The Role of the Exciplex State in Photoinduced Electron Transfer of Phytochlorin−[60]Fullerene Dyads

Abstract

The photoinduced electron transfer (ET) in five structurally different phytochlorin-fullerene dyads was studied in polar and nonpolar solvents using femtosecond fluorescence up-conversion and pump-probe transientabsorption techniques. Small changes in the structures of the dyads result in considerable changes in the ET properties and allow the determination of reorganization energies of the photoinduced reactions and electronic couplings between the initial and final states. After the excitation of the phytochlorin moiety to the second excited singlet state, the dyads relax rapidly to the first excited singlet state of phytochlorin. The first excited singlet state of phytochlorin is in equilibrium with an intramolecular exciplex state. In polar benzonitrile, the exciplex undergoes an electron transfer, and a complete-charge-separated (CCS) state is formed with a quantum yield close to unity. In contrast to the previously studied phytochlorin-fullerene dyads, the dyads in the present study form the CCS state also in nonpolar toluene with a yield influenced by minor changes in the molecular structure. The new dyads have a weaker phytochlorin-fullerene interaction due to longer separation distances between the two moieties. Therefore, the energies of the exciplex states are increased, and thus, their formation rates are reduced. In addition, the rates and yields of the complete charge separations are increased both in polar and nonpolar solvents. In benzonitrile, the reorganization energies for the transitions from the exciplex to the CCS and from the CCS to the ground state are 0.38 and 1.05 eV, respectively. The electronic couplings between the corresponding initial and final states of the two transitions mentioned above are 22 and 15 cm -1 .

Effect of Anion Ligation on Electron Transfer of Double-Linked Zinc Porphyrin−Fullerene Dyad

“…For all of the dyad solutions, two decay components were required to fit the results (Figure ). The biexponential decay of the S 1 state has been earlier observed for several porphyrin−, phthalocyanine−, and phytochlorin−fullerene dyads and attributed to an equilibrium between two consecutive intermediates in their excitation relaxation pathways. ,, …”

“…A strong emission of the intramolecular exciplex was observed in the steady state emission spectra above 770 nm for the dyad in anisole and TBAPF 6 /anisole (Figure ). The exciplex is a highly probable intermediate in this kind of compounds as proved for the porphyrin−, phthalocyanine−, and phytochlorin−fullerene dyads previously studied. ,− ,,, However, the exciplex emission was not observed in TBACl/anisole, which indicates that the ET rate is faster in the ligated dyad. Another reason for the lack of the exciplex emission could be that the S 1 state converts directly into the CS state, i.e., the formation of an exciplex is precluded by the ligation of chloride ion, which may cause distortion in the mutual orientation of ZnP and C 60 .…”

Effect of Mutual Position of Electron Donor and Acceptor on Photoinduced Electron Transfer in Supramolecular Chlorophyll–Fullerene Dyads

“…The central atom substitution and modification of the peripheral substituents of macrocycles can be used to change these properties for improving chromophore activity in donor–acceptor (DA) systems. Although nature has evolved chlorophyll to show excellence in the multitasking roles, the complicated purification of chlorophylls, instability of unmodified chlorophyll being exposure to light and limited amount of synthetic modification methods have limited its use in artificial systems. ,− However, in comparison to the semisynthetic chlorophyll analogues, the asymmetry of chlorophyll has its advantages over easily accessible symmetric synthetic porphyrines and phtalocyanines. It has been recently shown that the asymmetry enables precise supramolecular assemblies, e.g., derivation of chlorins for face-selective ligation and high order helix formation …”

Independence and inverted dependence on temperature of rates of photoinduced electron transfer in double-linked phthalocyanine-fullerene dyads

“…33 During the last decade we have studied a series of photoinduced electron transfer reactions of pheophytin-, phthalocyanine-, and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other. [36][37][38][39][40][41][42][43][44][45][46][47] The common feature for all of these compounds is that a p-p interaction in the D-A pair has an important role in the ET reaction. Practically in all studied molecules the donor acceptor pairs form an intramolecular exciplex as a transient before the formation of a CS state or a tight ion pair.…”