Photoinduced charge transfer in push-pull pyrazoline-based chromophores - Relationship between molecular structure and photophysical, photovoltaic properties

Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D - π - A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán's and Laurence's four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.