Tuning the Electronic Structure and Properties of Perylene

Article pubs.acs.org/JPCA

Tuning the Electronic Structure and Properties of Perylene− Porphyrin−Perylene Panchromatic Absorbers Javad Amanpour,†,# Gongfang Hu,†,# Eric J. Alexy,† Amit Kumar Mandal,‡ Hyun Suk Kang,‡ Jonathan M. Yuen,‡ James R. Diers,§ 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: Light-harvesting architectures that afford strong absorption across the near-ultraviolet to near-infrared region, namely, panchromatic absorptivity, are potentially valuable for capturing the broad spectral distribution of sunlight. One previously reported triad consisting of two perylene monoimides strongly coupled to a free base porphyrin via ethyne linkers (FbT) shows panchromatic absorption together with a porphyrin-like S1 excited state albeit at lower energy than that of a typical monomeric porphyrin. Here, two new porphyrin−bis(perylene) triads have been prepared wherein the porphyrin bears two pentafluorophenyl substituents. The porphyrin is in the free base (FbT-F) or zinc chelate (ZnT-F) forms. The zinc chelate (ZnT) of the original triad bearing nonfluorinated aryl rings also was prepared. The triads were characterized using static and time-resolved optical spectroscopy. The results were analyzed with the aid of molecular-orbital characteristics obtained using density functional theory calculations. Of the four triads, FbT is the most panchromatic in affording the most even distribution of absorption spectral intensity as well as exhibiting the largest wavelength span (380−750 nm). The triads exhibit fluorescence yields (0.35 for FbT-F in toluene) that are substantially greater than for the porphyrin benchmarks (0.049 for FbP-F). The singlet excited-state lifetimes (τS) for the triads in toluene decrease in the order FbT-F (2.7 ns) > FbT (2.0 ns) > ZnT (1.2 ns) ∼ ZnT-F (1.1 ns). The τS values in benzonitrile are FbT (1.3 ns) > FbT-F (1.2 ns) > ZnT-F (0.6 ns) > ZnT (0.2 ns). Thus, the free base triads exhibit relatively long (1.2−2.7 ns) excited-state lifetimes in both polar and nonpolar media. The combined photophysical characteristics indicate that FbT and FbT-F are the best choices for panchromatic light-harvesting systems. Collectively, the findings afford insights into the effects of electronic structure on the panchromatic behavior of ethynyl-linked porphyrin−perylene architectures that can help guide next-generation designs and utilization of these systems.

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INTRODUCTION

energy migrates smoothly toward the trap site, and (4) excitedstate quenching among the diverse absorbers that reduces the overall efficiency of energy delivery. A complementary approach entails strong electronic interactions among a small number of covalently linked chromophores (even dyads) to achieve panchromatic absorption. The absorption spectrum of such an entity is more than the sum of the parts. The harvested energy is delivered to the discrete lowest excited state of the architecture in analogy with internal conversion from upper to lower excited states of a simple single chromophore, as illustrated in Figure 1B. The two approaches are not mutually exclusive given that architectures can be envisaged in which discrete energy-transfer steps occur to a superchromophore, or absorption by a superchromophore

Collections of pigments that together afford broad, even panchromatic absorption and engender energy-transfer cascades occur in photosynthetic light-harvesting antennas and in a wide variety of synthetic constructs. The latter include covalently linked multipigment architectures,1−7 chromophores in multilayer films8−10 or surfactant assemblies,11 and synthetically modified native light-harvesting systems.12−15 Ideally in such systems the energy harvested through absorption of different colors of light by distinct pigments is funneled via the stepwise energy-transfer cascade, achieving irreversible vectorial flow to a target site. This process is illustrated in Figure 1A. Ultimately such an approach may be limited by a number of factors including (1) the ability to structurally accommodate on a given scaffold the number of different absorbers that may be needed to give the desired spectral coverage, (2) the overall size of the architecture, (3) the challenge of constructing an energy gradient among the many chromophores so that harvested © 2016 American Chemical Society

Received: July 8, 2016 Revised: August 19, 2016 Published: September 16, 2016 7434

DOI: 10.1021/acs.jpca.6b06857 J. Phys. Chem. A 2016, 120, 7434−7450

Article

The Journal of Physical Chemistry A

Figure 2. Absorption spectra of a previously studied31 porphyrin− perylene dyad (red) and benchmark porphyrin (blue) and perylene (green) in toluene.

Figure 1. Distinct approaches to panchromatic absorption. Energytransfer cascade among a set of discrete absorbers (panel A) versus internal conversion in a superchromophore (panel B).

words, the dyad exhibits panchromatic absorption across the visible region, yet the lowest excited singlet state exhibits tetrapyrrole-like characteristics, including a relatively long lifetime (several nanoseconds) in both polar and nonpolar media. We have extended the approach from dyads to arrays that include up to four perylenes per porphyrin.31 The triad with two perylenes disposed in a trans configuration about the porphyrin (FbT) is shown in Chart 1. With an increasing number of perylenes per porphyrin, the arrays exhibit further redistribution of intensity and a progressive bathochromic shift of the long-wavelength absorption band, reaching ∼800 nm for the pentad that contains four perylenes. We have also constructed dyads analogous to those shown in Figure 2 in which the perylene is attached at the meso- vs β-pyrrole position of a porphyrin, or the chromophore attached to the meso-position of a porphyrin is a boron-dipyrrin or terrylenemonoimide instead of a perylene.36 Such structural variations give rise to differences in the panchromatic absorption in terms of the evenness of the spectral distribution of intensity, and in the longest wavelength reached. While initial photophysical characterization has been carried out, an in-depth analysis of the origin of the panchromaticity has not yet been performed of any of the aforementioned ethynyl-linked tetrapyrrole constructs. One design feature not yet explored to tune the electronic properties of the tetrapyrrole constituent in panchromatic absorbers involves variation of the central metal ion and/or installation of electron-withdrawing substituents at the perimeter of the macrocycle. Herein, we have explored these

is followed by an energy-transfer cascade. Energy-transfer cascades among discrete absorbers can in principle be designed in a straightforward fashion, yet there are no a priori design guidelines for superchromophores that afford panchromatic absorption and a long-lived lowest singlet excited state (although a number of “black” absorbers are known16−28). A long-lived excited singlet state constitutes a proxy for the ability to carry out subsequent energy- and/or electron-transfer processes. Both approaches shown in Figure 1 are distinct from the use of multiple distinct cosensitizers in mesoporous films.29 We have recently begun exploring the second approach. For example, we recently prepared and characterized dyads of the type shown in Figure 2 wherein a perylene-monoimide dye (hereafter referred to as a perylene) and a porphyrin are strongly coupled via a direct ethyne linkage.30,31 The use of such ethyne linkages was first reported by Arnold32 and developed extensively since by Anderson33 and Therien.34 This linkage motif has been employed extensively to join two or more porphyrins35 but less so to attach a visible-absorbing chromophore to a tetrapyrrole as in Figure 2. Such dyads exhibit several striking characteristics (Figure 2): (1) The absorption spectrum of the dyad contains bands that are not observed for the porphyrin or perylene constituents. (2) Overall there is a net shift of intensity from the blue region (for the porphyrin) into and across the green to red regions of the spectrum. (3) The fluorescence emission of the dyad resembles that of a tetrapyrrole, albeit shifted bathochromically. In other 7435


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Chart 1. Previously Reported (FbT31) and New (ZnT, FbT-F, ZnT-F) Porphyrin−perylene Triads along with New Benchmark Porphyrins (FbP-F, ZnP-F) and Previously Prepared Benchmark Perylene-Monoimide (PMI-e37)

