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 λ_exc = 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 λ_exc = 440 nm leads, in addition to excimer formation, also to weak S₁ emission at 480 nm (t = 0.34 ns). Higher-energy excitation λ_exc < 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.