Triplet Sensitization Research Articles

Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes

Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findings can also be generalized to fulvenes other than the pentafulvenes, even though the effect is attenuated as the size of the fulvene increases. Our studies thus reveal that triplet-state aromaticity can greatly influence the properties of conjugated compounds in the T1 state.

Photochemical decomposition of amidobiphenyls in highly excited triplet states studied by stepwise two-color laser photolysis

Photochemical decomposition of amidobiphenyls (R–AN; R = Me, Et and PhCH 2) in highly excited triplet states ( T n with n over 2) was investigated in solution by using stepwise two-color laser photolysis techniques. Upon direct excitation, R–ANs underwent photodecomposition in excited singlet states ( S 1) with a quantum yield ( Φ 254) of ca. 0.015. Triplet sensitization using acetone (Ac) was employed for efficient formation of the lowest triplet states ( T 1) of R–AN and for determining molar absorption coefficients of the T n ← T 1 absorption. No photochemical reactions were found in the T 1 state. Upon 355-nm laser flash photolysis of the T 1 states of R–ANs, they underwent decomposition. The quantum yields ( Φ dec) for the decomposition of the molecules in the T1 states were determined to be ca. 0.08. The photodecomposition of the T 1 state was proposed to be responsible for CO–N bond cleavage in the T n state.

Layered Double Hydroxides with Intercalated Porphyrins as Photofunctional Materials: Subtle Structural Changes Modify Singlet Oxygen Production

This study presents new photofunctional materials producing singlet oxygen, 1O2, and investigates the interdependence between their structural and photophysical properties. These materials consist of Mg−Al layered double hydroxides (LDH) with intercalated photosensitizers, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) or Pd(II)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (PdTPPC). Powder X-ray diffraction and X-ray photoelectron spectroscopies were employed to characterize the host structure and confirm intercalation of porphyrins into the interlayer space. Because the kinetic parameters of the sensitizer triplet states predetermine the formation of 1O2, the excited-state kinetics of intercalated porphyrins were investigated by means of time-resolved diffuse reflectance. Comparison of the decay rates in the presence and absence of oxygen confirms that the triplet states of PdTPPC and TPPS in LDHs are quenched by oxygen. Photoproduction of 1O2 was monitored by time-resolved measurement of its lu...

Mechanistic Aspects of the Formation of Aldehydes and Nitriles in Photosensitized Reactions of Aldoxime Ethers

The photooxidation of a series of aldoxime ethers was studied by laser flash photolysis and steady-state (product studies) methods. Nanosecond laser flash photolysis studies have shown that chloranil (CA)-sensitized reactions of the O-methyl (1), O-ethyl (2), O-benzyl (3), and O-tert-butyl (4) benzaldehyde oximes result in the formation of the corresponding radical cations. In polar non-nucleophilic solvents such as acetonitrile, there are several follow-up pathways available depending on the structure of the aldoxime ether and the energetics of the reaction pathway. When the free energy of electron transfer (DeltaGET) becomes endothermic, syn-anti isomerization is the dominant pathway. This isomerization pathway is a result of triplet energy transfer from CA to the aldoxime ether. For substrates with alpha-protons (aldoxime ethers 1-3), the follow-up reactions involve deprotonation at the alpha-position followed by beta-scission to form the benziminyl radical (and an aldehyde). The benziminyl radical reacts to give benzaldehyde, the major product under these conditions. A small amount of benzonitrile is also observed. In the absence of alpha-hydrogens (aldoxime ether 4), the major product is benzonitrile, which is thought to occur via reaction of the excited (triplet) sensitizer with the aldoxime ether. Abstraction of the iminyl hydrogen yields an imidoyl radical, which undergoes a beta-scission to yield benzonitrile. An alternative pathway involving electron transfer followed by removal of the iminyl proton was not deemed viable based on charge densities obtained from DFT (B3LYP/6-31G*) calculations. Similarly, a rearrangement pathway involving an intramolecular hydrogen atom transfer process was ruled out through experiments with a deuterium-labeled benzaldehyde oxime ether. Studies involving nucleophilic solvents have shown that all aldoxime ethers reacted with MeOH by clean second-order kinetics with rate constants of 0.7 to 1.2 x 10(7) M(-1) s(-1), which suggests that there is only a small steric effect in these reactions. The steady-state experiments demonstrated that under these conditions no nitrile is formed. This is explained by a mechanistic scheme involving nucleophilic attack on the nitrogen of the aldoxime ether radical cation, followed by solvent-assisted [1,3]-proton transfer and elimination of an alcohol, similar to the results obtained for a series of acetophenone oxime ethers.

