Ultrafast time-resolved study of photophysical processes involved in the photodeprotection of p-hydroxyphenacyl caged phototrigger compounds

A combined femtosecond Kerr gated time-resolved fluorescence (fs-KTRF) and picosecond Kerr gated time-resolved resonance Raman (ps-KTR(3)) study is reported for two p-hydroxyphenacyl (pHP) caged phototriggers, HPDP and HPA, in neat acetonitrile and water/acetonitrile (1:1 by volume) solvents. Fs-KTRF spectroscopy was employed to characterize the spectral properties and dynamics of the singlet excited states, and the ps-KTR(3) was used to monitor the formation and subsequent reaction of triplet state. These results provide important evidence for elucidation of the initial steps for the pHP deprotection mechanism. An improved fs-KTRF setup was developed to extend its detectable spectral range down to the 270 nm UV region while still covering the visible region up to 600 nm. This combined with the advantage of KTRF in directly monitoring the temporal evolution of the overall fluorescence profile enables the first time-resolved observation of dual fluorescence for pHP phototriggers upon 267 nm excitation. The two emitting components were assigned to originate from the (1)pipi (S(3)) and (1)npi (S(1)) states, respectively. This was based on the lifetime, the spectral location, and how these varied with the type of solvent. By correlating the dynamics of the singlet decay with the triplet formation, a direct (1)npi --> (3)pipi ISC mechanism was found for these compounds with the ISC rate estimated to be approximately 5 x 10(11) s(-)(1) in both solvent systems. These photophysical processes were found to be little affected by the kind of leaving group indicating the common local pHP chromophore is largely responsible for the fluorescence and relevant deactivation processes. The triplet lifetime was found to be approximately 420 and 2130 ps for HPDP and HPA, respectively, in the mixed solvent compared to 150 and 137 ns, respectively, in neat MeCN. The solvent and leaving group dependent quenching of the triplet is believed to be associated with the pHP deprotection photochemistry and indicates that the triplet is the reactive precursor for pHP photorelease reactions for the compounds examined in this study.     

A theoretical investigation of P-hydroxyphenacyl caged phototrigger compounds: how water induces the photodeprotection and subsequent rearrangement reactions

Complete active-space self-consistent field (CASSCF) calculations with a (14,11) active space and density functional theory calculations followed by Car-Parrinello molecular dynamic simulations are reported for the p-hydroxyphenacyl acetate, diethyl phosphate, and diphenyl phosphate phototrigger compounds. These calculations considered the explicit hydrogen bonding of water molecules to the phototrigger compound and help reveal the role of water in promoting the photodeprotection and subsequent rearrangement reactions for the p-hydroxyphenacyl caged phototrigger compounds experimentally observed in the presence of appreciable amounts of water but not observed in neat nonproton solvents like acetonitrile. The 267 nm excitation of the phototrigger compounds leads to an instantaneous population of the S3(1pipi*) state Franck-Condon region, which is followed by an internal conversion deactivation route to the S1(1npi*) state via a 1pipi*/1npi* vibronic coupling. The shorter lifetime of the S1(1npi*) state (approximately 1 ps) starting from the FC geometry is terminated by a fast intersystem crossing at a 3pipi*/3npi* intersection with a structure of mixed pipi*/npi* excitation in the triplet state. The deprotection reaction is triggered by a proton (or hydrogen atom) transfer assisted by water bridges and emanates from this pipi*/npi* triplet state intersection. With the departure of the leaving group, the reaction evolves into a water-mediated post-deprotection phase where the spin inversion of pQM (X, 3A) leads to a spiroketone in the ground state by a cyclization process that is followed by an attack of water to produce a 1,1'-di-hydroxyl-spiroketone. Finally, the H atom of the hydroxyl in 1,1'-di-hydroxyl-spiroketon transfers back to the p-O atom aided by water molecules to generate the p-hydroxyphenyl-acetic acid final rearrangement product.     

含氮杂环化合物的固相光化学

本文研究了含氮杂环化合物同尿嘧啶醛、芳香醛酮、稠环化合物的固相光化学反应, 得到与液相光化学反应不同的结果。合成了十九种新化合物, 它们的结构由IR, ^1H NMR, MS和元素分析确定。用光谱方法表征了混晶的性质。X射线粉未衍射和差热分析表明, 吲哚同萘, 咔唑同蒽的混晶, 形成了分子化合物。对反应机理进行了初步探讨。     

Photo-excited intramolecular charge transfer and excited state intramolecular proton transfer behaviors for HPIBT system: A theoretical investigation

Given facile synthetic route and excellent photo stability, excited state intramolecular proton transfer (ESIPT)-active luminous materials have gained more and more attention. Here, we focus on photo-induced excitation process and the ESIPT reaction process for the novel 5-benzothiazol-2-yl-6-hydroxy-2-methyl-isoindole-1,3-dione (HPIBT) molecule. On the level of chemical geometries and infrared spectra, we verify that O─H⋯N of HPIBT should be enhanced. We find that a proton is likely to be attracted by enhanced electronic densities around N, that is, charge transfer impetus ESIPT trend. Combing potential energy curves and searching for transition state, we clarify the ultrafast ESIPT mechanism of HPIBT due to a low barrier, which legitimately explains previous experimental characteristics.     

