Linear and third-order non-linear spectroscopic investigations on novel dicyanodibenzodioxin dyes

A comprehensive spectroscopic and computational investigation reveals how intramolecular charge-transfer (ICT) dynamics and two-photon absorption (2PA) properties can be systematically tuned in a series of dicyanodibenzodioxin (D(CBO)) derivatives. Steady-state absorption and fluorescence spectroscopy, femtosecond transient absorption (TA), quantum-chemical calculations, and open-aperture Z-scan measurements were combined to establish clear structure-property relationships governing their nonlinear optical response. The molecular design strategy incorporated controlled variation of the donor-acceptor strength through substitution with electron-withdrawing (Br, -CN, -NO₂) and electron-donating (piperazine, N, N-dimethylpiperazine, pyrazine) groups to modulate the extent of charge delocalisation and ICT character. Steady-state absorption spectra exhibited insubstantial solvatochromism, whereas fluorescence spectra showed pronounced red shift with increasing solvent polarity, confirming the formation of dipolar ICT states in selected derivatives. Lippert-Mataga analysis revealed that the change in dipole moment (Δμ) between the ground and excited states governs excited-state stabilisation and correlates with variations in fluorescence lifetimes. Transient absorption measurements demonstrated polarity-dependent relaxation pathways and multi-exponential decay kinetics associated with ICT state formation. Time-dependent density functional theory (TD-DFT) calculations support the experimental observations, demonstrating substituent-dependent modulation of frontier molecular orbital localisation and excited-state charge redistribution. Open-aperture Z-scan experiments further established that the 2PA coefficients scale with the degree of charge localisation, with the nitro-substituted derivative 3d exhibiting the fastest excited-state dynamics and the highest nonlinear two-photon absorption coefficient. The findings elucidate critical structure-property correlations that enable rational tuning of ICT and NLO behaviour in D(CBO)-based π-conjugated systems, offering design principles for high-performance molecules for applications like bio-imaging and optoelectronics.