Photostability of 4,4′-Dihydroxythioindigo, a Mimetic of Indigo

Conical Intersections at Interfaces Revealed by Phase-Cycling Interface-Specific Two-Dimensional Electronic Spectroscopy (i2D-ES)

Conical intersections (CIs) hold significant stake in manipulating and controlling photochemical reaction pathways of molecules at interfaces and surfaces by affecting molecular dynamics therein. Currently, there is no tool for characterizing CIs at interfaces and surfaces. To this end, we have developed phase-cycling interface-specific two-dimensional electronic spectroscopy (i2D-ES) and combined it with advanced computational modeling to explore nonadiabatic CI dynamics of molecules at the air/water interface. Specifically, we integrated the phase locked pump pulse pair with an interface-specific electronic probe to obtain the two-dimensional interface-specific responses. We demonstrate that the nonadiabatic transitions of an interface-active azo dye molecule that occur through the CIs at the interface have different kinetic pathways from those in the bulk water. Upon photoexcitation, two CIs are present: one from an intersection of an optically active S2 state with a dark S1 state and the other from the intersection of the progressed S1 with the ground state S0. We find that the molecular conformations in the ground state are different for interfacial molecules. The interfacial molecules are intimately correlated with the locally populated excited state S2 being farther away from the CI region. This leads to slower nonadiabatic dynamics at the interface than in bulk water. Moreover, we show that the nonadiabatic transition from the S1 dark state to the ground state is significantly longer at the interface than that in the bulk, which is likely due to the orientationally restricted configuration of the excited state at the interface. Our findings suggest that orientational configurations of molecules manipulate reaction pathways at interfaces and surfaces.

Visible light-induced switching of soft matter materials properties based on thioindigo photoswitches

Thioindigos are visible light responsive photoswitches with excellent spatial control over the conformational change between their trans- and cis- isomers. However, they possess limited solubility in all conventional organic solvents and polymers, hindering their application in soft matter materials. Herein, we introduce a strategy for the covalent insertion of thioindigo units into polymer main chains, enabling thioindigos to function within crosslinked polymeric hydrogels. We overcome their solubility issue by developing a thioindigo bismethacrylate linker able to undergo radical initiated thiol-ene reaction for step-growth polymerization, generating indigo-containing polymers. The optimal wavelength for the reversible trans-/cis- isomerisation of thioindigo was elucidated by constructing a detailed photochemical action plot of their switching efficiencies at a wide range of monochromatic wavelengths. Critically, indigo-containing polymers display significant photoswitching of the materials' optical and physical properties in organic solvents and water. Furthermore, the photoswitching of thioindigo within crosslinked structures enables visible light induced modulation of the hydrogel stiffness. Both the thioindigo-containing hydrogels and photoswitching processes are non-toxic to cells, thus offering opportunities for advanced applications in soft matter materials and biology-related research.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Mechanistic Insights into the Photoisomerization of N,N′ ‐Disubstituted Indigos

N,N′‐disubstituted indigos are photoswitchable molecules that have recently caught the attention due to their addressability by red‐light. When alkyl and aryl groups are utilized as the N‐substituents, the thermal half‐lives of Z isomers can be tuned independently while maintaining the advantageous red‐shifted absorption spectra. To utilize these molecules in real‐world applications, it is of immense importance to understand how their molecular structures as well as the environment influence their switching properties. To this end, we probed their photoisomerization mechanism by carrying out photophysical and computational studies in solvents of different polarities. The fluorescence and transient absorption experiments suggest for more polar excited and transition states, which explains the bathochromic shifts of absorption spectra and shorter thermal half‐lives. On the other hand, the quantum chemical calculations reveal that in contrast to N‐carbonyl groups, N‐alkyl and N‐aryl substituents are not strongly conjugated with the indigo chromophore and can thus serve as a tool for tuning the thermal stability of Z isomers. Both approaches are combined to provide in‐depth understandings of how indigos undergo photoswitching as well as how they are influenced by N‐substituent and the chemical surroundings. These mechanistic insights will serve as guiding principles for designing molecules eyeing broader applications.

