Distinct Photophysical and Electronic Characteristics of Strongly

Article pubs.acs.org/JPCB

Distinct Photophysical and Electronic Characteristics of Strongly Coupled Dyads Containing a Perylene Accessory Pigment and a Porphyrin, Chlorin, or Bacteriochlorin Jieqi Wang,† Eunkyung Yang,‡ James R. Diers,§ Dariusz M. Niedzwiedzki,‡ Christine Kirmaier,‡ David F. Bocian,*,§ Jonathan S. Lindsey,*,† and Dewey Holten*,‡ †

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, United States § Department of Chemistry, University of California, Riverside, California 92521-0403, United States ‡

S Supporting Information *

ABSTRACT: The synthesis, photophysical, redox, and molecular-orbital characteristics of three perylene−tetrapyrrole dyads were investigated to elucidate characteristics favorable for use in next-generation light-harvesting assemblies. Each dyad contains a common perylene-monoimide that is linked at the 9-position via an ethynyl group to the meso-position of the tetrapyrrole. The tetrapyrroles include a porphyrin, chlorin, and bacteriochlorin, which have zero, one, and two reduced pyrrole rings, respectively. The increased pyrrole-ring reduction results in a progressive red shift and intensification of the lowest-energy absorption band, as exemplified by benchmark monomers. The direct ethyne linkage and accompanying strong perylene−tetrapyrrole electronic coupling in the dyads is evident by significant differences in optical absorption versus the sum of the features of the constituents. The perturbations decrease for the tetrapyrrole constituent along the series porphyrin > chlorin > bacteriochlorin. This trend is explained by the relative configurational mixing in the tetrapyrrole excited states and how the configuration-interaction energy (and not simply the energies of the configurations) is affected by coupling to the perylene. The perylene−tetrapyrrole electronic coupling is further evidenced in the redox and MO characteristics of the three dyads. All three dyads in nonpolar solvents exhibit relatively long singlet excited-state lifetimes (3.3−6.5 ns) and relatively large fluorescence quantum yields (0.14−0.40). Collectively, the physicochemical characteristics of the strongly coupled perylene−tetrapyrrole dyads render these architectures excellent candidates for light-harvesting materials with significant, even panchromatic, near-ultraviolet to near-infrared absorption.

I. INTRODUCTION A major challenge in artificial photosynthesis concerns the development of molecular architectures that absorb sunlight and funnel the resulting excited-state energy to a designated site. One approach entails the organization of a large number of pigments in a well-defined three-dimensional architecture. The intensity of a given absorption profile can be increased by incorporating multiple copies of a given chromophore; spectral coverage can be enhanced by use of chromophores with complementary absorption properties. Design criteria include control of interchromophore through-bond and through-space electronic interactions to provide the requisite absorption and energyfunneling while minimizing deleterious quenching processes.1 Tetrapyrrole chromophores form the basis for native antenna systems and are attractive for use in artificial light-harvesting arrays. The three major tetrapyrrole classes are porphyrin, chlorin, and bacteriochlorin, which possess zero, one, or two reduced (hydrogenated) pyrrole rings.2 All three chromophore classes are characterized by strong absorption in the nearultraviolet (NUV) region but weak absorption in the greenorange region; chlorins also afford moderately strong absorption © XXXX American Chemical Society

in the red region, whereas bacteriochlorins afford intense absorption in the near-infrared (NIR) region, the latter of comparable strength to the NUV features. Although the natural photosynthetic pigments are chlorins (e.g., chlorophyll a) and bacteriochlorins (e.g., bacteriochlorophyll a), most synthetic multitetrapyrrole arrays are built of porphyrins due to greater ease of synthesis. Many hundreds of such multiporphyrin arrays have been prepared and studied;3,4 significantly fewer contain chlorins,5 and only ∼20 arrays contain bacteriochlorins.6 Improvements in the overall light-harvesting efficiency can be achieved by the introduction of accessory pigments that absorb in regions where the tetrapyrroles are relatively transparent.7 Perylene-imides, members of the family of rylene dyes,8,9 are attractive in this regard and exhibit the following characteristics: (1) moderately large extinction coefficients in the 450−550 nm region,10,11 which improves utilization of the solar spectrum; (2) a long singlet excited-state lifetime,12,13 which allows efficient Received: May 21, 2013 Revised: June 25, 2013

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dx.doi.org/10.1021/jp405004d | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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Chart 1. Previously Studied Perylene−Porphyrin Dyadsa

