Bond-Breaking/Bond-Forming Reactions by Vibrational Excitation: Infrared-Induced Bidirectional Tautomerization of Matrix-Isolated Thiotropolone

A Competition among Three Ultrafast Photoinduced Events in Thiotropolone: Internal Conversion vs Intersystem Crossing vs ESIPT

Gas phase excited-state quantum wavepacket dynamics simulations of the thiotropolone demonstrate the ultrafast triplet formation upon photoexcitation to the dipole-allowed S2 state. The dominant relaxation pathway of the S2-T4 intersystem crossing, facilitated by the strong spin-orbit coupling and narrow energy gap, competes with the S2 to S1/S3 internal conversion. The wavepacket populated in T4 via the former pathway decays to lower triplet states. Computed potential energy profiles suggest proton transfer via S2, which might compete with internal conversion and intersystem crossing. The nonadiabatic population transfer from the S2 to S1/S3 states enables proton transfer via the latter states, resulting in multiple proton transfer pathways. Experimental investigations are necessary to shed light on the complex ultrafast photodynamics of thiotropolone.

Hydrogen-Bonded Thiol Undergoes Unconventional Excited-State Intramolecular Proton-Transfer Reactions

The chapter on the thiol-related hydrogen bond (H-bond) and its excited-state intramolecular proton-transfer (ESIPT) reaction was recently opened where compound 4'-diethylamino-3-mercaptoflavone (3NTF) undergoes ESIPT in both cyclohexane solution and solid, giving a 710 nm tautomer emission with an anomalously large Stokes shift of 12,230 cm-1. Considering the thiol H-bond to be unconventional compared to the conventional Pauling-type -OH or -NH H-bond, it is thus essential and timely to probe its fundamental difference between their ESIPT. However, thiol-associated ESIPT tends to be nonemissive due to the dominant nπ* character of the tautomeric lowest excited state. Herein, based on the 3-mercaptoflavone scaffold and π-elongation concept, a new series of 4'-substituted-7-diethylamino-3-mercaptoflavones, NTFs, was designed and synthesized with varied H-bond strength and 690-720 nm tautomeric emission upon ultraviolet (UV) excitation in cyclohexane. The order of their H-bonding strength was experimentally determined to be N-NTF < O-NTF < H-NTF < F-NTF, while the rate of -SH ESIPT measured by fluorescence upconversion was F-NTF (398 fs)-1 < H-NTF (232 fs)-1 < O-NTF (123 fs)-1 < N-NTF (101 fs)-1 in toluene. Unexpectedly, the strongest H-bonded F-NTF gives the slowest ESIPT, which does not conform to the traditional ESIPT model. The results are rationalized by the trend of carbonyl oxygen basicity rather than -SH acidity. Namely, the thiol acidity relevant to the H-bond strength plays a minor role in the driving force of ESIPT. Instead, the proton-accepting strength governs ESIPT. That is to say, the noncanonical thiol H-bonding system undergoes an unconventional type of ESIPT.

Low-Temperature Infrared Spectra and Ultraviolet-Induced Rotamerization of 5-Chlorosalicylaldehyde

5-Chlorosalicylaldehyde (abbreviated as 5CSA) is an important chemical used in the synthesis of fragrances, dyes, and pharmaceuticals. In this investigation, 5CSA isolated in solid N₂, at 10 K, and in its neat amorphous and crystalline phases, at 50 and 190 K, respectively, were investigated by infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. The systematic theoretical analysis of the 5CSA conformational landscape showed that the compound exhibits four different conformers, which were structurally characterized in detail. In the as-deposited low-temperature matrices of 5CSA, only the most stable conformer, the intramolecularly hydrogen-bonded form I, was found. The same was observed in the case of the investigated low-temperature amorphous and crystalline phases of 5CSA. Conformer I was successfully converted into a higher-energy conformer(II), where both aldehyde and hydroxyl groups are rotated by 180° relative to their position in the initial conformer, through narrowband ultraviolet (UV) (λ = 308 nm) in situ irradiation of the as-deposited N₂ matrix of 5CSA. The infrared spectra of both matrix-isolated conformers, as well as those of the neat amorphous and crystalline phases of 5CSA, were assigned and interpreted in comparative terms, allowing us to elucidate structurally and vibrationally relevant effects of the main intra- and intermolecular interactions operating in the different studied phases. Very interestingly, the observed UV-induced I → II rotamerization was found to take place in an exclusive basis, with no other photochemical processes being observed to occur upon UV irradiation, under the experimental conditions used in the present investigation.

