Synthesis and properties of pyrrolo[3,2-b]pyrrole-1,4-diones (isoDPP) derivatives

The synthesis of three pyrrolo[3,2-b] pyrrole-1,4-dione (isoDPP) derivatives is described, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b] pyrrole-2,5(1H, 4H)-dione 2, 1,4-diphenyl-3,6-di(thiophen-2-yl) pyrrolo[3,2-b]-pyrrole-2,5(1H, 4H)-dione 3, and 1,4-bis(4-(hexyloxy) phenyl)-3,6-di(thiophen-2-yl) pyrrolo[3,2-b] pyrrole-2,5(1H, 4H)-dione 7 in which the molecular structures differ in the aromatic ring (phenyl or thiophene) attached to the nitrogen atom. Thin films of 2, 3, and 7 could be formed by evaporation under vacuum. In the case of 2 and 3 GIWAXS measurements showed that the film structural ordering was similar to that measured in single crystals. In contrast GIWAXS showed that 7 had features associated with liquid crystalline materials. Time dependent density functional theory (TDDFT) calculations predicted that the transition between the lowest energy singlet excitation (S-1) and the ground state (S-0) would be optically forbidden due to the centrosymmetric geometries of compounds. Photophysical measurements showed that the compounds were weakly luminescent, with low radiative rates in solution of order 10(6) s(-1), which are consistent with the TDDFT predictions. Furthermore, photoinduced absorption (PIA) spectroscopy showed that there is a long-lived low energy state, which has been assigned as a triplet and provides a further non-radiative decay pathway for the excited state.