Ultrafast Excited-state Intramolecular Proton Research Articles

Ultrafast excited state intramolecular proton transfer and isomerization of long-chain linked Schiff bases.

The ultrafast proton transfer and the following dynamics for aromatic Schiff bases N,N'-bis(salicylidene)ethylenediamine (salen) and N,N'-bis(salicylidene)-1,4-butylenediamine (salbn) were investigated with experimental and theoretical methods. A dual emission property with a large Stokes shift in salen and salbn indicates that excited state intramolecular proton transfer occurs with photoexcitation. An efficient single proton transfer was confirmed within 200fs for both molecules. Subsequently, a fast twisted motion of the keto moiety carries cis-keto to a relaxed stable geometry in the S1 state. Following the twisted motion, the phenol ring at keto moiety further rotates to a conical intersection with the ground state and a cis-trans isomerization occurs. The isomerization rate is high, which dominates the competition with the radiative transition, resulting in weak emission intensity. It is confirmed that the length of alkyl chain affects the direction of phenol ring twisting and rotation during the whole subsequent relaxation of excited cis-keto tautomer. Compared with polar solvent acetonitrile, the barrier of isomerization is higher and the hydrogen bond on keto moiety is stronger in nonpolar solvent toluene. It makes fluorescence radiation channels competing with isomerism more likely to occur, contributing to the observed difference of enol/keto emission ratios of salen and salbn in toluene and acetonitrile.

Exploring the excited state multi-proton transfer path and the associated photophysical properties of P-TNS molecule by DFT and TDDFT theory

Recently, Chen et al. designed and synthesized a new multi-purpose dye probe P-TNS with bifunctional groups, which has an ultra-fast excited state intramolecular proton transfer characteristic (ESIPT) (Spectrochim Acta A Mol Biomol Spectrosc., 2021, 262: 120084.). The P-TNS molecules are very sensitive to hydrazine (N2H4) and cyanide (CN-), which makes it have a good practical application prospect in the field of probe detection. However, the corresponding optical properties and the related mechanisms have not been systematically investigated in experiments. In the present work, the proton transfer pathways as well as the photophysical properties changes in the transfer path of P-TNS with three intramolecular hydrogen bonds (HB1, HB2 and HB3) are revealed for the first time. First, the analysis of potential energy surface and potential energy curve determined the stable conformation of P-TNS molecule and its isomer structure, and clarified the reaction mechanism of ESIPT process. Subsequently, the analysis of the frontier molecular orbitals reveal that the charge difference is the driving force required for the proton transfer process from the microscopic level, which led to the determination that the HB1 structure without the charge difference could not complete the ESIPT process. Finally, the variation of hydrogen bond strength and the path of proton transfer are demonstrated from several perspectives of hydrogen bond parameters, hydrogen bond energy, infrared vibrational frequency (IR) and reduced density gradient (RDG) surface. This study essentially explains the photophysical properties and proton transfer pathways of a new multipurpose dye probes for organic luminescence with bifunctional groups, which are important for the fields of spectroscopy and measurement technology, and also provides ideas for the design and synthesis of new fluorescent probes.

ESIPT and AIE characteristics of three Schiff base derivatives and the relevant photophysical mechanism analyses

