Article pubs.acs.org/JPCC
Preferential Through-Space Charge Separation and Charge Recombination in V‑Type Configured Porphyrin−azaBODIPY− Fullerene Supramolecular Triads Venugopal Bandi,† Habtom B. Gobeze,† Paul A. Karr,‡ and Francis D’Souza*,† †
Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States Department of Physical Sciences and Mathematics, Wayne State College, 111 Main Street, Wayne, Nebraska 68787, United States
‡
S Supporting Information *
ABSTRACT: Electron transfer is one of the most fundamental and prevalent processes occurring in chemistry, physics, and biology. In donor− acceptor systems with one of the partners’ a photosensitizer, upon photoexcitation, transfer of an electron between the photoexcited and ground-state molecules occurs. Factors affecting the geometry, energetics, and dynamics of this process have been one of the intensively studied scientific topics, often by building model donor−acceptor conjugates or by utilizing natural systems. A wealth of information, applicable to almost all areas of modern science and technology, has been generated from these studies. In the present study, we demonstrate preferential through-space charge separation and charge recombination in supramolecular triads composed of porphyrin (free-base, zinc, or magnesium at the central cavity) as excited-state electron donor, BF2-chealted azadipyrromethene (azaBODIPY), and fullerene (C60) as electron acceptors. Because of spatial close proximity of the terminal porphyrin and fullerene entities of the triads as a consequence of the V-type configuration, photoinduced charge separation from the singlet excited porphyrin involves fullerene instead of energetically more favorable covalently linked azaBODIPY entity. Interestingly, charge recombination also follows this path of through-space instead of an electron migration from the fullerene anion radical to the covalently linked azaBODIPY entity. The present study highlights the importance of geometric disposition of donor and acceptor entities in governing not only the forward photoinduced electron transfer but also the dark reverse electron transfer in multimodular donor−acceptor conjugates, applicable toward light-energy-harvesting and building optoelectronic devices.
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devices.48−68 Conventionally, the synthetic analog of natural chlorophyll pigments, porphyrins, and phthalocyanines has extensively been used as light-energy-harvesting antenna and electron donor entities in these multimodular systems performing photoinduced energy- and electron-transfer reactions.31−52 Fullerene, C60, due to delocalization of charges within the spherical carbon structure of the rigid aromatic π-sphere has captured special attention as novel electron-acceptor molecule.69−76 The small reorganization energies of fullerenes in electron-transfer reactions result in fast charge separation and relatively slow charge recombination, yielding long-lived charge separated states.31−47,69−76 Recently, the BF2-chelated dipyrromethene (BODIPY) dyes derived from 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4-adiaza-s-indacene77−80 and their structural analog, BF2-chelated azadipyrromethene (azaBODIPY) dyes,81−84 have emerged as efficient and tunable light-absorption and luminescent species. A large number of BODIPY and azaBODIPY derivatives can readily be obtained by modification of peripheral carbon
INTRODUCTION Electron-transfer reactions are ubiquitous in nature and are of great importance to almost every discipline of chemical and biological science.1−7 Research on this topic has addressed some of the fundamental questions of modern science. Photoexcitation of either the donor or acceptor entities of the conjugate provides an additional driving force useful to experimentally verify the theoretical predictions of structural and energetic effects.8,9 The design of discrete donor−acceptor architectures that allow electron transfer in a controlled fashion began in the early 1980s and is continuing up to the present.10−30 The common approach on building such architectures have emphasized a biomimetic approach to the challenge by utilizing synthetic analogs of photosynthetic electron donor and acceptor molecules.22−28 Supramolecular multimodular donor−acceptor systems assembled using different photo- and redox-active species have been extensively studied in the last three decades to exploit the mechanistic details of electron transfer under various conditions.31−47 Such well-designed and assembled multimodular systems have been found to be useful in solar-fuel and solar-electricity generation and in building optoelectronic © XXXX American Chemical Society
Received: July 24, 2014
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dx.doi.org/10.1021/jp507439t | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Scheme 1. Pathways for Through-Bond and Through-Space Photoinduced Charge Separation and Charge Recombination in Porphyrin−AzaBODIPY−Fullerene Triads
Scheme 2. Synthetic Procedure Adapted for Porphyrin−AzaBODIPY−Fullerene Supramolecular Triads
positions,85−88 exhibiting attractive photophysical characteristics with strong absorption bands in the visible/near-IR region, making them comparable to those of porphyrin and phthalocyanines. Hence, these days BODIPY and azaBODIPY have been commonly employed to fulfill the role of lightharvesting antenna, electron donor, or acceptor molecules.89−107
In the present study, we have synthesized supramolecular triads composed of azaBODIPY covalently linked to a porphyrin and a fullerene in a V-type configuration, as shown in Scheme 1. The V-design brings the terminal donor and acceptor entities close enough to cause spatial interactions. Consequently, as shown by femtosecond transient absorption spectroscopy studies, the singlet excited porphyrin undergoes preferential electron transfer to the spatially close fullerene to B
dx.doi.org/10.1021/jp507439t | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
yield MP•+-azaBODIPY-C60•− radical ion-pair (reaction 1 in Scheme 1) instead of the covalently linked azaBODIPY, resulting in the formation of MP•+-azaBODIPY•−-C60 radical ion pair (reaction 4 in Scheme 1), although the covalently linked azaBODIPY in the triads is a better electron acceptor than the fullerene. Following the charge separation, the MP•+azaBODIPY-C60•− radical ion pair also prefers a through-space charge recombination (reaction 2 in Scheme 1) to yield ground-state MP-azaBODIPY-C60 instead of the thermodynamically feasible charge migration reaction to yield MP•+azaBODIPY•−-C60 radical ion pair (reaction 3 in Scheme 1). Please note that the reaction 5 in Scheme 1 is an uphill process. To the best of our knowledge, this is the first example to demonstrate geometric preference over thermodynamic feasibility of charge separation and charge recombination in multimodular donor−acceptor supramolecular conjugates.
