Understanding of Intramolecular Charge Transfer Dynamics of a Push–Pull Dimethylamino-phenylethynylphenyl-dicyanoimidazole by Steady-State and Ultrafast Spectroscopic Studies
Haraprasad Mandal, J. Laxmikanth Rao, Jiří Kulhánek, Filip Bureš, Prakriti Ranjan Bangal
Abstract
Photophysical behaviors of D−π–A compound 2-{4-[4-( N, N -dimethylamino)phenylethynyl]phenyl-1-methyl-1 H -imidazole-4,5-dicarbonitrile (DMAPPIDCN) were explored using steady-state absorption, fluorescence emission, and femtosecond time-resolved absorption and emission spectroscopic techniques at room temperature along with computational time-dependent density functional theory (TD-DFT) calculation. The spectroscopic studies were carried out in different solvents of varying polarities including binary solvent mixtures. The role of the solvent polarity, viscosity, and temperature on the relaxation mechanism of DMAPPIDCN is disclosed. The observed steady-state and time-resolved spectroscopic features were attributed to intramolecular charge transfer (ICT) dynamics. The ICT in DMAPPIDCN is rationalized to a sequential twisted motion of both N(CH 3 ) 2 and whole N, N -dimethylaminophenyl moieties around the molecular axis interconnecting the adjacent imidazolephenyl moiety leading to the TICT 1 and TICT 2 (σ*) states. The increased solvent polarity affected mostly the fluorescence emission spectra pointing to a significant increase in the excited state dipole moment. This result clearly reveals formation of the TICT 2 (σ*) involving efficient charge transfer from the ( N, N -dimethylamino)phenyl (DMAP) donor to the phenyl-1-methyl-1 H -imidazole-dicarbonitrile (PIDCN) acceptor in the excited state in a polar environment. In the TICT 2 (σ*) state, the planes of electron-withdrawing and electron-donating moieties are perpendicular with the angle (DMAP)C–C≡C being 141.1°. This nonplanar arrangement accounts for the observed large Stokes shift. Time-resolved fluorescence spectroscopic studies unveil the excited state relaxation processes confirming the increase in the nonradiative decay rate in aprotic medium with increase in the solvent dielectric constants. Femtosecond transient spectroscopic studies unambiguously confirmed the existence of well separated LE and TICT states and their ensuing kinetics in polar medium. In nonpolar solvents, DMAPPIDCN shows strong fluorescence which emits from the LE (ππ*) state, whereas in polar solvents, formation of two consecutive TICT states occurs from the LE (ππ*) in a sub picosecond to few picosecond time domain depending on polarity of the solvents and the non-radiative decay from the TICT states.