SOLVENT-DEPENDENT SWITCHING BETWEEN 2 DIPOLAR EXCITED-STATES IN A RIGIDLY EXTENDED TRICHROMOPHORIC SYSTEM

We present a novel, rigidly extended trichromophoric system (1) that c onsists of a central naphthalene (N) chromophore interposed between an N,N,4-trimethylaniline electron donor (D) and a 4-cyanobenzamide elec tron acceptor (A). Upon photoexcitation, quantitative charge transfer occurs to yield the fluorescent D+-N--A state. In non-polar solvents s uch as cyclohexane and benzene this is the predominantly formed charge -transfer (CT) state. In a slightly more polar solvent, such as dioxan e, a second electron transfer step occurs, resulting in the formation of the non-fluorescent 'giant dipolar' D+-N-A(-) state with > 95% effi ciency. Nanosecond transient absorption spectroscopy of 1 in cyclohexa ne and benzene shows rapid formation (k(isc) approximate to 0.7 x 10(9 ) s(-1)) of a triplet state localised on the naphthalene chromophore ( lambda(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-) (lambda(max) = 370 and 500 nm). The lifeti me of the D+-N-A(-) state is 6 ns in dioxane. As the absorption charac teristics of both excited states differ profoundly, this implies that a subtle change of the polarity of the surrounding medium induces a dr amatic change of the transient absorption spectra. Time-resolved micro wave conductivity (TRMC) measurements confirm the increase of the exci ted state dipole moment of 1 from 19 D for D+-N--A in cyclohexane to 4 2 D for D+-N-A(-) in dioxane.