measurements using selective excitation of the porphyrin in optically matched samples at 420 nm. Similarly, the Fb* fluorescence lifetime is basically the same in the dyad and reference porphyrin (τ ) 12.5 versus 13.3 ns; Table 1). These two results represent a notable difference from the behavior
observed in the preceding article for PDI-pep-Zn (and below for PDI-pep-Mg). For the two arrays containing a metalloporphyrin, charge-transfer quenching of the excited metalloporphyrin by the perylene dramatically reduces the lifetime and fluorescence yield of the excited metalloporphyrin relative to the isolated chromophores (Figure 1). Fluorescence Spectra, Quantum Yields, and Lifetimes for PDI-pep-Mg. Figure 2B (dashed line) shows the fluorescence spectrum of PDI-pep-Mg in toluene using predominant excitation of the perylene at 490 nm. The spectrum shows the pronounced emission bands of the Mg porphyrin at 610 and 665 nm, respectively. The spectrum also shows the perylene bands at 535 and 580 nm, with intensities that are a reasonable fraction of the porphyrin features. This behavior is much different than that observed for PDI-pep-Fb, where the porphyrin fluorescence overwhelms the perylene emission (Figure 2B versus 2A). Excitation at 490 nm of optically matched samples of PDI-pep-Mg and isolated PDI shows that the perylene emission in the dyad has Φf ∼ 0.001, which corresponds to the same ∼1000-fold reduction from the isolated dye (Φf ) 0.97) that was found for PDI-pep-Fb. Transient absorption measurements described below indicate that this quenching reflects about 50/50 energy- and hole-transfer to the porphyrin (Figure 1). Selective excitation of the Mg porphyrin in optically matched samples of PDI-pep-Mg and MgU′ in toluene shows that the porphyrin emission in the dyad has Φf ∼ 0.002, which is quenched ∼80-fold from Φf ) 0.16 for the isolated chromophore. The Mg* fluorescence lifetime of 0.14 ns in the dyad is also reduced ∼80-fold from the value of ∼10 ns in MgU′ (Table 1). Thus, unlike the unquenched Fb* in PDIpep-Fb, the Mg* excited state in PDI-pep-Mg must decay primarily by a pathway not present in the monomeric porphyrin. By analogy to the discussion given for PDI-pep-Zn,1 this quenching pathway must be electron transfer from Mg* to the ground-state perylene to give PDI--pep-Mg+ (Figure 1). Thus, this Mg* charge-transfer quenching pathway, combined with the even more quenched PDI* state and the Φf values of 0.16 and 0.97 in the respective monomers, gives rise to the comparable (within a factor of ∼2) porphyrin and perylene
8252 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Figure 3. Representative time-resolved absorbance difference spectra and kinetic profiles for PDI-pep-Fb in toluene at room temperature obtained using 130 fs 490 nm excitation flashes. The inset in panel A shows a blow up of the spectra at 30 ps and 2.9 ns with the same wavelength axis as the main panel, and with a (horizontal) ∆A ) 0 line. The inset to panel B shows the entire kinetic trace for which the data at early times are shown in the main panel. The solid line through the points is a dual-exponential fit with time constants of 2.8 and 450 ps.