Photochromism of a Spiropyran in Low-Temperature Matrices: Unprecedented Bidirectional Switching between a Merocyanine and an Allene Intermediate

Photochromism of spiropyrans has attracted much attention due to its potential in many light-controlled system applications. However, several fundamental aspects regarding the structure, energetics, and mechanistic details of the transformations of spiropyrans are still not well understood. Here, we report the study of the photochromism of a 6-hydroxy-spiropyran (HBPS) under conditions of matrix isolation, where monomers of the compound are frozen in a solidified noble gas (krypton, at 15 K). The structure of the matrix-isolated HBPS was first elucidated by infrared (IR) spectroscopy supported by density functional theory computations. Then, the photochromism of HBPS, from the colorless spiropyran to the colored merocyanine, was induced by ultraviolet (UV) irradiation at 310 nm. The analysis of the IR spectrum of the photoproduced species revealed the exclusive formation of the most stable merocyanine MC-TTC stereoisomer. Subsequent visible-light (550 nm) irradiation of MC-TTC generated a new colorless allenic isomeric species ALN, where the UV irradiation (310 nm) of ALN was found to convert this species back to MC-TTC. This constitutes an unprecedented bidirectional transformation between a colored merocyanine and a colorless allene species. The newly observed photoswitching reaction (or photochromism) occurs along an intramolecular hydrogen bond existing in both merocyanine and allenic species, thus suggesting that it might be generally feasible in the chemistry of spiropyrans. On the other hand, the usual assumption that, as a general rule, merocyanines photochemically revert to spiropyrans is not supported in this work.

IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

In this article, IR-induced and tunneling-driven reactions observed in cryogenic matrices are described in a historical perspective, the entangling of the two types of processes being highlighted. The story of this still ongoing fascinating scientific endeavor is presented here following closely our own involvement in the field for more than 30 years, and thus focuses mostly on our work. It is, because of this reason, also an incomplete story. Nevertheless, it considers a large range of examples, from very selective IR-induced conformational isomerizations to IR-induced bond-breaking/bond-forming reactions and successful observations of rare heavy atom tunneling processes. As a whole, this article provides a rather general overview of the major progress achieved in the field.

Inducing molecular reactions by selective vibrational excitation of a remote antenna with near-infrared light

We demonstrate here that selective vibrational excitation of a moiety, remotely attached in relation to the molecular reaction site, might offer a generalized strategy for inducing bond-breaking/bond-forming reactions with exquisite precision. As a proof-of-principle, the electrocyclic ring-expansion of a benzazirine to a ketenimine was induced, in a cryogenic matrix, by near-IR light tuned at the overtone stretching frequency of its OH remote antenna. This accomplishment paves the way for harnessing IR vibrational excitation as a tool to guide a variety of molecular structure manipulations in an exceptional highly-selective manner.

Effect of a Solid-Hydrogen Environment on UV-Induced Hydrogen-Atom Transfer in Matrix-Isolated Heterocyclic Thione Compounds

To shed more light on the mechanisms of UV-induced hydrogen-atom-transfer processes in heterocyclic molecules, phototautomeric thione → thiol reactions were investigated for thione compounds isolated in low-temperature Ar as well as in n-H₂ (normal hydrogen) matrices. These studies concerned thione compounds with a five-membered heterocyclic ring and thione compounds with a six-membered heterocyclic ring. The experimental investigation of 2-thioimidazole and 3-thio-1,2,4-triazole (thione compounds with a five-membered heterocyclic ring) revealed that for the compounds isolated in solid n-H₂ only trace amounts of thiol photoproducts were photogenerated; even though for the same compounds isolated in the solid Ar matrix, the thione → thiol photoconversion was nearly total. In contrast to that, for 3-thiopyridazine and 2-thioquinoline (thione compounds with a six-membered heterocyclic ring) isolated in solid n-H₂, the UV-induced thione → thiol conversion occurred with the yield reaching 25–50% of the yield of the analogous process observed for the same species isolated in solid Ar. The obtained experimental results allow us to conclude that the dissociation–association mechanism nearly exclusively governs the phototransformation in thione heterocycles with high barriers for tautomerization (such as thione compounds with a five-membered ring), whereas the strictly intramolecular hydrogen-atom shift contributes to the mechanism of hydrogen-atom transfer in thione heterocycles with lower barriers (such as thione compounds with a six-membered ring).

