Red-Purple Photochromic Indigos from Green Chemistry: Mono-tBOC or Di-tBOC N-Substituted Indigos Displaying Excited State Proton Transfer or Photoisomerization

Abstract

In indigo, excited state proton transfer (ESPT) is known to be associated with the molecular mechanism responsible for highly efficient radiationless deactivation. When this route is blocked (partially or totally), new deactivation routes become available. Using new green chemistry procedures, with favorable green chemistry metrics, monosubstitution and disubstitution of N group(s) in indigo, by tert-butoxy carbonyl groups, N-(tert-butoxycarbonyl)indigo (NtBOCInd) and N,N'-(tert-butoxycarbonyl)indigo (N,N'tBOCInd), respectively, were synthetically accomplished. The compounds display red to purple colors depending on the solvent and substitution. Different excited-state deactivation pathways were observed and found to be structure- and solvent-dependent. Trans-cis photoisomerization was found to be absent with NtBOCInd and present with N,N'tBOCInd in nonpolar solvents. Time-resolved fluorescence experiments revealed single-exponential decays for the two compounds which, linked to time-dependent density functional theory (TDDFT) studies, show that with NtBOCInd ESPT is extremely fast and barrierless-predicted to be 1 kJ mol-1 in methylcyclohexane and 5 kJ mol-1 in dimethylsulfoxide-, which contrasts with ∼11 kJ mol-1 experimentally obtained for indigo. An alternative ESPT, competitive with the N-H···O═C intramolecular pathway, involving dimer units is also probed by TDDFT and found to be consistent with the experimentally observed time-resolved data. N,N'tBOCInd, where ESPT is precluded, shows solvent-dependent trans-cis/cis-trans photoisomerization and is surprisingly found to be more stable in the nonemissive cis conformation, whose deactivation to S0 is found to be solvent-dependent.