Time-resolved resonance Raman study of the triplet states of p-hydroxyacetophenone and the p-hydroxyphenacyl diethyl phosphate phototrigger compound

Pico- and nanosecond time-resolved resonance Raman (TR3) spectroscopy have been utilized to study the dynamics and structure of p-hydroxyacetophenone (HA) and the p-hydroxyphenacyl-caged phototrigger compound p-hydroxyphenacyl diethyl phosphate (HPDP) in acetonitrile solution. Transient intermediates were detected and attributed to the triplet states of HA and HPDP. Nanosecond-TR3 measurements were done for two isotopically substituted HA molecules to help better assign the triplet state carbonyl C=O stretching and the ring related vibrational modes. The dynamics of formation and the spectral characteristics for the triplet states were found to be similar for the HA and HPDP. The temporal evolution at very early picosecond time scale indicates there is rapid intersystem crossing (ISC) conversion and subsequent relaxation of the excess energy of the initially produced energetic triplet state. B3LYP/6-311G** density functional theory (DFT) calculations were done to determine the structures and vibrational frequencies for both the triplet and ground states of HA and HPDP. The calculated spectra reproduce the experimental spectra and the observed isotopic shifts reasonably well and were used to make tentative assignments to all the experimentally observed features. The triplet states were found to have extensive conjugated pipi* nature with a single-bond-like carbonyl CO bond. We briefly compare the triplet structure and formation dynamics of HA and HPDP as well as the conformational changes upon going from the ground state to the triplet state. We discuss our present results in relation to the initial pathway for the p-hydroxyphenacyl photodeprotection process. We also compare and discuss the properties of the HA pipi* triplet state relative to the published results of other aromatic carbonyl compounds.     

A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system

Given the tremendous potential applications of excited state intramolecular proton transfer (ESIPT) systems, ESIPT molecules have received widespread attention. In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically study the excited state dynamical behaviors of salicyladazine (SA) molecules. Our simulated results show that the double intramolecular hydrogen bonds of SA are strengthened in the S₁ state via exploring bond distances, bond angles, and infrared (IR) vibrational spectra. Exploring the frontier molecular orbitals (MOs), we confirm that charge redistributions indeed have effects on excited state dynamical behaviors. The increased electronic densities on N atoms and the decreased electronic densities on O atoms imply that charge redistribution may trigger the ESPT process. Analyzing the constructed S₀‐state and S₁‐state potential energy surfaces (PESs), we confirm that only the excited state single proton transfer reaction can occur although SA possesses two intramolecular hydrogen bonds. In this work, we clarify the specific ESIPT mechanism, which may facilitate developing novel applications based on the SA system in future.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

A theoretical study on the acid catalysed hydration of excited state acetylene

That significant modification in the acid/base behavior of aromatic molecules can be induced by electronic excitation is common knowledge. A recent application of this phenomenon is the acid catalyzed photohydration of aromatic acetylenes: ArCCH. The energetics of proton transfer and subsequent hydration of acetylene, as indicated by ab initio MO methods, suggests that this property of enhanced excited state basicity is not uniquely characteristic of the Ar substituent.     

A theoretical and experimental investigation of the source-sample-detector geometry for an annular type radioisotope excited XRF spectrometer

In this study, the dependence of the characteristic X-ray intensities on the counting geometry has been investigated for a radioisotope excited XRF spectrometer. The collimation factor for the source-sample-detector geometry, which was prepared for an annular type¹⁰⁹Cd radioisotope source, has been determined both theoretically and experimentally. The discrepancy between the theoretical and experimental results is discussed in terms of possible sources of errors.     

A theoretical investigation of the excited states of the H2O2 molecule

Ab initio calculations for a series of excited states of the H₂O₂ molecule have been performed in order to discuss some recent photofragmentation experiments with laser light. For understanding these experiments the direction of the transition moments is of great interest, as well as the possibility of a HOO…H fragmentation.     

A theoretical study on the ground and excited state behaviors of TTBC related carbocyanine dyes

The effects of functional groups on the benzimidazole rings, length of the conjugated chain and alkyl groups bonded to the nitrogen atoms on the ground and excited state behaviors of the 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (TTBC or JC-1) have been analyzed via quantum chemical methods. DFT and TDDFT with B3LYP/6-31G(d,p) level of theory were used for the ground and excited state calculations, respectively. It has been found that TTBC has a very rigid geometry; no significant effect of functional groups has been predicted either as donor or acceptor on its optimum structure. However, the length of alkyl groups changes the structure of the molecule. It is possible to increase λ_max of TTBC based carbocyanine dye with NH₂, butyl/propyl and increasing polymethine chain length.     

