Photoelectrical properties and excited-state dynamics of a nitrostilbene-functionalized organosiloxane liquid crystal

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2025-03, Vol.329, Article 125549
• Main Authors: Velayutham, Thamil Selvi, Khyasudeen, M. Faisal, Zahid, N. Idayu, Gopal, Sanjeev R., Ibrahim, A. Ramadan, Abou-Zied, Osama K.
• Format: Article
• Language: English
• Subjects: Nitrostilbene chromophore; Organosiloxane liquid crystal; Photoconductivity; Photophysical properties; Ultrafast dynamics; UV irradiation
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2024.125549

[Display omitted] •We studied the photoelectrical properties of a nitrostilbene siloxane liquid crystal.•Time-resolved spectroscopy revealed heterogeneous environment and aggregate formation.•UV exposure boosts SNS conductivity, especially in the Smectic C phase.•Dielectric studies reveal broad temperature relaxation beyond typical Goldstone mode.•Findings suggest possible applications in advanced photonics and electronic devices. This study investigates the photophysical and electrical properties of 4-heptamethyl-trisiloxanyl-n-undecyloxy-4′-nitrostilbene (SNS), a low molar mass organosiloxane liquid crystal containing a nitrostilbene chromophore. Real-time monitoring of the absorption and fluorescence spectra of the nitrostilbene moiety was conducted in dichloromethane solution and thin films (in both crystalline and Smectic C (SmC) phases). A charge transfer excited state is formed following vibrational cooling and solvent interaction within 0.4 to 1.6 ps, which then relaxes to the ground state in 1.6 ns. Five distinct lifetime components were identified in thin films, attributed to the heterogeneous local environment and aggregate formation. Notable differences in excited-state lifetimes were observed between the crystalline and SmC phases, with slower dynamics in the former due to the rigidity of the crystalline phase. Photoconductivity under UV irradiation was examined in SmC, Smectic A (SmA), and isotropic phases, showing a significant increase in current response, particularly in the SmC phase. Polarizing optical microscopy revealed morphological changes post-UV exposure, such as reduced SmC domain size and decreased birefringence. Dielectric measurements indicated distinct relaxation peaks in SmC and SmA phases, reflecting more disordered molecular arrangements. The study reveals that UV exposure significantly enhances SNS conductivity, particularly within the SmC phase. This enhancement is likely attributed to UV excitation promoting the formation of an intramolecular charge transfer state within the trans-stilbene moiety. These findings provide valuable insights into the potential applications of nitrostilbene-functionalized organosiloxane liquid crystals in optoelectronic devices, such as light switches and photodetectors.