energy transfer from the perylene to acceptors; (3) high photostability,14,15 and (4) suitability for use in a modular building block approach.16−19 Among perylene-imides, the perylene-bis-imide dyes have been widely incorporated in molecular arrays as light-harvesting elements and/or electron acceptors in conjunction with tetrapyrrole chromophores, particularly porphyrins.8,9,16,17 By contrast, the perylenemonoimide dyes have been less extensively examined.11,13,19−28 Previously, we have examined a number of dyads that employ perylene-imides and porphyrins. The dyads were found to undergo either excited-state charge transfer (CT), with a range of efficiencies and lifetimes of the CT products, or excited-state energy transfer, with minimal CT (even in polar media). This diverse behavior could be realized through combinations of perylene type, perylene substituents, covalent linker, linker attachment sites to perylene and porphyrin, and porphyrin metalation state. The first set of dyads comprised a perylenediimide (PDI) joined at an N-imide position to the porphyrin at the meso position via a diphenylethyne (pep) linker (Chart 1A).29,30 The second set of dyads contained a perylenemonoimide (PMI) joined at the 9-position to the porphyrin at the meso position via a phenylethyne (ep) linker (Chart 1B).13,21 Subsequent studies encompassed three dyads, each with a different perylene-monoimide joined to a common zinc porphyrin at the p-aryl position (Chart 1C).28 The most recent study employed a dialkoxy-substituted perylene-monoimide joined at the 9-position to the porphyrin at the meso position via a phenylethyne (ep) linker (Chart 1D). The goal of the latter studies was to elucidate the most suitable type of perylenemonoimide for light-harvesting in conjunction with porphyrins. The incorporation of aryloxy groups both increases the solubility and renders the perylene chromophore more electron-rich and less susceptible to reduction upon photoexcitation of either the perylene or the porphyrin in the dyad. The attachment of the ethynylphenyl linker to the perylene 9-position affords extensive electronic communication given that the perylene has a lobe in the highest occupied molecular orbital (HOMO) at the 9position, to be contrasted with a node at the N-imide position.31,32 Each of the arrays shown in Chart 1 employs a meso-aryl porphyrin, which affords weak electronic coupling between the porphyrin and a chromophore attached to the aryl ring. By contrast, the use of meso-ethynylporphyrins enables strong electronic coupling. Such ethynylporphyrins, first reported by Arnold in 1978,33 became readily accessible in the early 1990s for use as building blocks in the construction of multiporphyrin arrays.34−37 The preparation of oligo(ethynylporphyrins) for studies of light-harvesting and other optical phenomena was pioneered by Therien and co-workers,37−48 whereas oligo(1,3butadiynylporphyrins) were developed in parallel by Anderson and co-workers.49 Numerous ethynyl-linked porphyrin−phthalocyanine dyads50−52 and related architectures53,54 also have been prepared. Such ethynyl-linked arrays typically show significant differences from the constituent monomers, including bathochromic shifts and intensity enhancements in the longwavelength absorption feature. Recent advances in the synthetic chemistry of chlorins and bacteriochlorins have opened the door to the study of lightharvesting with tetrapyrroles that absorb strongly in the red and NIR spectral regions.55,56 We have taken advantage of the new methodology to create new dyads that employ a previously studied perylene-monoimide and a porphyrin, chlorin, or bacteriochlorin (Chart 2). The diphenylethynyl or phenyl-

a

The dyads employed include a common zinc porphyrin and (A) a perylene-diimide, (B) a perylene-monoimide attached at the 9position, (C) perylene-monoimides, and (D) a perylene-monoimide attached at the 9-position.

ethynyl linkers utilized previously (e.g., Chart 1) have been replaced by a direct ethynyl linker. A chief motivation for the design of such dyads is to compare the light-harvesting efficacy and underlying electronic communication of a strongly absorbing accessory chromophore (i.e., perylene) with the porphyrin versus chlorin versus bacteriochlorin. In progressing from porphyrin to chlorin to bacteriochlorin, the decrease in singlet excited-state energy and increase in ease of ground-state oxidation modulate the propensities for excited-state electron transfer from tetrapyrrole to perylene. A second motivation was to examine the effects of the direct attachment of the perylene to the tetrapyrrole via an ethyne linker, which is expected to engender significant through-bond electronic communication. For comparison purposes, we also prepared and characterized benchmark monomers that contain an ethyne linker attached at B



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Chart 2. Strongly Coupled Perylene−Tetrapyrrole Dyads (Left Panel) and Benchmark Compounds (Right Panel)

spectra were obtained by laser desorption mass spectrometry (MALDI-MS) in the presence of a matrix [POPOP, 1,4-bis(5phenyloxazol-2-yl)benzene].57 Absorption and emission spectra were collected in THF or toluene at room temperature. Preparative size-exclusion chromatography (SEC)58 was performed using BioRad Bio-Beads SX-1 (200−400 mesh) beads. All commercially available materials were used as received. B. Synthetic Compounds. Three brominated tetrapyrrole macrocycles [5-bromo-10,15,20-trimesitylporphyrin (Br-P),59,60 15-bromo-17,18-dihydro-18,18-dimethyl-10-mesityl-5-p-tolylporphyrin (Br-C),61 and 15-bromo-5-methoxy-2,12-di-p-tolylbacteriochlorin (Br-BC)62,63], 9-ethynyl-1,6-bis(4-tert-butylphenoxy)-N-(2,5-di-tert-butylphenyl)-3,4-perylenedicarboximide (PMI-e-H),23,26,27 and three benchmark compounds (PMI-eTMS,23,26−28 Ph-e-P,64 Ph-e-BC62) were synthesized as described in the literature. The syntheses of the benchmark

the same position as in the dyads, but terminated by a phenyl or trimethylsilyl group. Herein, we present the synthesis and the photophysical, redox, and molecular-orbital (MO) characteristics of the three new perylene−tetrapyrrole dyads. The studies provide insights into the light-harvesting properties exhibited by the three distinct tetrapyrrole chromophores in conjunction with a common accessory pigment. The studies further indicate that dyads exhibit unexpected absorption characteristics and afford significant panchromatic light absorption across the NUV into the NIR spectral regions.

II. EXPERIMENTAL SECTION A. General. 1H NMR (300 MHz) spectra were recorded in THF-d8 at room temperature unless noted otherwise. Mass C



