Photochemistry of squaraine dyes. 1. Excited singlet, triplet, and redox states of bis[4-(dimethylamino)phenyl]squaraine and bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine

Abstract

Singlet and triplet excited states and the reduced and oxidized forms of two squaraine dyes, bis[4-(dimethylamino)- phenyllsquaraine (SQH) and bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine (SQOH), have been investigated by picosecond and nanosecond laser flash photolysis. Both singlet and triplet excited states of SQH and SQOH have distinct and intense absorption maxima in the visible region. The lifetime of ISQOH (T_s= 3.0 ns) is considerably higher than %QH ( T_s= 1.5 ns) as the substituent OH group restricts rotational relaxation of the excited state. Both squaraine dyes exhibit poor intersystem crossing efficiency (Φ_isc <0.001). However, the excited triplets of SQH and SQOH can be generated by triplet-triplet sensitization. The oxidized forms of these dyes, which were generated by photolytic and pulse radiolytic methods, exhibit absorption maxima in the vicinity of the ground-state absorption. The cation radical of SQOH can also be generated by charge transfer between excited dye and Ti0₂ semiconductor colloids.