LUMO.38−40 The electronic tuning provided by the structural variations in the new arrays affords insights into the origins of panchromatic absorption and further guides the design of nextgeneration panchromatic light-harvesting architectures.

two approaches via the synthesis of the three new triads shown in Chart 1 (triad FbT is also shown). The new constructs include replacing the free base porphyrin of FbT with the zinc chelate (ZnT), and for both macrocycles replacing the p-tolyl groups with pentafluorophenyl substituents at the two nonlinking meso-positions (FbT-F and ZnT-F). Two new porphyrin benchmarks that contain the pentafluorophenyl groups (FbP-F and ZnP-F) were also prepared (Chart 1). The three new triads and two new benchmarks were studied using static and time-resolved absorption and fluorescence spectroscopy to characterize the ground-state absorption spectra and excited-state photophysical properties in both toluene and benzonitrile. The results were analyzed with the aid of molecular-orbital (MO) characteristics (electron densities and energies) obtained from density functional theory (DFT) calculations. Note that the use of 2,6-diisopropyl versus 2,5-ditert-butyl groups for solubilization of the perylene units has insignificant effects on photophysics and energetics as verified by direct experimental comparison.37 The absence of a significant effect stems from the presence of a node at the perylene imide nitrogen atom in both the HOMO and the

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EXPERIMENTAL SECTION General Methods. All chemicals obtained commercially were used as received unless otherwise noted. Reagent-grade solvents and HPLC-grade solvents (toluene, CH2Cl2, hexanes) were used as received. THF was freshly distilled from sodium/ benzophenone ketyl and used immediately. Matrix-assisted laser-desorption mass spectrometry (MALDI-MS) was performed with the matrix 1,4-bis(5-phenyl-2-oxaxol-2-yl)benzene (POPOP)41 unless noted otherwise. 1H NMR spectra (300 MHz) were collected at room temperature. ESI-MS data are reported for the molecular ion or cationized molecular ion. Compounds 142 and 343,44 were prepared following literature procedures. All other compounds were used as received from commercial sources. Chromatography. Following a sequence of three-chromatography procedures,45 samples were first chromatographed 7436


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38 μmol) in THF/CH3OH (1:1, 2.0 mL) was stirred at room temperature for 16 h. The mixture was then concentrated and chromatographed [silica gel, hexanes/CH2Cl2 (3:1)] to afford a blue solid (7.5 mg, 100%): 1H NMR (THF-d8) δ 9.70 (d, J = 4.5 Hz, 4H), 8.94 (d, J = 4.5 Hz, 4H), 0.62 (s, 18H); 13C NMR (75 MHz, THF-d8) δ 153.4, 150.2, 133.0, 131.4, 108.3, 104.7, 102.4, 102.1, 0.093; ESI-MS obsd 896.0793, calcd 896.0822 (M+, M = C42H26F10N4Si2Zn); λabs (CH2Cl2) 437, 575, 607 nm. 5,15-Diethynyl-10,20-bis(pentafluorophenyl)porphyrin (2). A solution of the porphyrin FbP-F (35 mg, 42 μmol) in CH2Cl2 (18 mL) was treated dropwise with TBAF solution in THF (1.0 M, 0.13 mL, 0.13 mmol). The solution was stirred at room temperature and monitored by TLC [silica, hexanes/CH2Cl2 (1:1)]. After 30 min, methanol (20 mL) was added, and the solution was washed with brine and water. The organic layer was dried (Na2SO4) and concentrated to afford the title compound (28 mg, 98%). The compound was used in the following reaction without further purification: 1H NMR (THF-d8) δ 9.76 (d, J = 4.5 Hz, 4H), 9.04−9.05 (m, 4H), 4.93 (s, 2H), −2.35 (s, 2H); MALDI-MS obsd 691.6, ESI-MS obsd 691.0974, calcd 691.0975 [(M + H)+, M = C36H12N4F10]; λabs (toluene) 431, 529, 565 nm. 5,15-Bis[2-(N-(2,6-diisopropylphenyl)-1,6-bis(4-tertbutylphenoxy)perylene-3,4-dicarboximid-9-yl)ethynyl]10,20-bis(pentafluorophenyl)porphyrin (FbT-F). Following a standard procedure,45,49 a solution of toluene/triethylamine (5:1, 3.3 mL) was deaerated with argon for 1 h. A mixture of 2 (12 mg, 17 μmol), 3 (30 mg, 35 μmol), Pd2(dba)3 (4.8 mg, 3.2 μmol), and P(o-tol)3 (13 mg, 42 μmol) was placed into a Schlenk flask and evacuated under high vacuum for 10 min. The flask was refilled with argon thereafter, and a freeze− pump−thaw procedure was performed three times. The reaction mixture was then stirred for 3 h at 60 °C. The reaction mixture was allowed to cool to room temperature, concentrated, and chromatographed according to the threechromatography method to afford a black solid (24 mg, 60%): 1 H NMR (CDCl3) δ 9.77 (d, J = 4.8 Hz, 4H), 9.52 (dd, J = 7.8, 5.7 Hz, 4H), 8.98 (d, J = 8.4 Hz, 2H), 8.82 (d, J = 4.5 Hz, 2H), 8.39 (s, 2H), 8.38 (s, 2H), 8.28 (d, J = 8.7 Hz, 2H), 7.88 (t, J = 8.1 Hz, 2H), 7.49−7.42 (m, 10 H), 7.31 (d, J = 7.8 Hz, 4H), 7.15 (t, J = 9.0 Hz, 8H), 2.75 (pent, J = 6.9 Hz, 4H), 1.35 (d, J = 4.2 Hz, 36H), 1.17 (d, J = 6.6 Hz, 24H); MALDI-MS obsd 2241.7, calcd 2241.8304 [(M + H)+, M = C144H110N6F10O8]; λabs (toluene) 436, 536, 648, 718 nm. Zinc(II)-5,15-bis[2-(N-(2,6-diisopropylphenyl)-1,6-bis(4-tert-butylphenoxy)perylene-3,4-dicarboximid-9-yl)ethynyl]-10,20-bis(pentafluorophenyl)porphyrin (ZnTF). A solution of FbT-F (11 mg, 5.0 μmol) in CH2Cl2/ CH3OH (5:1, 25 mL) was treated with Zn(OAc)2 (4.6 mg) at room temperature for 40 h. The reaction was monitored by TLC [silica, hexanes/CH2Cl2 (1:1)]. Then, the reaction was quenched by the addition of water. The organic layer was dried (Na2SO4), concentrated to dryness, and chromatographed [silica gel, hexanes/CH2Cl2 (5:6)] to afford a black solid (10 mg, 90%): 1H NMR (300 MHz, CDCl3) δ 9.56 (d, J = 4.2 Hz, 4H), 9.52 (d, J = 7.2 Hz, 2H), 9.44 (d, J = 8.1 Hz, 2H), 8.84 (d, J = 4.8 Hz, 4H), 8.66 (d, J = 7.5 Hz, 2H), 8.37 (s, 2H), 8.35 (s, 2H), 8.00 (d, J = 8.7 Hz, 2H), 7.81 (t, J = 8.1 Hz, 2H), 7.50− 7.31 (m, 10H), 7.30 (d, J = 7.8 Hz, 4H), 7.20−7.12 (m, 8H), 2.71 (pent, J = 7.2 Hz, 4H), 1.36 (d, J = 7.2 Hz, 36H), 1.14 (dd, J = 6.9, 2.7 Hz, 24H); MALDI-MS obsd 2305.6 [(M + H)+]; ESI-MS obsd 2303.7453, calcd 2303.7433 [(M + H)+, M = C144H108F10N6O8Zn]; λabs (toluene) 429, 509, 545, 685 nm.

