Questioning the photophysical model for the indole chromophore in the light of evidence obtained by controlling the non-specific effect of the medium with 1-chlorobutane as solvent

Abstract

In this work, we substantiate the change in the emitting state of indole caused by the dipolarity increase in the solvent 1-chlorobutane, observed on lowering the temperature from 293 to 133 K, accompanied by no significant changes in the corresponding excitation and absorption spectra. No similar changes in indole emission were observed over the temperature range 293-133 K for solutions of indole in 2-methylbutane in the presence and absence of 0.5 M 1-chlorobutane. The solvatochromism of indole in 1-chlorobutane at temperatures from 293 to 133 K allowed us to estimate the dipole moment and polarizability of the emission state of the chromophore and to detect two states (S(1) and ): one, the S(1), involving no significant change and the other, the , exhibiting a substantial change in the dipole moment of the chromophore upon electronic excitation (viz. μ(S(1)) = 2.5 and vs. μ(S(0)) = 2.13 D). The former state, S(1), is the major contributor to the structured emission of indole at temperatures from 293 to 193 K, as is the latter, S'(1) , to its structureless, red-shifted emission over the range 193-133 K. Although the emission changes of indole, dissolved in 1-chlorobutane at temperatures from 293 to 133 K, are seemingly consistent with the widely accepted photophysical model for inversion of its (1)L(b) and (1)L(a) states as the polarity of the medium is increased, below 133 K the emission becomes structured and blue-shifted, two typical features of indole above 193 K. Also, below 123 K is not feasible to photo-select the (1)L(a) state in spite of this state being the first excited electronic state of indole under large dipolarity conditions. Therefore, the established photophysical model cannot hold under these conditions and a new one accounting for these experimental facts is proposed instead.

Fluorosolvatochromism of monomethyl indoles: further evidence in support of a new photophysical model for the indole chromophore

“…At this point, it is worth revisiting the fluorescence of indole in ClB (see Fig. 1 in the work of Catalán and Catalán). The fluorescence of indole at temperatures from 293 to 213 K in this solvent exhibits a clear‐cut peak with its 0–0 component at 290.4 nm; however, the peak disappears below 193 K and forms a shoulder similar to that in the fluorescence spectra of 1‐MeIndole in ClB (Fig.…”

“…When analyzing the behavior shown by 6‐MeIndole in ClB at the same temperatures, we conclude that not only their spectra displace hypsochromically their maximum by Δῦ = +684 cm −1 but also their vibronic structure increases significantly. This situation is similar to that present for indole in ClB, which shifts bathochromically its spectrum by Δῦ = +917 cm −1 . This spectral behavior of 6‐MeIndole and of Indole is immediately explained by the model of Eisinger and Navon, based on a strong rearrangement of some solvent molecules leading to an emission from 1 L a state that may be blocked, preventing that these solvent molecules move themselves responding to the strong dipolar demand of the compounds.…”

“…As can be seen, lowering the temperature of the solutions in 2MB leads to increasingly structured spectra exhibiting virtually no solvatochromic band shift but causes the bands in ClB to become essentially structureless and significantly redshifted. A comparison of the behavior of indole with that illustrated in Fig. (a) shows that the presence of a methyl group at position 3 also activates the fluorescence of the second electronic excited state in these compounds, which prevails below 293 K rather than below 193 K, as for indole.…”

“…For the reasons exposed, we think that an interpretation of this behavior based on the model of Eisinger and Navon should be discarded and the model recently proposed by us should be also open up to a possible structural change or to a possible aggregation of the compound.…”

“…Scheme shows five previously reported diagrams for the potential energy surfaces of the ground and the first two excited electronic states ( 1 L a and 1 L b ) of indole (i.e., those shaping its photophysical behavior), such as Scheme 1(a) of Weber, Scheme 1(b) of Konev et al , Scheme 1(c) of Eisinger and Navon, Scheme 1(d) of Park et al , and Scheme 1(e) of Catalán and Catalán; for a more detailed explanation of these models, see the work of Catalán and Catalán ref. .…”