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.     

On the unusual fluorescence properties of xanthone in water

Photo-excited xanthone is known to undergo ultrafast intersystem crossing (ISC) in the 1 ps time domain. Correspondingly, its fluorescence quantum yield in most solvents is very small ( approximately 10(-4)). Surprisingly, the quantum yield in water is 100 times larger, while ISC is still rapid ( approximately 1 ps), as seen by ultrafast pump probe absorption spectroscopy. Temperature dependent steady state and time resolved fluorescence experiments point to a delayed fluorescence mechanism, where the triplet (3)npi* state primarily accessed by ISC is nearly isoenergetic with the photo-excited (1)pipi* state. The delayed fluorescence of xanthone in water decays with a time constant of 700 ps, apparently by internal conversion between the (3)npi* state and the lowest lying triplet state (3)pipi*.     

On the intrinsic population of the lowest triplet state of thymine

The population of the lowest triplet state of thymine after near-UV irradiation has been established, on the basis of CASPT2//CASSCF quantum chemical calculations, to take place via three distinct intersystem crossing mechanisms from the initially populated singlet bright 1pipi* state. Two singlet-triplet crossings have been found along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV, involving the lowest 3npi* and 3pipi* states, respectively. Large spin-orbit coupling elements predict efficient intersystem crossing processes in both cases. Another mechanism involving energy transfer from the lowest 1npi* state with much larger spin-orbit coupling terms can also be proposed. The wavelength dependence measured for the triplet quantum yield of pyrimidine nucleobases is explained by the location and accessibility of the singlet-triplet crossing regions.     

A theoretical investigation of p-hydroxyphenacyl caged phototrigger compounds: an examination of the excited state photochemistry of p-hydroxyphenacyl acetate

Ab initio and density functional theory methods were employed to study the excited states and potential energy surfaces of the p-hydoxyphenacyl acetate (HPA) phototrigger compound. Complete active space (CAS) ab initio calculations predicted adiabatic electronic transition energies for the HPA-T(1)((3)npi), HPA-T(2)((3)pipi), HPA-S(1)((1)npi), HPA-T(3)((3)npi), HPA-S(2)((1)npi), HPA-S(3)((1)pipi) <-- HPA-S(0) transitions that were similar to and in agreement with those found experimentally for closely related aromatic ketones such as p-hydroxyacetophenone and results from similar calculations for other related aromatic carbonyl systems. The alpha or beta bond cleavage reactions from the S(1) excited state were both found to have relatively high barriers to reaction, and the S(1), T(1), and T(2) states are close in energy with the three S(1)((1)npi), T(1)((3)npi), and T(2)((3)pipi) surfaces intersecting at the same region. The calculations suggest that intersystem crossing (ISC) can occur very fast from the S(1) state to the nearby triplet states. This is consistent with results from ultrafast spectroscopy experiments that observe the S(1) state ISC occurs within about 1-2 ps to produce a triplet state for HPA and related pHP compounds. The alpha and beta bond cleavage reactions for the T(1) state of HPA are both predicted to have fairly high barriers and compete with one another. However, this is not completely consistent with experiments that observe the photodeprotection reactions (e.g. the beta bond cleavage) of HPA and some other pHP phototriggers in largely water containing solvents are predominant and occur very fast to release the leaving group. Comparison of the computational results with experimental results for HPA and related pHP compounds suggests that water molecules likely play an important part in changing the triplet state beta bond cleavage so that it becomes the predominant pathway and occurs very fast to give an efficient deprotection reaction. The results reported here provide new insight into the photophysics, reaction pathways, and photochemistry of the p-hydoxyphenacyl acetate and related pHP caged phototrigger compounds and also provide a benchmark for further and more sophisticated investigations in the future.     

Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine

The CASPT2/CASSCF method with the 6-311G basis set and an active space up to (14, 11) was used to explore the ultrafast internal conversion mechanism for excited 9H-adenine. Three minima, two transition states, and seven conical intersections were obtained to build up the two deactivation pathways for the internal conversion mechanism. Special efforts were made to explore the excited-state potential energy surfaces near the Franck-Condon region and determine the various barriers in the processes of deactivation. The barrier required from the 1pipi (1La) state to deactivate nonradiatively is found to be lower than that required from the 1pipi (1Lb) state. On 250 nm excitation, the 1pipi (1La) state is populated, and the transition from 1pipi (1La) to the lowest 1npi state involves very low barriers, which may account for the observed short (<50 fs) lifetime of the 1pipi excited state. The deactivation of the lowest 1npi state is required to overcome a barrier of 3.15 kcal/mol, which should be responsible for the 750 fs lifetime of the npi excited state. On 267 nm excitation, the vibrationally active 1pipi (1Lb) state is populated. Excitation at 277 nm prepares the 1pipi (1Lb) state without much excessive vibrational energy, which may be responsible for the observed >2 ps lifetime.     

