Solvent-dependent switching between two dipolar excited states in a rigidly extended trichromophoric system

Abstract

We present a novel, rigidly extended trichromophoric system (1) that consists of a central naphthalene (N) chromophore interposed between an N,N,4-trimethylaniline electron donor (D) and a 4-cyanobenzamide electron acceptor (A). Upon photoexcitation, quantitative charge transfer occurs to yield the fluorescent D⁺–N^––A state. In non-polar solvents such as cyclohexane and benzene this is the predominantly formed charge-transfer (CT) state. In a slightly more polar solvent, such as dioxane, a second electron transfer step occurs, resulting in the formation of the non-fluorescent ‘giant dipolar’ D⁺–N–A^– state with > 95% efficiency. Nanosecond transient absorption spectroscopy of 1 in cyclohexane and benzene shows rapid formation (k_isc≈ 0.7 × 10⁹ s^–1) of a triplet state localised on the naphthalene chromophore (λ_max= 440 nm). In dioxane and more polar solvents absorptions are observed that correspond to the donor radical cation (D⁺) and the acceptor radical anion (A^–)(λ_max= 370 and 500 nm). The lifetime of the D⁺–N–A^– state is 6 ns in dioxane. As the absorption characteristics of both excited states differ profoundly, this implies that a subtle change of the polarity of the surrounding medium induces a dramatic change of the transient absorption spectra. Time-resolved microwave conductivity (TRMC) measurements confirm the increase of the excited state dipole moment of 1 from 19 D for D⁺–N^––A in cyclohexane to 42 D for D⁺–N–A^– in dioxane.