with adsorption column chromatography (flash silica, Baker) to remove catalysts and ligands from coupling reactions. Then, preparative-scale size exclusion chromatography (SEC) was performed using BioRad Bio-Beads S-X1 in a preparative scale glass column (4.3 × 53 cm) with HPLC grade toluene (gravity flow, ∼0.2 mL/min). The third column chromatography (flash silica, Baker) was performed with HPLC-grade CH2Cl2 and hexanes unless noted otherwise. Analytical-scale SEC was performed to monitor the preparative purification of the samples.36,46 Zinc(II)-5,15-bis[2-(1,6-bis(4-tert-butylphenoxy)-N(2,5-di-tert-butylphenyl)-3,4-perylene-3,4-dicarboximid9-yl)ethynyl]-10,20-di-p-tolylporphyrin (ZnT). A solution of FbT (29.2 mg, 13.6 μmol) in CH2Cl2 (3.0 mL) was treated with a suspension of Zn(OAc)2·2H2O (63.5 mg, 0.272 mmol) in MeOH (0.30 mL) and stirred at room temperature for 4 h. Due to incomplete conversion, another portion of Zn(OAc)2· 2H2O (150 mg, 0.683 mmol) was added, and the reaction was continued overnight. The crude reaction mixture was concentrated. The resulting mixture was treated with CH2Cl2, and the resulting suspension was filtered through a cotton plug. The filtrate was treated with aqueous saturated NaHCO3 and extracted with CH2Cl2. The organic extract was dried (Na2SO4) and concentrated to yield a dark red solid (27.2 mg, 91%): 1H NMR (CDCl3) δ 9.80 (d, J = 4.66 Hz, 4H), 9.26 (d, J = 7.50 Hz, 2H), 9.08 (d, J = 8.16 Hz, 4H), 8.96 (d, J = 4.60 Hz, 4H), 8.26 (s, 2H), 8.15 (d, J = 8.19 Hz, 2H), 8.10 (s, 2H), 8.04 (d, J = 7.50 Hz, 4H), 7.60−7.70 (m, 6H), 7.53 (d, J = 8.50 Hz, 2H), 7.36−7.44(m, 10H), 7.01−7.06 (m 6H), 6.88−6.92 (m, 4H), 2.77 (s, 6H), 1.28−1.34 (m, 72H); MALDI-MS (CHCA as matrix) obsd 2207.9, calcd 2207.9321 [(M + H)+, M = C150H131N6O8Zn]; λabs (toluene) 440, 483, 515, 648, 708 nm. 5,15-Bis(pentafluorophenyl)-10,20-bis(2trimethylsilylethynyl)porphyrin (FbP-F). Following a standard procedure47,48 with modification, a solution of dipyrromethane 1 (0.31 g, 1.0 mmol) in degassed CH2Cl2 (60 mL) was treated with 3-(trimethylsilyl)-2-propynal (0.18 mL, 1.2 mmol), and then dropwise with BF3·OEt2 (15 μL, 0.14 mmol), whereupon the solution turned blue. After stirring for 1 h at room temperature, TLC analysis [silica, hexanes/CH2Cl2 (2:1)] showed the reaction to be complete. DDQ (0.33 g, 1.5 mmol) was then added, and the mixture was stirred for 20 min at room temperature. The reaction was quenched by the addition of triethylamine (0.20 mL, 1.5 mmol). The mixture was passed through a pad of silica gel (CH2Cl2). The filtrate was concentrated to afford a purple solid (0.30 g). A minimum amount of hexanes was added to the crude mixture, which then was sonicated (benchtop sonication bath) for 30 s. The mixture was filtered. The filter cake was washed with hexanes several times until the washings turned to purple from green. The filter cake was dissolved in CH2Cl2 and dried to afford the title compound in 90% purity on the basis of NMR data (45 mg). The filtrate was concentrated to dryness and chromatographed [silica gel, hexanes/CH2Cl2 (5:1 to 3:1)] to afford the desired title compound as a purple solid (25 mg). The two portions were combined (70 mg, 17%): 1H NMR (CDCl3) δ 9.67 (d, J = 4.8 Hz, 4H), 8.76 (d, J = 4.8 Hz, 4H), 0.61 (s, 18H), −2.28 (s, 2H); MALDI-MS obsd 835.5; ESI-MS obsd 835.1753, calcd 835.1766 [(M + H)+, M = C42H28N4F10Si2]; λabs (toluene) 437, 537, 576 nm. Zinc(II)-5,15-bis(pentafluorophenyl)-10,20-bis(2trimethylsilylethynyl)porphyrin (ZnP-F). A mixture of porphyrin FbP-F (7.0 mg, 8.4 μmol) and Zn(OAc)2 (7.0 mg, 7437


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Figure 3. Absorption spectra of free base (A) and zinc chelated (B) triads containing two pentafluorophenyl groups (red), benchmark porphyrins (blue), and perylene (green) in toluene. Spectra of the benchmark porphyrins are multiplied by 0.5 to downscale the intense near-UV Soret (B) band; the inset is multiplied by 3 to reveal the weak Q bands between 500 and 700 nm. Spectra are normalized so that the integrated intensity (300− 900 nm when plotted in wavenumbers) for a triad is the sum of integrated intensity of the porphyrin benchmark plus two times the integrated intensity of the perylene benchmark. The vertical tick marks are incremented by an estimated molar absorptivity of 50 000 M−1 cm−1 (see dotted horizontal line) as described in the Experimental Section.

Photophysical Properties. All studies were performed at room temperature for dilute (μM) argon-purged samples in toluene or benzonitrile. Absorption spectra were acquired using a Shimadzu UV-1800 spectrometer. Static emission spectra were acquired using a Spex-Horiba Nanolog spectrofluorimeter with 2−4 nm excitation and detection bandwidths and corrected for instrument spectral response. The Φf values were determined for samples having A ≤ 0.1 at λexc (typically in the Soret region) using replicate measurements with an integrating sphere (Horiba, Quanti-Phi). The τS values were determined by two methods. The first method used a stroboscopic fluorescence decay apparatus with an ∼1 ns Gaussian instrument response function (Laser Strobe TM-3; Photon Technology International) and samples excited in the blue-to-green spectral regions by a dye laser pumped by a nitrogen laser. The second method utilized transient absorption spectroscopy employing ∼100 fs excitation flashes from an ultrafast laser system (Spectra Physics) and acquisition of difference spectra (360−900 nm) using a white-light pulsed laser (∼1 ns rise time) in 100 ps time bins with variable pump− probe spacing up to 0.5 ms (Ultrafast Systems, EOS). Transient absorption measurements were made using excitation both on the long-wavelength side of the Soret absorption (420−440 nm) and in the Q-band region (530−570 nm) with the exact wavelength depending on compound. Time profiles of ΔA at ∼1.5 nm spacing across the 450−800 nm region were analyzed at a number of individual wavelengths and also globally. The excited-state lifetimes obtained from the transient absorption and fluorescence measurements were the same to within the experimental uncertainty on the reported average value. Transient absorption studies were also used to determine the yield of S1 → T1 intersystem crossing (Φisc) by comparing the extent of bleaching of the ground-state Bx, Qx, and Qy absorption bands (relative to the featureless excited-state