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phenylethynylchlorin (Ph-e-C) and target dyads (PMI-e-P, PMI-e-C, PMI-e-BC) are described in the Supporting Information. C. Photophysical Properties. Photophysical studies were performed as described previously65,66 and utilized dilute (μM) Ar-purged solutions at room temperature. Samples for Φf measurements had A < 0.1 at λexc. A τS value greater than 1 ns was determined using time-resolved fluorescence and was consistent with that obtained by transient absorption studies. A τS value less than 1 ns was derived only by transient absorption. The latter experiments utilized 0.5 μJ, ∼130 fs excitation pulses focused to 1 mm, and Ar-purged solutions (2-mm path) with A ∼ 0.15 at λexc (near an absorption peak). D. Electrochemistry. Electrochemical studies were performed in a cell housed in a glovebox.67 The butyronitrile or CH2Cl2 (Burdick and Jackson) solvent contained 0.1 M tetrabutylammonium hexafluorophosphate (Aldrich; recrystallized three times from methanol and dried at 110 °C in vacuo). The E1/2 values were obtained with square wave voltammetry (frequency 10 Hz) under conditions where the ferrocene couple has a potential of +0.19 V. E. MO Characteristics. Density functional theory (DFT) calculations were performed as described previously65 with Spartan ‘10 for Windows version 1.2.0 using the hybrid B3LYP functional and basis set 6-31G*.68 Equilibrium geometries were optimized using the Spartan default parameters. MO images used an isovalue of 0.016. Time-dependent DFT (TDDFT) calculations were performed using Gaussian ’09 version B.01 64-bit for Linux using OpenSUSE version 11.4 or 12.1.69 Geometries were optimized at the B3LYP/6-31G* level. TDDFT single-point calculations were performed at the B3LYP/6-31G* level.

Scheme 1. Synthesis of Perylene−Tetrapyrrole Dyads

III. RESULTS A. Synthesis. The joining reaction to create the perylene− tetrapyrrole dyad requires a perylene−ethyne and a bromo− tetrapyrrole. Each brominated tetrapyrrole (Br-P, Br-C, Br-BC) was used with ethynyl−perylene PMI-e-H in an anaerobic, copper-free, Sonogashira coupling reaction,70 which has been used previously to prepare perylene−porphyrin dyads.28 Each dyad was obtained in pure form after purification by column chromatography on silica gel and preparative SEC (Scheme 1). The dyads were characterized by 1H NMR spectroscopy and mass spectrometry in addition to static absorption and fluorescence spectroscopy. The benchmark monomers shown in Chart 2 were prepared in similar manner. Each of the monomers is connected to a phenyl group via an ethynyl linker attached at the same position as that in the dyads. Benchmark chlorin Ph-e-C was prepared in 85% yield by coupling of chlorin Br−C with phenylacetylene (see Supporting Information). B. Photophysical Properties. 1. Electronic Ground-State Absorption Spectra. Figure 1 shows ground-state absorption spectra (solid lines) of the perylene−tetrapyrrole dyads and benchmark perylene and tetrapyrrole monomers in toluene. Similar spectra are observed in benzonitrile (Figure S1). In both figures, the left column of panels is for the porphyrin, the middle column is for the chlorin, and the right column is for the bacteriochlorin. The peak positions and relative intensities of the features are listed in Table S1. The spectrum of each dyad (in either toluene or benzonitrile) is not simply the sum of the component parts. The overall differences between the spectrum of a dyad versus the benchmark is greatest for the porphyrin, intermediate for the

chlorin, and least (but significant) for the bacteriochlorin. In addition, the long-wavelength absorption band is bathochromically shifted in the dyad versus the tetrapyrrole benchmark. The long-wavelength feature is the Qx(0,0) band for the porphyrin, and the Qy(0,0) band for the chlorin and bacteriochlorin. The position of this band (and the fluorescence band) defines the energy of the lowest singlet excited state. For each dyad versus the tetrapyrrole benchmark, the position of this band is as D



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Figure 1. Absorption spectra (solid) and fluorescence spectra (dashed) of tetrapyrrole benchmark (top panels), perylene benchmark (middle row of panels) and perylene−tetrapyrrole dyad (bottom panels) for samples in toluene. The tetrapyrroles are porphyrins (left panels), chlorins (middle column of panels), and bacteriochlorins (right of panels).

The spectra for the compounds in benzonitrile exhibit similar characteristics (Figure S1). The absorption spectra of the three dyads in both toluene and benzonitrile solvents indicate that the lowest singlet excited state retains substantial tetrapyrrole character but with differences compared to the benchmark monomer depending on the tetrapyrrole class. 2. Fluorescence Spectra. Figure 1 shows fluorescence emission spectra (dashed or dotted lines) for the perylene− tetrapyrrole dyads, perylene benchmark, and tetrapyrrole benchmarks in toluene. Analogous data for the compounds in benzonitrile are given in Figure S1. The fluorescence spectral shapes are similar to those of the tetrapyrrole benchmark in all cases except for the PMI-e-C and PMI-e-BC in benzonitrile, for which the fluorescence is extremely weak (as quantitated below). The shift between the fluorescence maximum and the longwavelength absorption maximum is 3−11 nm spanning the benchmarks and dyads and the two solvents. For all the dyads in toluene (Figure 1) and PMI-e-P in benzonitrile (Figure S1 left panels), the same fluorescence spectrum is observed upon nominal excitation of the tetrapyrrole in the Soret (or Qx) region or upon nominal excitation of the perylene at 510−520 nm (dotted versus dashed spectra for the dyads). Furthermore, essentially the same emission intensity is observed upon correction for absorbance at the excitation wavelength. Similarly, the excitation spectrum of the PMI-e-C fluorescence (λdet = 693 or 790 nm) matches the absorption spectrum. Additionally, little perylene emission is observed even upon excitation at wavelengths (510−520 nm) where the perylene is expected to have the most significant contribution to the absorbance, despite the fact that the perylene has an ∼0.9 inherent (i.e., in the benchmark) fluorescence yield. The latter observation holds even for PMI-e-C and PMI-e-BC in benzonitrile, for which tetrapyrrole emission is very weak (Figure S1).