Triplet formation of 4-thiothymidine and its photosensitization to oxygen studied by time-resolved thermal lensing technique

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.     

Direct observation of electronic relaxation dynamics in adenine via time-resolved photoelectron spectroscopy

We present femtosecond time-resolved photoelectron spectra of adenine in a molecular beam, recorded at pump wavelengths of 250, 267, and 277 nm. This leads to initial excitation of the bright S2(pipi*). Close to the band origin (277 nm), the lifetime is several picoseconds. Higher vibronic levels (267 and 250 nm excitation) show much shorter lifetimes of t < 50 fs, and we observe strong coupling between S2(pipi*) and S1(npi*). Rapid internal conversion (t < 50 fs) populates the lower lying S1(npi*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(pisigma*) state, previously proposed as an ultrafast relaxation pathway from S2(pipi*).     

Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational study

Previous studies have established that the major pathway for the first singlet excited state of 1-nitronaphthalene is intersystem crossing to the triplet manifold. In this contribution we present determinations of the decay of the S1 state of this compound in several solvents to establish the time scale of the multiplicity change as a function of the polarity and hydrogen-bonding ability of the solvent environment. The measurements were made with the femtosecond frequency up-conversion technique to follow the weak spontaneous molecular emission which precedes triplet formation. Our results show that in all environments the S1 lifetime is 100 fs or less, making 1-nitronaphthalene the organic compound with the fastest multiplicity change ever measured. We also show that the bathochromic shifts observed for the first absorption band imply changes in the relative energies of the singlet and triplet manifolds, which in turn manifest in a 2-fold increase of the fluorescence lifetime in cyclohexane compared with the polar solvents. Additionally, we performed excited-state calculations at the TD-DFT/ PBE0/6-311++G(d,p) level of theory with the PCM model for solvation. The TD-DFT theory identifies the presence of upper triplet states which can act as receiver states in this highly efficient photophysical pathway. Together, the experimental and theoretical results show that the dynamics of the S1 state in 1-nitronaphthalene represent an extreme manifestation of El-Sayed's rules due to a partial (n-pi*) character in the receiver triplets which are nearly isoenergetic with S1, determining a change in the molecular spin state within 100 fs.     

A theoretical characterization of the photoisomerization channels of 1,2-cyclononadienes on both singlet and triplet potential-energy surfaces

The ground-, (1)(pipi*)-, and (3)(pipi*)-state potential-energy surfaces of 1,2-cyclononadiene and isomeric C(9)H(14) species, as well as 1-methyl-1,2-cyclononadiene and isomeric C(10)H(16) species were all mapped using CASSCF and the 6-31G(d) basis set. Theoretical results were found to be in good agreement with the available experimental observations for both 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene isomerization reactions under singlet and triplet direct or sensitized irradiation. Extremely efficient decay occurs from the first singlet excited state to the ground state through at least three different conical intersections (surface crossings). The first of these crossing points is accessed by a one-bond ring closure. From this conical intersection point (CI-A or CI-C), some possible subsequent ground-state reaction paths have been identified: 1) intramolecular C--H bond insertion to form the bicyclic photoproduct and 2) intramolecular C--H bond insertion to form tricyclic photoproducts. An excited state [1,3]-sigmatropic shift leads to the second conical intersection (CI-B or CI-E), which can give a three-bond cyclononyne species. Besides these, in the singlet photochemical reactions of 1-methyl-1,2-cyclononadiene, excited-state, one allenic C--H bond insertion leads to a third conical intersection (CI-D). Possible ground-state reaction pathways from this structure lead to the formation of a diene photoproduct or to transannular insertion photoproducts. Moreover, in the case of triplet 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene photoisomerization reactions, both chemical reactions will adopt a 1,3-biradical (T(1)/S(0)-1, T(1)/S(0)-2, and T(1)/S(0)-3), which may undergo intersystem crossings leading to the formation of tricyclic or bicyclic photoproducts. The results obtained allow a number of predictions to be made.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Vertical excitation energies for ribose and deoxyribose nucleosides

Vertical excitation energies for DNA and RNA nucleosides are determined with electron structure calculations using the time-dependent density functional theory (TDDFT) method at the B3LYP/6-311++G(d,p) level for nucleoside structures optimized at the same level of theory. The excitation energies and state assignments are verified using B3LYP/aug-cc-pVDZ level calculations. The nature of the first four excited states of the nucleosides are studied and compared with those of isolated bases. The lowest npi* and pipi* transitions in the nucleoside remain localized on the aromatic rings of the base moiety. New low-energy npi* and pisigma* transitions are introduced in the nucleosides as a result of bonding to the ribose and deoxyribose molecules. The effect on the low-lying excited state transitions of the binding to phosphate groups at the 5'- and 3',5'-hydroxyl sites of the uracil ribose nucleoside are also studied. Some implications of these calculations on the de-excitation dynamics of nucleic acids are discussed.     

