Absorption and fluorescence properties of tetracoordinated borates: energy versus electron transfer in spirointeractions

Bicyclic borates can have two π-planes oriented perpendicular to each other through four tetrahedral BO bonds and exhibit spirointeractions as revealed by the presence of broad charge transfer bands, although absorption peaks of the contributory moieties were hardly modified. Their fluorescence spectra demonstrate that the spirointeractions allow intramolecular quenching of two moieties by electron transfer but not by energy transfer. The overall phenomenon related to the spiroconjugation indicates a new pattern of excited-state interaction distinguishable from that of exciplex formation in intermolecular quenching and electron transfer through rigid σ-bond systems.