follows: porphyrin (689 versus 668 nm), chlorin (672 versus 660 nm), bacteriochlorin (768 versus 757 nm). Thus the greatest wavelength shift (+21 nm) and energy shift (+456 cm−1) is for the porphyrin. The position of the NUV Soret maximum is modestly shifted in each dyad versus the benchmark, with the magnitude and direction differing among the tetrapyrrole classes. This band is the coalesced Bx(0,0) plus By(0,0) band for the porphyrin and either Bx(0,0) or By(0,0) or a mixture for the chlorins and bacteriochlorins, and corresponds to the third or fourth singlet excited state. For each dyad versus the tetrapyrrole benchmark, the position of this band is as follows: porphyrin (429 versus 434 nm), chlorin (430 versus 426 nm), bacteriochlorin (380 versus 388 nm). There is a substantial shift in intensity from the Soret region to the Qx region (and intermediate wavelength Qy region) in the porphyrin-based dyad compared to that of the benchmark (Figure 1C versus 1A). The B(0,0)/Qx(0,0) peak-intensity ratio is reduced 15-fold to 2.4 for dyad PMI-e-P from 37 for benchmark Ph-e-P. The shift in oscillator strength into the absorption from the ground state to the lowest singlet excited state (Qx in this case) reflects not only the decrease in peakintensity ratio but also the increased breadth of the composite Soret profile in the dyad versus the porphyrin benchmark. The shift in absorption strength from the NUV to the visible region in the dyad versus the benchmark is less pronounced for the chlorin (Figure 1F versus 1D). The B(0,0)/Qy(0,0) peakintensity ratio is reduced about ∼2-fold to 1.8 for dyad PMI-e-C from 3.8 for benchmark Ph-e-C. There is essentially no shift in absorption intensity from the NUV to the NIR in the dyad versus benchmark for the bacteriochlorin (Figure 1I versus 1G). The B(0,0)/Qy(0,0) peak-intensity ratio is ∼1 in both dyad PMI-eBC and benchmark Ph-e-BC. E



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3. Fluorescence Quantum Yields and Singlet Excited-State Lifetimes. The fluorescence quantum yields (Φf) and lifetime of the lowest singlet excited state S1 (τS) of the benchmark compounds in toluene or benzonitrile are listed in Table 1. The

are reduced approximately 2-fold from the values in toluene (Table 1). The Φf (0.23) for the chlorin constituent of dyad PMI-e-C in toluene is 1.6-fold larger than the value (0.14) for the benchmark chlorin. The corresponding τS (3.4 ns) of the chlorin in PMI-e-C is 4.1-fold smaller than the value (14 ns) for the benchmark. The Φf (0.008) and τS (120 ps) for the chlorin constituent of PMI-eC in benzonitrile are dramatically reduced compared to the values for the same dyad in toluene. The Φf (0.16) for the bacteriochlorin constituent of dyad PMI-e-BC in toluene is modestly (16%) smaller than the value (0.19) for the benchmark. Similarly, the τS (4.3 ns) of the bacteriochlorin in the dyad is modestly (19%) smaller than the value (5.3 ns) for the benchmark. The Φf (0.001) and τS (30 ps) for the bacteriochlorin constituent of PMI-e-BC in benzonitrile are dramatically reduced compared to the values in toluene. 4. Transient Absorption Studies. Representative ultrafast transient absorption spectra for dyad PMI-e-P in toluene are shown in Figure 2A,B, and associated kinetic profiles are shown

Table 1. Photophysical Properties of Perylene−Tetrapyrrole Dyads and Benchmarksa compound Benchmarks PMI-e-TMS Ph-e-P Ph-e-C Ph-e-BC Dyads PMI-e-P PMI-e-C PMI-e-BC

solvent

Φf

τS (ns)

(kf)−1 (ns)

(knr)−1(ns)

toluene PhCN toluene PhCN toluene PhCN toluene PhCN

0.95 0.85 0.14 0.14 0.34 0.38 0.19 0.19

5.6 5.0 14 13 16 16 5.3 5.2

5.9 5.9 100 93 47 42 28 27

112 33 16 15 24 26 6.5 6.4

toluene PhCN toluene PhCN toluene PhCN

0.40 0.21 0.23 0.008 0.16 0.001

6.5 3.3 3.4 0.12 4.3 0.03

16 16 15 15 27 30

11 4.2 4.4 0.1 5.1 0.03

a

All data were obtained at room temperature in Ar-purged toluene or benzonitrile (PhCN). The typical error in Φf and τS is ±10%.

Φf = 0.85−0.95 and τS= 5.0−5.6 ns for the benchmark perylene PMI-e-TMS are consistent with values for this compound28 and other perylene dyes13,29,30 published previously. Similarly, the Φf and τS values for the benchmark porphyrin (0.14, 13−14 ns), chlorin (0.34−0.38, 16 ns), and bacteriochlorin (0.19, 5.2−5.3 ns) in toluene and benzonitrile are consistent with values for other tetrapyrroles in these three classes reported previously.65,71,72 The Φf and τS for each dyad (PMI-e-P, PMI-e-C, PMI-e-BC) in toluene and benzonitrile are also listed in Table 1. On the basis of the absorption and fluorescence spectra, the S1 state has substantial tetrapyrrole character, although with properties that may be altered by the attached perylene to an extent depending on the dyad and medium. Values are not tabulated for the excited perylene constituent, which effectively represents an upper excited state of the dyad. Compared to the benchmark, perylene fluorescence is reduced by ∼100-fold or more (vide supra) and the nominal perylene excited-state lifetime is reduced from ∼5 ns to chlorin Ph-e-C (+0.55 V) > bacteriochlorin Ph-e-BC (+0.33 V), corresponding to increasing ease of oxidation. The potential for tet ) does not differ as the first tetrapyrrole reduction (ERed1 substantially between the three classes, as is commonly found.73 The perylene benchmark PMI-e-TMS exhibits a first oxidation potential (+0.95 V)28,13 that is more positive than that for the benchmark for all three tetrapyrrole classes, corresponding to a greater difficulty of oxidation. The first reduction of the perylene benchmark (−1.12 V) occurs at a less negative potential than the benchmarks for all three tetrapyrrole classes, corresponding to a greater ease of reduction. These findings are relevant to the direction of possible CT processes. D. MO Characteristics. DFT calculations were performed to examine the MO characteristics of the molecules. The electrondensity distributions and energies of the two highest-energy occupied MOs (HOMO-1 and HOMO) and two lowest-energy unoccupied MOs (LUMO and LUMO+1) for the perylene and tetrapyrrole benchmarks are shown in Figure 4 and those (along with HOMO-2 and LUMO+2) for the perylene−tetrapyrrole dyads in Figure 5. The energies of these MOs and the LUMO− HOMO energy gap are collected in Table S2 of the Supporting Information. Inspection of Figures 4 and 5 indicate that the MOs of each dyad have electron-density distributions on the tetrapyrrole constituent generally similar to those of related orbitals of the tetrapyrrole benchmark. For a given dyad, pairs of some orbitals appear to be bonding and antibonding combinations of perylene and tetrapyrrole orbitals, such as the LUMO and LUMO+1 of each dyad. Although many of the orbitals of each dyad have significant electron density on both the perylene and tetrapyrrole, there are notable exceptions depending on the tetrapyrrole class. For example, the HOMO-1 of PMI-e-P has somewhat less density on the perylene (versus porphyrin) than the other orbitals, whereas the HOMO-1 of PMI-e-C is pure chlorin while the HOMO for PMI-e-BC is essentially pure bacteriochlorin. Such differences likely contribute to the