fluorescence in this dyad (Figure 2B). The absence of Fb* quenching in PDI-pep-Fb gives an ∼65-fold larger amount of porphyrin fluorescence in this array for the same amount of perylene emission (Figure 2A). Time-Resolved Absorption Spectra for PDI-pep-Fb. Figure 3A shows representative transient absorption difference spectra for PDI-pep-FbP in toluene following excitation with a 130 fs flash at 490 nm. Photons at this wavelength primarily (but not exclusively) pump the perylene component. The spectrum at 0.2 ps can be ascribed in a straightforward manner to the photoexcited perylene PDI* based on comparison with the static absorption and fluorescence features of PDI in the dyad (Figure 2A) and in the PDI-m reference compound.1 Briefly, the feature at 490 nm is bleaching of the PDI (1,0) ground-state absorption band; the 530 nm feature contains equal contributions of bleaching of the (0,0) band together with PDI* (0,0) stimulated emission (emission stimulated by the whitelight probe pulse occurring at approximately the position of the static fluorescence band); the feature at 580 nm is (0,1) stimulated emission. The spectrum at 30 ps (Figure 3A and inset) shows that the substantial PDI bleachings at 530 and 490 nm have largely (but not completely) decayed, whereas the PDI* stimulated emission at 580 nm (and the contribution at 530 nm) is completely absent. These residual perylene bleachings can be assigned to a small amount of PDI--pep-Fb+ formed by hole transfer from PDI* to the Fb porphyrin, by analogy with the situation for PDIpep-Zn (Figure 1).1 However, the major fraction of the PDI* decay occurs by energy transfer to the ground-state Fb porphyrin to produce Fb*. The formation of the latter species is evident
Yang et al. from bleaching of the Fb porphyrin ground-state Q-bands at 515, 550, and 595 nm together with combined Q(0,0) bleaching and Fb* stimulated emission feature at 650 nm.9 By 2.9 ns, the PDI bleachings at 530 and 490 nm have disappeared, indicating that the small PDI--pep-Fb+ fraction present at 30 ps has decayed by charge recombination. However, predominant Fb porphyrin ground-state bleachings and Fb* stimulated-emission characteristics all remain and with approximately the same amplitudes as at 30 ps. These observations imply that (1) Fb* is unquenched by the presence of the perylene and has the same long lifetime as in the isolated pigment (in agreement with the fluorescence lifetime and yield measurements) and (2) charge recombination of PDI--pep-Fb+ largely (probably >90%) produces Fb* rather than the ground state. This latter conclusion is also consistent with the relatively fast PDI--pep-Fb+ charge-recombination rate (vide infra). The PDI*-pep-Fb f PDI--pep-Fb+ hole-transfer pathway is estimated to have a yield of 15% (Table 1). This estimate is made by first subtracting the 2.9 ns Fb* spectrum from the combined Fb* + PDI--pep-Fb+ spectrum at 30 ps and then comparing the resulting amplitudes of the 530 and 490 nm PDI features of PDI--pep-Fb+ with those due to PDI* at 0.2 ps (noting that the 530 nm feature in the latter spectrum is 50/50 PDI bleaching and stimulated emission). By difference, the yield of the PDI*-pep-Fb f PDI-pep-Fb* energy-transfer process is 85%. Figure 3B shows a representative time profile for PDI-pepFb in toluene. The combined PDI bleaching and PDI* stimulatedemission feature at 530 nm decays largely with a time constant of 2.8 ( 0.4 ps. Similar values are found for the decay of PDI bleaching at 490 nm, the PDI* stimulated emission at 580 nm, and the growth of Fb* absorption at 465 nm (not shown). The average of these results gives a PDI* lifetime of 2.9 ( 0.3 ps. The residual PDI bleachings at 530 and 490 nm associated with PDI--pep-Fb+ decay with an average time constant of 450 ( 50 ps, which reflects the lifetime of the charge-separated species (Figure 3B inset). Qualitatively similar transient absorption results were found for PDI-pep-Fb in acetonitrile (Figure 4 and Table 1). The 0.2 ps PDI* spectrum is the same as that in toluene and appears to decay with slightly shorter PDI* lifetime (average value 2.6 ( 0.3 ps). The spectrum shown at 7 ps has qualitatively the same Fb* + PDI--pep-Fb+ characteristics as the 30 ps spectrum in toluene; the Fb* spectrum at long times is also similar in the two solvents. The differences are as follows: (1) The yield of PDI*-pep-Fb f PDI--pep-Fb+ hole transfer is increased to 25% (and the PDI*-pep-Fb f PDI-pep-Fb* energy-transfer yield is correspondingly reduced to 75%), as assessed by the same spectral-amplitude analysis described above for the dyad in toluene. (2) A shorter PDI--pep-Fb+ charge-recombination time of 50 ( 5 ps is determined from the decay of the small PDI bleachings at 530 and 490 nm remaining after the PDI* decay (Figure 4B and Table 1). Time-Resolved Absorption Spectra for PDI-pep-Mg in Toluene. Figure 5 shows representative transient absorption data for PDI-pep-Mg in toluene. The perylene component was again preferentially excited using 130 fs flashes at 490 nm. The 0.2 ps PDI* spectrum and the 2.5 ( 0.4 ps PDI* lifetime are comparable to those found for PDI-pep-Fb (Figure 3 and Table 1). The 15 ps spectrum for PDI-pep-Mg (like that at 30 ps for PDI-pep-Fb in toluene) represents the products of the two PDI* decay channels (Figure 1). However, the nonoverlap of the perylene and porphyrin ground-state absorption and fluorescence features for this dyad (see Figure 2B and vide
Effects of Porphyrin Metalation State
Figure 4. Representative time-resolved absorbance difference spectra and kinetic profiles for PDI-pep-Fb in acetonitrile at room temperature. The other details are similar to those for Figure 3.
