Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer–Fullerene Donor–Acceptor Systems

Abstract

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor–acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1–4, 6) appended to C60 were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5–25 ps) electron transfer to C60 followed by slower (200–650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor–acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C60-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In this marvelous process, sunlight is harvested by the antenna complexes and the excitation energy is funneled to the reaction center (RC), where multistep electron-transfer reactions occur to generate a potential that can drive chemical reactions ( Figure 1). [34][35][36] Through a survey of the literature, one finds that tetrapyrrole macrocycles, such as porphyrins [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] and phthalocyanines, [54][55][56][57][58][59][60][61]…”

“…These studies are important not only for photochemical conversion of solar energy into fuels, [21][22][23][24][25][26][27][28][29][30][31][32][33] but also for the construction of various optoelectronic devices. [34][35][36] Through a survey of the literature, one finds that tetrapyrrole macrocycles, such as porphyrins [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] and phthalocyanines, [54][55][56][57][58][59][60][61][62][63][64][65] have extensively been used as lightharvesting and electron-transfer units in artificial photosynthetic systems to perform photoinduced energy and electron transfer due to their close resemblance to natural pigments, namely, chlorophyll and bacteriochlorophylls, which act as light-harvesting antenna and electron donors in the electrontransfer cascade.…”

Electron transfer reactions in sub-porphyrin–naphthyldiimide dyads

“…The values of the reorganization energies and electronic couplings are reasonable when compared to systems with similar donor-acceptor distances reported in the literature. A zinc-porphyrin-C 60 dyad had an electronic coupling for a charge separation of 44 cm À1 and a reorganization energy of 0.85 eV (in toluene) 28,50 while a zinc-porphyrin-gold-porphyrin dyad with a slightly larger donor-acceptor separation had corresponding values of 19 cm À1 and 1.1 eV (in dichloromethane). 51 Obviously, the exact nature of the bridging structure and the stereoelectronic effects plays a fairly large role as should be evident from this study, preventing a quantitative comparison.…”

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

“…11–12 The complicated excited state dynamics which depend on both structural and electronic effects are frequently further complicated by solvent interactions and/or coupling between the electronic and nuclear processes, making these processes difficult to study. 13–14 However, advances in time-resolved optical spectroscopy down to femtosecond time-scales relevant for many photoinduced electron transfer processes 4, 15 combined with theoretical calculations of key electronic factors, e.g. electronic coupling strengths, have provided significant insight into important processes such as long-range electron transfer in supramolecular and heterogeneous donor-bridge-acceptor systems.…”