Recently, three novel Schiff base derivatives (L1-3) were experimentally found to show water-induced aggregation-induced emission (AIE) and ultrafast excited state intramolecular proton transfer (ESIPT) characteristics, which have potential applications in fields of both optical material and fluoride ion detection. However, the corresponding optical properties have not been recognized completely and the relevant theoretical mechanism is lacking in analysis. In this work, based on the constructed relaxed potential energy curves, we investigated the stable configurations and clarified the proton transfer reaction mechanism for L1-3 with different substituents. The results show that the proton transfer associated with the interconversion from enol to keto isomer in S0 state is increasingly activated as substituted from electron-withdrawing group to electron-donating group, while the reverse trend is observed in S1 state. Above phenomenon can be ascribed to the change in strength of intramolecular hydrogen bond contributed to the reaction, which is demonstrated by hydrogen-bond parameters and interaction region indicator (IRI) analyses. Based on the optimized geometries, the single-fluorescence emission is found following the dual-absorption. The calculated fluorescence peaks (545 nm for L1, 536 nm for L2, 704 nm for L3) show little difference from those of experiment, which are reassigned to the ESIPT induced formation of keto isomer instead of enol form. In addition, the mechanism of water-induced AIE is clarified by increasing radiative rate and orbital overlap. To essentially comprehend the photophysical properties of organic luminogens with AIE and ESIPT characteristics is of fundamental interests in molecular electronics and photonics, and is also helpful to design new optical materials.

Sensing mechanism of fluorescent sensor to Cu2+ based on inhibiting ultra-fast intramolecular proton transfer process

A novel and efficient chemosensor 1 for detecting Cu2+ has recently been developed. However, the photophysical properties of chemosensor 1 and its response mechanism to Cu2+ are still unclear. Herein, the density functional theory and the time-dependent density functional theory approaches are implemented to investigate the excited state behavior of chemosensor 1 and its sensing mechanism for Cu2+ is revealed. Through constructing the potential energy curve with the dihedral angle of hydroxide radical as a variable, the irreversibility of the adjustment of the hydrogen proton direction is determined. This feature provides a favorable geometric configuration condition for the formation of intramolecular hydrogen bond. Moreover, the reduced density gradient analysis and topological analysis are performed to visualize the hydrogen bond strength, it is found that the hydrogen bond is enhanced in first singlet excited state (S1) compared with that in ground state (S0). The chemosensor 1 has only a low potential barrier in the S1 state, indicating that it could undergo an ultra-fast excited state intramolecular proton transfer (ESIPT) process. Furthermore, the reaction sites of chemosensor 1 and Cu2+ is theoretically predicted by the electrostatic potential analysis and the coordination mode of 1 + Cu2+-H+ is confirmed. Thus, we verify that the deprotonation inhibits the ESIPT behavior and leads to fluorescence quenching to achieve the recognition of chemosensor 1 to Cu2+. In addition, the binding energy of Cu2+ with chemosensor 1 is greater than that of Mg2+ and Zn2+, the high selectivity of chemosensor 1 to Cu2+ is illustrated. Our investigation clarifies the sensing mechanism of chemosensor 1 to Cu2+ based on inhibiting ultra-fast ESIPT process, which provides a theoretical basis for the development of new metal ion sensors.

Ultrafast excited state intramolecular proton transfer (ESIPT) mechanism for 2,6-bis(benzothiazolyl-2-yl)phenol: A theoretical investigation

We theoretically probe into photo-induced hydrogen bonding effects between S0 and S1 states as well as excited state intramolecular proton transfer (ESIPT) behavior for 2,6-bis(benzothiazolyl-2-yl)phenol (DBTP) system. By analyzing primary geometrical parameters involved in hydrogen bond, we confirm O36-H37···N35 of DBTP should be strengthened in the first excited state. By constructing potential energy surfaces for DBTP along with ESITP reactional orientation, we affirm that DBTP molecule undergoes the ultrafast ESIPT process. Once excited to the S1 state, DBTP quickly transfers its proton forming DBTP-PT tautomer. That is why no fluorescence of DBTP could be reported in experiment.