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S₁ absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm^-1. Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible nπ* contribution to the S₁' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S₁' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

Evidence of IR-Induced Chemistry in a Neat Solid: Tautomerization of Thiotropolone by Thermal, Electronic, and Vibrational Excitations

Thiotropolone isolated in argon and xenon matrices (as monomers) or in a neat solid (as the crystalline or amorphous state) at low temperature was found to exist only in the thione-enol form. Visible light irradiation (λ ≥ 400 nm) leads to thione-enol → thiol-keto tautomerization in matrices and under neat solid conditions at 15 K. The assignment of the IR spectra of the two thiotropolone tautomers (thione-enol and thiol-keto) was carried out with the support of B3LYP/6-311+G(2d,p) computations. The thiol-keto form generated in situ in a neat solid was found to tautomerize back to the thione-enol upon annealing up to 100 K. Gaussian-4 (G4) computations estimate that such a tautomerization process has an energy barrier of ∼25 kJ mol^-1, which is consistent with the observations. Moreover, it was found that narrowband IR irradiation of the thiol-keto form in a neat solid, at the frequency of its CH stretching overtones/combination modes, also induces tautomerization to the thione-enol form. Such a result constitutes an important demonstration of vibrationally induced chemistry under neat solid conditions.

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

A new Schiff base compound, 3-(5-bromo-2-hydroxybenzylideneamino)phenol (abbreviated as BHAP) was synthesized and characterized by ¹H- and ¹³C- nuclear magnetic resonance and infrared spectroscopies. DFT/B3LYP/6-311++G(d,p) calculations were undertaken in order to explore the conformational space of both the E- and Z- geometrical isomers of the enol-imine and keto-amine tautomers of the compound. Optimized geometries and relative energies were obtained, and it was shown that the most stable species is the E-enol-imine form, which may exist in four low-energy intramolecularly hydrogen-bonded forms (I, II, V, and VI) that are almost isoenergetic. These conformers were concluded to exist in the gas phase equilibrium with nearly equal populations. On the other hand, the infrared spectra of the compound isolated in a cryogenic argon matrix (10 K) are compatible with the presence in the matrix of only two of these conformers (conformers II and V), while conformers I and VI convert to these ones by quantum mechanical tunneling through the barrier associated with the rotation of the OH phenolic group around the C–O bond. The matrix isolation infrared spectrum was then assigned and interpreted with help of the DFT(B3LYP)/6-311++G(d,p) calculated infrared spectra for conformers II and V. In addition, natural bond orbital (NBO) analysis was performed on the most stable conformer of the experimentally relevant isomeric form (E-enol-imino conformer V) to shed light on details of its electronic structure. This investigation stresses the fundamental structural relevance of the O–H···N intramolecular H-bond in o-hydroxyaryl Schiff base compounds.

Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide

As sulfur‐containing organic molecules thioamides and their isomers are conceivable intermediates in prebiotic chemistry, for example, in the formation of amino acids and thiazoles and resemble viable candidates for detection in interstellar media. Here, we report the characterization of parent thioformamide in the solid state via single‐crystal X‐ray diffraction and its photochemical interconversion to its hitherto unreported higher energy tautomer thiolimine in inert argon and dinitrogen matrices. Upon photogeneration, four conformers of thiolimine form, whose ratio depends on the employed wavelength. One of these conformers interconverts due to quantum mechanical tunneling with a half‐life of 30–45 min in both matrix materials at 3 and 20 K. A spontaneous reverse reaction from thiolimine to thioformamide is not observed. To support our experimental findings, we explored the potential energy surface of the system at the AE‐CCSD(T)/aug‐cc‐pCVTZ level of theory and computed tunneling half‐lives with the CVT/SCT approach applying DFT methods.