supra) allows particularly clear assignment of features to Mg* (560 nm Q(0,1) bleaching; 610 nm combined Q(0,0) bleaching and stimulated emission; 650 nm Q(0,1) stimulated emission) and PDI--pep-Mg+ (PDI bleachings at 530 and 490 nm; absence of the 580 nm PDI* stimulated emission present at 0.2 ps). The clear separation of the perylene and porphyrin features allows a yield of 50% for the PDI*-pep-Mg f PDI--pepMg+ hole-transfer process to be estimated by direct comparison of the amplitude of the PDI bleachings (relative to the broad transient absorption) at 15 ps versus 0.2 ps (again noting that the 530 nm feature at the early time is 50% stimulated emission). By difference, the yield of the PDI*-pep-Mg f PDI-pepMg* energy-transfer process is 50%. These yields are significantly changed compared to the other two dyads in toluene (Table 1). At 1 ns after excitation, the PDI--pep-Mg+ features present at 15 ps (which include PDI bleachings at 530 and 490) have increased in amplitude, whereas the 650 nm Mg* stimulated emission and 465 nm excited-state absorption have decreased. These observations reflect electron transfer from Mg* to the ground-state perylene to form more PDI--pep-Mg+ (i.e., in addition to that coming from PDI*). Note again how the contrasting spectral data for PDI-pep-Mg and PDI-pep-Fb in Figures 5A and 3A readily demonstrate the clear absence of the analogous quenching of the excited porphyrin in the latter array in toluene. In particular, the Fb* features present at 30 ps do not decay until well past 2.9 ns, and the PDI--pep-Fb+ features (arising from PDI* hole transfer) do not increase in amplitude by formation of more of this species by electron transfer from Fb*. A Mg* lifetime of 140 ( 40 ps is assessed from the average of kinetic measurements in the above-mentioned spectral features (Figure 5B), in good agreement with time constant of 0.14 ns determined from the porphyrin fluorescence decay (vide
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8253
Figure 5. Representative time-resolved absorbance difference spectra and kinetic profiles for PDI-pep-Mg in toluene at room temperature. The other details are similar to those for Figure 3.
supra). A high yield of the PDI-pep-Mg* f PDI--pepMg+ hole-transfer process is indicated by the findings that between 15 ps and 1 ns the bleaching of the porphyrin Q(1,0) ground-state absorption band at 560 nm does not decay and the Q(0,0) feature at 650 nm decreases only partially as the Mg* stimulated-emission contribution disappears. However, the 650 nm ground-state bleaching contribution associated with the Mg+ component of PDI--pep-Mg+, and the related PDI bleachings at 530 and 490 nm do not change appreciably to the 3.5 ns limit of the measurements. Thus, the lifetime of PDI-pep-Mg+ in toluene is >10 ns. Time-Resolved Absorption Spectra for PDI-pep-Mg in Acetonitrile. The 0.2 ps PDI* spectrum and the average 2.4 ( 0.3 ps PDI* lifetime for PDI-pep-Mg in the polar solvent acetonitrile (Figure 6) are both comparable to those observed in toluene (Figure 5). Similarly, the 15 ps combined Mg* + PDI--pep-Mg+ spectrum in acetonitrile is similar to that observed in toluene, except that the yield of the charge-transfer product is slightly increased in the polar solvent (Table 1). However, the evolution of the spectrum past 15 ps is much different in the two solvents (Figure 6A versus 5A). In acetonitrile, all of the spectral features associated with both Mg* and PDI--pep-Mg+ present at 15 ps have decayed completely by 2.4 ns. In toluene, the PDI--pep-Mg+ features have increased dramatically by 1 ns as the Mg* features have decayed (vide supra). A time constant of 330 ( 50 ps is found for decay of the Mg* excited-state absorption at 460 nm in acetonitrile (Figure 6B, inset). This value is the same as the Mg* lifetime of 0.3 ( 0.1 ns measured by fluorescence decay (Table 1). The Mg* lifetime for PDI-pep-Mg in acetonitrile is thus somewhat longer than the value of 140 ps in toluene but again considerably shorter than the ∼10 ns lifetime of the MgU′ reference compound. This lifetime reduction, like the comparably reduced fluorescence yield in the dyad versus the monomer in acetonitrile (Table 1), indicates that the PDI-pep-Mg* f PDI--pep-Mg+ electron-transfer process occurs nearly quan-
8254 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Figure 6. Representative time-resolved absorbance difference spectra and kinetic profiles for PDI-pep-Mg in acetonitrile at room temperature. The other details are similar to those for Figure 3, except in this case the main trace and inset in panel B show data at different wavelengths, as indicated. The data before and during the flash are not shown for clarity.
