Two-Laser Photochemistry of Phenothiazine in Solution: Ionization and Bond Cleavage from Upper States

Abstract

The one (308 nm)-and two (308 plus 480 nm)-laser photochemistry of phenothiazine in nitrogen-saturated acetonitrile and cyclohexane was investigated. The lowest excited triplet state (TI) of phenothiazine was efficiently generated following 308-nm excitation in both acetonitrile and cyclohexane. In cyclohexane, generation of the phenothiazine neutral radical via homolytic N-H bond cleavage from SI accompanied intersystem crossing. In acetonitrile, photoionization via the singlet manifold yielding the cation radical competes with intersystem crossing and homolytic bond cleavage. An additional route to the neutral radical in acetonitrile is deprotonation of the cation radical. Generation of upper triplet states (Tn) by 480-nm excitation of T1 resulted in permanent bleaching of the T-T absorption in both solvents. The triplet bleaching quantum yield was 0.120 in cyclohexane, but only 0.004 in acetonitrile, indicating that relaxation from Tn to TI is more efficient in acetonitrile. Enhanced neutral radical production in cyclohexane and enhanced production of the neutral and cation radicals in acetonitrile were observed concurrently with T1 depletion. In cyclohexane, the chemical efficiency of two-laser radical production was 0.90, indicating that the majority of Tn states that do not relax to T1 undergo bond cleavage. In acetonitrile, neutral and cation radical chemical efficiencies were 0.53 and 0.45, respectively. Thus, bond cleavage and ionization from Tn are competitive.

Complexation and Chiral Drug Recognition of an Amphiphilic Phenothiazine Derivative with β‐Cyclodextrin

“…27 Under strong irradiation conditions, the phenothiazine derivates in aqueous solution form oxidation products, even at low concentrations of drug. 28,29 In our case, we have observed that the degradation product shows a slight fluorescence band around 364 nm. The use of fresh and stirred solutions assures that its contribution to the overall PTZ fluorescence can be considered negligible at the experimental conditions.…”

Proton transfer in phenothiazine photochemical oxidation: Laser flash photolysis and fluorescence studies

“…1. It is assigned to 3 PTH * (E T , 60.4 kcal/mol) [29] because of its drastically reduced lifetime in the presence of dissolved oxygen [19,30]. The energy transfer experiments with anthracene (E T , 42.6 kcal/mol) and ferrocene (E T , 38.0 kcal/mol) also support this conclusion.…”

“…In acetonitrile, the pK a of PTH +Å is estimated low (pK a % 1.0) relative to that of diphenylamine (pK a = 1.8), as determined by M. Jonsson [26], it then mainly decays by deprotonation to give the long-lived PT Å . In addition, the energy of exciting photon of 355 nm and also that of S 1 state is higher than the N-H bond dissociation energy (3.47 eV) [30], photodissociation of the N-H bond is energetic possible. Since the radical ions yields are low due to low excitation energy of 355 nm pulses, their absorptions appear only as shoulders.…”

“…PTH is well known to yield its cation and neutral radical due to its photoionization and the following deprotonated processes [19,31,32]. The available evidence in the literature suggests that the absorption at 390 nm should be attributed to PT Å [30]. However, the cation radical (PTH +Å ) absorption at 520 nm was not detected at low laser power due to the overlapping absorption of triplet.…”

Intramolecular Singlet−Singlet and Triplet−Triplet Energy Transfer in Adamantyl-Linked Trichromophores

“…The laser flash photolysis system has been described in detail elsewhere. 47 Briefly, for kinetic studies and transient absorption spectra, solutions were prepared SCHEME 1 at concentrations sufficiently large to give absorbances in the range 0.6-0.8 at the excitation wavelength. Unless otherwise noted, the solutions, contained in a reservoir, were continuously purged with a stream of nitrogen and were caused to flow through a specially constructed quartz cell (7 mm × 7 mm) by means of a peristaltic pump.…”