Terahertz Time-Domain Spectroscopy and Density Functional Theory Analysis of Low-Frequency Vibrational Modes of a Benzoxazolium-Coumarin Donor-pi-Acceptor Chromophore

TL;DR

This study combines terahertz spectroscopy and density functional theory to identify and analyze low-frequency vibrational modes in a donor-pi-acceptor chromophore, revealing their connection to intramolecular charge transfer motions.

Contribution

It demonstrates the effectiveness of THz-TDS combined with DFT in characterizing vibrational modes of complex organic molecules.

Findings

Distinct low-frequency modes identified at specific THz frequencies

DFT calculations agree well with experimental data

Vibrational modes linked to charge transfer motions

Abstract

To elucidate low-frequency vibrational modes, we investigate a benzoxazolium--coumarin (BCO+) donor-pi-acceptor derivative using transmission terahertz time-domain spectroscopy (THz-TDS). The retrieved complex refractive index reveals distinct modes at 0.62, 0.85, 1.30, 1.81, and 2.07 THz. Gas-phase density functional theory (DFT) agrees well with these features and enables assignment of specific intramolecular motions. Together, THz-TDS and DFT identify the characteristic low-frequency modes of BCO+ and suggest their connection to intramolecular charge transfer-relevant nuclear motions, highlighting that THz-TDS can serve as a sensitive probe of vibrational signatures in donor-pi-acceptor systems.