Theoretical insights into elaborating and regulating excited state dynamics for the novel 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,2a]pyridine system in polar and nonpolar solvents

ABSTRACTGiven the importance of excited state relaxation in photochemical and photophysical behaviours, in this work, we mainly focus on the excited state dynamical behaviours for the novel 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,2a]pyridine (6-CN-HPIP) system in different aprotic solvents. We find the ultrafast excited state intramolecular proton transfer (ESIPT) behaviour without potential energy barrier for 6-CN-HPIP in nonpolar solvents. Furthermore, we also explore the excited state intramolecular hydrogen bonding interactions and confirm the hydrogen bond O-HN of 6-CN-HPIP should be strengthened based photo-excitation process. It provides the possibility of ESIPT process. And the charge redistribution resulting from photo-excitation around hydrogen bonding moieties further reveals the ESIPT tendency for 6-CN-HPIP molecule. It is worth mentioning that the increased electronic densities around proton-acceptor moiety play important roles in attracting hydrogen proton, which directly promotes ESIPT reaction. Via constructing potential energy curves, we clarify the ESIPT dynamical process for 6-CN-HPIP system and present a novel mechanism that nonpolar aprotic solvents facilitate ESIPT behaviour for 6-CN-HPIP. We hope that novel applications and developments could be promoted for 6-CH-HPIP and its derivatives through regulating and controlling ESIPT behaviours in future.

Ultrafast excited state hydrogen atom transfer in salicylideneaniline driven by changes in aromaticity.

We investigated two important unresolved issues on excited state intramolecular proton transfer (ESIPT) reactions, i.e., their driving force and the charge state of the transferred species by means of quantum chemical topology. We related changes in the aromaticity of a molecule after electron excitation to reaction dynamics in an excited state. Additionally, we found that the conveyed particle has a charge intermediate between that of a bare proton and a neutral hydrogen atom. We anticipate that the analysis presented in this communication will yield valuable insights into ESIPT and other similar photochemical reactions.

Excited state intramolecular proton transfer and substituent effect of 10-hydroxybenzo[h]quinoline: A time-dependent density functional theory study

The excited state intramolecular proton transfer (ESIPT) and the substituent effect of 10-hydroxybenzo[h]quinoline (HBQ) compounds are investigated using the time-dependent density functional theory (TDDFT) method. With the spectra and potential energy curve calculations we have demonstrated the occurrence of an ultrafast excited state intramolecular proton transfer reaction in HBQ compounds. The HBQ-a and HBQ-b in the enol conformations can convert into the keto tautomers in the excited state S1. The significant Stokes shift of HBQ-a is observed, as large as 300nm, which is much larger than that of HBQ-b. The calculated molecular orbital gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for HBQ-a is 3.33eV, smaller than the 3.79eV for HBQ-b. The calculations demonstrate that it is much easier to take place the ESIPT reaction for HBQ-a than HBQ-b. The reaction mechanism of ESIPT is analyzed with theoretical potential energy curves and the ESIPT reaction energy barriers of 3.73kcal/mol for HBQ-a and 17.75kcal/mol for HBQ-b are obtained. These results clearly indicate that the substituent with electron-withdrawing groups in the hydrogen-accepting moiety in HBQ-a facilitate the proton transfer in the excited state.

The origin of ultrafast proton transfer: Multidimensional wave packet motion vs. tunneling

We investigate the reaction kinetics of ultrafast excited state intramolecular proton transfer (ESIPT) and discuss the possible origins of the process: tunneling of the reactive proton, vibrationally enhanced tunneling, and multidimensional wave packet dynamics of the entire system. Comparison of the measured kinetics for the protonated and the deuterated form of 2-(2′-hydroxyphenyl)benzothiazole (HBT) to numerical simulations allows us to ascribe the characteristic 50fs time found for the ESIPT solely to a ballistic wave packet motion along skeletal coordinates that mainly affect the donor acceptor distance. Tunneling is not found to be decisive.

Chemometric analysis of femtosecond transient absorption spectroscopy data: Study of the photochromism of anils

Chemometric methods are applied for the purpose of extracting relevant information from transient absorption spectroscopy data probing the photochromism of molecules from the family of salicylidene aniline. The process consists of an ultrafast excited state intramolecular proton transfer occurring from an enol form which is then followed by a cis-trans isomerization to finally reach a trans-keto photo-product. This work focuses on the potential of combining multivariate curve resolution for modeling pure profiles and two dimensional correlation spectroscopy data analysis for providing information on the dynamics of spectral features. The results obtained for one derivative of salicylidene aniline provide information regarding the number of species created after the proton transfer and characterization of their absorption spectra and their kinetics in the picosecond time scale. The spectral resolution of two cis-keto* forms is proposed for the first time. It is also found that both cis-keto* species are involved in the formation of the trans-keto photo-product. The main precursor of the trans-keto photo-product is the cis-keto* form which has the shortest characteristic time.

