Photoinduced electron transfer dynamics in porphyrin donor dyads

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Abstract

Intramolecular photoinduced electron transfer (PET) processes occurring in dyads with a free base porphyrin-tetraazaanthracene donor (P) and either a tetracyanonaphthoquinidodimethane (TCQ) or benzoquinone (BQ) acceptor linked by a rigid six σ-bond polynorbornane bridge ([6]) have been investigated. For P[6]BQ, PET in the polar solvent benzonitrile (ɛs = 25.9) occurs with a rate constant (kPET) of 1.6 × 108 s−1 but is not evident in solvents less polar than tetrahydrofuran (ɛs = 7.52). For P[6]TCQ, highly efficient forward PET occurs in both polar and non-polar solvents (kPET > 2 × 1010 s−1). For P[6]TCQ the lifetime of the resulting charge-separated state decreases markedly with increasing solvent polarity. The results are discussed in the context of the likely mechanisms for electronic coupling and current theories for PET processes in such linked molecular systems.

Introduction

Photoinduced electron transfer (PET) reactions are ubiquitous in nature and are primary processes occurring in many photoexcited synthetic molecular and supramolecular systems. The roles of the donor and acceptor structure, intermolecular separation, relative orientation and surrounding environment on electron transfer dynamics remain topics of intense experimental and theoretical investigation [1], [2], [3], [4], [5]. Control of PET reactions is an essential requirement for the efficient operation of many molecular devices and light energy conversion and storage systems [6].

Of particular importance is acquiring an understanding of the factors that can affect the electronic coupling and driving force for electron transfer within a photoexcited donor–acceptor pair. In previous work we have demonstrated for a range of linked electron donor–acceptor molecular systems with interchromophore separations exceeding orbital overlap, the importance of through-bond interactions and the role of bridge structure on electronic coupling mechanisms [7], [8], [9], [10], [11]. In the present work we investigate PET dynamics in a covalently linked dyad bearing a free base porphyrin-tetraazaanthracene (P) donor and either a benzoquinone (BQ) or tetracyanonaphthoquinidodimethane (TCQ) acceptor. Connecting the two chromophores by a rigid polynorbornane bridge of six σ-bonds in length (molecules P[6]BQ and P[6]TCQ, Scheme 1) allow the donor–acceptor separation and orientation to be completely fixed. The geometry of the two systems is illustrated by the AM1 optimized geometry of P[6]TCQ, shown in Fig. 1. The different acceptors provide molecular systems with a large difference in PET driving force allowing the role of solvent on the PET dynamics to be investigated.

Section snippets

Experimental

Tetrakis(3′,5′-di-tert-butylphenyl)porphyrintetraazaanthracene[6]benzoquinone (P[6]BQ) was prepared as described previously [12]. Tetrakis(3′,5′-di-tert-butylphenyl)porphyrintetraazaanthracene[6]tetracyanonaphthoquinidodimethane (P[6]TCQ) was prepared from P[6]BQ in 73% yield using a previously reported method for the conversion of norbornane and bicyclo[2.2.2]octane fused naphthoquinones to 5,12-bis(dicyanomethylidene)naphthalene systems (Scheme 1) [12]. m.p. > 300 °C. 1H NMR (CDCl3, 300 MHz): δ

Results

An estimate of the driving force for photoinduced charge separation (ΔGCS) in P[6]BQ and P[6]TCQ in various solvents can be determined from the Weller equation [13],ΔGCS=e(Eox(D)Ered(A))E00Xwhere the term X accounts for the finite donor–acceptor separation (Rc), ionic radii (r+, r) and solvent dielectric constant (ɛs),X=e24πε0εsRc+e28πε01r++1r1εref1εs

The oxidation potential of the porphyrin donor (Eox(D)) determined by cyclic voltametry is 1.1 V (vs. SCE in MeCN) while the reduction

Discussion

The results demonstrate the high efficiency of the PET process for the TCQ acceptor compared to BQ. The fluorescence quenching data are also generally consistent with the predictions of the Weller calculations reported in Table 1 (using the appropriate 7.32 Å donor–acceptor separation), that PET is thermodynamically feasible for P[6]TCQ in all solvents under investigation while only in the more polar solvents should charge separation occur with BQ as an acceptor. It is interesting to note that

Conclusion

The P[6]TCQ molecular dyad studied in this work represents a molecular system where very efficient forward PET mediated by through-bond coupling occurs in both polar and non-polar solvents. Charge recombination exhibits a strong solvent dependence with the extended lifetimes observed in non-polar solvents characteristic of Marcus inverted region behaviour. The ratio of forward to back electron transfer rates (kPET/kCR = 2500) for P[6]TCQ in the non-polar solvent iso-pentane is substantial for a

Acknowledgements

The corresponding authors (KPG and MNP-R) acknowledge financial support from the Australian Research Council (ARC) under the Discovery Grant Program, SJL acknowledges the award of an ARC Postdoctoral Fellowship and TDMB acknowledges the award of an Australian Postgraduate Award. TG is grateful to the University of Melbourne for the provision of a Visiting Scholars Award. MNP-R thanks the Australian Partnership for Advanced Computing and the Australian Centre for Advanced Computing and

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1

Present address: Department of Spectroscopy, Indian Association for the Cultivation of Science, Calcutta 700032, India.

2

Present address: School of Chemistry, Monash University, Clayton 3800, Australia.

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