Spectroscopy and dynamics of dehydrobenzo[12]annulene derivatives possessing peripheral carboxyphenyl groups: theory...

In this work, we report on the results of theoretical and experimental studies of a series of dehydrobenzoannulene (DBA) derivatives (Nu-T12 [5,6,11,12,17,18-hexadehydrotribenzo[a,e,i]cyclododecene], T12-COOMe [5,6,11,12,17,18-hexadehydro-2,3,8,9,14,15-hexakis(4-methoxycarbonylphenyl)tribenzo[a,e,i]cyclododecene] and T12-COOH [5,6,11,12,17,18-hexadehydro-2,3,8,9,14,15-hexakis(4-carboxyphenyl)tribenzo[a,e,i]cyclododecene]) in N,N'-dimethylformamide (DMF) solutions. The theoretical and experimental findings show that the S₀ → S₁ transition of these molecules is forbidden. Time-resolved spectroscopy measurements determined a lifetime of ∼100 ps of the transition from the first electronical excited (S₁) state. These molecules also emit through charge transfer (CT) species, with lifetimes of ∼1 and ∼4.5 ns. In addition to this, Nu-T12 and T12-COOMe in DMF solutions exhibit an emission from their triplet state in 35 and 24.5 ns, respectively. However, T12-COOH strongly interacts through H-bonds with DMF molecules, leading to the formation of new species having a proton-transferred character, whose emission spectrum is red-shifted and its lifetime from the S₁ state is ∼25 ns. Using nanosecond (ns) flash photolysis, we also observed the presence of non-emissive triplet states, in addition to the emissive ones. The theoretical calculations suggest that this non-radiative triplet state originates from a CT structure of the emissive triplet one. The new findings presented here elucidate the photobehaviour of three DBA derivatives of relevance to crystalline Hydrogen-Bonded Organic Framework (HOF) materials. The photophysical data provide a strong basis to explore and to better understand the photodynamics of HOF crystals.