Ultrasensitive ultraviolet-visible 20fs absorption spectroscopy of low vapor pressure molecules in the gas phase

We describe an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution. The tunable UV pump and the visible (450-740 nm) probe pulses are generated by two independently tunable noncollinear optical parametric amplifiers, providing a temporal resolution of 20 fs. A homebuilt low gain photodetector is used to accommodate strong probe pulses with a shot noise significantly lower than the overall measurement noise. A matched digitizing scheme for single shot analysis of the light pulses at kilohertz repetition rates that minimizes the electronic noise contributions to the transient absorption signal is developed. The data processing scheme is optimized to yield best suppression of the laser excess noise and thereby transient absorbance changes down to 1.1 x 10(-6) can be resolved. A collinear focusing geometry optimized for a 50 mm interaction length combined with a heatable gas cell allows us to perform measurements on substances with low vapor pressures, e.g., on medium sized molecules which are crystalline at room temperature. As an application example highlighting the capability of this instrument, we present the direct time-domain observation of the ultrafast excited state intramolecular proton transfer of 2-(2(')-hydroxyphenyl)benzothiazole in the gas phase. We are able to compare the resulting dynamics in the gas phase and in solution with a temporal precision of better than 5 fs.

Spectroscopic and photophysical studies of the hydroquinone family of photochromic Schiff bases analyzed over a 17-orders-of-magnitude time scale

The hydroquinone family of photochromic Schiff bases has been studied by means of stationary and time-resolved spectroscopic absorption and emission techniques in the UV-Vis spectral range in the temporal range from 100 fs to 1 h. The studies have revealed that besides the ultrafast excited state intramolecular proton transfer reaction there is also another deactivation channel from the initially excited state. For the symmetric molecule with two intramolecular hydrogen bonds, the efficiency of the proton transfer reaction has been found to be at least ten times reduced when compared to that of the asymmetric molecule with one intramolecular hydrogen bond. The long-lived transient species absorbing in the UV range and coexisting with the photochrome have been observed in differently interacting solvents. Evidence for different conformers of almost all of the tautomers involved in the photochromic cycle has been also found.

Photo-physical properties of 1-hydroxy-2-naphthaldehyde: A combined fluorescence spectroscopy and quantum chemical calculations

The ground and excited state properties of 1-hydroxy-2-naphthaldehyde (HN12) towards proton transfer reaction have been elaborately investigated on the basis of steady state absorption and emission, time-resolved fluorescence spectroscopy and quantum chemical calculations by ab initio (Hartree–Fock) and density functional theory (DFT) methods. The molecule HN12 exists as closed (intramolecular hydrogen bonded) and open conformer and their anions, and the hydrogen bonded solvated cluster in the ground state. The closed conformer on excitation undergoes photo-induced keto-enol tautomerism across the pre-existing intramolecular hydrogen bond (IMHB) and shows a red shifted emission band for the keto tautomer. Theoretically a significant change of some structural parameters such as O d–H 1 bond length, O d⋯H 1⋯O a bond angle and charge distribution at the proton translocation site in the excited state favours intramolecular proton transfer process. Calculated potential energy curves along the proton transfer coordinate at the pre-existing hydrogen bonding site by DFT level account well for the feasibility of a barrierless ultrafast excited state intramolecular proton transfer reaction in HN12 molecule.

