Time‐resolved study of the photochemical reaction of 4‐dimethylaminobenzonitrile with carbon tetrachloride

et al. 2004

J. Org. Chem.

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.

Section: Resultssupporting

confidence: 75%

Time-Resolved Resonance Raman Study of the Triplet States of p-Hydroxyacetophenone and the p-Hydroxyphenacyl Diethyl Phosphate Phototrigger Compound

Zuo

et al. 2004

J. Org. Chem.

“…The slightly longer time scale observed here for the early picosecond triplet spectral evolution (with 8 and 11 ps time constants in the neat and mixed MeCN, respectively) than the ∼2 ps ISC conversion from the fs-TRF measurement is likely due to the combined consequences of the triplet formation and the relaxation of the excess energy of the initially formed energetic triplet state produced from the rapid ISC conversion. Characteristic evidences for the excess energy relaxation are the early time bandwidth narrowing and frequency upshift of the picosecond Raman features (see Figures a and a) and that the temporal changes of this bandwidth and frequency are found to occur with similar time constants to the intensity build-up of the triplet TR 3 features. ,− The triplet energy of MAP in the solution phase was determined to be ∼25 000 cm -1 . Since ISC is the predominant way to dissipate the excitation energy absorbed by the singlet excited state, the 267 nm pump pulse used here can introduce a great amount of excess energy (∼12000 cm -1 ) into the initially generated triplet.…”

Section: Resultsmentioning

confidence: 79%

“…Characteristic evidences for the excess energy relaxation are the early time bandwidth narrowing and frequency upshift of the picosecond Raman features (see Figures 4a and 5a) and that the temporal changes of this bandwidth and frequency are found to occur with similar time constants to the intensity build-up of the triplet TR 3 features. 10, [55][56][57][58][59] The triplet energy of MAP in the solution phase was determined to be ∼25 000 cm -1 . 13 Since ISC is the predominant way to dissipate the excitation energy absorbed by the singlet excited state, the 267 nm pump pulse used here can introduce a great amount of excess energy (∼12000 cm -1 ) into the initially generated triplet.…”

Section: Resultsmentioning

confidence: 99%

Time-Resolved Resonance Raman and Density Functional Theory Study of Hydrogen-Bonding Effects on the Triplet State of p-Methoxyacetophenone

Chan

et al. 2005

J. Phys. Chem. A

Picosecond and nanosecond time-resolved resonance Raman spectroscopy combined with density functional theory calculations have been performed to characterize the structure, dynamics, and hydrogen-bonding effects on the triplet state of the phototrigger model compound p-methoxyacetophenone (MAP) in cyclohexane, MeCN, and 50% H2O/50% MeCN (v:v) mixed solvent. Analogous work has also been done to study the corresponding ground state properties. The ground and triplet states of MAP were both found to be associated strongly with the water solvent molecules in the 50% H2O/50% MeCN solvent system. A hydrogen-bond complex model involving one or two water molecules bonded with the oxygen atoms of the MAP carbonyl and methoxy moieties has been employed to explore the hydrogen-bond interactions and their influence on the geometric and electronic properties for the ground and triplet states of MAP. Among the various hydrogen-bond configurations examined, the carbonyl hydrogen-bond configuration involving one water molecule was calculated to lead to the most stable hydrogen-bond complex for both the ground and the triplet states with the strength of the hydrogen-bond interaction being stronger in the triplet state than the ground state. The increased carbonyl located hydrogen-bond strength in the triplet state results in substantial modification of both the electronic and the structural conformation so that the triplet of the hydrogen-bond complex can be considered as a distinct species from the free MAP triplet state. This provides a framework to interpret the differences observed in the TR3 spectral and triplet lifetime obtained in the neat MeCN solvent (attributed to the free MAP triplet state) and the 50% H2O/50% MeCN solvent (due to the triplet of the hydrogen-bond complex). Temporal evolution at early picosecond times indicates rapid ISC conversion, and subsequent relaxation of the excess energy of the initially formed energetic triplets occurs for both the free MAP and the hydrogen-bond complex. The triplet of the carbonyl hydrogen-bond complex appears to be generated directly from the corresponding ground state complex and it does not dissociate back to the free triplet state within the triplet state lifetime. We briefly discuss the influence of the carbonyl hydrogen-bond effect on the pi pi* triplet reactivity for MAP and closely related compounds.

“…It has been reported for several molecules that relaxation of excess energy produces band changes of such as those observed here in the TR 3 spectra on a very similar time scale. [68][69][70][71][72] The availability of this excess energy implies that the lowest triplet of HPA is substantially lower in energy than the singlet. The energy separation between the 1 nπ* and 3 nπ* states was estimated to be ∼1800 cm -1 for acetophenone 25,53 and that between 3 nπ* and 3 ππ* to be ∼2500 cm -1 for p-hydroxyacetophenone.…”

Section: Resultsmentioning

confidence: 99%

Time-Resolved Resonance Raman Study of the Triplet State of the p-Hydroxyphenacyl Acetate Model Phototrigger Compound

Chan

et al. 2004

J. Phys. Chem. B

Picosecond and nanosecond time-resolved resonance Raman spectroscopy have been used to study the dynamics and structure of the phototrigger compound p-hydroxyphenacyl acetate (HPA) in acetonitrile solution. An intermediate was observed that exhibited significant quenching by oxygen. This intermediate was identified and attributed to the ππ* triplet state of HPA. Temporal evolution at early picosecond times indicates rapid intersystem crossing conversion and subsequent relaxation of the excess energy of the initially formed triplet state. B3LYP/6-311G** density functional theory (DFT) calculations were done to determine the structures and vibrational frequencies for both the HPA triplet and ground states. The experimental and theoretical results were compared and discussed in relation to the initial pathway for the p-hydroxyphenacyl deprotection reaction and the nature of the triplet state relative to other aromatic carbonyl compounds. The present results demonstrate the utility of applying ultrafast vibrational spectroscopy to study the mechanism of the photorelease process in phototrigger compounds.