Solvent Mediated Excited State Proton Transfer in Indigo Carmine

Abstract

Excited state proton transfer (ESPT) is thought to be responsible for the photostability of biological molecules, including DNA and proteins, and natural dyes such as indigo. However, the mechanistic role of the solvent interaction in driving ESPT is not well understood. Here, the electronic excited state deactivation dynamics of indigo carmine (InC) is mapped by visible pump−infrared probe and two-dimensional electronic−vibrational (2DEV) spectroscopy and complemented by electronic structure calculations. The observed dynamics reveal notable differences between InC in a protic solvent, D 2 O, and an aprotic solvent, deuterated dimethyl sulfoxide (dDMSO). Notably, an acceleration in the excited state decay is observed in D 2 O (<10 ps) compared to dDMSO (130 ps). Our data reveals clear evidence for ESPT in D 2 O accompanied by a significant change in dipole moment, which is found not to occur in dDMSO. We conclude that the ability of protic solvents to form intermolecular H-bonds with InC enables ESPT, which facilitates a rapid nonradiative S 1 → S 0 transition via the monoenol intermediate.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

“…Excited-state proton transfer (ESPT) to solvent serves as a model system for the ground-state reaction and therefore remains as an active topic in chemical sciences. [10][11][12][13][14][15] ESPT to solvent is oen modeled according to the two-step Eigen-Weller model presented in Scheme 1. 16,17 The rst step consists of a short-range proton transfer from an excited photoacid (ROH*) to solvent producing contact ion pairs (CIP*).…”

Deep in blue with green chemistry: influence of solvent and chain length on the behaviour of N- and N,N′- alkyl indigo derivatives

“…It is also worth noting that in the studied solvents intermolecular excited proton transfer (to the solvent) is precluded, a mechanism that has been proposed to exist with indigo carmine. 33,34 However, intermolecular (but involving two NC 1 Ind molecules), i.e. dimeric, species are predicted from TDDFT (see the discussion at the end of this section), which may explain the very fast ESPT in these compounds.…”

Synthesis and photophysical studies of an indigo derivative:N-octyl-7,7′-diazaindigo

“…In case of organic aprotic solvents, such as DMSO or DMF, the intramolecular H-bonds retain the molecular planarity, preventing the ESPT process. 44 Considering the previous discussion, the deactivation of the keto form of compounds 1 and 2 in DMF is expected to occur through an internal conversion (IC) process to the ground state, without passing by the enol form. DFT calculations were carried out to corroborate this assumption (vide infra).…”

“…Moreover, the negative signal in the 525-600 nm range is attributed to the ground state bleaching (GSB) and, The fast deactivation of the excited state of the highly soluble indigo carmine derivative has been studied in detail in previous articles. 43,44 The deactivation dynamics is strongly dependent on the aprotic/protic nature of the solvent with an excited state proton transfer (ESPT) as the main deactivation mechanism mediated by the formation of intermolecular H-bonds in protic solvents as H 2 O. In case of organic aprotic solvents, such as DMSO or DMF, the intramolecular H-bonds retain the molecular planarity, preventing the ESPT process.…”