Enol-keto tautomerism of aromatic photochromic Schiff base N,N′-bis(salicylidene)-p-phenylenediamine: Ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale

A photochromic symmetric Schiff base, N,N'-bis(salicylidene)-p-phenylenediamine, is proposed as a probe for the study of solvent dependent enol-keto tautomerism in the ground and excited states. The ground state equilibrium between the enol-keto tautomers is found to depend mainly not on polarity but on the proton donating ability of the solvent. Upon selective excitation of each of these tautomers, the same excited state of a keto tautomer is created: in enol, after the ultrafast excited state intramolecular proton transfer (ESIPT), reaction, and in keto tautomer, directly. Then some part (<30%) of excited molecules are transferred to the photochromic form in its ground state. The evidence of another ultrafast deactivation channel in the excited enol tautomer competing with ESIPT has been found. The solvent does not influence the ESIPT dynamics nor the efficiency of the creation of the photochrome.

Keto–enol tautomerism of two structurally related Schiff bases: Direct and indirect way of creation of the excited keto tautomer

Femtosecond time-resolved absorption spectra of two structurally related, internally H-bonded Schiff bases are reported. The 2-hydroxynaphthylidene-1′-naphthylamine (HNAN) stable as an enol tautomer undergoes an ultrafast excited state intramolecular proton transfer, while the 2-hydroxynaphthylidene-(8′-aminoquinoline) (HNAQ), stable as a keto structure, reveals unusual relaxation routes after electronic excitation. In particular, the rise of the bleaching band with the characteristic time of ∼700 fs was found and attributed to a gradual population of the S 1 fluorescent state from a ‘hot’ excited state. The results accompanied by TDDFT calculations are used to construct the diagram of relaxation routes of an excited HNAQ molecule.

Direct observation of the nuclear motion during ultrafast intramolecular proton transfer

The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(2′-hydroxyphenyl)benzothiazole, 2-(2′-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV–visible pump-probe spectroscopy with 30 fs resolution. The proton transfer is observed in real time and a characteristic ‘ringing’ of the molecule in a small number of vibrational modes is found after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor–acceptor distance and an electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the acceptor site and tunneling has not to be considered.

An ultrafast excited state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA) and its “double” analogue salicylaldehyde azine (SAA). A controversial case

Two simple, structurally related photochromic Schiff bases, salicylideneaniline (SA) and salicylaldehyde azine (SAA) were studied in femto– and picosecond time domains. In both systems an ultrafast excited state intramolecular proton transfer (ESIPT) reaction was stated with the characteristic time below 50 fs. For SA this result is in contrast to the recent data published by Mitra and Tamai (S. Mitra and N. Tamai, Chem. Phys. Lett., 1998, 282, 391; S. Mitra and N. Tamai, Chem. Phys., 1999, 246, 463; S. Mitra and N. Tamai, Phys. Chem. Chem. Phys., 2003, 5, 4647), reporting on the corresponding time as long as 200–300 fs. The kinetics of decay of keto-tautomers in S1 states was followed by the transient absorption (410 nm and 470 nm for SA and SAA, respectively) and stimulated emission bands. About 10–30% of excited molecules give birth to the long-lived ground states of photochromic forms.

Intramolecular proton transfer in the first excited singlet state of 4-methyl-2,6-diformylphenol: effect of non-polar and weakly polar aprotic solvents

Steady state and transient emission studies reveal that an ultrafast excited state intramolecular proton transfer (ESIPT) reaction takes place in the first excited singlet state of 4-methyl-2,6-diformylphenol (MFOH). The large Stokes shifted emission orignates from the proton transferred form of the intramolecularly hydrogen-bonded closed conformer of MFOH in the S 0 state. From the solvent dependence of non-radiative decay rate constant k f nr, an enol tautomer is suggested for the fluorescing species. The change in luminescence at 77 K from yellow fluorescence to blue phosphorescence in n-hexane and chloroform is attributed due to the formation of open contormer. The T 1 state of MFOH in n-hexane is purely 3nπ ∗ in character but in chloroform it is 3ππ ∗.