Effect of Sensitizer Protonation on Singlet Oxygen Production in Aqueous and Nonaqueous Media

The yield of singlet molecular oxygen, O2(a(1)Delta(g)), produced in a photosensitized process can be very susceptible to environmental perturbations. In the present study, protonation of photosensitizers whose chromophores contain amine functional groups is shown to adversely affect the singlet oxygen yield. Specifically, for bis(amino) phenylene vinylenes dissolved both in water and in toluene, addition of a protic acid to the solution alters properties of the system that, in turn, result in a decrease in the efficiency of singlet oxygen production. In light of previous studies on other molecules where protonation-dependent changes in the yield of photosensitized singlet oxygen production have been ascribed to changes in the quantum yield of the sensitizer triplet state, Phi(T), and to possible changes in the triplet state energy, E(T), our results demonstrate that this photosystem can respond to protonation in other ways. Although protonation-dependent changes in the amount of charge-transfer character in the sensitizer-oxygen complex may influence the singlet oxygen yield, it is likely that other processes also play a role. These include (a) protonation-dependent changes in sensitizer aggregation and (b) nonradiative channels for sensitizer deactivation that are enhanced as a consequence of the reversible protonation/deprotonation of the chromophore. The data obtained, although complicated, are relevant for understanding and ultimately controlling the behavior of photosensitizers in systems with microheterogeneous domains that have appreciable pH gradients. These data are particularly important given the use of such bi-basic chromophores as two-photon singlet oxygen sensitizers, with applications in spatially resolved singlet oxygen experiments (e.g., imaging experiments).

Modification of photochemical pathways of sensitized oxidation of phenylthioacetic acid: Effects of solvent and tetrabutylammonium salt

The mechanisms of triplet-sensitized electron transfer from thioether-containing aromatic carboxylic acids were investigated using steady state and laser flash photolysis in acetonitrile and aqueous solutions. Benzophenone (BP) and 4-carboxybenzophenone were used as the triplet sensitizers, and phenylthioacetic acid and its tetrabutylammonium salt were used as the electron-donating quenchers. In aqueous solution, (BP −⋯>S +) radical pairs (formed in the electron-transfer quenching of the triplet state of BP) decayed mainly via a charge-separation reaction leading to the formation of ketyl radical anions (BP −) and sulfur-centered radical zwitterions (>S +). The later transients underwent fast decarboxylation leading to the formation of alkyl-type radicals. In acetonitrile solutions, however, proton transfer between the radical ions was observed with the formation of ketyl radicals (BPH ). It was shown that the carboxylic and methylene groups in the phenylthioacetic acid moiety within the radical-ion pair are responsible for the proton-transfer reactions. The presence of counter cations from the tetrabutylammonium salt of phenylthioacetic acid can significantly modify the decay pathways of the photochemically produced radical-ion pairs in acetonitrile. In this case, the reaction leads to the formation of [BP −⋯N +(C 4H 9) 4] transients that can undergo Hofmann elimination leading to the formation of an alkene and tributylamine. These results indicate that the photochemical pathways (primary and secondary reactions) for the sensitized oxidation of phenylthioacetic acid depend on its state of ionization (that can be modified by the solvent used), properties of solvent, and the presence of associated counter cations from the tetrabutylammonium salt of phenylthioacetic acid.