titatively in the polar solvent, just as it does in toluene (vide supra). Thus, the major difference between the behavior past 15 ps for PDI-pep-Mg in acetonitrile versus toluene (Figure 6A versus 5A) is that charge recombination of PDI--pepMg+ is significantly faster in acetonitrile compared to >10 ns in toluene. Close inspection of the kinetic trace at 530 nm in Figure 6B shows a very small component with a time constant of ∼100 ps that follows the dominant, fast decay of the PDI bleaching and stimulated emission. This finding may indicate that the PDI--pep-Mg+ lifetime could be as short as ∼100 ps in acetonitrile. However, this spectral change is very small and is convoluted with the broad, weak Mg* excited-stated state absorption in this region. Therefore, an upper limit for the PDI--pep-Mg+ lifetime of 5a >0.5a >10 10 < 0.4
0.05a ∼1.5 NA NA NA
The value assumes that >90% of the PDI--pep-Fb+ decay occurs by charge recombination to PDI-pep-Fb* and E1/2(Zn) > E1/2(Mg),13,14 there are uncertainties in the exact values of the potentials to be used. These uncertainties arise for the following reasons: (1) The experiments on the dyads were performed in toluene or acetonitrile, whereas the redox potentials for the dyads and monomeric porphyrins typically have been measured in other solvents. (2) For a given monomeric porphyrin13 (and for related porphyrin arrays8,14), there are significant (>100 mV) variations in the measured redox potentials depending on solvent (polarity, metal-coordination ability, etc.) and other experimental conditions. Therefore, for the purpose of the qualitative analysis of the results on the dyads presented below, average values of the redox differences spanning these variations are used. In particular, we assume that the Fb porphyrin oxidation occurs at a potential ∼150 mV more positive than the Zn porphyrin oxidation and likewise for the Zn porphyrin relative to the Mg porphyrin. Therefore, PDI--pep-Fb+ and PDI--pep-Mg+ in toluene are placed 2.2 and 1.9 eV, respectively, above the ground state (Figure 7A,E). It is also estimated that the chargeseparated species will be stabilized by 0.27 eV in acetonitrile versus toluene.15 The solvent effect on the spectroscopic excitedstate energies of the metalloporphyrins (∼0.03 eV) is negligible in comparison. Differences in the Dominant Photophysical Processes for the Three Dyads. PDI-pep-Fb. This dyad exhibits all of the favorable properties of a light-input unit described above. These characteristics are as follows: (1) The strong perylene (0,0) absorption at 530 nm complements the weaker porphyrin features at 515 and 550 nm, and the perylene 490 nm band lies between the porphyrin Q and Soret bands. (2) Predominant excitation of the perylene at 490 nm gives ultrafast (∼3 ps) and highly efficient (∼85%) energy transfer to the porphyrin forming Fb*. Furthermore, most of the minor (∼15%) fraction of PDI* that decays by hole transfer to the porphyrin also leads, in short order, to Fb* (Figure 7A). The latter is so because charge recombination within the resultant PDI--pep-Fb+ species appears to predominantly form Fb*, rather than the typical deactivation to the ground state. The PDI--pep-Fb+ f PDI-pep-Fb* process is indicated by the transient absorption data (vide supra) and the fact that the charge recombination rates of (500 ps)-1 in toluene and (50 ps)-1 in acetonitrile seem too rapid to represent deactivation over an energy span of ∼2.3 eV to the ground state, especially when compared with the analogous processes in the other two dyads (vide infra). (Note that the faster disappearance of PDI--pepFb+ in acetonitrile versus toluene also indicates that the decay is not proceeding primarily by thermal repopulation of PDI*, and in any event, this latter process would have the same end effect, namely, forming Fb*.) Thus, the already high (85%) yield of PDI*-pep-Fb f PDI-pep-Fb* energy transfer is supplemented by the relatively rapid, sequential PDI*-pep-Fb f PDI--pep-Fb+ f PDI-pep-Fb* route for most of the remaining 15%, thus affording an essentially quantitative yield of Fb* after excitation of the perylene. (3) Once formed, the excited Fb porphyrin has essentially the same lifetime and fluorescence yield as in the isolated
