Pyrrolo[1,2-a]quinoxaline: A Combined Experimental and Computational Study on the Photophysical Properties of a New Photofunctional Building Block

We report a detailed photophysical exploration of medicinally important pyrrolo[1,2-a]quinoxalines. Pyrrolo[1,2-a]quinoxaline (QHH), 2,4-diphenylpyrrolo[1,2-a]quinoxaline (QPP), 2-phenyl-4-(thiophen-2-yl)pyrrolo[1,2-a]quinoxaline (QPT), and 4-phenyl-2-(thiophen-2-yl)pyrrolo[1,2-a]quinoxaline (QTP) were synthesized and studied using various photophysical techniques. Preliminary studies revealed the environmental responsiveness of these systems along with the involvement of aggregation-induced emission (AIE). Molecules QPP and QTP displayed significant fluorescence enhancement via AIE when compared to QHH and QPT. The fluorophores could be used for bioimaging with subcellular localization, specifically on the lysosomes. TDDFT studies revealed that both S0-S1 and S0-S2 transitions dominate the higher-wavelength peaks in the absorption spectra of the nonrigid PQNs. A complete charge delocalization was seen in the fluorescent S1 state of the rigid unsubstituted analogue. In contrast, in its 2,4-disubstituted analogues, electronic charge clouds on the benzene and thiophene rings adjacent to the fused pyrrole of the PQN moieties were absent. The S1 states of these latter nonrigid molecules undergo ISC with higher triplets (T2/T3) through further twisting along the C-C bonds, accompanied by high charge transfer with a 1.5- to 3.0-fold increase in the dipole moments. These triplets were identified as the phosphorescent states, while triplet T1 seems to be responsible for ROS generation, aided by aggregation. Computational results were validated by phosphorescence studies, which showed the lowest intensity for the completely unsubstituted QHH, while the highest intensity was observed in the case of QPP. Subsequent studies also revealed the ROS generation capability of these systems, as indicated by the computational results. This work establishes the versatility of the pyrrolo[1,2-a]quinoxaline core and puts forward a plausible mechanism responsible for its photophysical features.