absorption) for the T1 state at long times compared to that due to S1 right after the flash. The contribution of stimulated emission (to S1 spectra) was taken into account for studies in the Qy region. Absorption Spectral Analysis. The spectra in Figures 3 and 4 are presented in a manner to provide the most consistent comparisons among arrays regarding the distribution of absorption spectral intensity for triads versus benchmarks (Figure 3) and among the different triads (Figure 4). The steps, rationale, and assumptions in the analysis affording the presentation in Figure 3 follows that used in recent studies of other perylene−porphyrin panchromatic absorbers, including that used to obtain the analogous Figure 2.31,36 The steps are as follows: (1) The total absorption intensity (300−900 nm) was obtained for each compound (triad, porphyrin benchmark, perylene benchmark) when integrations were performed on spectra plotted in wavenumbers, which is linear with energy. (2) The total intensity for the benchmark porphyrin (FbP-F, ZnP-F) was normalized to that for a related porphyrin FbP-Ph (Figure 2), which was determined31 to have 3.75-fold the intensity for perylene benchmark PMI-e used previously (Figure 2) and herein (Chart 1 and Figure 3). (3) Prior spectral comparisons among a set of compounds containing 1− 4 perylenes per porphyrin (including triad FbT in Chart 1),31 and among sets of compounds with variation in the chromophore (perylene, terrylene, boron-dipyrrin) and porphyrin attachment site,36 give consistent results among the compounds and with measured molar absorptivities using a value of 342 000 M−1 cm−1 at the near-UV maximum of FbPPh and 45 000 M−1cm−1 at the visible (green) maximum of PMI-e. (4) The spectra in Figure 2 were normalized so that the total intensity for the triad is the sum of the intensity of the porphyrin benchmark plus twice the perylene benchmark, consistent with the molecular composition (Chart 1). (5) 7438


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Scheme 1. Preparation of the Pentafluorophenyl-Substituted Porphyrin Building Block

Figure 4. Absorption spectra (solid) and fluorescence spectra (dashed) of the four triads in toluene. Fluorescence spectra used excitation in the Soret region (410−450 nm). Absorption spectra are normalized to the total intensity (300−900 nm) obtained from integration of spectra plotted in wavenumbers. Emission intensities are scaled to the long-wavelength absorption band.

triad FbT was metalated with zinc acetate to generate the zincchelated triad ZnT in 91% yield (Chart 1).

Consequently, the absorption spectra for all four triads in Figure 3 have the same total integrated intensity, which is reasonable considering that they differ only in porphyrin metalation state and fluorination of two aryl rings. Molecular Orbital Calculations. DFT calculations were performed with SPARTAN’ 10 for Windows v 1.2.0 using the hybrid B3LYP functional and basis set 6-31G*.50 All the equilibrium geometries were fully optimized using the default parameters of the Spartan program.

Scheme 2. Synthesis of the Pentafluorophenyl-Substituted Porphyrin−Perylene Triads

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RESULTS AND DISCUSSION Synthesis. The construction of the porphyrin−perylene arrays was based on the copper-free palladium-catalyzed Sonogashira coupling reaction46,49 of individual building blocks. The porphyrin building block is a trans-A2B2-porphyrin bearing two pentafluorophenyl groups and two meso-ethynyl groups. Condensation47,48 of the pentafluorophenyldipyrromethane 142 and 3-trimethylsilylpropynal in the presence of BF3·OEt2 followed by oxidation with DDQ afforded trimethylsilyl (TMS)-protected meso-ethynylporphyrin FbP-F in 17% yield (Scheme 1). Deprotection of FbP-F with tetrabutylammonium fluoride (TBAF) generated the diethynylporphyrin 2 in 98% yield. The diethynylporphyrin, FbP-F, serves as a benchmark compound for photophysical studies. Zinc chelation afforded the zinc porphyrin benchmark ZnP-F quantitatively. The bromo-perylene building block 343,44 derives from chemistry originally pioneered by Langhals.51−55 The copperfree Sonogashira coupling46,49 of the porphyrin 2 and perylene 3 followed by chromatography45 generated the triad FbT-F in 60% yield. Subsequent metalation of the free base triad afforded the zinc-chelated counterpart ZnT-F in 90% yield (Scheme 2). For comparison purposes, the known31 porphyrin−perylene

Absorption Characteristics. The absorption spectra of the pentafluorophenyl-substituted triad containing a free base porphyrin (FbT-F) along with benchmark porphyrin FbP-F and perylene PMI-e in toluene are shown in Figure 3A. The spectra for the analogous zinc-chelated triad (ZnT-F) and benchmark porphyrin (ZnP-F) plus the perylene (PMI-e) common to the triads are given in Figure 3B. The spectra are 7439


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The Journal of Physical Chemistry A Table 1. Spectral Properties of Triads and Arraysa compound

solvent

λab1 (nm)

Iab1

λab2 (nm)

Iab2

λab3 (nm)

FbT-F

toluene PhCN toluene PhCN toluene PhCN toluene PhCN

435 440 427 451 431 436 439 447

0.61 0.74 0.32 0.53 0.70 0.79 0.96 0.081

466 509 509 517 475 480 484 490

0.58 0.61 0.46 0.57 0.62 0.63 0.58 0.58

536 539 544 553 536 540 547 553

toluene PhCN toluene PhCN toluene

437 438 443 445

1.0 1.0 1.0 1.0

537 537 535 540 492

0.048 0.048 0.007 0.011 0.63

576 577 576 585 526

ZnT-F FbT ZnT

FbP-F ZnP-F PMI-ec

Iab3

λab4 (nm)

Triads 1.0 648 1.0 650 1.0 640 1.0 660 1.0 636 0.98 641 0.99 665 0.75 685 Benchmarks 0.11 618 0.10 617 0.040 612 0.043 624 1.0

Iab4

λab5 (nm)

Iab5

λem1 (nm)

λem2 (nm)

0.66 0.77 0.28 0.37 0.74 0.78 0.40 0.42

718 720 685 711 728 737 707 739

0.51 0.78 0.63 0.93 0.80 1.0 1.0 1.0

727 733 707 734 744 755 730 775

806 810 779 808 824 835 808 875

0.17 0.26 0.12 0.31 0.15 0.21 0.19 0.50

172 246 454 385 295 323 446 629

0.027 0.027 0.038 0.060

675 674

0.019 0.020

677 675 615 627 556

755 753 684 693 597

0.84 0.73 1.3 0.84 0.57

44 44 80 77 1314

Iem2 Iem1

Δab‑emb (cm−1)

a All measurements were performed at room temperature. For each compound, each absorption-band position (λi) and intensity (Ii) is given relative to the most intense band in the spectrum for that compound. bStokes shift between the long-wavelength absorption peak and the emission maximum. cFrom ref 30.

