Excimer formation in the mixed dimers of naphthalene and 1-methoxynaphthalene in a supersonic jet

Electronic spectra for the S-1<--S-0 transition of jet-cooled mixed cluster s of naphthalene and 1-methoxynaphthalene have been measured by laser-induc ed fluorescence excitation and dispersed fluorescence spectroscopy. Under j et-cooled conditions, naphthalene forms two isomeric 1:1 mixed dimers with 1-methoxynaphthalene, which exhibit significant differences in excimer form ation dynamics from the locally excited states. One of the isomeric dimers emits excimer fluorescence when excited to the electronic origin of the low est excited state but the other shows a significant barrier to formation of the excimer. The energy gap between the S-1 states of naphthalene and 1-me thoxynaphthalene is similar to 350 cm(-1). The observation of excimer emiss ion, particularly when excited to the S-1 origin of one of the isomeric dim ers, supports our earlier proposal that for naphthalene-methoxynaphthalene mixed dimers energy exchange or exciton resonance interactions are not sign ificant for the stabilization of the mixed excimers. The charge-transfer (C T) interaction and overlap between pi -molecular orbitals of two molecules at a suitable geometry are considered to be the primary factors for the sta bility of the excimers. At higher naphthalene vapour pressure we have obser ved a mixed trimer of 2:1 composition of naphthalene and 1-methoxynaphthale ne. The trimer does not emit excimer fluorescence when excited with additio nal vibrational energy in S-1 up to 650 cm(-1). The observation indicates t hat the geometry of the trimer is very different from the stack configurati on essential for formation of the excimer.