Substituent-Controlled Excited-State Decay in Carboxyl-Functionalized BODIPY Dyes: A Two-Channel SF-TDDFT Study

Carboxyl-functionalized boron-dipyrromethene (BODIPY) dyes are promising fluorophores for bioimaging and bioconjugation because carboxylic-acid groups provide chemically addressable functional sites while retaining strong visible absorption and emission. Here, the excited-state electronic structures and decay mechanisms of a tetramethyl-BODIPY reference dye (CB-1) and four carboxyl-functionalized derivatives (CB-2–CB-5) were investigated using spin-flip time-dependent density functional theory (SF-TDDFT), charge-density-difference analysis, vibronic-coupling calculations, and potential-energy-surface analysis in toluene. Benchmark calculations showed that SF-TDDFT/B3LYP-D3 reproduced the experimental absorption ordering and yielded calculated absorption maxima within 17–23 nm of experiment. The lowest singlet excited states of all derivatives are dominated by HOMO→LUMO π→π∗ transitions and exhibit small electron–hole separation distances (D=0.063–0.108 Å), indicating predominantly locally excited rather than strongly charge-transfer character. CB-5, containing a thiophene substituent, displays the lowest excitation energy (2.24 eV) and the most red-shifted calculated absorption maximum (553 nm), arising from stabilization of the unoccupied orbital manifold and extension of π-conjugation. Radiative rates range from 1.59×108 to 1.97×108 s−1, whereas direct internal-conversion rates calculated from the relaxed S1 minima range from 0.0844×108 to 0.377×108 s−1. The representative MECI-access rates estimated for the NEB pathways are too small to compete with the calculated radiative and direct internal-conversion rates. The calculated fluorescence quantum yields are high (0.81–0.96) and reproduce the experimental fluorescence-efficiency order in toluene: CB-5 > CB-4 > CB-1 > CB-2 > CB-3. Within the singlet-state decay model examined here, the calculated fluorescence-yield differences are governed primarily by competition between radiative emission and direct vibronic internal conversion from the relaxed S1 minimum, whereas the representative MECI-access pathways identified by NEB are too slow to compete. Because intersystem crossing (ISC) rates and all possible crossing trajectories were not evaluated explicitly, this mechanistic interpretation is restricted to the decay channels considered in the present calculations.