the triad and benchmarks does not imply an assignment for the former based on the latter. Similar comparisons can be made regarding the spectrum of zinc chelated triad ZnT-F versus benchmarks ZnP-F and PMI-e (Figure 3B). The main differences relative to the free base analogues described above are as follows: (1) For the porphyrin monomer ZnP-F, the Soret maximum is even more intense (and sharper) than that of FbP-F. This change is most likely due to a greater coincidence of the underlying Bx and By components. This increased overlap is associated with the higher symmetry of the zinc versus free base porphyrin monomers, which also underlies the number of Q bands being reduced to principally two. The latter pair are Q(1,0) at 576 nm and Q(0,0) at 612 nm, spaced by 1330 cm−1. (2) For triad ZnT-F, a dramatic shift of absorption strength from the nearUV to visible region also occurs, but more intensity is grouped into a relative sharp band at ∼544 nm and a broad asymmetric feature at ∼685 nm. The peak position of the latter feature is shifted bathochromically by ∼70 nm from the Q(0,0) band of benchmark ZnP-F, but displays a hypsochromic shift of ∼35 nm from the longest-wavelength band for the analogous free base triad FbT-F (Table 1). Figure 4 reproduces the absorption spectra of fluorinated triads FbT-F and ZnT-F and shows the spectra of the nonfluorinated analogues FbT studied previously31 and ZnT studied here (Chart 1). The spectral data are listed in Table 1. Comparison of ZnT versus FbT follows that just given for ZnT-F versus FbT-F in that the (composite) feature with a maximum in the vicinity of 650 nm for the free base macrocycle apparently shifts bathochromically and melds to some degree with the longest wavelength feature, which lies at 707 nm for ZnT and 728 nm for FbT. The overall spectrum for ZnT is somewhat “compressed” relative to that for FbT in that the manifold of overlapping features between ∼400 and ∼570 nm moves to somewhat longer wavelength whereas the absorption at the red end moves in the opposite direction. The same overall effect occurs for ZnT-F versus FbT-F. Of the four triads, the nonfluorinated free base array FbT is the most panchromatic in affording the most even distribution of

plotted to reveal the redistribution of porphyrin intensity from the near-UV to green-to-red regions for triads versus benchmarks. The steps affording the presentation in Figure 3 (like Figure 2) follow those described for prior studies of perylene−porphyrin panchromatic absorbers,31,36 as summarized in the Experimental Section. Consequently, the integrated absorption intensity of the entire spectrum for a triad is the sum of the intensity of the porphyrin benchmark plus twice that of the perylene benchmark, consistent with the molecular composition (Chart 1). Generally similar absorption spectra are observed in benzonitrile. Table 1 summarizes the peak positions and relative intensities for the triads and benchmarks in toluene and benzonitrile. The absorption spectrum of fluorinated monomer FbP-F shows the typical characteristics of a free base porphyrin (Figure 3A). The features are the intense near-UV Soret band at 437 nm (with underlying By and Bx components) and a series of weak visible bands (inset): Qy(2,0) at 506 nm, Qy(1,0) at 537 nm, Qy(0,0) at 576 nm, Qx(1,0) at 618 nm, and Qx(0,0) at 675 nm. The average vibronic spacing is ∼1320 cm−1, which is typical for most tetrapyrroles.56 The perylene benchmark PMIe shows a (0,0) band at 526 nm and a vibronic satellite at 492 nm spaced by ∼1310 cm−1. The spectrum of triad FbT-F is clearly not the sum of the spectra of the two benchmarks (Figure 3A). The spectrum of FbT-F contains features not present for the constituents and shows a dramatic redistribution of absorption strength from the porphyrin near-UV region into and across the visible wavelengths. Whereas monomer FbP-F has ε ∼ 340 000 M−1 cm−1 at the near-UV maximum and an average ε ∼ 12 000 M−1 cm−1 for the visible bands, triad FbT-F has a violet-blue molar absorptivity similar to the ε ∼ 60 000 M−1 cm−1 average for the features that span the green-to-red regions. Additionally, the reddest band for FbT-F at 718 nm is shifted bathochromically by 43 nm from the longest-wavelength Q-band of FbP-F. Because the precise electronic/vibronic origins of the spectral features for the triad are uncertain, the wavelength maxima in Table 1 are listed simply as λ1 to λ5. The alignment of these positions in columns according to rough spectral positions for 7440


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The Journal of Physical Chemistry A Table 2. Excited-State Properties of Triads and Benchmarksa compound

solvent

ES1b (eV)

τS (ns)

FbT-F

toluene PhCN toluene PhCN toluene PhCN toluene PhCN

1.71 1.70 1.78 1.71 1.68 1.66 1.71 1.63

2.7 1.2 1.1 0.6 2.0 1.3 1.2 0.2

toluene PhCN toluene PhCN toluene

1.83 1.83 2.02 1.98 2.43

13.3 13.0 2.1 2.2 5.6

ZnT-F FbT ZnT

FbP-F ZnP-F PMI-ed

Φf

Φisc

Triads 0.35 0.31 0.20 0.11 0.25 0.66 0.16 0.30 0.26c 0.22 0.18c 0.08 0.30 0.25 0.023 0.005 Benchmarks 0.049 0.71 0.039 0.77 0.043 0.92 0.040 0.90 0.95

Φic

kf−1 (ns)

kisc−1 (ns)

kic−1 (ns)

0.34 0.66 0.090 0.52 0.52 0.74 0.45 0.97

7.7 6.0 4.4 3.8 7.7 7.2 4.3 8.7

9 11 2 2 9 16 5 46

8 2 12 1 4 2 3 0.2

0.24 0.19 0.037 0.060

270 330 49 55 5.9

19 17 2 2

55 68 57 37

All measurements were performed at room temperature. The typical errors (percent of value) are τS (±7%), Φf (±5%), Φisc (±15%), Φic (±20), kf (±10%), kisc (±20%), and kic (±25%). bAverage energy of the long-wavelength absorption and emission maxima. cCorrected from 0.41 (toluene) and 0.24 (benzonitrile) reported in a prior communication (ref 31). dFrom ref 30. a

Figure 5. Transient absorption data for triads FbT-F (A−C) and ZnT-F (D−F) in toluene acquired using 100 fs excitation flashes at 435 or 426 nm, respectively. Panels A and D show measured TA spectra at select times. Panels B and E give amplitude spectra (spectra of the (pre-exponential factors) obtained from global analysis using fitting of the kinetics with a function consisting of two exponentials plus a constant convolved with a Gaussian instrument-response function. Panels C and F give ground-state absorption spectra and fluorescence spectra for comparison.

1. The spectra for benchmarks FbP-F and ZnP-F (in toluene and benzonitrile) typically have a (0,0)/(1,0) intensity ratio of 0.80−1.3 compared to 0.12−0.19 for the triads. For typical monomeric porphyrins, a significant fraction of the emission (and absorption) intensity in the vibronic satellite feature(s) arises from Herzberg−Teller (vibronic) coupling in which intensity is borrowed from the intense near-UV transitions.56,57 For the triads, the dramatic diminution in the near-UV radiative strength depletes the source of the vibronic intensity borrowing and thereby diminishes the intensity in the (0,1) fluorescence (and absorption) features. Thus, the fluorescence spectra of the porphyrin-based triads in Figure 4 are similar to those of monomeric chlorins and bacteriochlorins,58,59 in which the intensity of the vibronic satellites is more Franck−Condon

absorption spectral intensity as well as exhibiting the largest wavelength span. Fluorescence Spectra. Emission spectra for the four triads are given in Figure 4 (dashed lines). The spectra are quite similar to one another in showing a prominent origin band and a vibronic satellite to longer wavelength that has ∼15% of the peak intensity on the average. The Stokes shift between the (0,0) fluorescence and absorption bands is in the range ∼170− 450 cm−1 for the compounds in toluene and ranging ∼250− 630 cm−1 in benzonitrile. The fluorescence spectra of the porphyrin benchmarks are given in the Supporting Information. The emission spectral characteristics for the triads and benchmarks are listed in Table 1. Key aspects of the spectra are as follows. 7441


