Absorption and Fluorescence Properties of Oligothiophene Biomarkers from Long-Range-Corrected Time-Dependent Density Functional Theory

TL;DR

This study uses long-range-corrected TDDFT to accurately predict absorption and fluorescence properties of oligothiophene biomarkers, highlighting the importance of distance-dependent exchange for excited-state calculations.

Contribution

It demonstrates the effectiveness of LC-TDDFT with an optimized range parameter in modeling charge-transfer excitations in oligothiophenes, outperforming conventional hybrid functionals.

Findings

LC-TDDFT accurately describes charge-transfer excitations.

Optimized 'mu' parameter improves excitation energy predictions.

Conventional hybrid functionals are less consistent for these properties.

Abstract

The absorption and fluorescence properties in a class of oligothiophene push-pull biomarkers are investigated with a long-range-corrected (LC) density functional method. Using linear response time-dependent density functional theory (TDDFT), we calculate excitation energies, fluorescence energies, oscillator strengths, and excited-state dipole moments. To benchmark and assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled cluster singles and doubles (CC2) calculations. When using a properly-optimized value of the range parameter, "mu", we find that the LC technique provides an accurate description of charge-transfer excitations as a function of biomarker size and chemical functionalization. In contrast, we find that re-optimizing the fraction of Hartree Fock exchange in conventional hybrid functionals still yields an inconsistent description of excitation energies and oscillator strengths for the two lowest excited states in our series of biomarkers. The results of the present study emphasize the importance of a distance-dependent contribution of exchange in TDDFT for investigating excited-state properties.