Contributions from Excited-State Proton and Electron Transfer to the Blinking and Photobleaching Dynamics of Alizarin and Purpurin

Abstract

The blinking and photobleaching dynamics of alizarin (1,2dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) are investigated using single-molecule spectroscopy. The time-dependent emission of alizarin and purpurin on glass under N 2 is analyzed using the change point detection (CPD) method to compile on-and off-event distributions. The number of distinct emissive events per molecule is about four times higher for alizarin relative to purpurin, consistent with an excited-state intramolecular proton transfer (ESIPT) process to populate an emissive tautomer state. To elucidate the mechanism for blinking (i.e., switching between on and off events), maximum likelihood estimation (MLE), goodness-of-fit tests based on the Kolmogorov−Smirnov (KS) statistic, and the loglikelihood ratio (LLR) tests are used to establish the best fits to the on-and offinterval probability distributions. For both alizarin and purpurin the on intervals are log-normally distributed, and off intervals are Weibull distributed, consistent with a dispersive electron-transfer (ET) kinetics model for blinking (i.e., involving Gaussian-like distributions of activation barriers to ET). Further analysis of the blinking dynamics reveals that ET to a long-lived dark state most often precedes molecular photobleaching, where extended residency in the dark state increases the probability of photobleaching. Based on these findings, mechanisms for the blinking and photobleaching of alizarin and purpurin are proposed. The ability of alizarin to undergo ESIPT enables fast excited-state decay and decreases the probability of ET. In contrast, purpurin exhibits faster injection and slower back ET relative to alizarin, leading to increased photobleaching via a dark radical cation state.

Visible‐light Homogeneous Photocatalytic Conversion of CO2 into CO in Aqueous Solutions with an Iron Catalyst

“…These results are also in accordance with recent electrochemical studies, which showed that Fe– p ‐TMA is one of the few molecular catalysts to keep its high activity towards CO 2 in aqueous solutions . One major reason for the catalytic activity leveling off is photosensitizer instability, as shown in Figure , where the main absorption peak of PP − (at 510 nm) starts decreasing after a couple of hours, reaching a 60 % lessening after 47 h. PP − photobleaching has otherwise been described . Adding more PP − to the solution mixture after two days of irradiation almost fully restored the catalytic activity (Figure ), with a TON of 120 for CO and retention of 95 % catalytic selectivity at the end of a four‐day experiment (Table , entry 6).…”

Photochemistry of Artists’ Dyes and Pigments: Towards Better Understanding and Prevention of Colour Change in Works of Art

“…Advances in the fundamental photophysical processes enabled to gain further insights into the stability of this dye by single-molecule spectroscopy. [69] Authors proposed that the existence of ad ark radical cation state formed by electron transfer (ET) would promote irreversible chemical transformation of the molecule when returning to the ground state, resulting in the colour fading. In the excited state there will be ak inetic competition between an unsafe ET deactivation pathway and as afe ESPT process that will decay with…”

A TD‐DFT investigation of the photo‐induced excited state intramolecular proton transfer dynamics for the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) system

“…Based on DFT//TDDFT/B3LYP/6‐31G(d) level in toluene solvent, we construct the PESs for the Ia system in both S 0 and S 1 states with fixing both O1―H2 and O4―H5 bond lengths. Herein, we want to mention that although previous work has reported that the TDDFT theoretical level might not provide the sufficiently accurate ordering of closely spaced excited states, theoretical results have shown the reliability of this method to offer qualitative energetic pathways for ESIPT behaviors . Specifically, the constructed PESs of both S 0 and S 1 states are through fixing O1―H2 and O4―H5 ranged from 0.9 to 2.1 Å in step of 0.1 Å (see Figure ).…”