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absorption difference spectrum, and little change elsewhere. The time evolution over the first tens of picoseconds thus gives rise to a kinetic component with a time constant of 15−20 ps for both triads, and an asymmetric derivative-like feature centered at ∼700 nm in the amplitude spectrum (Figure 5B and E). The positive longer-wavelength wing of this spectral shape is associated with the enhancement/shift of the stimulated emission, with the bleaching presumably remaining relatively constant if no molecules return to the ground state during this period. Although this fast evolution likely occurs primarily within the S1-state, relaxations from the upper states to S1 may contribute. In this regard, a similar 15−20 ps component is found using excitation flashes in the green− yellow regions (rather than in the blue) that excite the triads at lower levels in the excited-state manifold. Similar excited-state dynamics are observed for FbT-F and ZnT-F in benzonitrile and for nonfluorinated analogues FbT and ZnT in both solvents. Following this initial fast phase, the spectrum decays fairly uniformly to a lower overall level without substantial change in shape. The associated time constant of ∼3 ns for FbT-F and ∼1 ns for ZnT-F reflects the lifetime of the S1 state. The decay of this excited state occurs in part by return of molecules to S0 by fluorescence and internal conversion (diminishing bleaching of the ground-state absorption bands), and in part by intersystem crossing to produce the triplet excited state (T1). The S1 decay is virtually complete by 7 ns, and thus T1 is responsible for the measured difference spectrum at that time (Figure 5A and D) and for the nondecaying (infinity time for this experiment) amplitude spectrum derived from global analysis (Figure 5B and E). Comparison of the magnitudes of the measured ground-state bleachings at long time (T1 state) versus short time (S1 state) shows that the triplet yield is about 2-fold greater for ZnT-F than FbT-F (Table 2). The Φisc values of 0.66 for ZnT-F and 0.31 for FbT-F in toluene when coupled with the associated τS afford kisc values of (2 ns)−1 versus (9 ns)−1, respectively. The more facile intersystem crossing for the zinc chelated triad is derived from heavy-atom (zinc) enhancement of the spin−orbit coupling that underlies the process. Enhanced kisc is similarly found for benchmark ZnP-F versus FbP-F [(2 ns)−1 versus (19 ns)−1] and for the two triads and benchmarks in benzonitrile (Table 2). The kisc value is also greater for nonfluorinated zinc triad ZnT versus free base triad FbT in toluene [(5 ns)−1 versus (9 ns)−1]. However, for this pair the enhanced kisc for the zinc chelate is not sufficient to give a higher Φisc (0.25 versus 0.22) because the radiative rate constant is similarly enhanced [(4.3 ns)−1 versus (7.7 ns)−1]. The ∼2-fold increase in kf for ZnT [(4.3 ns)−1] versus FbT [(7.7 ns)−1] is comparable to that found for ZnT-F [(4.4 ns)−1] versus FbT-F [(7.7 ns)−1] (Table 2). This increase in the rate constant for S1 → S0 spontaneous emission implies a related increase in the S0 → S1 absorption via the connection between the Einstein coefficients.60 The long-wavelength absorption feature of both zincated triads is indeed more intense than that for the two free base triads (Figure 4). However, it is difficult to assess the percentage of the feature that derives from S0 → S1 absorption for the zinc-containing triads because of overlap with a transition to the next excited state that is spaced apart for the free base triads (vide supra). The internal-conversion rate constants (kic) are consistently ∼25% greater for the free base versus zinc chelated arrays, although the differences are close to experimental uncertainty.

rather than Herzberg−Teller in origin, due in these cases to a larger B−Q spacing that gives an unfavorable energy denominator for vibronic mixing. 2. The absorption-fluorescence Stokes shifts are ∼4−10-fold larger for the triads than the benchmarks (Table 1). This finding suggests that there is a greater change in nuclear coordinates (chromophore and medium) following excitation for the triads versus benchmarks. 3. The fluorescence profile for each triad deviates substantially from mirror symmetry with the absorption contour (Figure 4). This finding suggests that the weak (1,0) vibronic satellite associated with the longest-wavelength absorption transition is buried under larger features in the orange-red spectral region. The corollary is that the prominent absorption feature at ∼650 nm for FbT-F and Fb-T that apparently moves to longer wavelength and overlaps with the longer-wavelength band for ZnT-F and ZnT (Figure 4) is not simply a vibronic satellite of the S0 → S1 transition but primarily reflects a transition to a higher electronic state. 4. The fluorescence−excitation spectrum of each triad is identical within experimental error to the absorptance (1− transmittance) spectrum. Excitation within any absorption band leads to quantitative internal conversion to the tetrapyrrole-like S1 excited state of the triadic architecture. Excited-State Properties. The measured excited-state properties of the triads and benchmarks are the lifetime (τS) of the lowest singlet excited state (S1), the S1 → S0 fluorescence quantum yield (Φf), and the yield of S1 → T1 intersystem crossing (Φisc). The yield of S1 → S0 internal conversion is obtained by the difference Φic = 1 − Φf − Φisc. The fluorescence (kf), intersystem crossing (kisc), and internal conversion (kic) rate constants are obtained from these data via the calculation kx = Φx/τS, where x = f, isc, or ic. These data are collected in Table 2. The S 1 lifetime was generally determined by both fluorescence decay and transient absorption (TA) studies, and the values averaged. The latter measurements also afforded the intersystem-crossing (triplet) yield Φisc and revealed early time excited-state relaxation dynamics pertinent to the spectral data (e.g., Stokes shifts) described in the prior section. Thus, the TA data are briefly described before turning to comparisons of the yields and rate constants for the decay pathways of the S1 excited state of the triads and benchmarks. Representative TA spectra at selected times following excitation of triad FbT-F with 100 fs blue flashes are shown in Figure 5A. The data were globally analyzed by fitting the time profiles with a function consisting of two exponentials plus a constant convolved with the instrument response function. The resulting amplitude spectra (spectra of the pre-exponential factors) are shown in Figure 5B. For comparison, ground-state absorption spectra and fluorescence spectra are given in Figure 5C. Analogous data for triad ZnT-F are shown in Figure 5D−F. The spectrum at early time (0.5 or 2 ps) for FbT-F and ZnTF is dominated by bleaching of all ground-state absorption bands, likely along with some S1 → S0 stimulated emission near the same position as bleaching of the S0 → S1 long-wavelength absorption band. By 50 ps, the stimulated emission is enhanced and is positioned to longer wavelength than the bleaching of the long-wavelength absorption band (Figure 5A and D), and at a wavelength consistent with the spontaneous fluorescence spectrum (Figure 5C and F). This change between 0.5 and 50 ps has the overall appearance of a bathchromic shift of the composite negative-going feature on the red end of the 7442


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Figure 6. Electron densities and energies of triad FbT-F, benchmark porphyrin FbP-F, and benchmark perylene with hydrogen substituted for the TMS group of PMI-e for the calculations. The color-coding used for the energies and energy-positions of the orbitals of triad FbT-F are as follows (left to right): blue, orbitals that are primarily porphyrin HOMO-1 or LUMO+1 in character; red, orbitals that are a mixture of the porphyrin HOMO or LUMO and perylene HOMO or LUMO; green, orbitals that are primarily perylene HOMO or LUMO in character.

