Pyranine and N1,N1,N3,N3,N6,N6-hexaethyl-8-hydroxypyrene-1,3,6-trisulfonamide (EtHPTA-OH) with example of excited-state proton transfer.
The excitation of photoacids produces powerful proton donors that can trigger proton-coupled electron transfer, PCET, reactions. Coupled to time-resolved optical measurements, photoacids are poised to be a powerful tool for the study of PCET reaction mechanisms. However, to the knowledge of the authors, only one photoacid-initiated PCET reaction has been reported in the literature.
In work published in Physical Chemistry Chemical Physics, Christian Lennox and his collaborators in the Jillian Dempsey Group, describe a novel pyranine derivative, EtHPTA-OH, which was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of EtHPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of EtHPTA-OH was found to be highly photoacidic, pKa* = 8.74 in acetonitrile.
A series of aniline and pyridine bases were used to investigate excited-state proton transfer, ESPT, from singlet EtHPTA-OH, and rate constants for singlet quenching via ESPT were determined, kq = 5.18 × 109 to 1.05 × 1010 M−1 s−1. EtHPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet–triplet annihilation, TTA, process to reform singlet EtHPTA-OH on timescales of up to 80 μs. Detection of ESPT photoproducts on timescales comparable to that of TTA singlet regeneration provides strong evidence for photoacidic behavior stemming from the regenerated singlet EtHPTA-OH.
This exciting reactivity warrants investigation into new systems to able to harness it to leverage interesting and productive chemistry. Ideally, neutral sulfonamide pyranine derivatives such as EtHPTA-OH will prove useful in the study of photoacid-initiated PCET, an underexplored area of research that merits further investigation.