Yang et al. chromophore. In other words, the characteristics of Fb* are unchanged by charge-transfer reactions with the perylene. If operable, such reactions would be detrimental for use of this accessory pigment for light-harvesting purposes. Thus, Fb* in the PDI-pep-Fb unit can emit light or transfer the energy to a subsequent energy acceptor in larger arrays. In this connection, we have previously demonstrated that fast and efficient energy transfer occurs in dyads and triads containing free base porphyrins and phthalocyanines.16 PDI-pep-Zn. This dyad has almost the same fast rate (∼(3 ps)-1) and high yield (80%) of energy transfer from PDI* to the porphyrin as occurs in PDI-pep-Fb (Figure 7C). The resultant excited porphyrin then decays by efficient (80%) PDIpep-Zn* f PDI--pep-Zn+ electron transfer with a rate constant of ∼(500 ps)-1. This same charge-transfer product is also formed from the remaining 20% of the excited perylene by PDI*-pep-Zn f PDI--pep-Zn+ hole transfer. Thus, the PDI--pep-Zn+ product forms with an overall yield of 84% (0.2 + 0.8 × 0.8) through excitation of the perylene and 80% when the porphyrin is excited directly. In this fashion, the perylene accessory pigment complements the light-absorption properties of the porphyrin while maintaining the efficient formation of the charge-transfer product (with an effective rate constant of ∼(500 ps)-1 in both toluene and acetonitrile (Figure 7C,D)). The PDI--pep-Zn+ charge-transfer product has a very long lifetime in toluene (>10 ns). This deactivation time can be shortened substantially through an increase in the solvent polarity (to 99%) PDI-pep-Mg* f PDI--pep-Mg+ electron transfer with a rate constant of (140 ps)-1 in toluene. The electron-transfer process is slightly slower ((350 ps)-1) and slightly less efficient (97%) in acetonitrile. Accordingly, if it is desirable to have ultrafast formation of the charge-transfer product, this can be achieved in ∼2.5 ps with a 50% yield using excitation of the perylene in PDI-pepMg. Quantitative formation of the charge-separated species can be achieved with a time constant of 140 ps in toluene through direct excitation of the porphyrin. In either case, the chargetransfer product has a long lifetime (>10 ns) in toluene. Like PDI-pep-Zn, the charge-recombination time is shortened substantially with an increase in solvent polarity (to λ (inverted region); in this latter regime, the fall off in rate with increasing |-∆G| is not as steep as in the normal region when quantum corrections are applied.17,18 Using the standard formulation, we estimate the total reorganization energies of λ ∼ 0.4 eV for the charge-transfer and charge-recombination processes of the perylene-porphyrin dyads in toluene and λ ∼ 1.1 eV in acetonitrile.19 Thus, in toluene, the fastest rates of charge transfer in the dyads are expected to occur for processes with energy gaps at ∼0.4 V (all other things being equal). The rate should fall off as |-∆G| becomes progressively less than ∼0.4 V (in the normal region) and (less steeply) as |-∆G| becomes larger than ∼0.4 V (in the inverted region). Similar effects should occur about the optimum value of ∼1.1 eV in acetonitrile. The combined data for the three perylene-porphyrin dyads in both toluene and acetonitrile are generally well explained within the standard framework, as shown in Figure 8. This figure also shows several curves generated using a simple, commonly used single-quantum-mode expression.18a In generating these curves, reorganization energies in accord with the estimated values mentioned above were used, along with a typical highfrequency mode of 0.16 eV (∼1300 cm-1) that gives rise to