A polymer rigidity probe based on ultralong organic room temperature phosphorescence of a new skeleton benzo[4,5]imidazo[1,2-a]pyridine

Herein, we report a new organic phosphorescence backbone, benzo[4,5]imidazo[1,2-a]pyridine (BNPy). BNPy-1 and BNPy-2 are obtained by introducing a benzene ring to each end of BNPy. BNPy itself shows no ultralong organic room temperature phosphorescence (UORTP) activity. Interestingly, for both BNPy-1 and BNPy-2, having one more benzene ring activates their UORTP properties notably. Firstly, photophysical properties of the three molecules were studied in solution and in pure powder state. At 77 K, BNPy-1 and BNPy-2 both demonstrated intense intrinsic phosphorescence with >460 ms and >670 ms ultralong lifetimes in solution, while BNPy showed weak intrinsic phosphorescence with <65 ms lifetimes, suggesting that having one more benzene ring has a significant impact on ultralong phosphorescence. Pure powders of BNPy-1 and BNPy-2 displayed red-shifted ultralong phosphorescence at 77 K, assigned to aggregate phosphorescence. Secondly, all three molecules were doped into a PMMA film and PVA film separately and the phosphorescence behavior of the doped polymer films was investigated. BNPy exhibited no phosphorescence in both films at room temperature; BNPy-1 and BNPy-2 showed intense yellow and green phosphorescence, respectively. TD-DFT calculations and SOC values showed that the BNPy skeleton possesses a high ISC efficiency but having one more benzene ring has not significantly enhanced the ISC efficiency. It is proposed that having one more benzene ring greatly reduces the non-irradiative rate via enlarging the size of BNPy-1 and BNPy-2. However, only the intrinsic phosphorescence of BNPy-1 can be activated by the powder matrix DMAP, indicating that charge transfer and charge recombination occur efficiently between BNPy-1 and DMAP. More interestingly, copolymerization of methyl acrylate (MA) and acrylic acid (AA) with BNPy-1/BNPy-2 enabled ultralong room temperature phosphorescence with a duration time of over 10 s. The phosphorescence intensity and lifetime greatly depended on the matrix rigidity of the AA/MA copolymers, which was determined by the weight ratio of AA. In particular, for BNPy-1, the changing trend of the phosphorescence lifetime with the AA ratio correlated well with the changing trend of the matrix T_g, which visually reflected the matrix rigidity. Thus, BNPy-1 can be used as an efficient polymer rigidity (or T_g) probe. It is noteworthy that BNPy-1 is a pure organic phosphorescence molecule, devoid of a naphthalene structure. We believe that this study will provide deep insight into the structure–performance relationship of organic phosphorescence systems.