To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives

Intramolecular Photoredox Reaction Mechanism of Naphthoquinone Compounds: Combined Time-Resolved Spectroscopies and DFT Calculations

Time-resolved spectroscopies and DFT calculations were utilized to investigate the photoredox mechanisms of naphthoquinone compounds. 5-Methoxy-8-tetrahydropyrane-1,4-naphthoquinone (NQ) and 2-methyl-3-(3-methylbut-2-en-1-yl) 1,4-naphthoquinone (MNQ) were excited to singlet excited species (labeled NQ(S₁) and MNQ(S₁), respectively). NQ(S₁) underwent intersystem crossing to produce a triplet NQ, which further underwent hydrogen atom transfer to form a biradical intermediate. The biradical underwent electron transfer to form a zwitterion, followed by cyclization and proton transfer to generate a photoproduct. MNQ(S₁) underwent a 1,4-proton transfer process to produce a quinone methide intermediate (1,3-QM) with zwitterionic character, which tautomerized to 1,2-QM. Then, 1,2-QM underwent electrocyclization. The substituent on the parent naphthoquinone is the key factor leading to the different reaction processes for NQ and MNQ.

Inside/Outside: Post-Synthetic Modification of the Zr-Benzophenonedicarboxylate Metal-Organic Framework

The Zr‐based metal–organic framework, Zr‐bzpdc‐MOF, contains the photoreactive linker molecule benzophenone‐4,4′‐dicarboxylate (bzpdc) which imparts the possibility for photochemical post‐synthetic modification. Upon irradiation with UV light, the keto group of the benzophenone moiety will react with nearly every C−H bond‐containing molecule. Within this paper, we further explore the photochemical reactivity of the Zr‐bzpdc‐MOF, especially with regard to which restrictions govern internal versus external reactions. We show that apart from reactions with C−H bond‐containing molecules, the MOF reacts also with water. By studying the reactivity versus linear alcohols we find a clear delineation in that shorter alcohol molecules (up to butanol as a borderline case) react with photoexcited keto groups throughout the whole crystals whereas longer ones react only with surface‐standing keto groups. In addition, we show that with the alkanes n‐butane to n‐octane, the reaction is restricted to the outer surface. We hypothesize that the reactivity of the Zr‐bzpdc‐MOF versus different reagents depends on the accessibility of the pore system which in turn depends mainly on the size of the reagents and on their polarity. The possibility to direct the post‐synthetic modification of the Zr‐bzpdc‐MOF (selective modification of the whole pore system versus surface modification) gives additional degrees of freedom in the design of this metal–organic framework for shaping and for applications.

Does the photoredox reaction affect the photorelease of anthraquinone protected benzaldehyde? A time-resolved spectroscopic study

Time-resolved spectroscopy studies coupled with the results from density functional theory (DFT) computations were utilized to unravel the photodeprotection reaction mechanism of Aqe-diol-PPG and the photoredox of photoproduct Aqe-diol.

Time-Resolved Spectroscopic Observation and Characterization of Water-Assisted Photoredox Reactions of Selected Aromatic Carbonyl Compounds

In recent years, unusual and efficient self-photoredox reactions were detected for selected benzophenone derivatives (BPs) and anthraquinone derivatives (AQs) in aqueous environments by Wan and co-workers, where the carbonyl undergoes reduction to the corresponding alcohol and a side-chain alcohol group undergoes oxidation to the corresponding carbonyl. To unravel the photoredox reaction mechanisms of these types of BPs and AQs in aqueous environments, we have utilized a combination of time-resolved spectroscopy techniques such as femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy to detect and characterize the electronic absorption and vibrational spectra of the intermediates and transient species from the femtosecond to microsecond time region after they are generated in the photoredox reactions. With the assistance of density functional theory calculations to simulate the electronic absorption and Raman spectra, the structural and kinetic information on the key reactive intermediates is described. Furthermore, the reaction pathways were calculated by finding the transition states connecting with the reactant and product complexes to better understand the photoredox reaction mechanism. In this Account, we summarize some of our time-resolved spectroscopic observations and characterization of water-assisted photoredox reactions of selected BPs and AQs. In the strong hydrogen-donor solvent isopropanol, the commonly studied photoreduction reaction for aromatic carbonyls via an intermolecular hydrogen atom tranfer process was observed for BPs and AQs. The photoredox reactions for the investigated BPs and AQs in aqueous environments occur on the triplet excited-state surface. Under moderately acidic aqueous conditions, the photoredox reactions for BPs and AQs are triggered by a proton transfer (PT) pathway. In neutral aqueous solutions, AQs may also undergo proton-coupled electron transfer (PCET) leading to the photoredox reaction, while BPs generate the ketyl radical species. Both BPs and AQs prefer the photohydration reaction in high-proton-concentration aqueous solutions (pH 0). The PT and PCET processes were found to offer more possibilities for the aromatic carbonyl compounds to lead to new photochemical reactions like the unusual photoredox reactions associated with BPs and AQs described here. Clear characterization of the photophysical pathways and the photochemical reactions of representative aromatic carbonyl compounds in aqueous environments not only provides fundamental information to better understand the photochemistry of carbonyl-containing compounds but also will facilitate the development of applications of these systems, like photochemical synthesis, drugs, and photolabile protecting groups. In addition, the importance of water molecules in the photochemical reactions of interest here may also lead to further understanding of how water influences the photochemistry of related carbonyl-containing compounds in aqueous environments.

Time-Resolved Spectroscopic Study on the Photoredox Reaction of 2-(p-Hydroxymethyl)phenylAnthraquinone

In this work, we report a combined time-resolved spectroscopic and density functional theory computational study on 2-(p-hydroxymethyl)phenylanthraquinone (PPAQ) in which the benzyl alcohol moiety is significantly farther away from the AQ ketone group compared to the compound 2-(1-hydroxyethyl) 9,10-anthraquinone (HEAQ) so as to investigate the photophysical and photochemical reactions of PPAQ in several solvents, especially for the photoredox reaction in a pH 2 aqueous solution. The results here indicate that PPAQ undergoes the photoredox reaction via a two-step pathway and that the low efficiency of the photoredox reaction of PPAQ compared to the related HEAQ molecule is caused by the longer distance between the benzyl alcohol moiety and the AQ ketone moieties.