Protoisomerization of Indigo and Isoindigo Dyes Confirmed by Gas-Phase Infrared Ion Spectroscopy

Abstract

Gas-phase infrared multiple-photon dissociation (IRMPD) spectra are recorded for the protonated dye molecules indigo and isoindigo by using a quadrupole ion trap (QIT) mass spectrometer coupled to the free electron laser for infrared experiments (FELIX). From their fingerprint IR spectra (600-1800 cm^-1) and comparison with quantum-chemical calculations at the density functional level of theory (B3LYP/6-31++G(d,p)), we derive their structures. We focus particularly on the question of whether trans-to-cis isomerization occurs upon protonation and transfer to the gas phase. The trans-configuration is energetically favored in the neutral forms of the dyes in solution and in the gas phase. Instead, the cis-isomer is lower in energy for the protonated forms of both species, but indigo is also notorious for not undergoing double-bond trans-to-cis isomerization, in contrast to many other conjugated systems. The IR spectra suggest that protoisomerization from trans to cis indeed occurs for both dyes. To estimate the extent of isomerization, on-resonance kinetics are measured on diagnostic and common vibrational frequencies to determine the ratio of cis-to-trans isomers. We find ratios of 65-70% cis and 30-35% trans for indigo versus 75-80% cis and 20-25% trans for isoindigo. Transition-state calculations for the isomerization reactions have been carried out, which indeed suggest a lower barrier for protonated isoindigo, qualitatively explaining the more efficient isomerization.

Scheme 1. Schematic Showing Neutral (a) trans-Indigo and (b) Isoindigo

Isoindigo is a structural isomer of indigo and has a 5-membered cyclic amide (γ-lactam) arrangement. Oxygen atoms are identical (O1 = O2) in both isomers. DFT optimized structures are also shown with important bond distances (in Å) indicated.

Scheme 2. Protonation on One of the Two Identical Oxygen Atoms in Indigo Allows for Mesomeric Structures Having a Single Bond Connecting the Two Subunits, Suggesting More Facile Trans-to-Cis Isomerization of the Protonated Species (Similar Resonance Structures Can Be Drawn for Isoindigo)

Figure 1

(a) Gas-phase IRMPD spectrum of protonated indigo (magenta trace) with band centers for the main experimental bands indicated. The experimental spectrum is compared with computed spectra for (b) the cis- and (c) trans-isomer. The gray trace in (a) represents the aggregate spectrum of cis- and trans-isomers assuming fractional populations as suggested by the kinetic measurements described in the text. Computed intensities in km mol^–1 refer to the stick spectra. The optimized structures along with their relative Gibbs energies are shown. Atomic distances shown are in Å.

Figure 2

(a) Experimental IRMPD spectrum of protonated isoindigo (magenta trace). (b) Overlay of the experimental spectrum with the calculated IR spectrum for the cis-isomer and (c) with that of the trans-isomer. The gray trace in (a) represents a composite computed spectrum with 77% cis and 23% trans, as suggested by the kinetic measurements described in the text. Optimized structures along with the relative Gibbs energies are shown as well as some relevant atomic distances (in Å).

Figure 3

(a) Photofragmentation decay of mass-isolated protonated isoindigo as a function of the number of IR FEL pulses at fixed frequencies diagnostic for the trans-isomer (1745 cm^–1, ●) and for the cis-isomer (1686 cm^–1, ★, ☆: replicate data). Data points indicated with ▷ were taken with the laser set at 763 cm^–1, where both isomers absorb leading to complete depletion of the precursor ion population. (b) Results of similar experiments for protonated indigo, performed at frequencies resonant with trans (1725 cm^–1, ●, ○: replicate), and with both cis + trans (1604 cm^–1, ★, ☆ replicate, and 745 cm^–1, ▷). Each data point is obtained from 10 averaged mass spectra. Solid curves are single-exponential decay fits to a limited range of the experimental data points, except for trans-isoindigo where all data points are included.

Figure 4

Computed TS barriers for trans-to-cis isomerization and optimized TS geometries of protonated isoindigo (a) and indigo (b). Values given for the TS barriers are in kJ mol^–1 relative to the trans-form of the protonated molecules. For the protonated systems, unlike for the neutral molecules, cis is the minimum-energy isomer. Although both cis-isomers are positioned at the same energy in the plot, protonated cis-isoindigo is 46.3 kJ mol^–1 more stable than protonated cis-indigo as a consequence of the stronger shared-proton interaction induced by the smaller OO distance (see structures in Figures 1 and 2).