Laser flash photolysis studies on dynamic behaviour of triplet naphthalenophanes sensitized by triplet benzophenone

Abstract

The formation and decay processes of triplet naphthalenophanes ( ³ NP*) sensitized by triplet benzophenone ( ³ BP*) have been investigated by means of laser photolysis techniques. It was found that the dynamic behaviour of ³ NP* markedly depends on their molecular conformations. For the cases of NP with tetramethylene chains (tNP), triplet energy transfer from ³ BP* occurred to produce triplet tNP ( ³ tNP*). The triplet–triplet (T–T) absorption spectrum of syn-tNP having full-overlapped naphthalene moieties (parallel form) consisted of local (430 nm) and excimer triplet (580 nm) absorption bands. For anti-tNP with partially overlapped naphthalene moieties (antiparallel form), only the local T–T absorption was observed. The efficiency (ϕ _TET ) of triplet energy transfer was determined to be unity for syn-tNP while 0.76 for anti-tNP. The rate constants for the quenching of ³ BP* (k _q ) and triplet energy transfer (k _TET ) were determined. At high BP concentrations, it was shown that the decay processes of ³ tNP* mainly proceeded via a triplet exciplex between ³ tNP* and BP regardless of their molecular structures. For the cases of NP with a cyclobutane ring (cNP, tilted form), after triplet energy transfer from ³ BP*, the bond dissociation of the cyclobutane ring of ³ cNP* subsequently took place to give the corresponding triplet biradicals.