Figure 3. Representative transient absorption kinetic profiles for PMI-eP in toluene (A,B) and PMI-e-BC in benzonitrile (PhCN) (C,D) and fits to a function consistent of the instrument response, three exponentials, and a constant.

benzonitrile (0.5), PMI-e-C in toluene (0.4), PMI-e-BC in toluene (0.3). For chlorin dyad PMI-e-C in benzonitrile, the (nominal tetrapyrrole) τS is reduced to 120 ps (Table 1), and the state formed from S1 decays completely to the ground state with τ = 35 ps. For bacteriochlorin dyad PMI-e-BC in benzonitrile, τS is further reduced to 30 ps, and the state formed from S1 is reduced to 10 ps (Figures 2C,D and 3C,D). There is also a shorter (∼5 ps) component that likely represents a combination of electronic, vibrational, conformational, and solvent relaxation leading to S1. In both cases, the assignment of the longer time constant (120 vs 35 ps; 30 vs 10 ps) as the S1 lifetime (τS) is made because the reverse assignment gives a radiative rate constant kf (Table 1) that is much greater than that for the dyad in toluene, inconsistent with the similarity in absorption spectra in the two media (Figures 1 and S1). Regardless, the S1 lifetime for PMI-eC and PMI-e-BC in benzonitrile is orders of magnitude shorter than the value in toluene (Table 1), and the state formed from S1

Table 2. Redox Properties of Perylene and Tetrapyrrole Benchmarks and Dyads.a compound Benchmarks PMI-e-TMS Ph-e-P Ph-e-C Ph-e-BCb Dyads PMI-e-P PMI-e-C PMI-e-BC

Etet Ox2(V)

Eper Ox1(V)

Etet Ox1(V)

Eper Red1(V) −1.12

+0.95 +1.16 +0.97 +0.66 +0.95c +0.86c +0.68

Eper Red2(V)

−1.18 −1.17 −1.22

+0.69d +0.57 +0.24

Etet Red2(V)

−1.64 −1.36 −1.49 −1.31

+0.70 +0.55 +0.33 +0.95c +0.86c +1.05

Etet Red1(V)

−1.31 −1.39 −1.36

−1.78 −1.88 −1.78 −1.82e −1.73e −1.75

−1.82e −1.73e −1.98

a

All potentials (measured in V) were measured in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate except for Ph-e-BC, which was measured in butyronitrile containing 0.1 M tetrabutylammonium hexafluorophosphate. The typical error is ±0.02 V. The potentials are adjusted so that the ferrocene couple has a value of 0.190 V under the conditions of the measurement. The categorizaion of redox potentials as either perylene or tetrapyrrole is a zeroth-order description (due to strong perylene−tetrapyrrole interactions and mixed MO compositions) used to facilitate comparions. bData from ref 74. cThe observed potential may represent overlapping of the first oxidation of the perylene and the second oxidation of the tetrapyrrole. dSmall waves are also observed at +0.28 and −0.07 V. eThe observed potential may represent overlapping second reductions of the perylene and tetrapyrrole. G



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Figure 4. Frontier MOs of perylene and tetrapyrrole benchmarks. TMS was replaced by H in the calculations for PMI-e-TMS.

benchmarks. Thus, the configurational mixing in the (nominal tetrapyrrole) S1 excited state of PMI-e-P and PMI-e-C that is present in the respective porphyrin and chlorin benchmarks is now virtually absent. In parallel (vide supra), the absorption spectrum of PMI-e-P has significant absorption strength shifted from the Soret bands (Bx, By) to the visible bands (both Qx and Qy), and the same is true but to a lesser extent for PMI-e-C, with little changes in the nominal tetrapyrrole absorptions for PMI-eBC (Figure 1). The relationships between MO characteristics, configurational mixing, and the spectra of the perylene− tetrapyrrole arrays are discussed further below. Note that the TDDFT calculations are used here only to provide an additional assessment (consistent with four-orbitalmodel simulations) of trends in the configurational mixing in the S1 excited states of the benchmarks and arrays. The TDDFT calculations are not used to predict the NUV−NIR absorption spectra. Even for sparsely substituted tetrapyrrole monomers, the energies of states responsible for the main optical transitions are