the Franck-Condon progressions in the optical spectra (Figure 2) and an electronic coupling of 0.004 eV (∼30 cm-1). Inspection of this figure reveals the following: (1) The PDIpep-Por* f PDI--pep-Por+ electron-transfer reactions (Por implies Mg or Zn) all lie in the normal region (squares). (2) The PDI*-pep-Por f PDI--pep-Por+ hole-transfer reactions are much faster because they lie closer to (if not in) the activationless region (circles). (3) The PDI--pep-Por+ f PDI-pep-Por charge-recombination processes all lie in the inverted region (triangles). (4) The PDI--pep-Fb+ f PDIpep-Fb* charge-recombination reaction is in the normal region (half-filled squares). The faster rates of charge recombination in acetonitrile compared to toluene reflect the fact the processes in the polar solvent do not lie as deeply in the inverted region. The two sources of this effect are (a) the larger λ associated with the
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8257
Figure 8. Rate versus free energy profiles for the charge-transfer and charge-recombination processes for the three dyads in toluene (A) and acetonitrile (B) at room temperature. The processes are PDI*-pepPor f PDI--pep-Por+ hole transfer (circles), PDI-pep-Por* f PDI--pep-Por+ electron transfer for Por ) Zn, Mg (open and filled squares), PDI--pep-Por+ f PDI-pep-Por* charge recombination for Por ) Fb) (half-filled square), and PDI--pep-Por+ f PDI-pepPor charge recombination (triangles). The processes for PDI-pep-Zn are given by filled symbols, those for PDI-pep-Mg are given by open symbols, and those for PDI-pep-Fb are given by half-filled symbols. The curves are simulations using a simple single-quantum-mode expression for the electron-transfer rate18a using a 0.16 eV vibration and an electronic coupling of 0.004 eV. The curves use the reorganization energies (internal and solvent) of (0.4 and 0.1; solid), (0.4 and 0.2; dotted), and (0.3 and 0.2; dashed) in panel A and (0.4 and 0.7; solid), (0.4 and 0.6; dotted), and (0.3 and 0.7; dashed) in panel B.
dielectric properties of the solvent and (b) the smaller |-∆G| between the charge-separated species and the ground state because of increased stabilization of the former entity in the polar medium. Finally, it should be noted that the only deviation from the general trend among the reactions20 can be easily rationalized in terms of the uncertainties in relevant redox potentials of the porphyrins and other assumptions in the modeling. For example, we have assumed constant electronic couplings and reorganization energies for each reaction (hole transfer, electron transfer, and charge recombination) in a given solvent. These assumptions can also account for deviations of the data points from the curves generated using the simple expression for the electron-transfer rate. It is clear from Figure 8 that the various charge-transfer and charge-recombination processes for the porphyrin-perylene dyads are qualitatively well described by the standard ideas and formulations for nonadiabatic charge-transfer processes. This knowledge together with the other insights gained from the comparative studies on the three PDI-pep-Por dyads provides a strong foundation for further manipulating the properties of these systems. Furthermore, the comprehensive studies of all aspects of the photodynamics of these dyads has already afforded a group of well-characterized molecules that can be used as components for light-harvesting and light-induced switching units in molecular photonics applications. These concepts will be expanded upon in a forthcoming article that
