Aggregation-Enhanced Excimer Emission of Tetraarylethene Linkers in Ladderphanes

Tetraarylethene (TAE) has been used as linkers in polynorbornene- and polycyclobutene-based double-stranded ladderphanes and as pendants in related single-stranded polymers. The adjacent TAE linkers in these polymers are separated by 4.5-5.5 angstrom. Upon irradiation at 315 nm in dichloromethane (DCM), TAE excimer emission is observed at 493 nm with quantum yield Phi = 0.005-0.015. When MeOH/DCM (3/1) is used as the medium, Phi(s) of hexyloxy-substituted TAE ladderphanes increase to 0.45-0.48 due to aggregation-enhanced excimer emission (AEEE). TAE-appended single-stranded polynorbornene has Phi = 0.21 under the same conditions. The neighboring TAE chromophores in these single-stranded polymers may not be eclipsed and therefore might be less favored for excimer formation. Entangling of hexyloxy groups, inter alia, may be responsible for AEEE. Replacement of a hexyloxy group by a methoxy group leads to a reduction in the quantum yield by 70% due to lack of entangling. The overall process that is responsible for AEEE involves excimer formation, aggregation of polymers, plus entangling of long-chain alkyl groups.

Automated summary

The use of TAE in polymers depends on the distance between TAE linkers and the solvent conditions, resulting in differences in the quantum yield of the formed excimer. Entangling of hexyloxy groups may affect the formation of the excimer.