Rigid-Core Oligothiophene-S,S-dioxides with High Photoluminescence Efficiencies both in Solution and in the Solid State

The photoluminescence (PL) frequencies and quantum efficiencies of dithieno[3,2-b;2',3'-d]thiophene-4,4-dioxide (1), its 3,5-dimethyl derivative (2), and the corresponding 4-oxide (3) are reported and discussed in relation to their single-crystal X-ray structures. The peculiar packing modalities of dioxides 1 and 2, based on dimeric units originated by short S . . .O contacts between molecules related by an inversion center, cause an unusually high bathocromic shift of PL frequencies from solution to solid state. This effect is accompanied by a marked decrease in solid-state PL efficiencies (12% and 16%) compared to those in solution (75% and 77%). In monoxide 3 the loss of local symmetry inherent to the change SO2 --> SO deeply modifies the self-assembly and PL properties, and the PL efficiency in the solid state is close to that in solution. Ab initio calculations on the ground and excited states of compound 1 were performed and compared to those of a conformationally mobile counterpart. Oligomers containing dithienothiophene 2 as internal core were synthesized and found to be characterized by high PL efficiencies in the solid-state (up to 48%) as well as in solution (up to 85%). The current-voltage and luminance-voltage plots of an electroluminescent diode fabricated with one of these new rigid-core oligothiophene-S,S-dioxides are reported.