character of excited states responsible for the photophysical properties. Studies using various theoretical approaches, including the four-orbital model, ab initio calculations, and TDDFT calculations indicate that the S1 and S2 (e.g., Qy and Qx) excited states of monomeric tetrapyrroles are comprised of various mixtures of the [HOMO → LUMO], [HOMO-1 → LUMO+1], [HOMO-1 → LUMO] and [HOMO → LUMO+1] configurations.71,75−80 Consistent with this view, TDDFT calculations (at the B3LYP/6-31G* level) indicate that all four configurations contribute to the S1 excited state of porphyrin Ph-e-P (30%, 9%, 21%, 39%) and chlorin Ph-e-C (28%, 8%, 40%, 23%) while the S1 state of Ph-e-BC is 90% [HOMO → LUMO]. In contrast, calculations for the three perylene−tetrapyrrole dyads (PMI-e-P, PMI-e-C, and PMI-e-BC) show that the S1 state is dominated by the HOMO→LUMO configuration, with contributions of 93%, 98%, and 95%, respectively. Spectral simulations using the fourorbital model79,80 suggest a similar trend in dyads versus H



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Figure 5. Frontier MOs of perylene−tetrapyrrole dyads.

electronic properties of the resulting constructs are affected by the coupling, given that the porphyrin, chlorin, and bacteriochlorin differ in a systematic manner. The most profound overt difference among the three tetrapyrroles is the absorption spectrum: the little (porphyrin) or substantial (chlorin) red absorption of the benchmark is enhanced in the dyad, and the strong NIR absorption of the bacteriochlorin is retained. In all dyads and media except for the chlorin and bacteriochlorin dyad in benzonitrile, strong coupling to perylene retains (or enhances) modest fluorescence yields and long singlet excited-state lifetimes relative to the benchmarks. To our knowledge,

typically underestimated by 0.2−0.6 eV, and band-intensity ratios (e.g., Q/B) are in error by an order of magnitude in many cases (although predicted reasonably well in others). Additionally, the TDDFT calculations become less reliable when excited states contain CT character,81 which is exhibited by the perylene−tetrapyrrole dyads.

IV. DISCUSSION The electronic coupling between the perylene and tetrapyrrole constituents of the dyads is substantial owing to direct linkage via the ethynyl group. A key objective was to understand how the I



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comparisons of electronic communication − and light-harvesting − among porphyrins, chlorins, and bacteriochlorins, have heretofore not been examined. Before turning to analysis of the observations, it is useful to describe the electronic states of a dyad with strong perylene− tetrapyrrole electronic interactions. In the strong-coupling limit, the lowest electronic states are derived from linear combinations of four basis configurations: the lowest energy locally excited perylene and tetrapyrrole configurations (Per* and Tet*) and the two CT configurations (Per−Tet+ and Per+Tet−). Given the absorption spectra of the benchmarks (Figures 1 and S1 and Table S1), Per* is higher in energy than Tet*, which shifts to lower energy along the series porphyrin > chlorin > bacteriochlorin. The redox data (Table 2) indicates Per−Tet+ will be the lowest energy of the two CT configurations, and the photophysical data are most consistent with Per−Tet+ lying above Tet* for all cases except for the chlorin and bacteriochlorin dyad in benzonitrile (vide inf ra). In the latter two cases, Per−Tet+ should be stabilized by both the polar solvent (relative to toluene) and tetrapyrrole oxidation that is progressively more facile along the series porphyrin < chlorin < bacteriochlorin. Admixtures of the four basis configurations (Per+Tet−, Per−Tet+, Per*, and Tet*) give rise to four excited states of the dyad. The composition of each state (and its energy) will depend on the relative energies of the four configurations, which in turn depend on the tetrapyrrole and medium. In cases where Tet* is the lowest configuration, the lowest excited state will have preferential (but not solely) Tet* character and can be referred to as the tetrapyrrole-weighted S1 excited state. To the extent to which Per+Tet− contributes, this S1 state may have some net shift of electron density from tetrapyrrole to perylene (e.g., CT character). In cases where the combination of tetrapyrrole and medium results in the Per+Tet− configuration being lower than Tet*, the lowest excited state would (i) have more substantial (even predominant) CT character, (ii) have very limited absorption oscillator derived from the Tet* contribution, and (iii) be referred to as a CT-like state. In such a case, the second lowest excited state, at higher energy than the CT-like state, would have significantly more Tet* than Per+Tet− character and would again be referred to as a tetrapyrrole-weighted S1 excited state. The following sections are organized as follows. First, we consider the rate constants for excited-state processes in the monomeric constituents, and particularly how the rate constants change in the series porphyrin, chlorin, and bacteriochlorin. We then turn to the rate constants for excited-state processes in the dyads, with consideration of the possibility of CT-contributions to the decay of the tetrapyrrole-weighted S1 excited state. Finally, the redox, spectral, and photophysical properties are interpreted in terms of the MO characteristics of the perylene−tetrapyrroles. A. Rate Constants for Excited-State Processes in the Benchmark Compounds. The lowest singlet excited state (S1) of each benchmark monomer (porphyrin, chlorin, bacteriochlorin, perylene) decays by S1→S0 spontaneous fluorescence, S1→S0 internal conversion and S1→T1 intersystem crossing with rate constants kf, kic, and kisc. The value of kf and a value for the total nonradiative decay rate constant (kic + kisc) are obtained from the expressions kf = Φf /τS and knr = (1 − Φf)/τS, respectively. These values are listed in Table 1 as the corresponding time constant in units of nanoseconds. The magnitude of kf roughly doubles at each step along the series Ph-e-P (93−100 ns)−1 < Ph-e-C (42−47 ns)−1 < Ph-e-BC (27−28 ns)−1 for the benchmarks in toluene and benzonitrile.

