MOLECULAR-CONFORMATION EFFECTS ON THE RELAXATION PROCESSES IN THE EXCITED-STATE OF NAPHTHALENOPHANES

The photophysical processes of naphthalenophanes having different molecular structures have been studied by measurements of emission spectra, emission quantum yields, intersystem crossing (ISC) quantum yields, and lifetimes. The molecular conformations of naphthalenophanes are divided into three groups by means of MM2 calculations: (a) parallel forms (1 and 2), (b) antiparallel form (3), and (c) tilted forms (4 and 5). The singlet excimer state 1EX* formed via the Franck-Condon excited singlet state 1(R*-R) or 1(R* AND R) is produced very rapidly. Intramolecular excimer emissions are observed significantly for naphthalenophanes except for the antiparallel form 3. From emission polarization measurements, it is found that the 1EX* state has 1L(a)-like character for parallel and antiparallel forms and 1L(b)-like character for tilted forms. The relaxation processes in the excited state of naphthalenophanes depend on their molecular structures. For 1 and 2, the ISC process from 1EX* produced via 1(R*-R) to the local triplet state (3LE*, 3L(a)) of a naphthyl moiety competes with the decay processes from 1EX* to the ground state. For 3, the 3LE* state is formed via both the locally excited singlet state (1LE*, 1L(b)) and 1EX*. For 4 and 5, 1EX* is mainly produced from 1(R* AND R), which decays to the ground state, and the ISC process from 1EX* to 3LE* (3L(a)) is very inefficient. Local phosphorescences from 3LE* (3L(a)) are observed for 1-3 but not (or scarcely) for 4 and 5. However, no triplet excimer emission for 1-5 is observed upon direct excitation.