Effect of peripherical groups on the two-photon brightness of (E)-3-benzyl-6-bromo-2-styryl-3H-imidazo[4,5-b]pyridine derivatives

•Study of withdrawing and donating groups on the two-photon brightness of imidazo[4,5-b]pyridines.•Two-photon absorption modeled by using sum-over-states approach.•Fluorescence emission triggered through two-photon excitation at 775 nm.•High two-photon absorption cross section (around 130 GM) of imidazo[4,5-b]pyridines derivatives.•Fluorescence quantum yields results displaying different behaviors according to the peripherical group. Here, a study of the effect of several electron-withdrawing (F, Br, CF3, Cl) and electron-donating (OMe, OTBDMS, CH3, S) on the two-photon brightness (2 PB) of nine imidazo[4,5-b]pyridine derivatives was developed. The two-photon absorption cross-section (2PACS) spectra were acquired by the tunable femtosecond Z-scan technique from 470 nm up to 700 nm, and hence the 2 PB was evaluated at the 2PACS peak (around 540 nm) and 700 nm. Transition dipole moment, the difference of state dipole moments, anisotropy coefficient, and fluorescence lifetime were figured out as a result of the Gaussian decomposition of one-photon absorption spectra, solvatochromism, fluorescence anisotropy, and Strickler Berg equation, respectively. Besides the photophysical analysis, the photophysical parameters were used as input parameters to phenomenologically model the 2PACS applying the Sum-Over-States approach regarding a three-level energy system. Results have shown a 2PACS peak of around 130 GM for all imidazo[4,5-b]pyridine derivatives, and values of 2PACS at 700 nm ranged from 4 up to 14 GM. Results show that for Br-π-electron withdrawing (Br-π-EW) molecular arrangement, higher values of 2 PB can be reached, mainly through the control of ϕ, which demonstrated to be more sensible than the 2PACS to the addition of different peripherical groups.