This progressive increase in kf for S1→S0 (spontaneous) fluorescence along the series porphyrin < chlorin < bacteriochlorin is as expected, via the relationships between the Einstein coefficients, given that the oscillator strength for (stimulated) S0→S1 absorption increases in the same order. The increased S0→S1 absorption strength is readily observed from the intensity of the long-wavelength (Q-band) absorption normalized to the Soret absorption (top panels in Figures 1 and S1, and Table S1). The net nonradiative rate constant varies among the benchmarks for the three tetrapyrrole classes (Table S1). The values of knr derived for the compounds in toluene and benzonitrile are (15−16 ns)−1 for the porphyrin Ph-e-P, (24− 26 ns)−1 for the chlorin Ph-e-C, and (6.4−6.5 ns)−1 for bacteriochlorin Ph-e-BC. The overall greater knr for the bacteriochlorin can be ascribed in part to enhanced internal conversion associated with the lower energy of the S1 state of Phe-BC compared to Ph-e-C and Ph-e-P, along with some differences in intersystem crossing.65,72 An increased rate constant for internal conversion is expected due to the energygap law for nonradiative decay, which deals with the energy dependence of a Franck−Condon factor.82 Despite the increased knr of the bacteriochlorin compared to the chlorin or porphyrin, the long S1 lifetime (∼5 ns) remains quite favorable for driving processes such as excited-state energy or charge transfer in arrays. B. Rate Constants for Excited-State Processes of the Tetrapyrrole in the Dyads. The excited-state dynamics on the PMI-e-C (+0.57 V) > PMI-e-BC (+0.24 V). These values follow the trend noted above for the benchmarks, namely porphyrin > chlorin > bacteriochlorin. Furthermore, the Etet Ox1 values for the porphyrin and chlorin in the dyads are within 0.02 V of those for the benchmarks, while the value for the bacteriochlorin becomes slightly less positive (+0.24 V versus +0.33 V), indicating a slightly greater ease of oxidation in the dyad versus benchmark. Inspection of the redox potentials suggests that CT involving tetrapyrrole oxidation and perylene reduction to form Per−Tet+ is more favorable than the reverse process to produce Per+Tet−. This finding is consistent with our prior studies of PMI-based per porphyrin arrays.13,28 The calculated Etet Ox1 − ERed1 values using the dyads (and benchmarks) are 1.87 V (1.82 V) for the perylene−porphyrin, 1.74 V (1.67 V) for the perylene−chlorin, and 1.46 V (1.45 V) for the perylene−bacteriochlorin (Table 3).

(0.40 versus 0.14; Table 1). In turn, the enhanced kf and Φf in dyad versus benchmark parallel the substantial shift in absorption oscillator strength from the Soret region to the visible (Qy and Qx) region (Figure 1C versus 1A). The parallelism follows because kf for (spontaneous) fluorescence translates, via the Einstein coefficients, to enhanced (stimulated) S0→S1 absorption in dyad versus benchmark. Additionally, the intensified S1 absorption band shifts to ∼700 nm (Figure 1C). The lesser but still notable enhancement in kf for the chlorin dyad versus benchmark (Table 1) is also manifested in increased red-region absorption (Figure 1F versus 1D). Consequently, the perylene−porphyrin and perylene−chlorin dyads have overall NUV to NIR absorption spectra that are more similar to those of the bacteriochlorin systems (Figures 1 and S1 bottom panels) when compared to the substantial distinctions of the three tetrapyrrole benchmarks (Figure 1 top panels). Of course, the bacteriochlorin dyad retains the favorable lowest energy absorption (Qy) in the NIR region. This absorption and strong NUV (Soret) counterpart of bacteriochlorin spectra are not compromised by the complementary substantial visible absorption derived from the perylene (Figures 1I and S1I). The overall nonradiative S1 decay rate (knr) for each dyad relative to the benchmark is only modestly (∼1.5-fold) greater for porphyrin (PMI-e-P) and bacteriochlorin ((PMI-e-BC) in toluene, ∼5-fold greater for chlorin (PMI-e-C) in toluene and ∼4-fold greater for porphyrin in benzonitrile. Transient absorption studies (e.g., Figure 4A and B) show that as S1 decays in these four cases, the long-lived state(s) (τ > 10 ns) that form do so with a modest quantum yield (0.3−0.5) and exhibit a feature (∼950 nm) not seen in the tetrapyrrole benchmarks (Supporting Information), suggesting perylene involvement. Comparison with the 0.3−0.5 yield of this S1 decay pathway with the respective Φf (0.16−0.4) suggests that S1→S0 internal conversion has a yield of 0.3−0.5 in these four cases. Possible state(s) formed from the tetrapyrrole-weighted S1 include the tetrapyrrole or perylene triplet with some mixed character, a CTlike state in which electron density has shifted from tetrapyrrole to perylene, or a combination thereof. Regardless, the formation of the long-lived state(s) does not compromise the high Φf (0.16 − 0.40) and relatively long τS (3.3−6.5 ns) for PMI-e-P in toluene and benzonitrile and PMI-e-C and PMI-e-BC in toluene. In the cases of chlorin (PMI-e-C) and bacteriochlorin (PMIe-BC) in benzonitrile, the τS of the tetrapyrrole-weighted S1 excited state is dramatically reduced (120 and 30 ps, respectively), and the state formed from it (with near unity yield) in turn rapidly deactivates to the ground state (35 and 10 ps, respectively). In these cases, it is likely that the state formed from the tetrapyrrole-weighted S1 has substantial tetrapyrrole→ K