Such a finding would be consistent with the anticipated greater Franck−Condon factor for internal conversion associated with the high-energy N−H vibrations in the free base macrocycles; such vibrations are obviously absent in the corresponding zinc chelates. The composite kf, kic, and kisc values afford singlet excitedstate lifetimes for the triads in toluene that decrease in the order FbT-F (2.7 ns) > FbT (2.0 ns) > ZnT (1.2 ns) ∼ ZnT-F (1.1 ns). The τS value decreases for each triad in benzonitrile, for which the order is FbT (1.3 ns) > FbT-F (1.2 ns) > ZnT-F (0.6 ns) > ZnT (0.2 ns). The reductions in τS for the triads in the polar versus nonpolar solvent derives primarily from an increase in kic (Table 2). The increase is larger for the zinc versus free base triads, and this could be connected in part with axial ligation of the benzonitrile solvent molecules with the metal ion. The overall result is that all four triads have a long (>1 ns) excited-state lifetime in toluene, and such a value is retained for the free base triads in polar media. Thus, the

excited-state properties in conjunction with the spectral characteristics described above indicate that the two free base triads FbT and FbT-F are the best choices in terms of composite properties for panchromatic light-harvesting systems. Although not directly relevant to light-harvesting, the fluorescence yields of the triads are noteworthy because they are substantially greater than those for the porphyrin benchmarks. We have pointed out this effect previously regarding perylene−tetrapyrrole dyads.30,31 For example, in toluene the Φf values are 0.35 for FbT-F and 0.25 for ZnT-F compared to 0.049 for FbP-F and 0.043 for ZnP-F (Table 2). The enhanced fluoresence yields for the triads follow from the aforementioned increase in kf, which occurs in concert with the shift of intensity into the S0 → S1 absorption band (Figure 3) versus the benchmarks. A difference in fluorescence yield is not roughly proportional to changes in kf for the triads because Φf = kf • τS = kf/(kf + kic + kisc) and kf makes a substantial 7443


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Figure 7. Electron densities and energies of the MOs of triad ZnT-F, benchmark porphyrin ZnP-F, and benchmark perylene with hydrogen substituted for the TMS group of PMI-e for the calculations. The color-coding is as in Figure 6.

with energies given in blue, red, or green font for reasons discussed below. Although there are differences in detail, all four triads share similar MO characteristics that will be described first for FbT-F. Then notable differences among the triads will be highlighted by way of comparison. Examination of the bottom half of Figure 6 shows that the HOMO of benchmark FbP-F (−5.49 eV) and the HOMO of perylene (−5.22 eV) mix in the porphyrin−bis(perylene) triad FbT-F to give three orbitals. One such orbital (−5.04 eV) is at higher energy than the HOMOs of the benchmarks and is the HOMO of the triad. The second orbital (−5.71) lies lower in energy than the HOMOs of the benchmarks and is the HOMO-4 of the triad. Both triad orbitals have substantial electron density spread across the porphyrin and both perylenes, and differ in characteristics such as the number of nodes associated with the ethyne bridges and attachment sites. The energies of these two orbitals are indicated in red font. The third orbital derived from the benchmark HOMOs is displaced to the middle right and has electron density predominantly on the two perylenes. Its energy (−5.33 eV), indicated in green

contribution to the total decay rate (and lifetime) of S1 for the triads, unlike typical porphyrin monomers (where k isc dominates). It is interesting in this regard that the three S1 decay routes have roughly equal yields (Φf = 0.35, Φisc = 0.31, Φic = 0.34) for FbT-F in toluene, which is quite uncommon for a porphyrinic system. Molecular Orbital Characteristics. Diagrams showing electron-density distributions and energies of the frontier MOs for triads FbT-F, ZnT-F, FbT, and ZnT are shown in Figures 6−9, respectively. The column of orbitals on the left of each figure gives the four frontier MOs of the porphyrin benchmark, which are the highest occupied MO (HOMO), the lowest unoccupied MO (LUMO), and the HOMO-1 and LUMO+1. These four orbitals are shown because linear combinations of the excited-state configurations arising from one-electron promotions among these orbitals generally underlie the optical spectra of (monomeric) tetrapyrroles.61,62 The HOMO and LUMO of the perylene benchmark (common to all four triads) are shown at the right of each figure. Three columns of orbitals are shown for each triad in the horizontal middle of each plot 7444


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Figure 8. Electron densities and energies of the MOs of triad FbT, benchmark porphyrin FbP, and benchmark perylene with hydrogen substituted for the TMS group of PMI-e for the calculations. The color-coding is as in Figure 6.

font, is only slightly (0.11 eV) below that of the HOMO of the perylene benchmark and is the HOMO-2 of the triad. The fourth triad filled orbital is the HOMO-3 (−5.78 eV) and is displaced middle left with its energy indicated in blue font. This orbital is basically the HOMO-1 of the porphyrin benchmark down-shifted in energy by only 0.01 eV. The porphyrin HOMO-1 has nodes at the meso-carbon attachment sites of the ethyne linker and thus does not mix significantly with the perylene HOMO. A similar pattern is seen for the empty orbitals of triad FbT-F (and the benchmarks) in the top half of Figure 6. The porphyrin LUMO (−2.95 eV) mixes with the LUMO of the two perylenes (−2.67 eV) to give the triad split pair (−3.16 and −2.59 eV; red font) that lie at higher and lower energy with density spread across all constituents, along with the intervening orbital (−2.80 eV; green font) with density primarily on the perylenes. The porphyrin LUMO+1 (−2.79 eV) remains unaltered (−2.79 eV; blue font) in the triad. The patterns of four filled and four empty frontier MOs for zinc chelated triad ZnT-F (Figure 7) are similar to those just

described for FbP-F (Figure 6). A notable characteristic for ZnT-F is that the HOMO (−5.09 eV) has no appreciable electron density on the zinc ion, whereas the benchmark ZnP-F places considerable density on the metal in the a2u(π)-like HOMO (−5.54 eV). The same comparisons hold for the nonfluorinated counterparts FbT (Figure 8) and ZnT (Figure 9) with a few notable differences. In the absence of the stabilizing pentafluorophenyl substituents, the frontier MOs of porphyrin benchmarks FbP and ZnP are elevated in energy with respect to those of FbP-F and ZnP-F. A consequence of the different energy relationship of the HOMOs and the LUMOs of the constituents is that the exact order of the frontier orbitals of FbT and ZnT is not always the same as for FbT-F and ZnT-F. For example, although the HOMO/HOMO-4 and LUMO of all triads are orbitals that have mixed perylene−porphyrin−perylene parentage, the LUMO+4 and LUMO+3 of FbT and ZnT are reversed compared to FbT-F and ZnT-F such that for the nonfluorinated triads the LUMO+4 primarily has the character of the benchmark porphyrin LUMO+1 (Figures 8 and 9). 7445


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Figure 9. Electron densities and energies of the MOs of triad ZnT, benchmark porphyrin ZnP, and benchmark perylene with hydrogen substituted for the TMS group of PMI-e for the calculations. The color-coding is as in Figure 6.

