Picosecond time-resolved absorption and emission studies of pyrazolotriazole azomethine dyes

Abstract

The temperature- and viscosity-dependent photochemistry of three pyrazolotriazole azomethine dyes, which can undergo syn–anti isomerisation, has been studied using ps absorption, single photon counting (SPC) and steady-state emission techniques. Emission intensities are viscosity dependent, with ϕ_F increasing from ca. 0.005 to ca. 1 in di-n-butylphthalate as the glass-transition temperature is approached. SPC fluorescence decay kinetics are non-monoexponential for all dyes both in rigid organic glasses at 77 K and in solution. Reasonable fits to two exponentials can be obtained with a dominant (ca. 80–90%) decay process of τ≈ 4 ns and a secondary component of τ≈ 1 ns at 77 K in rigid glasses. At 297 K in 96 : 4 glycerol–ethanol the dominant fluorescence decay process (ca. 75%) has τ⩽ 10 ps, and there is a secondary component with τ≈ 60 ps. Picosecond transient absorption studies show three transient states. One is assigned to a fluorescent state because of a stimulated emission band; the lifetime is viscosity dependent with τ 60 ps in glycerol–ethanol (96 : 4) at 4 °C, but only ca. 2 ps at 297 K. A second transient is associated with strong bleaching at the dye absorption maximum, an increase in absorption in the blue spectral region, and τ≈ 6 ps; this is tentatively assigned to a twisted excited singlet state. The third state with τ≈ 3 ps is associated with a weak bleaching at the dye absorption maximum and an absorption increase above ca. 630 nm; this is tentatively assigned to a twisted ground state which relaxes to the syn- and anti-ground-state conformations. A comparison of kinetic and steady-state data suggests partitioning between a highly emitting and zero-emitting state within a few ps of excitation. This is supported by ps absorption studies which show the simultaneous formation of both the fluorescent state and the transient assigned to the twisted excited state within the laser pulse.