Competitive Relaxation Pathways of Dibenzophenanthroline Isomer Emission: Charge-Transfer, Excimer Formation.

The photophysical properties and excited-state dynamics of the nonlinear heteroacene derivative DBP6 (bis-phenylethynyl-substituted dibenzophenanthroline) were systematically investigated through steady-state spectroscopy, time-resolved fluorescence measurements, and density functional theory (DFT) calculations. This study focuses on the competitive relaxation pathways after excitation to the lowest excited band of phenanthroline, that is, charge-transfer (CT), and intramolecular benzene excimer formation, which govern its dual-emission behavior. After excitation of the lowest excited band of dibenzophenanthroline, with λexc = 500 nm DBP6 in DCM exhibits dominant excimer emission at 570 nm with a long-lived fluorescence lifetime of 12 ns. This emission is attributed to stable π-π interactions between side phenyl rings. In contrast, excitation λexc = 440 nm leads, in addition to excimer formation, also to weak S1 emission at 480 nm (t = 0.34 ns). Higher-energy excitation λexc < 370 nm reveals ultrafast CT state formation as an intermediate bridging S1 depopulation and excimer generation. Solvent polarity-dependent studies demonstrate a progressive red-shift in S1 emission (455 nm in toluene to 515 nm in ethanol) confirming CT state stabilization in polar environments. DFT simulations corroborate experimental results, predicting S1 emission at 495 nm. By varying excitation wavelength and solvent polarity, DBP6's dual emission can be tuned, highlighting its potential for ratiometric sensing, stable organic light-emitting diodes (OLEDs), and energy conversion systems. This work advances the understanding of nonlinear heteroacenes and provides a framework for designing optoelectronic materials with tailored excited-state interactions.