Figure 10 summarizes for all four triads the energies of the split pairs of filled or empty perylene−porphyrin−perylene orbitals (red lines and font) that derive from the porphyrin HOMO or LUMO (black lines and font). The spacing (red) and average energy (blue) is indicated along the vertical dotted line that connects each split pair of triad MOs. The splitting between filled triad orbitals (HOMO and HOMO-4) is 0.65− 0.70 eV and that between the pair of empty triad orbitals (LUMO and LUMO+3 or LUMO+4) is 0.52−0.59 eV. Such a similarity in orbital mixing/splitting is reasonable given the structural similarity between the four triadsthey share the same linker motif that employs direct ethyne bridges from a porphyrin meso-position to the polyaromatic core of a perylene. Evidently the change of the porphyrin from free base to zinc chelate or the use of pentafluorophenyl versus ptolyl groups at nonlinking meso-positions has only marginal effect on the net MO interactions, but do contribute to the detailed spectra and photophysical properties (vide supra). Regarding potential connection to spectral properties, the LUMO−HOMO gap (in eV) for the triads (Figures 6−10)

follows the order ZnT-F (1.97) > ZnT (1.92) > FbT-F (1.86) > FbT (1.83). The energy ordering of the long-wavelength absorption maximum is the same: ZnT-F (1.81) > ZnT (1.75) > FbT-F (1.73) > FbT (1.70). Basically the same order [ZnT-F (1.78) > ZnT (1.71) ∼ FbT-F (1.71) > FbT (1.68)] for the S1 energy is obtained in the usual way from the average energies of the absorption and fluorescence maxima for the triads in toluene (Table 2), which includes a variable absorption− fluorescence Stokes shift. Regardless, one would expect parallel trends between LUMO−HOMO energy gap and the S0 → S1 energy derived from spectra if the S1 excited-state configuration of a chromophore derives simply from a one-electron promotion from HOMO to LUMO. However, this simple situation is certainly not the case for monomeric porphyrins, for which the S1 excited state is a ∼50/50 mixture of the HOMO → LUMO and HOMO-1 → LUMO+1 configurations. These two configurations for porphyrins are typically close in energy (giving a small energy denominator for electronic mixing) and have a substantial configuration−interaction energy of ∼0.4 eV.61,62 7446


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Figure 10. MO diagram showing the split pair of triad filled/empty orbitals (red) that result from interaction of the porphyrin HOMO/LUMO (black) with the perylene HOMO/LUMO (green). The vertical arrows between a pair of triad orbitals indicate the energy splitting (red italics) and average energy (blue italics). All values are given in eV.

We have discussed previously for ethyne-linked perylene− porphyrin dyads (e.g., Figure 1) that one effect of this attachment motif, the same as that used for the triads studied here, is an apparent reduction in configuration−interaction energy, presumably from the changes in the electron density in the porphyrinic system such as are illustrated in Figures 6−9.30 Diminished electronic mixing reduces transition dipole−dipole constructive interference that makes the near-UV (Bx and By) bands so strong and reduces the dipole−dipole cancellation that makes the visible (Qx and Qy) bands so weak for monomeric porphyrins. This effect likely contributes to the panchromatic absorption properties of tightly coupled perylene−porphyrin arrays including the triads. The expansion of the number of excited-state configurations involving the triad orbitals derived from the porphyrin HOMO and LUMO (Figures 6−10), in principle, could give rise to added (at least twice as many) features in the absorption spectrum relative to the porphyrin benchmarks. The relative orientations of perylene and porphyrin and associated effects on the perylene−porphyrin coupling could also play a role.63 Regarding photophysical properties, it was noted above that the triads all have relatively long (1.1−2.7 ns) S1 lifetimes (and high fluorescence yields) in toluene and that the τS remains >1 ns for FbT-F and Fb-T in benzonitrile, dropping to lower values only for the zinc chelates in the polar solvent (Table 2). Examination of the MO diagrams in Figures 6−9 shows no obvious differences between the four triads to which this difference for free base and zinc chelates can be attributed. All of the energy gaps between a filled orbital with perylene− porphyrin−perylene character and an unoccupied orbital with mainly perylene character or vice versa have similar energies

(2.1−2.3 eV) with no obvious trend. It is the case that such excited-state configurations give displacement of charge in the architecture (but not a dipole moment owing to symmetry), and likely have little radiative probability. If such configurations are stabilized in the polar medium and contribute more to the S1 state, they could enhance nonradiative decay and shorten the lifetime. Perhaps such an effect when coupled with axial ligation of the benzonitrile molecules to the metal ion is the source of reduced τS values in the zinc-chelated triads. Otherwise, the panchromatic arrays show robust photophysical properties that augur well for use in molecular-based light-harvesting systems. Summary and Outlook. A building block approach has been used to construct triads composed of two perylenes and one porphyrin joined via ethyne linkers. The electronic properties of the porphyrin have been tuned by choice of metal chelation (free base or zinc chelate) and by installation of pentafluorophenyl rather than p-tolyl substituents at the nonlinking meso-positions. DFT calculations show that HOMO of the tetrapyrrole couples with the HOMO of the two perylenes to give a trio of linear-combination orbitals, the highest and lowest of which have mixed porphyrin/perylene character with the middle orbital having density most on the perylenes. The same mixing occurs for the LUMOs of the porphyrin and perylenes. The HOMO-1 and LUMO+1 of the porphyrin lack electron density at the site of the ethynyl linker to the perylene, do not couple appreciably to the perylenes to give composite orbitals, and thus retain similar energies in the triad as in the benchmark. The lowest singlet state of each porphyrin−bis(perylene) triad has tetrapyrrole-like excited-state properties with a relatively long singlet excited-state lifetime (1.2−2.7 ns) but 7447


The Journal of Physical Chemistry A with an enhanced fluorescence yield (0.25−0.35) in toluene. These Φf and τs values are reduced only about 2-fold for triads FbP-F and Fb-P that contain a free base porphyrin in benzonitrile, but more so for the zinc chelated analogues. The two free base triads are the most panchromatic in affording the most even distribution of absorption spectral intensity, with the spectrum of FbP being the most uniform and exhibiting the largest wavelength span (380−750 nm). The time-resolved absorption data show that all absorption bands of the triads bleach effectively instantly upon excitation, with some initial (15−20 ps) excited-state relaxation dynamics that likely occur mainly within the S1 excited state. The combined electronic-structure, spectral, and excited-state characteristics demonstrate that tight electronic coupling between the constituents in the triads effectively leads to a superchromophoric system. Excitation at any wavelength in the panchromatic absorption spectrum produces an electronic state of the architecture that leads to rapid and essentially quantitative formation of the lowest singlet excited state via normal internal conversion. These characteristics show that the porphyrin−bis(perylene) triads achieve the second of the two strategies outlined in the Introduction for panchromatic lightharvesting systems. This superchromophore approach is complementary to the more common strategy to assemble a number of chromophores, each absorbing in different spectral regions followed by an energy-transfer cascade. Finally, the approach embodied by the ethyne-linked (bis)perylene−porphyrins described herein has certain favorable characteristics for utilization in synthetic light-harvesting and energy-conversion schemes: (1) Fewer components and a smaller system are required to achieve the desired spectral coverage. (2) Systems can be designed that retain robust excited-state lifetimes in polar media (including FbP-F and FbP), which can be more difficult when many chromophores with diverse electronic properties are employed to achieve the desired spectral range. (3) The architectures are under synthetic control so that properties such as a linker for attachment (to a peptide, catalyst, surface, etc.) can be incorporated to direct flow of energy (or electrons) to a target site. Such characteristics will be explored in next-generation light-harvesting and energy conversion designs.

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ACKNOWLEDGMENTS

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REFERENCES

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, under Award Number DE-FG02-05ER15661. J.A. was supported by the Ministry of Science, Research and Technology, Islamic Republic of Iran. Mass spectra were obtained at the NCSU Mass Spectrometry Facility located in the Department of Chemistry. 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 *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b06857. Complete citations for truncated references. Characterization data for all new compounds; full-page displays of Figures 6−9; absorption and fluorescence spectra of porphyrin benchmarks in toluene (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 951-827-3660. *E-mail: [email protected]. Tel: 919-515-6406. *E-mail: [email protected]. Tel: 314-935-6502. Author Contributions #

Equal contributions by Javad Amanpour and Gongfang Hu.

Notes

The authors declare no competing financial interest. 7448


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