Infrared-induced coherent vibration of a hydrogen-bonded system: Effects of mechanical and electrical anharmonic couplings

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.

Multidimensional Widefield Infrared-Encoded Spontaneous Emission Microscopy: Distinguishing Chromophores by Ultrashort Infrared Pulses

Photoluminescence (PL) imaging has broad applications in visualizing biological activities, detecting chemical species, and characterizing materials. However, the chemical information encoded in the PL images is often limited by the overlapping emission spectra of chromophores. Here, we report a PL microscopy based on the nonlinear interactions between mid-infrared and visible excitations on matters, which we termed MultiDimensional Widefield Infrared-encoded Spontaneous Emission (MD-WISE) microscopy. MD-WISE microscopy can distinguish chromophores that possess nearly identical emission spectra via conditions in a multidimensional space formed by three independent variables: the temporal delay between the infrared and the visible pulses (t), the wavelength of visible pulses (λvis), and the frequencies of the infrared pulses (ωIR). This method is enabled by two mechanisms: (1) modulating the optical absorption cross sections of molecular dyes by exciting specific vibrational functional groups and (2) reducing the PL quantum yield of semiconductor nanocrystals, which was achieved through strong field ionization of excitons. Importantly, MD-WISE microscopy operates under widefield imaging conditions with a field of view of tens of microns, other than the confocal configuration adopted by most nonlinear optical microscopies, which require focusing the optical beams tightly. By demonstrating the capacity of registering multidimensional information into PL images, MD-WISE microscopy has the potential of expanding the number of species and processes that can be simultaneously tracked in high-speed widefield imaging applications.

Exploring the Origins of Spectral Signatures of Strong Hydrogen Bonding in Protonated Water Clusters

The effects of anharmonicity on the spectral features of strong ionic hydrogen bonds are explored through reduced dimensional studies of the couplings between the hydrogen bonding OH and the donor-acceptor OO stretching vibrations in protonated water clusters with 2-4 water molecules. Specifically, this study focuses on how the anharmonicities and couplings in these ions are reflected in the vibrational spectra by exploring the intensities of the transitions to states with excitation in both the OH and the OO stretching vibrations and changes in the frequency of the OO stretching vibration when the OH stretching vibration is excited. These questions are addressed through the application of several approximate treatments that are based on an adiabatic separation of the high-frequency OH and low-frequency OO stretching vibrations as well as low-order expansions of the potential and dipole surfaces. While an adiabatic approximation captures most of the trends found in the spectra and from an analysis of the two-dimensional model, a vibrational Franck-Condon approach fails to capture the intensities of these transitions. Of the terms in the expansion of the dipole moment function, those that are proportional to Δr_OH and Δr_OH² are found to provide the largest contributions to the calculated intensities of the transitions involving excitation of both the OH and the OO stretches. This leads to the conclusion that the intensities of these transitions encode information about the frequency and anharmonicity of the OH stretching vibration and how they are affected by changes in the OO distance. The anharmonicity of the potential also leads to changes in the OO stretching frequency with excitation of the OH stretching vibration. The direction of this change in frequency encodes additional information about the strength of the ionic hydrogen bond.

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.

Polariton chemistry: controlling molecular dynamics with optical cavities

Molecular polaritons are the optical excitations which emerge when molecular transitions interact strongly with confined electromagnetic fields. Increasing interest in the hybrid molecular-photonic materials that host these excitations stems from recent observations of their novel and tunable chemistry. Some of the remarkable functionalities exhibited by polaritons include the ability to induce long-range excitation energy transfer, enhance charge conductivity, and inhibit or accelerate chemical reactions. In this review, we explain the effective theories of molecular polaritons which form a basis for the interpretation and guidance of experiments at the strong coupling limit. The theoretical discussion is illustrated with the analysis of innovative applications of strongly coupled molecular-photonic systems to chemical phenomena of fundamental importance to future technologies.

Electrical anharmonicity in hydrogen bonded systems: complete interpretation of the IR spectra of the Cl-H[combining right harpoon above] stretching band in the gaseous (CH3)2OHCl complex

Following the previous developments to simulate the fully infrared spectra of weak hydrogen bond systems within the linear response theory, an extension of the adiabatic model is presented here. A general formulation including the electrical anharmonicities in the calculation of the damped autocorrelation function of weak H-bonds is adopted to facilitate the support of the additional properties, and thus the IR spectra of the Cl-H[combining right harpoon above] stretching band in the gaseous (CH₃)₂OHCl complex. We have explored the origins of the broadening of the Cl-H[combining right harpoon above] stretching band. We found that the main features of the lineshape are attributed to electrical anharmonicity as a consequence of the large mixed second derivatives of the dipole moment with respect to the Cl-H[combining right harpoon above] bond and of the intermonomer elongations . In addition to providing more accurate theoretical band shapes, inclusion of the electrical anharmonicity in the present model paves the way for a more complete interpretation by generating three new Franck-Condon superposed distributions.

