Solid-state absorption and luminescence spectroscopy of nitronyl nitroxide radicals

Abstract

Low-temperature absorption and luminescence spectra of three crystalline nitronyl nitroxides are presented. For 2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide and 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, luminescence with resolved vibronic structure is observed between 700 nm and 1100 nm. The modes forming the resolved structure have frequencies of approximately 600 cm⁻¹ and 1400 cm⁻¹. They are characterized by Raman spectroscopy and electronic structure calculations. The intensity distribution within the vibronic progressions indicates small structural changes between the ground and emitting states. The lowest-energy absorption bands observed between 450 nm and 700 nm of the two luminescent compounds and of 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide also show resolved structure, but the overall width of these absorption band systems is larger by a factor of 2 than that of the luminescence spectra. This difference is rationalized in terms of overlapping electronic transitions to at least two excited states arising from the (SOMO − 1)¹(SOMO)² and (SOMO)⁰(SOMO + 1)¹ electron configurations, supported by transition energies and oscillator strengths from density functional calculations.