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RESULTS AND DISCUSSION Syntheses of Porphyrin−AzaBODIPY−Fullerene Supramolecular Triads. The syntheses of the supramolecular triads involved a multistep procedure, as shown in Scheme 2, while the details are given in the Experimental Section. In brief, the synthesis of the BF2-chelated [5-(4-hydroxyphenyl)-3phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol2-ylidene]amine, 1a, was performed according to our previously published method.46 This compound was subsequently reacted with 4-formyl benzoic acid in dimethylformamide (DMF) in the presence of 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDCl) at 0 °C to obtain mono benzaldehyde functionalized azaBODIPY, 1b. In a separate experiment, stoichiometric amounts of pyrrole, benzaldehyde and 4-formyl benzoic acid ester were reacted in propionic acid to obtain mono(carboxyesterphenyl)triphenyl porphyrin, 1c. Compound 1c was subsequently hydrolyzed to obtain 1d, followed by chromatographic purification. Next, compounds 1b and 1d were reacted in DMF using EDCl to obtain azaBODIPYporphyrin dyad, 1e. Furthermore, compound 1e was reacted with fullerene, C60, in the presence of sarcosine in toluene according to Prato’s method108 to obtain compound 1. Finally, compound 1 was metalated using either zinc acetate or MgBr2· (OEt)2 under appropriate experimental conditions, followed by chromatographic separation to obtain the zinc and magnesium derivatives (compounds 2 and 3) of the triad. The structural integrity of the newly synthesized compounds was established from 1H NMR, mass, electrochemical, and optical techniques. Steady-State Absorption and Fluorescence Measurements. The newly synthesized triads were studied in nonpolar toluene and polar benzonitrile to probe solvent effects and their influence over energy and electron-transfer events. In general, nonpolar solvents promote light-induced energy transfer, while the polar solvents promote light-induced electron transfer.69−76 Figure 1 shows the absorption spectra of the investigated compounds along with the controls (H2P, ZnP, and MgP, where P = meso-tetraphenylporphyrin and azaBODIPY) in benzonitrile; similar spectra were also obtained in toluene. The most intense band of azaBODIPY was located at 657 nm, while two additional weak bands were also present at 312 and 482 nm. The free-base porphyrin triad, 1, revealed the Soret band at 422 nm and the visible bands at 516, 551, and 664 nm due to porphyrin and appended azaBODIPY optical transitions. The fulleropyrrolidine band was overlapped with the azaBODIPY band in the 315 nm region. The zinc and magnesium metalated
Figure 1. Absorption spectra of indicated compounds in benzonitrile. The concentration of each compound was 4.6 μM.
triads, 2 and 3, revealed their characteristic bands involving all three entities. The visible bands of ZnP in 2 were located at 556 and 598 nm (Figure 1a), while these bands for MgP in 3 were located at 558 and 608(sh) nm (Figure 1b). The Soret and visible bands of porphyrin in the triads were red-shifted by 1 to 2 nm compared with their control pristine MPs. The azaBODIPY band was red-shifted by ∼7 nm and appeared at 664 nm. These results suggest existence of some intramolecular interactions between the entities of the triad independent of the metal ion in the central cavity. Figure 2a shows the fluorescence spectra of triads 1−3 in toluene, excited at their respective Soret band position. Similar spectra were obtained when the samples were excited using wavelength corresponding to the most intense Q-band. The free-base porphyrin emission located at 652 and 718 nm and that of zinc porphyrin located at 597 and 645 nm were found to be almost all quenched with the appearance of a new emission band at 694 nm, corresponding to singlet emission of azaBODIPY. These results suggest excited-state quenching involving singlet−singlet energy transfer from 1MP* to azaBODIPY as one of the quenching mechanisms.109 Furthermore, excitation spectra recorded for the triads by holding the emission monochromator to the emission wavelength of azaBODIPY and scanning the excitation wavelength revealed peaks of both azaBODIPY and MP, confirming excitation transfer. These observations are similar to that reported recently for covalently linked zinc-porphyrin− azaBODIPY dyads and triads, wherein excitation transfer in nonpolar toluene or slightly polar o-dichlorobenzene was observed.99 Interestingly, as shown in Figure 2b, in polar benzonitrile, the emission spectra recorded by exciting the triads at the Soret or Q-band position of the porphyrins revealed negligible amounts of azaBODIPY emission (
