Triplet-State Dissolved Organic Matter Quantum Yields and Lifetimes from Direct Observation of Aromatic Amine Oxidation

Abstract

Excited triplet state chromophoric dissolved organic matter (³CDOM*) is a short-lived mixture of excited-state species that plays important roles in aquatic photochemical processes. Unlike the study of the triplet states of well-defined molecules, which are amenable to transient absorbance spectroscopy, the study of ³CDOM* is hampered by it being a complex mixture and its low average intersystem crossing quantum yield (Φ_ISC). This study is an alternative approach to investigating ³CDOM* using transient absorption laser spectroscopy. The radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), formed through oxidation by ³CDOM*, was directly observable by transient absorption spectroscopy and was used to probe basic photophysical properties of ³CDOM*. Quenching and control experiments verified that TMPD^•+ was formed from ³CDOM* under anoxic conditions. Model triplet sensitizers with a wide range of excited triplet state reduction potentials and CDOM oxidized TMPD at near diffusion-controlled rates. This gives support to the idea that a large cross-section of ³CDOM* moieties are able to oxidize TMPD and that the complex mixture of ³CDOM* can be simplified to a single signal. Using the TMPD^•+ transient, the natural triplet lifetime and Φ_ISC for different DOM isolates and natural waters were quantified; values ranged from 12 to 26 μs and 4.1-7.8%, respectively.