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 work shows that terahertz time-domain spectroscopy (THz-TDS) can resolve discrete low-frequency vibrational modes in a benzoxazolium–coumarin donor–π–acceptor chromophore. By combining THz-TDS with gas-phase density functional theory (DFT), five principal IR-active modes are assigned to torsional and bridge-related motions that modulate intramolecular charge transfer (ICT). The study finds a uniform solid-state blue shift relative to gas-phase predictions, highlighting environment-induced stiffening, and reveals a weak unassigned feature likely due to condensed-phase effects. Together, these results establish THz-TDS as a sensitive probe of ICT-relevant dynamics in D–π–A systems and provide a framework for extending this approach to related chromophores and time-resolved studies of vibronically mediated charge transfer.

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.

Figures (5)

• Figure 1: Schematic of the home-built THz--TDS setup in transmission geometry.
• Figure 2: THz--TDS overview for the $\mathrm{BCO}^{+}$ pellet: (a) time-domain waveforms reference (blue dotted) vs. sample (red solid), (b) FFT spectra reference (blue dotted) vs. sample (red solid) defining the 0.43--2.51 $\mathrm{THz}$ analysis window, (c) magnitude of the complex transmission $|Q(\nu)|$, where the thick solid magenta curve shows the mean and the thin blue curves indicate the upper and lower uncertainty bounds at each frequency, and (d) unwrapped phase of $Q(\nu)$ with the thick green curve as the mean phase and thin red curves as the corresponding phase uncertainty bounds used to retrieve the complex refractive index.
• Figure 3: Retrieved complex refractive index for $\mathrm{BCO}^{+}$ from THz--TDS: (a) real refractive index $n(\nu)$, plotted as a thick solid blue curve for the mean spectrum with thin red curves indicating the upper and lower uncertainty bounds at each frequency, and (b) extinction coefficient $\kappa(\nu)$, plotted as a thick solid red curve for the mean spectrum with thin blue curves indicating the corresponding uncertainty bounds at each frequency, together forming error bands for both optical constants.
• Figure 4: Experimental extinction coefficient $\kappa(\nu)$ (blue solid) overlaid with the scaled DFT spectrum (red dashed).
• Figure 5: (a) UV--Vis absorption (blue solid) and steady-state fluorescence ($\lambda_{\mathrm{ex}}=530\,\mathrm{nm}$; red dashed) spectra of $\mathrm{BCO}^{+}$ in DMSO at 298 K. (b) HOMO--LUMO isosurfaces (isovalue $0.02$); the HOMO is localized on the donor with partial backbone delocalization, and the LUMO on the acceptor, consistent with D--$\pi$--A ICT.