Two-dimensional vibrational-electronic spectroscopy

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (νCN) and either a ligand-to-metal charge transfer transition ([Fe(III)(CN)6](3-) dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific νCN modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems.

Measuring Coherently Coupled Intramolecular Vibrational and Charge-Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy

We demonstrate Fourier transform (FT) 2D vibrational-electronic (2D VE) spectroscopy employing a novel mid-IR and optical pulse sequence. This new femtosecond third-order nonlinear spectroscopy provides the high time and frequency resolutions of existing 2D FT techniques; however, resulting 2D VE spectra contain IR and electronic dipole moment cross terms. We use 2D VE spectroscopy to help understand the vibrational-electronic couplings in the cyanide-bridged transition-metal mixed valence complex [(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide. The amplitudes of the cross-peaks in the 2D VE spectra reveal that three of the intramolecular cyanide stretching vibrations lying along the charge-transfer axis are coherently coupled to the metal-to-metal charge-transfer electronic transition with differing strengths. Analysis of the 2D VE line shapes reveals positive and negative correlations of the cyanide stretching modes with the charge-transfer transition depending on the physical orientation of the vibration in the molecule and its interaction with the solvent. The insights found thus far into the vibronic couplings in the mixed valence model system indicate that the 2D VE technique will be a valuable addition to the existing multidimensional spectroscopy toolbox.

Excited-state intramolecular hydrogen transfer (ESIHT) of 1,8-dihydroxy-9,10-anthraquinone (DHAQ) characterized by ultrafast electronic and vibrational spectroscopy and computational modeling

We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and methanol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump-probe spectroscopic data. Upon photoabsorption at 400 nm, the S2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S2) and 1,10-quinone (S1) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1-50 ps time scale. Transient UV/vis pump-probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S2 and S1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S2/S1 relaxation to either the ground or triplet state.

Vibrational dynamics of acetate in D2O studied by infrared pump-probe spectroscopy

Solute-solvent interactions between acetate and D(2)O were investigated by vibrational spectroscopic methods. The vibrational dynamics of the COO asymmetric stretching mode in D(2)O was observed by time-resolved infrared (IR) pump-probe spectroscopy. The pump-probe signal contained both decay and oscillatory components. The time dependence of the decay component could be explained by a double exponential function with time constants of 200 fs and 2.6 ps, which are the same for both the COO asymmetric and symmetric stretching modes. The Fourier spectrum of the oscillatory component contained a band around 80 cm(-1), which suggests that the COO asymmetric stretching mode couples to a low-frequency vibrational mode with a wavenumber of 80 cm(-1). Based on quantum chemistry calculations, we propose that a bridged complex comprising an acetate ion and one D(2)O molecule, in which the two oxygen atoms in the acetate anion form hydrogen bonds with the two deuterium atoms in D(2)O, is the most stable structure. The 80 cm(-1) low-frequency mode was assigned to the asymmetric stretching vibration of the hydrogen bond in the bridged complex.

Coherent nuclear dynamics in ultrafast photoinduced structural change of bis(diimine)copper(I) complex

The photoinduced structural change of a prototype metal complex, [Cu(dmphen)(2)](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), was studied by ultrafast spectroscopy with time resolution as high as 30 fs. Time-resolved absorption measured with direct S(1) excitation clearly showed spectral changes attributable to the D(2d) (perpendicular) → D(2) (flattened) structural change occurring in the metal-to-ligand charge transfer singlet excited state ((1)MLCT) and the subsequent S(1) → T(1) intersystem crossing. It was confirmed that the two processes occur with time constants of ~0.8 ps (structural change) and ~10 ps (intersystem crossing), and their time scales are clearly well-separated. A distinct oscillation of the transient absorption signal was observed in the femtosecond region, which arises from the coherent nuclear motion of the perpendicular S(1) state that was directly generated by photoexcitation. This demonstrated that the perpendicular S(1) state has a well-defined vibrational structure and can vibrate within its subpicosecond lifetime. In other words, the S(1) state stays undistorted in a short period, and the coherent nuclear motion is maintained in this state. Time-dependent density functional theory (TDDFT) calculations gave consistent results, indicating a very flat feature and even a local minimum at the perpendicular structure on the S(1) potential energy surface. The vibrational assignments of the S(1) nuclear wavepacket motion were made on the basis of the TDDFT calculation. It was concluded that photoexcitation induces a(1) vibrations containing the Cu-ligand bond length change and a b(1) vibration attributed to the ligand-twisting motion that has the same symmetry as the flattening distortion. Ultrafast spectroscopy and complementary quantum chemical calculation provided an overall picture and new understanding of the photoinduced structural change of the prototypical metal complex.