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

Abstract

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.

Hydrogen‐Bonding Effects on Excited States of Para ‐Hydroxyphenacyl Compounds

“…Based on this, the first species seen in Figure 13.2 (0-1000 ps spectra) is assigned to the HA triplet state. It is interesting to note that a comparison of the triplet spectra here in water solution in Figure 13.2 with those reported in acetonitrile solvent [23] shows some distinct differences in the frequencies of many of the vibrational features as well as spectral profiles of some Raman bands that appear to be indicative of hydrogenbonding effects of the water solvent molecules on the triplet excited state. To distinguish the H-bonded triplet species from the normal free HA triplet species (denoted as 3 HA hereafter), the H-bonded HA triplet species is denoted as 3 HA 0 .…”

“…Examination of Figure 13.2 shows that the 400 nm or 416 nm probe wavelength TR 3 spectra observe two different transient species in water solvent, and this differs from the corresponding TR 3 spectra for HA in acetonitrile solution, where only one transient determined to be the triplet state of HA was seen [23]. A femtosecond-time-resolved fluorescence (fs-TRF) investigation found that, for HA and p-HP phototrigger compounds, the intersystem crossing (ISC) rates are extremely fast, with time constants of about 1-2 ps, with the conversion rates in water and water-mixed acetonitrile being similar to those measured in neat acetonitrile [24].…”

“…HA-13 C and HA-D 4 isotopically labelled compounds (displayed in Scheme 13.1 of Section 13.3) were synthesized and characterized as previously detailed in Ref. [23], and spectroscopic-grade acetonitrile and deionized water were employed as solvents to make samples for the TR 3 measurements with pH 9.0 (acetate þ phosphate þ borate, I ¼ 0.1 M) used in the aqueous samples. The HPA, HPDP and HPPP phototriggers were synthesized using the procedures detailed in Refs [24] and [25].…”

“…Each TR 3 spectrum presented in this chapter was acquired by first obtaining a pump-probe spectrum and then subtracting appropriately scaled pump only and probe only spectra from it. Details of the experimental equipment and methods employed for the ns-and ps-TR 3 experiments are the same as those described in Refs [23] to [28].…”

“…A previous TR 3 study on HA found that the free 3 HA triplet has a lifetime of $40 ns in acetonitrile, and the reader is referred to Ref. [23] for details of the vibrational assignments of the 3 HA TR 3 spectra. Examination of Figure 13.2 shows that the 3 HA 0 species decays within 10 ns in water and directly converts into another species with new Raman bands at, for example, 1167, 1357, 1481 and 1599 cm À1 (the features in red as shown in Figure 13.2).…”