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

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.

💡 Research Summary

This paper investigates the low‑frequency vibrational landscape of a prototypical donor‑π‑acceptor (D‑π‑A) chromophore, benzooxazolium‑coumarin (BCO⁺), by combining transmission terahertz time‑domain spectroscopy (THz‑TDS) with gas‑phase density functional theory (DFT) calculations. The experimental THz‑TDS setup employs a 790 nm, 70 fs Ti:sapphire regenerative amplifier to generate and detect THz pulses via a <110>-cut ZnTe crystal. A pressed pellet of BCO⁺ (thickness ≈ 459 µm) is measured in transmission under dry nitrogen, yielding time‑domain waveforms of ≈ 15 ps duration and a usable frequency window of 0.43–2.51 THz (≈ 66 GHz resolution). By applying a Fresnel transmission model to the complex transmission function Q(ν), the authors retrieve the complex refractive index n(ν)+iκ(ν). The extinction coefficient κ(ν) exhibits five distinct absorption peaks at 0.62, 0.85, 1.30, 1.81, and 2.07 THz, with a weaker, unassigned feature near 1.02 THz.

Parallel DFT calculations are performed with Gaussian 16 at the B3LYP‑D3/6‑311++G(d,p) level, including Grimme’s D3 dispersion correction. Geometry optimizations converge to true minima (no imaginary frequencies). Harmonic vibrational frequencies and IR intensities are computed, then uniformly scaled by a factor of 1.24 to account for systematic solid‑state hardening. After scaling, the calculated modes (PS₁–PS₅) align closely with the experimental peaks: PS₁ (0.62 THz) corresponds to inter‑ring torsion/libration about the D‑π‑A bridge; PS₂ (0.84 THz) is a ring libration with minor bridge involvement; PS₃ is a doublet (1.22 THz and 1.34 THz) representing coupled skeletal deformation and bridge twist, which merges experimentally into a single band at 1.30 THz; PS₄ (1.79 THz) involves localized bridge shear/ring deformation; PS₅ (2.07 THz) is a higher‑frequency coupled ring deformation. The calculated relative IR intensities match the observed peak strengths, confirming the assignments.

The authors discuss the relevance of these low‑frequency modes to intramolecular charge transfer (ICT). UV‑Vis absorption (λ_max = 535 nm) and fluorescence (λ_max = 620 nm) in DMSO reveal a large Stokes shift (≈ 85 nm, 0.318 eV), characteristic of a strongly ICT‑active D‑π‑A system. Frontier orbital analysis shows the HOMO localized on the coumarin donor and the LUMO on the benzooxazolium acceptor, confirming the push‑pull nature. The torsional and bridge‑twist motions (PS₁–PS₃) can transiently modulate the planarity of the D‑π‑A scaffold, altering orbital overlap and thus the ICT gap. Consequently, resonant excitation of these THz modes with intense THz pulses could dynamically control charge‑transfer efficiency, suggesting a pathway toward THz‑driven modulation of optoelectronic properties.

The weak, unassigned 1.02 THz feature is attributed to condensed‑phase effects such as symmetry breaking or weak intermolecular coupling that activate otherwise IR‑silent modes, highlighting the sensitivity of THz‑TDS to solid‑state interactions. Overall, the study demonstrates that THz‑TDS, when paired with accurate DFT modeling, provides a powerful probe of the subtle vibrational motions that underlie electronic communication in D‑π‑A molecules. The findings open avenues for designing THz‑responsive materials, sensors, and nonlinear optical devices where low‑frequency nuclear motions are harnessed to tune electronic functions.