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Thus, the trend is the same using the redox potentials for the dyads and the benchmarks. In particular, nominal ground-state electron transfer from tetrapyrrole to perylene would be easiest for the bacteriochlorin and hardest for the porphyrin. per The calculated Etet Ox1 − ERed1 values for the dyads versus benchmarks show a shift of +0.05 V for the perylene−porphyrin, +0.07 V for the perylene−chlorin, and +0.01 V for the perylene− bacteriochlorin. This comparison suggests that the formation of the dyad makes slightly more difficult the ground-state electron transfer from the porphyrin to the perylene or from the chlorin to the perylene, but has little effect on the propensity for electron transfer from the bacteriochlorin to the perylene. This finding parallels the above-noted differences in ground-state absorption spectra (Figure 1), wherein differences in spectral characteristics (spanning the NUV to NIR) for the dyad versus the benchmark are less for the bacteriochlorin than for the chlorin and porphyrin. per Table 3 also gives calculated Etet Ox1 − ERed1 values for nominal S1 excited-state tetrapyrrole-to-perylene electron transfer (i.e., CT). For each tetrapyrrole (in the dyad or for the benchmark) the S1 oxidation potential is calculated using the ground-state (S0) Etet Ox1 value and the S1 excited-state energy obtained from the absorption and emission spectra in toluene (Figure 1 and per Table 3). The S1 Etet Ox1 − ERed1 values calculated using data for dyads (and benchmarks) are +0.08 V (−0.03 V) for the porphyrin, −0.09 V (−0.20 V) for the chlorin, and −0.14 V (−0.18 V) for the bacteriochlorin. These values would be shifted to more negative (free) energy by the Coulomb-interaction energy. The stabilization would be nominally the same for the porphyrin, chlorin, and bacteriochlorin dyads due to the comparable structures (e.g., perylene−tetrapyrrole distance), but would be greater for dyads in benzonitrile than in toluene by roughly 0.4 eV. The latter estimate is obtained by using the simple Coulomb interaction e2/(εsr), where e2 = 14.45 eV·Å, r is the perylene−tetrapyrrole center-to-center separation (∼13 Å), and εs is the static dielectric constant (2.38 for toluene and 26.0 for benzonitrile). Collectively, these expectations are generally consistent with the above-noted experimental results on the contribution of CT to the excited-state properties of dyads that incorporate a tetrapyrrole from the three classes (Table 1). D. Comparison of Molecular Orbital and Redox Properties. Studies of the effects of substituent type and macrocycle position of substitution for synthetic chlorins72 and bacteriochlorins65 have shown that the potential for the first oxidation generally tracks the energy of the HOMO whereas the potential of the first reduction generally tracks the energy of the LUMO. The relationships are extended here to include the effects of the attached perylene and variation of the tetrapyrrole macrocycle (porphyrin, chlorin, bacteriochlorin) in both benchmarks and dyads. Tables 2 and S2 give the data, which are plotted in Figure 6. As noted above, the potentials for the nominal tetrapyrrole first oxidation in both dyads and benchmarks vary among the three tetrapyrrole classes (Table 2). Close inspection of the MO energies shows that the HOMO energies vary as well (Figures 4 and 5 and Table S2). The correlation of the two is shown at the lower right of Figure 6. In progressing from bacteriochlorin to chlorin to porphyrin the tetrapyrrole becomes harder to oxidize and the HOMO energy becomes more negative. Figure 6 also shows that the still harder oxidation of the perylene (diamond) correlates with a still more negative HOMO energy. The ground-state redox properties of the benchmarks suggest that in the dyad the perylene should be easier to reduce than the

Figure 6. MO energy versus redox potentials for the first oxidation (right) or first reduction (left) of the porphyrin (square), chlorin (circle), bacteriochlorin (triangle) and perylene (diamond) in the benchmarks (closed symbols) and dyads (open symbols). For the dyads, the open symbols refer to tetrapyrrole oxidation and reduction, and the open symbols with vertical lines are for perylene reduction. The orbital energies plotted are the HOMO for first oxidations, LUMO for first reductions of benchmarks and perylene in dyads, and LUMO+1 for first tetrapyrrole reduction in the dyads.

tetrapyrrole, given that the potential for the perylene first reduction should reflect the LUMO energy whereas the potential for the tetrapyrrole first reduction should reflect the LUMO+1 energy. The MOs in Figure 5 show that the situation is more complex than this simple view because the LUMO and LUMO +1 for each dyad have considerable electron density on both the tetrapyrrole and perylene constituents. However, at least for PMI-e-P and PMI-e-C, the LUMO is weighted more toward the perylene, and the LUMO+1 is weighted more toward the tetrapyrrole, which is in the direction expected on the basis of redox properties and MO energies of the benchmarks. For simplicity, Figure 6 (upper left) plots the potential for the nominal first reduction of the perylene versus LUMO energy (open symbols with vertical line) and of the tetrapyrrole versus LUMO+1 energy (open symbols). The plot reflects the findings that the assigned first reductions of the perylene and tetrapyrrole do not change substantially ( chlorin > bacteriochlorin. To the extent that the perylene-monoimide can be viewed as a superauxochrome for direct attachment to tetrapyrroles, this moiety may serve better in this role than a second tetrapyrrole itself. This view follows from the observations and apparent ability of the perylene to both dramatically shift intensity from the NUV to the red and NIR absorptions of the tetrapyrrole and to provide strong complementary absorption in the intermediate visible regions normally only weakly covered, thereby giving intense absorption spanning the NUV to NIR regions of the solar spectrum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.F.B.); [email protected] (J.S.L.); [email protected] (D.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy to D.F.B. (DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661), and J.S.L. (DE-FG02-96ER14632). Transient absorption studies were performed in the Ultrafast Laser Facility of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001035.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis methods and chemical characterization data; absorption spectra of dyads and benchmarks in benzonitrile; tabulated spectral characteristics of the benchmarks and dyads; and additional transient absorption data. This material is available free of charge via the Internet at http://pubs.acs.org. N



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