8258 J. Phys. Chem. B, Vol. 105, No. 34, 2001 describes changes in the perylene, the linker, and in the site of attachment of the linker to perylene. Experimental Section General. 1H NMR spectra were collected at 300 MHz. Products were analyzed by fast-atom bombardment (FAB) or by laser desorption mass spectrometry in the absence of a matrix (LD-MS). Absorption and emission spectra were routinely obtained in toluene. Unless otherwise indicated, all reagents were obtained from Aldrich Chemical Co., Milwaukee, WI, and all solvents were obtained from Fisher Scientific. PDI-pep-Fb. A solution of PDI-pep-Zn (25 mg, 0.017 mmol) in CH2Cl2 (5 mL) was treated with TFA (250 µL), and the solution was stirred for 2 h (with monitoring by fluorescence spectroscopy). The crude reaction mixture was dissolved in CH2Cl2 (50 mL), washed with NaHCO3 (100 mL), washed with water (100 mL), dried (Na2SO4), evaporated to dryness, and chromatographed (silica, CH2Cl2/hexanes, 5:1). Recrystallization (ethanol) afforded a purple solid (16 mg, 67%). 1H NMR (CDCl3) δ -2.63 (s, 2 H), 1.24 (s, 9 H), 1.27 (s, 9 H), 1.79 (s, 18 H), 2.56 (s, 9H), 7.00 (d, J ) 2.1 Hz, 2 H), 7.20 (m, 8 H), 7.40 (m, 3 H), 7.55 (d, J ) 8.7, 1H), 7.82 (d, J ) 8.1 Hz, 2 H), 7.88 (d, J ) 8.1 Hz, 2 H), 8.15 (d, J ) 8.1 Hz, 2 H), 8.57 (s, 5 H), 8.64 (m, 5 H), 8.73 (m, 5 H); LD-MS obsd 1421.2, 1443.3 [M+ + 23]; FAB-MS obsd 1416.62, calcd exact mass 1416.62 (C99H80N6O4); λabs 424, 460, 492, 528, 548 (s), 589 nm; λem 651, 721 nm. PDI-pep-Mg. Following a standard procedure,6 a mixture of PDI-pep-Fb (8.1 mg, 5.7 µmol) and MgI2 (200 mg, 0.72 mmol) in CH2Cl2 at room temperature was stirred for 5 min. Then N,N-diisopropylethylamine (125 µL, 0.72 mmol) was added, and the reaction mixture was stirred for 2 h (with monitoring by fluorescence spectroscopy). The crude reaction mixture was dissolved in CH2Cl2 (50 mL), washed with water (2 × 100 mL), dried (Na2SO4), evaporated to dryness, and chromatographed (alumina, CH2Cl2/hexanes, 2:1). Recrystallization (ethanol) afforded a purple solid (7.4 mg, 90%). 1H NMR (THF-d8) δ 1.15-1.38 (m, 18 H), 1.81-1.87 (m, 18 H), 2.552.61 (m, 9 H), 7.00-7.24 (m, 8 H), 7.39-7.47 (m, 1 H), 7.507.62 (m, 4 H), 7.82 (d, J ) 8.7 Hz, 2 H), 7.93 (d, J ) 7.8 Hz, 2 H), 8.24 (d, J ) 8.1 Hz, 2 H), 8.50-8.85 (m, 15 H); LD-MS obsd 1442.3, 1375.2 [M+ - 2(C(CH3)3)], 1568.1 [M+ + 126]; FAB-MS obsd 1438.59, calcd exact mass 1438.59 (C99H78N6O4Mg); λabs 428, 487, 527, 563, 604 nm; λem 611, 665 nm. Characterization. The electrochemical and spectroscopic methods employed are described in the preceding paper.1 Acknowledgment. This research was supported by a grant from the NSF (CHE-9707995 and CHE-9988142) (D.F.B., D.H., and J.S.L). Partial funding for the Mass Spectrometry Laboratory for Biotechnology at North Carolina State University was obtained from the North Carolina Biotechnology Center and the NSF. P.S. thanks Cochin University of Science and Technology for a sabbatical leave. References and Notes (1) Prathapan, S.; Yang, S. I.; Miller, M. A.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 8237. (2) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, 63-65. (3) Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem. Soc. 1998, 120, 5118-5119. (4) Just, E. M.; Wasielewski, M. R. Superlattices Microstr. 2000, 28, 317-328. (5) (a) Yang, S. I.; Prathapan, S.; Miller, M. A.; Seth, J.; Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D., in press. (b) Yang, S. I.; Prathapan,
Yang et al. S.; Miller, M. A.; Seth, J.; Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D. Manuscript in preparation. (6) Lindsey, J. S.; Woodford, J. N. Inorg. Chem. 1995, 34, 10631069. (7) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D.; Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines 1999, 3, 117-147. (8) Hascoat, P.; Yang, S. I.; Lammi, R. K.; Alley, J.; Bocian, D. F.; Lindsey, J. S.; Holten, D. Inorg. Chem. 1999, 38, 4849-4853. (9) Note that the small amplitudes of the Fb*-associated features at 30 ps compared to the PDI* features at 0.2 ps are simply a result of the larger extinction coefficients in the ground-state absorption bands (and associated natural radiative decay rate for fluorescence) of PDI versus the porphyrin.1 The difference does not represent substantial deactivation of PDI* to the ground-state