  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.
