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.

A Blueprint for Transforming Indigos to Photoresponsive Molecular Tools

“…In 2021, the de Melo group reported a comparative study between N-Boc-N'-H-indigos and N,N'-di-Boc-indigos and confirmed the existence of ESPT in the former class. [48] The authors noted the photodecomposition of N,N'-di-Boc-indigo in various solvents, as previously observed in the work by…”

“…These results can be explained by the resonance with the indigo carbonyl in competition with the resonance from the indigo nitrogen. In 2021, the de Melo group reported a comparative study between N ‐Boc‐ N′ ‐H‐indigos and N,N′ ‐di‐Boc‐indigos and confirmed the existence of ESPT in the former class [48] . The authors noted the photodecomposition of N,N′ ‐di‐Boc‐indigo in various solvents, as previously observed in the work by Głowacki and co‐workers [45c] …”

“…By combining it with a photobase generator under UV light, they could achieve in situ deprotection to form the parent indigo on a thin film surface. On the other hand, by replacing DMAP with a strong base, sodium hydride (NaH), de Melo and co‐workers were able to prepare mono‐ N ‐Boc‐indigo (as a mixture with the N,N′ ‐Boc‐indigo) [48] …”

“…On the other hand, by replacing DMAP with a strong base, sodium hydride (NaH), de Melo and co-workers were able to prepare mono-N-Boc-indigo (as a mixture with the N,N'-Boc-indigo). [48] A further application of N-acylindigos is to attach acyl substituents containing two α-hydrogens and a α-aryl group (Scheme 17). This structure is prone to undergo further condensation with the adjacent indigo carbonyl group to form an annulated structure, as shown by Engi in 1914 andPosner in 1924.…”

Excited state deactivation mechanisms in Shikonin rationalized from its naphthoquinone parent structures

“…3) accounts for the keto and enol emission of the naphthoquinone derivatives. Absence of concentration dependence of E2 emission intensity, allows us to rule out intermolecular proton transfer involving a dimer of this molecule, 22,26 and, together with the use of aprotic solvents, also discards solvent assisted ESIPT.…”