Photosensitized Z– E isomerization of cinnamate by covalently linked 2-(3′,4′-dimethoxybenzoyl)benzyl moiety via triplet–triplet energy transfer

( Z)- and ( E)-2-(3′,4′-Dimethoxybenzoyl)benzyl cinnamate ( 3Z and 3E) and ( E)- N-(2-(3′,4′-dimethoxybenzoyl)benzoxycarbonymethyl) cinnamide ( 4E) have been prepared. The photosensitized isomerization of cinnamates via intramolecular triplet–triplet (T–T) energy transfer from the triplet 2-dimethoxybenzophenone (DMBP) moiety was observed. The phosphorescence of the DMBP moiety is quenched by the attached cinnamate, indicating efficient T–T energy transfer from triplet DMBP to the cinnamate. On the basis of the intramolecular phosphorescence quenching and the intermolecular quenching experiments, it can conclude that T–T energy transfer to the E isomer is more efficient than to the Z isomer, and faster than the overall decay rate of the sensitizer triplets, and the non-isomerization radiationless decays of triplet 2, or triplet cinnamate moiety in 3 and 4 are faster than the isomerization. The latter be an important reason for low quantum yields of isomerization.

Highly Efficient Triplet Chain Isomerization of Dewar Benzenes: Adiabatic Rate Constants from Cage Kinetics

Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewar benzene reactants, R, to the corresponding benzene products, P. Considerable chain amplification is maintained even at high conversion. All relevant rate constants of this triplet chain reaction were extracted from laser flash photolysis plus steady-state photolysis experiments. The crucial rate constant ka for adiabatic isomerization of the triplet reactant to triplet product (R* --> P*) cannot be directly measured because it is so large relative to the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with the sensitizer, e.g., S/R* --> S + R*, and the in-cage processes, S/R* --> S/P* --> S*/P, in nonviscous and viscous solvents. These measurements yielded ka values of (approximately 4-9) x 10(9) s(-1), which suggests that only a small (approximately 3 kcal/mol) energy barrier exists along the potential energy surface from R* to P*. Steady-state data indicated that the chain-terminating rate constant R* --> R is negligibly small, an ideal condition for chain amplification. Triplet energy transfer from a series of sensitizers to the Dewar benzene derivatives shows a nonclassical falloff in rate constants with decreasing sensitizer triplet energy, suggesting energy transfer to thermally distorted configurations having lower singlet-triplet energy gaps. As a result of distorted geometries of R* and P*, the chain-propagating energy transfer from P* to R proceeds with a rate constant of only approximately 2 x 10(7) M(-1) s(-1), despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photons due to the high-energy content of the reactant together with the large chain length.

Stepwise Laser Photolysis Studies of β-Bond Cleavage in Highly Excited Triplet States of Biphenyl Derivatives Having C−O Bonds

Photochemical profiles of beta-bond dissociation in highly excited triplet states (Tn) of biphenyl derivatives having C-O bonds were investigated in solution, using stepwise laser photolysis techniques. The lowest triplet states (T1) were produced by triplet sensitization of acetone (Ac) upon 308-nm laser photolysis. The molar absorption coefficients of the T1 states were determined using triplet sensitization techniques. Any photochemical reactions were absent in the T1 states. Upon 355-nm laser flash photolysis of the T1 states, they underwent fragmentation, because of homolysis of the C-O bond in the Tn states from the observations of the transient absorption of the corresponding radicals. The quantum yields (Phidec) for the decomposition of the T1 states upon the second 355-nm laser excitation were determined. Based on the Phidec values and the bond dissociation energies (BDEs) for the C-O bond fission, the state energies (ERT) of the reactive highly excited triplet states (TR) were determined. It was revealed that (i) the Phidec was related to the energy difference (DeltaE) between the BDE and the ERT, and (ii) the rate (kdis) of beta-cleavage in the TR state was formulated as being simply proportional to DeltaE. The reaction mechanism for beta-bond cleavage in the TR states was discussed.

Photodimerization of acenaphthylene within a nanocapsule: excited state lifetime dependent dimer selectivity

Direct excitation of acenaphthylene molecules included in a syn fashion within the octa acid nanocapsule dimerizes quantitatively to a syn dimer, and upon triplet sensitization, yields both syn and anti dimers probably by reacting within and outside the capsule.

