CONFORMATIONAL DYNAMICS AND CHARGE SEPARATION IN THE EXCITED-STATE OFBRIDGED DONOR-ACCEPTOR COMPOUNDS

The process of intramolecular charge separation was studied in three n early identical donor-bridge-acceptor compounds (1-3) under jet-cooled conditions. Each consists of a vinylcyanonaphthalene group as a power ful electron acceptor and an anilino derivative as an electron donor, separated by a rigid hydrocarbon bridge. The only difference between t he three compounds is the substituent at the para position of the anil ino group, which directly influences the ionization potential of the d onor. For compound 1, which has the weakest donor of the three, it is found that excitation at (or very near) the electronic origin of the a cceptor chromophore results exclusively in local emission, while excit ations with an excess energy >60 cm(-1) lead to a distinct red shift. Comparison with 2 and 3 shows that the red-shifted band can be attribu ted to a charge-separated state with an extended conformation, in whic h an electron has been transferred across three sigma bonds. Interesti ngly, this charge separation process appears to be more efficient in 2 and 3 than in 1, resulting in complete quenching of the local emissio n even at the lowest energy excitations. These observations are interp reted in terms of intramolecular radiationless transition theory and a comparison is made with the behaviour of (semi)flexibly bridged syste ms in which charge separation was shown earlier to be linked to a majo r, electrostatically driven reorganization.