Excited-State Symmetry Breaking in a Multiple Multipolar Chromophore Probed by Single-Molecule Fluorescence Imaging and Spectroscopy

Excited-state symmetry breaking (ESB) has attracted much attention because it is often observed in symmetric multipolar chromophores designed as two-photon absorption/emission materials. Herein, we report an ensemble and single-molecule fluorescence imaging and spectroscopy investigation of ESB in hexakis[4-(p-dioctylaminostyryl)phenylethynyl]benzene(DB6), a two-photon absorber possessing a C₆-symmetric π-D₆ structure (π = hexaethynylbenzene, D = (p-dioctylaminostyryl)phenyl group) consisting of three equivalent D-π-D moieties. Ensemble and single-molecule measurements and theoretical calculations revealed that DB6 undergoes a photoabsorption process with two orthogonal transition dipole moments, whereas it fluoresces with a single transition dipole moment after one- or two-step ESB upon photoexcitation, depending on the environmental polarity. In nonpolar solvents and polymer films, one of the three D-π-D sites becomes planar, and the excited state is localized on this moiety: a [D^δ+-π^δ--D^δ+]* quadrupolar state is formed. In polar solvents, the symmetry is further broken within the planarized D-π-D moiety, and the excited state is localized on one of the two D-π sites; i.e., a D-[π^δ--D^δ+]* dipolar state is generated. Hence, DB6 can behave like a multichromophore with multiple emission sites in the molecule, which was demonstrated by stepwise photobleaching under photon antibunching conditions.

Unraveling the Excited-State Dynamics of Eosin Y Photosensitizers Using Single-Molecule Spectroscopy

The intersystem crossing and dispersive electron-transfer dynamics of eosin Y (EY) photosensitizers are probed using single-molecule microscopy. The blinking dynamics of EY on glass are quantified by constructing cumulative distribution functions of emissive ("on") and nonemissive ("off") events. Maximum likelihood estimation (MLE) and goodness-of-fit tests based on the Kolmogorov-Smirnov (KS) statistic are used to establish the best fit to the blinking data and differentiate among competitive photophysical processes. The on-time probability distributions for EY in N₂ and air are power-law distributed after ∼1 s, with fit parameters that are significantly modified upon exposure to oxygen. By extending the statistically principled MLE/KS approach to include an onset time for log-normal behavior, we demonstrate that the off-time distribution for EY in N₂ is best fit to a combination of exponential and log-normal functions. The corresponding distribution for EY in air is best fit to a log-normal function alone. Furthermore, power law and log-normal distributions are observed for an individual molecule in air, consistent with dynamic fluctuations in the rate constant for dark-state population and depopulation. These observations support the interpretation that dispersive electron transfer (i.e., the Albery model) from the first excited singlet state (S₁) of EY to trap states on glass is predominately responsible for blinking in oxic conditions. In anoxic environment, both triplet-state blinking and dispersive electron transfer from S₁ and the excited triplet state (T₁) contribute to the excited-state dynamics of EY.