Photoinduced electron transfer of double-bridged phthalocyanine–fullerene dyads

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Abstract

Three new double-bridged phthalocyanine–fullerene dyads were studied with spectroscopic and electrochemical methods. The two-linker strategy was applied for the first time to phthalocyanine–fullerene dyads. Photoinduced electron transfer (ET) of the dyads was found to occur from the excited state of phthalocyanine both in polar and non-polar environment. Examination of ET energetics revealed that the phthalocyanine–fullerene dyads exist mainly in an extended conformation as opposed to a face-to-face orientation common for the similar double-bridged porphyrin–fullerene dyads.

Graphical abstract

Three new double-bridged phthalocyanine–fullerene dyads were studied with spectroscopic and electrochemical methods. Photoinduced electron transfer (ET) of the dyads was found to occur both in polar and non-polar environment. Examination of ET energetics revealed that the phthalocyanine–fullerene dyads exist mainly in an extended conformation as opposed to a face-to-face orientation.

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Introduction

Photoinduced ET properties of porphyrin–fullerene dyads have been studied thoroughly by many research groups during the past decade [1], [2], [3], [4], [5], [6]. However, porphyrin absorbs efficiently only in a rather narrow spectral region around 430 nm, which is a major drawback considering applications in photonic devices. Replacing porphyrin with a phthalocyanine chromophore extends the absorption range of the dyad considerably matching better with e.g., the spectrum of white light and thus solar irradiation. Despite the obvious advantage of phthalocyanines as electron donors in donor–acceptor (D–A) systems, they have been studied to a modest degree as dyads [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], because of their more demanding synthesis compared to porphyrins.

Studies on covalently linked D–A dyads have demonstrated the importance of close proximity of the donor and acceptor moieties in achieving efficient ET [5], [17], [18]. Especially porphyrin–fullerene dyads with a face-to-face orientation realized with two-linker strategies have been found to undergo ET rapidly [19], [20], [21]. Such dyads can also be modified to form solid films in which the molecules are oriented controllably, which is of interest when building photovoltaic devices [22], [23].

Two-linker strategy is now applied for the first time to phthalocyanine–fullerene dyads. Compared to double-bridged porphyrin–fullerene dyads studied earlier [19], [24], the phthalocyanine–fullerene dyads H2PcF-1, H2PcF-2, and ZnPcF reach the complete charge-separated state from the first excited singlet state (S1) of phthalocyanine without an observable exciplex intermediate in both polar and non-polar environment. This behaviour can be explained both by the different orientation of the donor and acceptor in the present phthalocyanine–fullerene dyads compared to several porphyrin–fullerene dyads or by the short lifetime of the exciplex state in the former case.

Section snippets

Studied compounds

Structures of the studied compounds are presented in Fig. 1. Synthesis of the molecules is described in the electronic supplementary information. The structure of H2PcF-1 has been determined as the one presented in Fig. 1, but for dyads ZnPcF and H2PcF-2 there are a few possible regioisomers with different attachment positions of the linkers. The asterisks in the structure of ZnPc (Fig. 1) mark the two possible attachment points of one of the substituents resulting in altogether 13 possible

Absorption and fluorescence

For the free-base phthalocyanine compounds (Fig. 2) the Q band at 700 nm was split in two while only one sharp peak was observed for ZnPc and ZnPcF. The maximum of the band was clearly red-shifted to 737 nm for H2PcF-2 in toluene compared to 716 nm for H2PcF-1, because of the different substituents of the phthalocyanine ring (butoxy or tert-butyl groups). The Q bands were slightly broader for ZnPcF, H2PcF-1, and H2PcF-2 compared to the corresponding references indicating an interaction between the

Conclusions

For the phthalocyanine–fullerene dyads CT was found to occur from S1 of phthalocyanine both in polar and non-polar solvents and CR was significantly slower in non-polar environment. The rates of CT and CR for ZnPcF, H2PcF-1, and H2PcF-2 were of the same order of magnitude. Analysing the energetics of CT revealed that the phthalocyanine–fullerene dyads studied exist mainly in extended orientations. Further attempts to synthesize phthalocyanine–fullerene dyads with face-to-face orientations are

Acknowledgement

This work was supported by the National Technology Agency of Finland.

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