Abstract
The singlet and triplet excited states of 4-hydroxybenzophenone (4BPOH) undergo deprotonation in the presence of water to produce the anionic ground-state, causing fluorescence quenching and photoactivity inhibition. The same process does not take place in an aprotic solvent such as acetonitrile. In acetonitrile, 4BPOH is fluorescent (interestingly, one of its fluorescence peaks overlaps with peak C of humic substances), it yields singlet oxygen upon irradiation and induces the triplet-sensitised transformation of phenol (with a rate constant of (6.6 ± 0.3) × 10(7) M(-1) s(-1) (μ ± σ) between phenol itself and a triplet 4BPOH). The 4BPOH shows an intermediate behaviour in a partially protic solvent such as 2-propanol, where some deprotonation of the excited states is observed. In acetonitrile/2-propanol mixtures (at least up to 50% of 2-propanol) there is also some evidence of alcohol oxidation by the 4BPOH triplet state, while the experimental data are silent concerning such a possibility in pure 2-propanol. Considering that benzophenones are important components of chromophoric dissolved organic matter (CDOM) in surface waters, the present findings could have significance for the photoactivity of the hydrophilic surface layers vs. the hydrophobic cores of CDOM.
Highlights
Benzophenones are important chromophores and photosensitisers present in chromophoric dissolved organic matter (CDOM) in natural waters, and in humic substances.[1,2]
A major feature of benzophenones is the fact that the energy levels of their first excited singlet (S1) and triplet (T1) states are very near. This issue, combined with the fact that their S1 and T1 are strongly coupled by spin–orbit interactions, favours the inter-system crossing (ISC) from S1 to T1 that has elevated quantum yields
The π–π* configurations of both S1 and T1 are associated with longer excited-state lifetimes, lower ISC quantum yields and reduced photochemical reactivity.[5,6,7,8,9]
Summary
Benzophenones are important chromophores and photosensitisers present in chromophoric dissolved organic matter (CDOM) in natural waters, and in humic substances.[1,2] Recent evidence has provided additional arguments in favour of an important role played by benzophenones in the optical and photochemical properties of CDOM and humic materials.[3,4]A major feature of benzophenones is the fact that the energy levels of their first excited singlet (S1) and triplet (T1) states are very near. The concentration of dissolved oxygen was assessed based on the published data of its solubility in acetonitrile.[44] From the plotted data (see Fig. ESI1†) one obtains a second-order reaction rate constant of (3.6 ± 0.1) × 109 M−1 s−1 between 34BPOH* and O2, with the probable formation of 1O2 that is typical of triplet-state reactivity.[45]
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Schedule a callMore From: Photochemical & photobiological sciences : Official journal of the European Photochemistry Association and the European Society for Photobiology
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