competing with PDI*-pep-Fb f PDI-pepFb* energy transfer; the high efficiency of the latter process is also born out by the static fluorescence measurements. (10) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68, 13731380. (11) (a) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 3996-3997. (b) Lammi, R. K.; Wagner, R. W.; Ambroise, A.; Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 5341-5352. (c) Lammi, R. K.; Ambroise, A.; Wagner, R. W.; Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Chem. Phys. Lett. 2001, 341, 35-44. (12) We have shown previously that the redox properties of the porphyrins joined by diarylethyne linkers are essentially unchanged from the values for the isolated components.12a,b We have found this to be true for the redox properties of the components of PDI-pep-Zn.1 This result reflects the relatively weak electronic interactions in the dyads. (a) 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. (b) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey, J. S.; Holten, D.; Bocian, D. F. J. Mater. Chem. 1997, 7, 1245-1262. (13) Felton, R. H. In The Porphyrins; Dolphin, D., Ed; Academic Press: New York, 1978; Vol. V, 53-125. (14) (a) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592. (b) Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194-11207. (15) A difference in the stabilization of the charge-separated species in acetonitrile versus toluene is estimated to be 0.27 eV using the simple Coulomb interaction e2/(sr), where e2 ) 14.45 eV‚Å), r is the porphyrinperylene center-to-center separation (∼21 Å), and s is the static dielectric constant (2.38 for toluene and 37.4 for acetonitrile). (16) (a) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 9090-9100. (b) Yang, S. I.; Li, J.; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2000, 10, 283-296. (c) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2000, 65, 6634-6649. (17) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322. (b) Jortner, J. J. Am. Chem. Soc. 1980, 102, 6676-6686. (18) (a) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-447. (b) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. Soc. 1985, 107, 5562-5563. (c) Gunner, M. R.; Robertson, D. E.; Dutton, P. L. J. Phys. Chem. 1986, 90, 3783-3795. (19) The internal reorganization energy for oxidation of the porphyrin is small because of the large size of the molecule and its π-electron system and is generally estimated to have a value of ∼0.1 eV.13 The reorganization energy for reduction of the perylene could be larger, like quinones,18c because of bond-length changes associated with the keto groups (see Scheme 1). Thus, the total internal reorganization energy for charge transfer (and charge recombination) involving both components of the dyads is likely to be λi ) 0.2-0.4 eV. The solvent reorganization energies were estimated using the standard expression λs ) e2[(2rD)-1 + (2rA)-1 - (RDA)-1](op-1 - s-1), where e2 is the square of the electron charge (14.45 eV‚Å), rD and rA are the effective porphyrin and perylene radii (∼7 and ∼6 Å, respectively), RDA is the porphyrin-perylene center-to-center separation (∼21 Å), s is the static dielectric constant (2.38 for toluene and 37.4 for acetonitrile), and op ) n2, where n is the refractive index (1.492 for toluene and 1.342 for acetonitrile). Insertion of the parameters into the expression gives solvent-reorganization energies of λs ∼ 0.05 eV for toluene and ∼0.7 eV for acetonitrile. Thus, the total reorganization energy is estimated to be λ ∼ 0.4 eV for the charge-transfer and charge-recombination reactions in toluene and λ ∼ 1.1 eV in acetonitrile. (20) The only pair of processes that deviate slightly from the general trends exhibited by the others is PDI*-pep-Fb f PDI--pep-Fb+ hole transfer and the special PDI--pep-Fb+ f PDI-pep-Fb* charge recombination in toluene (half-filled circle and square in Figure 8A); the trend is followed by that pair in acetonitrile (Figure 8B).
