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

Abstract

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.

Origin of Kinetic Dispersion in Eosin-Sensitized TiO2: Insights from Single-Molecule Spectroscopy

“…For example, the molecule shown in Figure 2d exhibits 10 distinct intensities, including six on and six off events and an υ event of 0.070 s −1 . The corresponding t on range from 150 ms to 23.43 s and t off values span from 70 ms to 131.02 s. The observation that the blinking dynamics of EY demonstrate multiple emissive intensities is consistent with prior single-molecule studies of xanthene and anthraquinone dyes on glass 50,57 and TiO 2 , 24,25 which has been attributed to dynamic fluctuations in excitation, emission, and/or non-radiative decay. Overall, the substrate-dependent emission behavior of EY is consistent with a blinking mechanism that involves injection to TiO 2 to populate a non-emissive radical cation state of the dye.…”

“…These t off are much longer than the reported T 1 lifetime of EY (i.e., 55 μs in water, 54 3.6 ms in polymethyl methacrylate, 55 and 1 ms on alumina 56 ), suggesting that another dark state is operative. The blinking dynamics of EY on glass, which has been attributed to electron transfer to trap states, 57 is quite differentwith molecules exhibiting longer t on and smaller υ event on glass as compared to TiO 2 . For example, the molecule shown in Figure 2d exhibits 10 distinct intensities, including six on and six off events and an υ event of 0.070 s −1 .…”

Blinking-Based Multiplexing: A New Approach for Differentiating Spectrally Overlapped Emitters

“…R6G and QD are commonly employed in single-molecule experiments and are known to exhibit, on average , qualitative differences in blinking. However, several studies have shown the blinking behavior of R6G and QD on glass to be quite dispersed (i.e., with single-emitter events ranging from milliseconds to >100 s in duration and blinking statistics that are lognormally and power-law distributed). − This photophysical heterogeneity, which originates from dispersive charge-transfer processes arising from both static and dynamic variations in structure and energetics of the emitter and its local environment, is sufficiently broad to be representative of a complex “real world” sample for this initial investigation. To evaluate the BBM concept, we quantify the blinking statistics of hundreds of R6G and QD emitters to develop a classification metric.…”

The Weyl Semimetals MIrTe4 (M = Nb, Ta) as Efficient Catalysts for Dye-sensitized Hydrogen Evolution

“…[1][2][3] In typical dye-sensitized HER, a dye photosensitizer and an electrocatalyst are employed together with a sacrificial electron donor. [4], [5] One of the most commonly used photosensitizer dyes for HER is Eosin Y (EY, 2',4',5',7'-tetrabromofluorescein), which undergoes photoexcitation on a comparatively fast timescale of ps-ns. [5] Next, (in case of reductive quenching of the excited state) the photosensitizer/dye accepts electrons from the sacrificial donor.…”

“…[4], [5] One of the most commonly used photosensitizer dyes for HER is Eosin Y (EY, 2',4',5',7'-tetrabromofluorescein), which undergoes photoexcitation on a comparatively fast timescale of ps-ns. [5] Next, (in case of reductive quenching of the excited state) the photosensitizer/dye accepts electrons from the sacrificial donor. [6][7][8][9] In the following step, a designated HER catalyst then transfers the photogenerated electron from the photosensitizer to the surface-adsorbed protons/hydrogen to generate H2 gas at longer timescales (~ms-s) [5,[10][11][12] and consecutively the photosensitizer/dye reverts back to the ground state.…”