Synthesis of Bridged Indigos and Their Thermoisomerization and Photoisomerization Behaviors

Bridged indigos were synthesized by bridging the two nitrogen atoms in the indigo structure with a carbon chain, and their properties were carefully examined. These bridged indigos have intrinsic planar chirality, and the enantiomers were separated using chiral high-performance liquid chromatography. When the chiral bridged indigos were subjected to thermo- and photoisomerization, the corresponding (Z)-indigo was not observed at all, and racemization was observed. This phenomenon is caused by the low activation energy of inversion due to the 1.5 bond order of the double bond of the indigo skeleton and the large energy difference between the ground states of (E)-indigo and (Z)-indigo.

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

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.

Xylindein: Naturally Produced Fungal Compound for Sustainable (Opto)electronics

Organic semiconductors are of interest for (opto)electronic applications due to their low cost, solution processability, and tunable properties. Recently, natural product-derived organic pigments attracted attention due to their extraordinary environmental stability and unexpectedly good optoelectronic performance, in spite of only partially conjugated molecular structure. Fungi-derived pigments are a naturally sourced, sustainable class of materials that are largely unexplored as organic semiconductor materials. We present a study of the optical and electronic properties of a fungi-derived pigment xylindein, which is secreted by the wood-staining fungi Chlorociboria aeruginosa, and its blends with poly(methyl methacrylate) (PMMA) and crystalline nanocellulose (CNC). Optical absorption spectra of xylindein revealed the presence of two tautomers whose structures and properties were established using density functional theory. Pronounced pigment aggregation in polar solvents and in films, driven by intermolecular hydrogen bonding, was also observed. The pigment exhibited high photostability, electron mobility up to 0.4 cm2/(V s) in amorphous films, and thermally activated charge transport and photoresponse with activation energies of ∼0.3 and 0.2 eV, respectively. The dark and photocurrents in xylindein:PMMA blends were comparable to those in pristine xylindein film, whereas blends with CNC exhibited lower currents due to inhomogeneous distribution of xylindein in the CNC.

Macroscale Biomolecular Electronics and Ionics

The conduction of ions and electrons over multiple length scales is central to the processes that drive the biological world. The multidisciplinary attempts to elucidate the physics and chemistry of electron, proton, and ion transfer in biological charge transfer have focused primarily on the nano- and microscales. However, recently significant progress has been made on biomolecular materials that can support ion and electron currents over millimeters if not centimeters. Likewise, similar transport phenomena in organic semiconductors and ionics have led to new innovations in a wide variety of applications from energy generation and storage to displays and bioelectronics. Here, the underlying principles of conduction on the macroscale in biomolecular materials are discussed, highlighting recent examples, and particularly the establishment of accurate structure-property relationships to guide rationale material and device design. The technological viability of biomolecular electronics and ionics is also discussed.

Synthesis, Photophysical, Electrochemical, and Halochromic Properties of peri-Naphthoindigo

A facile synthesis of peri-naphthoindigo (PNI) was reported for the first time from simple precursor. Installation of a chromophore at the peri-position of naphthalene is very unique in terms of synthetic challenges and properties. PNI exists in monoenol form, undergoes halochromism in acidic medium, and displays a wide and strong absorption band (ε = 33390 M^-1cm^-1) with maxima at 632 nm (chloroform). The dye undergoes oxidation and reduction at +0.30 and -0.58 V (vs Fc/Fc⁺), respectively, in chloroform.