Triplet excited state characters and photosensitization mechanisms of alpha-terthienyl: a theoretical study

The triplet excited (T(1)) state characters of alpha-terthienyl (alpha-T) have been investigated using density functional theory calculations, based on which, its photosensitization mechanisms were explored. Primarily, the direct oxidation to the DNA bases by the T(1) state alpha-T through the electron transfer is not thermodynamically feasible. Secondly, 1O2 can be photogenerated both in benzene and water through the direct energy transfer from the T(1) state alpha-T to 3O2, while O2(.-) can only be formed in water through the electron transfer from the T(1) state alpha-T or alpha-T(-) to 3O2.     

A global investigation of excited state surfaces within time-dependent density-functional response theory

This work investigates the capability of time-dependent density functional response theory to describe excited state potential energy surfaces of conjugated organic molecules. Applications to linear polyenes, aromatic systems, and the protonated Schiff base of retinal demonstrate the scope of currently used exchange-correlation functionals as local, adiabatic approximations to time-dependent Kohn-Sham theory. The results are compared to experimental and ab initio data of various kinds to attain a critical analysis of common problems concerning charge transfer and long range (nondynamic) correlation effects. This analysis goes beyond a local investigation of electronic properties and incorporates a global view of the excited state potential energy surfaces.     

The photochemistry of lipoic acid: photoionization and observation of a triplet excited state of a disulfide.

Under short-wavelength UV irradiation, lipoic acid (LipSS) and its reduced form, dihydrolipoic acid (DHLA), undergo photoionization processes through a bi- or monophotonic pathway. After ionization, the LipSS radical cation (LipSS*+) and radical anion (LipSS*-) are generated. LipSS*- can be converted to equimolar amounts of LipSS and DHLA through second-order decay. Triplet acetone can be quenched by LipSS and DHLA with a rate close to the diffusion-controlled limit. The mechanism was further confirmed by continuous irradiation experiments. When LipSS is directly irradiated with UVA light, the first excited triplet state of LipSS is observed, with a lifetime tau=75 ns. Characteristic reactions include triplet energy transfer to oxygen and beta-carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.     

A theoretical investigation about the excited state behavior for 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole: The water‐assisted excited state proton transfer process

In this work, based on density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods, we theoretically investigate the excited‐state process of the 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole (2HPB) system in acetonitrile and water solvents. Since acetonitrile is an aprotic solvent, it has no effect on the solvent‐assisted excited‐state proton transfer (ESPT) process. Therefore, the 2HPB molecule cannot transfer the proton in acetonitrile, which is consistent with previous experimental observation. On the other hand, 2HPB can combine one water molecule (which is a protic solvent), forming the 2HPB–H₂O complex in the S₀ state. After photoexcitation, the intermolecular hydrogen bonds O1 H2···O3 and O3 H4···N5 both get strengthened in the S₁ state, which leads to the possibility of a water‐assisted ESPT process. Further, the charge redistribution reveals the tendency of ESPT. By exploring the potential energy curves for the 2HPB–H₂O complex in water, we confirm that a stepwise double proton transfer process occurs in the S₁ state. Water‐assisted ESIPT can occur along O1 H2···O3 or O3 H4···N5 because of their similar potential barriers. Based on the stepwise ESPT mechanism, we reinterpret the absorption and fluorescence spectra mentioned in the experiments and confirm the rationality of the water‐assisted ESPT process.     

A theoretical investigation of the enol content of acetic acid and the acetate ion in aqueous solution

Ab initio molecular orbital calculations and statistical Monte Carlo simulations employing a combined quantum and molecular mechanical potential were used to determine the enol contents of acetic acid and the acetate ion in aqueous solution. A pK_E of 19.3 ± 0.3 was predicted for the keto-enol equilibrium of acetic acid, and 21.8 ± 0.8 for the acetate ion in water. The results are found to be in good accord with Guthrie's calculations based on disproportionation reactions and kinetic data. Combining with the experimental pK_a value of acetic acid, we obtained pK_a^k = 26.6 for ionization of acetic acid as a carbon acid in water, and pK_a^E = 7.3 for ionization of the enol of acetic acid.     

On the ground state of titanium phosphide, TiP: a theoretical investigation

Using multireference variational and coupled cluster methods in conjunction with very large core-correlation-consistent basis sets, we have confirmed that the ground state of TiP is of (2)Sigma(+) symmetry with the first excited state A (2)Delta no more than 3.5 kcal/mol higher. We also report full potential energy curves, dissociation energies, bond lengths, dipole moments, and the usual spectroscopic constants.