Effects of Liquid Crystal Environment on the Spectroscopic and Photophysical Properties of Well-Known Reacting Systems 2,3-Dimethylindole (DMI) and 9-Cyanoanthracene (9CNA)

Abstract

Electrochemical and steady-state and time-resolved spectroscopic studies on a disubstituted indole, 2,3-dimethylindole (DMI), and well-known electron acceptor 9-cyanoanthracene (9CNA) in liquid crystal (LC) 4-(n- pentyl)-4'-cyanobiphenyl (5CB) environment demonstrate entirely different spectroscopic and photophysical behaviors from those observed earlier by our research group with the same reacting systems in isotropic media n-heptane and acetonitrile (ACN). From the UV-vis absorption spectral measurements of the donor DMI in the presence of the acceptor 9CNA in liquid crystal medium (in 5CB) in various temperatures above the nematic-isotropic phase transition from 308 to 313 K (pseudo-ordered domain), it was observed that the lower energy lying absorption band of DMI situates in a longer wavelength region than the corresponding band observed in isotropic medium n-heptane or ACN. The possibility of the photochromic effect is discussed. In this band, the degree of mixing of the two closely spaced electronic states (1)L(a) (S(2)) and (1)L(b) (S(1)) of DMI was very prominent in the ordered LC environment (5CB) whereas in isotropic medium the dominant contribution for the formation of the lower energy band system primarily originates from the (1)Lb (S(1)) state, as evidenced from the steady-state polarization measurements. Both steady-state fluorescence quenching and time-resolved fluorescence studies clearly demonstrate in favor of the presence of only the static mode in LC environment. The situation differs in isotropic media where the dynamic process possesses the key role in the quenching mechanism. Expectedly, the transient absorption measurements by the nanosecond laser flash photolysis technique show a lack of formation of transient ionic species in the pseudo-ordered domain of 5CB. On the contrary, in isotropic solvents n-heptane and ACN, the transient absorption spectra measured by the same nanosecond laser flash photolysis technique exhibit the broad band of 9CNA radical anion at around 560 nm (9CNA-) and the band of neutral radical of DMI at 540 nm. It is inferred that the charge-separation reactions occurring within the present intermolecular systems could be stopped significantly by changing the nature of the environment from the isotropic to the LC's pseudo-ordered domain which situates closely above the nematic (N)-isotropic (I) phase transition temperature. From the steady-state and time-resolved investigations, it is revealed that, due to the hysteresis phenomenon, the nematic phase properties persist over a wide temperature range well within pseudo-ordered domain to some extent into the isotropic phases. The investigations with the different donor-acceptor inter- and intramolecular systems in 5CB and some other LC's environment are underway.