Photoinduced ω-bond dissociation of m-halomethylbenzophenones studied by laser photolysis techniques and DFT calculations. Substituted position effects

Photochemical profiles of omega-cleavage of carbon-X (X = Br and Cl) bonds in m-bromo- and m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo omega-bond cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds. From these observations, it was inferred that the omega-bond of these m-halomethylbenzophenones (m-HMBP) cleaves in the lowest excited singlet state (S(1)(n,pi(*))) upon direct excitation. Upon triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption for an Ac-m-BMBP system, and an efficiency of the C-Br bond cleavage in the lowest triplet state (T(1)(n,pi(*))) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an Ac-m-CMBP system. Absence of C-Cl bond cleavage in the triplet state of m-CMBP indicated the reactive state of m-CMBP for omega-cleavage is only the S(1)(n,pi(*)) state. Based on the efficiencies and DFT calculations for excited state energies, photoinduced omega-bond dissociation of m- and p-HMBPs was characterized.

Photochemical Reactions of 1,3,3‐Trimethylbicyclo[2.2.2]octa‐5,7‐dien‐2‐ones

AbstractThe photochemistry of 1,3,3‐trimethylbicyclo[2.2.2]octa‐5,7‐dien‐2‐ones 1a–f containing multiple chromophoric moieties under various conditions is described. Upon direct irradiation, dienones 1a–f in benzene, acetonitrile, and methanol were found to aromatize through photoelimination of dimethylketene, which was trapped with 3‐phenylprop‐2‐en‐1‐ol (12). Triplet sensitization of dienone 1a in acetone and in acetone/acetophenone mixed solvents afforded aromatic compound 2a, whilst bicyclic systems 1b–c were found to be unreactive on sensitization. In the cases of dienones 1d–f in acetone, aromatic compounds 2d–f and di‐π‐methane (DPM) photoproducts 3d–f and 4f were observed, but in the acetone/acetophenone mixed solvent system only DPM photoproducts were detected. Plausible mechanisms for these photochemical reactions are described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Upconverted Emission from Pyrene and Di-tert-butylpyrene Using Ir(ppy)3 as Triplet Sensitizer

Metal-to-ligand charge-transfer sensitized upconverted fluorescence in noncovalent triplet energy transfer assemblies is investigated using Ir(ppy)3 as the sensitizer (ppy=2-phenylpyridine) and pyrene or 3,8-di-tert-butylpyrene as the triplet acceptor/annihilator. Upconverted singlet fluorescence from pyrene or 3,8-di-tert-butylpyrene resulting from triplet-triplet annihilation (TTA) is observed following selective excitation of Ir(ppy)3 in deaerated dichloromethane solutions using 450-nm laser pulses. In both systems, the TTA process is confirmed by the near quadratic dependence of the upconverted fluorescence intensity on incident light power, measured by integrating the upconverted delayed fluorescence kinetic traces as a function of incident excitation power. At the relatively high concentrations of pyrene that were utilized, pyrene excimer formation was detected by its characteristic broad emission centered near 470 nm. In essence, selective excitation of Ir(ppy)3 ultimately resulted in the simultaneous sensitization of both singlet pyrene and pyrene excimers, and the latter degrades the energy stored in the pyrene singlet excited state. Furthermore, in the case of di-tert-butylpyrene/Ir(ppy)3, the formation of excimers is successfully blocked because of the presence of the sterically hindering tert-butyl groups. The current work demonstrates that sensitized TTA is indeed accessible to chromophore systems beyond those previously reported, suggesting the generality of the approach.

Effects of Localized Triplet Exciton on Reactivity of Photoinduced ω-Bond Dissociation in Naphthyl Phenyl Ketones Having π,π* Lowest Triplet (T1) States Studied by Laser Flash Photolysis

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.

