Shedding new light on quadrupolar 1,4-dihydropyrrolo[3,2- <i>b</i> ]pyrroles: impact of electron-deficient scaffolds over emission

In this work, we disclose a series of seven quadrupolar centrosymmetric 1,4-dihydropyrrolo[3,2-<i>b</i>]pyrroles (DHPPs) linked to the two peripheral, strongly electron-accepting heterocycles such as benzoxadiazole, benzothiadiazole and benzoselenadiazole. This represents the first study probing the influence of electron-deficient heterocycles, rather that small electron-withdrawing substituents, on photophysics of DHPPs. These new acceptor-donor-acceptor hybrid dyes exhibit an appreciable combination of photophysical properties including absorption maxima in the range of 470-620 nm, and emission in the range of 500-720 nm with fluorescence quantum yields reaching 0.88. We discovered that the presence of two 7-nitro-benzoxadiazolyl substituents at positions 2 and 5 of DHPP core, evokes a strong fluorescence in non-polar solvents shifted to 639 nm. This is the most bathochromically shifted emission for quadrupolar, centrosymmetric chromophore bearing exclusively biaryl linkages. Interestingly, 1,2,4,5-tetraaryl-1,4-dihydropyrrolo[3,2-<i>b</i>]pyrrole (TAPP) possessing 4-benzothiadiazolyl groups is strongly emitting in the crystalline state (fluorescence quantum yield = 0.43). The combined photophysical and crystallographic studies point towards existence of intermolecular hydrogen bonds which modify the dihedral angles between the donor and acceptor moieties as a primary reason for this strong emission. Small structural alteration <i>via</i> the replacement of two 2,1,3-benzoxadiazole scaffolds with 2,1,3-benzoxadiazole-2-oxide moieties causes >10³ decrease in the fluorescence intensity. Computational studies point out to strong charge transfer originating from exceptionally large dihedral angles as the pivotal reason of this phenomenon. Although internal conversion originating from the charge-transfer state is the prevailing non-radiative deactivation mechanism, intersystem crossing also plays a role. The rational design of DHPPs that enables modulation of emission will advance their applicability.