Indigoid Photoswitches: Visible Light Responsive Molecular Tools

Indigoid photoswitches comprise a class of chromophores that are derived from the parent and well-known indigo dye. Different from most photoswitches their core structures absorb in the visible region of the spectrum in both isomeric states even without substitutions, which makes them especially interesting for applications not tolerant of high-energy UV light. Also different from most current photoswitching systems, they provide highly rigid structures that undergo large yet precisely controllable geometry changes upon photoisomerization. The favorable combination of pronounced photochromism, fast and efficient photoreactions, and high thermal bistability have led to a strongly increased interest in indigoid photoswitches over the last years. As a result, intriguing applications of these chromophores as reversible triggering units in supramolecular and biological chemistry, the field of molecular machines, or smart molecules have been put forward. In this Account current developments in the synthesis, mechanistic understanding of light responsiveness, advantageous properties as phototools, and new applications of indigoid photoswitches are summarized with the focus on hemithioindigo, hemiindigo, and indigo as key examples. Many methods for the synthesis of hemithioindigos are known, but derivatives with a fourth substituent at the double bond could not easily be prepared because of the resulting increased steric hindrance in the products. Recent efforts in our laboratory have provided two different methods to prepare these highly promising photoswitches in very efficient ways. One method is especially designed for the introduction of sterically hindered ketones while the second one allows rapid structural diversification in only three high-yielding synthetic steps. Given the lesser prominence of indigoid photoswitches, mechanistic understanding of their excited state behavior and therefore rational design opportunities for photophysical properties are also much less developed compared to, for example, azobenzenes or stilbenes. By testing different substitution patterns, we were able to produce strongly beneficial property combinations in hemithioindigo, hemiindigo, or indigo photoswitches, for example, red-light responsiveness together with very high thermal bistability of the switching states. This is of particular importance for photopharmacological and biological applications of these switches to reduce the damage from high-energy light and to enable deep penetration of the light into tissues. An additional ground state twisting in hemithioindigo allowed us to control the type of light-induced bond rotation simply by the polarity of the solvent. With the aid of time-resolved spectroscopy and quantum yield measurements, we could show that in apolar cyclohexane exclusive double bond rotation takes place while in polar DMSO sole single bond rotation is observed. Such precise control over geometrical changes is of great interest for the construction of future sophisticated molecular machinery. In this field, we have introduced hemithioindigo photoswitches as novel core structure for molecular motors providing very fast directional motions upon irradiation with visible light. The mechanism of the directional rotation adheres to a four-step process, which could directly be observed in situ with a slower second-generation motor. Further applications of indigoid photoswitches were made in our laboratory in the realms of photocontrolled folding and host-guest chemistry as well as in molecular digital information processing showcasing the great versatility and enormous future promise of indigoid photoswitches.

Kristall- und Molekülstruktur von „Urindigo“ (4,4,4′,4′-Tetramethyl-2,2′-bipyrrolidinyliden-3,3′-dion) und die Strukturverwandtschaft zum Indigo

The crystal structure of 4,4,4′,4′-tetramethyl-2,2′-bipyrrolidinyliden-3,3′-dione (3) („urindigo“ = primordial indigo) was determined by X-ray diffraction. The molecule has thetrans-configuration as indigo itself. Within the parent grouptrans-(OC)(HN)C=C(CO)(NH) the distances and angles of3are very similar to that in indigo: C=C (136.6 pm), C–N (140.9 pm), C–C (145.6 pm), C=O (122.6 pm). The intramolecular N–H···O bonds in3and indigo are similar whereas the intermolecular hydrogen bonds in the former are substantially weaker. The latter observation finds its interesting parallel in the markedly smaller difference between the Vis absorptions of the gaseous and the solid primordial indigo in comparison to that of indigo. The present results support the early view by Lüttke and Klessinger that thetrans-(OC)(HN)C=C(CO)(NH) fragment is the parent chromophor of indigo dyes.

The photoformation of a phthalide: a ketene intermediate traced by FSRS

Femtosecond stimulated Raman spectroscopy, transient absorption and quantum chemistry are combined to unravel the complex path of phthalide photoformation.

Synthesis and photophysics of thioindigo diimines and related compounds

We report the synthesis and comprehensive characterization of diamine and diimine derivatives of the fluorescent compound thioindigo. Diamines 1 were obtained by metal-mediated amine condensation reaction with thioindigo. Oxidation of the products of the coupling reaction provided the diimines 2. X-ray crystal structures, cyclic voltammetry, and spectral and photophysical data of the compounds are presented. X-ray crystal structures demonstrate a planar structure for the diimine derivatives and a twisted conformation for the diamines. The diamine compounds 1 absorb in the UV (λmax 324-328 nm), significantly blue-shifted from the absorption spectrum of thioindigo. Diamines 1 exhibit moderate fluorescence (ΦF = 0.25, 0.045). A transient triplet state is observed in laser flash photolysis (LFP) experiments of 1, with lifetimes 1 order of magnitude longer than those of thioindigo. The diimine compounds 2 absorb at longer wavelengths (λmax 495-510 nm) than the diamines but are still slightly blue-shifted from thioindigo, with molar extinction coefficients 17-70% higher compared to thioindigo. The diimine compounds are not emissive, and LFP studies indicate transient species with microsecond lifetimes. Quenching experiments and transient absorption spectra are consistent with trans-cis isomerization.