Excited-State Proton Transfer in Indigo

Abstract

Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle point, which is enhanced by tunneling through the barrier. From the temperature dependence of the steady-state and time-resolved fluorescence data, the activation energy for the process was found to be ∼11 kJ mol for Ind and ∼5 kJ mol for NHxInd, in close agreement with the values calculated by TDDFT: 12.3 kJ mol (Ind) and 3.1 kJ mol (NHxInd). From time-resolved data, the rate constants for the ESPT process in dimethyl sulfoxide were found to be 9.24 × 10 s (Ind) and 7.12 × 10 s (NHxInd). The proximity between the two values suggests that the proton transfer mechanism in indigo is very similar to that found in NHxInd, where a single proton is involved. In addition, with NHxInd, the TDDFT calculations, together with the viscosity dependence of the fast component, and differences in the activation energy values between the steady-state and time-resolved data indicate that an additional nonradiative process is involved, which competes with ESPT. This is attributed to rotation about the central carbon-carbon bond, which brings the system close to a conical intersection (CI). The CI is of the sloped type, where the seam is reached through an OH stretching vibration.

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“…Indigo, or indigotin (2,2′-bis(2,3-dihydro-3-oxoindolyliden), is an organic dye which owes its blue colour to the presence of a doubly cross-conjugated system of two electron donor groups (the amino groups) and two electron acceptors groups (the carbonyl groups). In solution the molecule exhibits a positive solvatochromism (ranging from 603 nm in chloroform to 611 nm in dimethylformamide) [46,47], and in the solid state the absorption is further red-shifted (661 nm) as the result of the presence of extended intermolecular hydrogen bonding [34,[48][49][50], as shown in Figs. 1a and 2a.…”

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…Its structure was first reported by Baeyer in 1883 . Its enormous photostability due to efficient excited‐state proton transfer (ESPT) prevents photoisomerization . However, trans – cis photoisomerization becomes possible after N‐alkyl or N‐acyl substitution of the indigo system (Figure a).…”

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“…The vat dying redox chemistry foreshadows the reversible redox chemistry that indigoids can support, an important feature when considering application as an organic electronic component. A significant feature of indigoids is the presence of ultrafast photoinduced proton transfer in the exited state, resulting in rapid (<10 ps) deactivation of the excited state . Because of this, indigos have very low fluorescence and phosphorescence quantum yields on the order of 1 × 10 −3 .…”