Structural Control of the Photodynamics of Boron−Dipyrrin Complexes

Boron−dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward
aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved
optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low
fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%)
by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an
o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes
(∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60° with
respect to the dipyrrin framework whereas the angle is ∼80° for mesityl or o-tolyl groups. The calculated
potential energy surface for the phenyl-substituted complex indicates that the excited state has a second,
lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin,
which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative
probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap
factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex
bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability
involving conformations at or near the initial Franck−Condon form of the excited state. These combined
results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class
of widely used dyes.