Laser photolysis studies of β-bond cleavage of phenacyl phenyl sulfide in acetonitrile

Photo-induced β-bond dissociation of phenacyl phenyl sulfide (PPS) has been investigated in acetonitrile by laser photolysis techniques. Direct excitation of PPS at 295 K provided the acetylmethyl and phenylthiyl radicals with a quantum yield (Φrad) of 0.18, whereas triplet sensitization using xanthone revealed an efficiency for β-cleavage of triplet PPS (α rad) of ≥0.64. From disagreement between the Φrad and α rad values, it was concluded that both the lowest excited single and triplet states are reactive for β-bond dissociation in PPS. The photochemical processes of excited PPS, including β-cleavage, are discussed in detail.

Enhancement of Singlet Oxygen Sensitization of Tetraphenylporphyrin by Silylation

Abstract Photophysical and singlet oxygen sensitization processes of 5,10,15,20-tetrakis(4-trimethylsilylphenyl)porphyrin have been studied. It was found out that the quantum yield (ΦΔ) of singlet oxygen production increased by the silylation of 5,10,15,20-tetraphenylporphyrin (TPP). The silylation does not affect the photophysical processes of TPP but enhances the sensitization efficiency of singlet oxygen by the triplet sensitizer (fΔT). This enhancement can be explained by the suppression of the process from the triplet encounter complex composed of the triplet sensitizer and ground-state oxygen to the ground-state sensitizer and oxygen.

Magnetic field effects on the reaction of a triplet-born radical pair consisting of two equivalent sulphur-centre radicals under ultrahigh fields of up to 28 T

A water-cooled pulsed magnet having a room temperature bore of diameter 20 mm was constructed. Magnetic field effects on a triplet sensitization reaction of p-aminophenyl disulfide (APDS) with xanthone (Xn) were studied in a sodium dodecylsulfate (SDS) micellar solution by a nanosecond laser flash photolysis technique under ultrahigh magnetic fields of up to 28 T. Upon irradiation of the SDS micellar solution containing APDS and Xn, triplet radical pairs of p-aminophenylthiyl radicals were generated. The yield of the escaped p-aminophenylthiyl radical was found to decrease by 6 ± 2% at 28 T compared with that at 0 T.

A Main‐Chain Liquid‐Crystalline Oligomer Prepared by in situ Photopolymerization of an LC Monomer Having Cinnamate Moieties

AbstractSummary: A liquid‐crystalline (LC) compound, having a cinnamate moiety on each end of the molecule, was synthesized and irradiated with UV light in its LC phase in the presence of a triplet sensitizer. Various measurements of the irradiated sample revealed that the linearly structured LC oligomers were formed by [2+2] cycloaddition of the cinnamate moieties, and that the resultant cyclobutane units dominantly assumed an anti head‐to‐head configuration.Schematic structures of the LC oligomer obtained by photopolymerization.imageSchematic structures of the LC oligomer obtained by photopolymerization.

The Effect of Solvent Polarity on the Balance between Charge Transfer and Non-Charge Transfer Pathways in the Sensitization of Singlet Oxygen by ππ* Triplet States

A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.

Reactive states for β-bond dissociation of α-phenoxylacetophenones studied by laser flash triplet sensitization techniques

Photoinduced β-bond dissociation of α-phenoxylacetophenones (PAs) substituted in the phenoxyl moiety has been investigated in polar media by means of laser flash photolysis and triplet sensitization techniques to clarify the spin multiplicity of the reactive states, excited lowest singlet (S 1) or triplet (T 1) state. The electronic character of the T 1 state of PAs was found to be of π,π * from transient absorption spectra at 77 K. Any photochemical intermediates were absent in transient absorption upon direct photolysis and triplet xanthone sensitization of PAs having electron withdrawing substituent groups at 295 K. Hydroxy-PAs and methoxy-PAs undergo β-cleavage upon both direct excitation and triplet sensitization, providing the corresponding phenoxyl radicals. From disagreement between the quantum yields of the radical formation upon direct excitation and the efficiency of β-cleavage in the triplet state determined by triplet sensitization, it was concluded that the S 1 and T 1 states are both reactive for β-bond dissociation in PAs. The non-radiative deactivation process of triplet PAs was suggested to be due to efficient quenching of the T 2(n,π *) state by the β-phenyl ring. The deactivation profiles of excited PAs are discussed.