A three-state model for the photophysics of adenine

An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/pipi* La)(CI), reached from the initially populated 1(pipi* La) state along a path which is found to be barrierless only in 9H-adenine, while for the 7H tautomer the presence of an intermediate plateau corresponding to an NH2-twisted conformation may explain the absence of ultrafast decay in 7-substituted compounds. A secondary picosecond decay is assigned to a path involving switches towards two other states, 1(pipi* Lb) and 1(npi*), ultimately leading to another conical intersection with the ground state, (gs/npi*), with a perpendicular disposition of the amino group. The topology of the hypersurfaces and the state properties explain the absence of secondary decay in 9-substituted adenines in water in terms of the higher position of the 1(npi*) state and also that the 1(pipi* Lb) state of 7H-adenine is responsible for the observed fluorescence in water. A detailed discussion comparing recent experimental and theoretical findings is given. As for other nucleobases, the predominant role of a pipi*-type state in the ultrafast deactivation of adenine is confirmed.     

Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: direct measurements by femtosecond fluorescence up-conversion

Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S1 decays is accounted by the pi-pi* nature of the S1 and T1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.     

Excited state proton transfer in guanine in the gas phase and in water solution: a theoretical study

Theoretical investigations were performed to study the phenomena of ground and electronic excited state proton transfer in the isolated and monohydrated forms of guanine. Ground and transition state geometries were optimized at both the B3LYP/6-311++G(d,p) and HF/6-311G(d,p) levels. The geometries of tautomers including those of transition states corresponding to the proton transfer from the keto to the enol form of guanine were also optimized in the lowest singlet pipi* excited state using the configuration interaction singles (CIS) method and the 6-311G(d,p) basis set. The time-dependent density function theory method augmented with the B3LYP functional (TD-B3LYP) and the 6-311++G(d,p) basis set was used to compute vertical transition energies using the B3LYP/6-311++G(d,p) geometries. The TD-B3LYP/6-311++G(d,p) calculations were also performed using the CIS/6-311G(d,p) geometries to predict the adiabatic transition energies of different tautomers and the excited state proton transfer barrier heights of guanine tautomerization. The effect of the bulk aqueous environment was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. The excited state geometries including that of transition states were found to be largely nonplanar. The nonplanar fragment was mostly localized in the six-membered ring. Geometries of the hydrated transition states in the ground and lowest singlet pipi* excited states were found to be zwitterionic in which the water molecule is in the form of hydronium cation (H3O(+)) and guanine is in the anionic form, except for the N9H form in the excited state where water molecule is in the hydroxyl anionic form (OH(-)) and the guanine is in the cationic form. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in the bulk water solution. The explicit inclusion of a water molecule in the proton transfer reaction path reduces the barrier height drastically. The excited state barrier height was generally found to be increased as compared to that in the ground state. On the basis of the current theoretical calculation it appears that the singlet electronic excitation of guanine may not facilitate the excited state proton transfer corresponding to the tautomerization of the keto to the enol form.     

Mechanism of the photochemical process of singlet oxygen production by phenalenone

Phenalenone (PN) is a very efficient singlet oxygen sensitiser in a wide range of solvents. This work uses ab initio quantum chemical calculations (CASSCF/CASPT2 protocol) to study the mechanism for populating the triplet state of PN responsible for this reaction, the (3)(π-π*) state. To describe in detail this reaction path, the singlet and triplet low-lying excited states of PN have been studied, the critical points of the potential energy surfaces corresponding to these states located and the vertical and adiabatic energies calculated. Our results show that, after the initial population of the S(2) excited state of (π-π*) character, the system undergoes an internal conversion to the (1)(n-π*) state. After populating the dark S(1) state, the system relaxes to the (1)(n-π*) minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the (3)(π-π*) state. Although the population of the minimum of this triplet state is strongly favoured, a conical intersection with the (3)(n-π*) surface opens an internal conversion channel to this state, a path accessible only at high temperatures. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state. Our computational results satisfactorily explain the experimental findings and are in very good agreement with the experimental data available. In the case of the frequency of fluorescence, this is the first time that these data have been theoretically predicted in good agreement with the experimental results.     

硫代乙酰胺光化学反应机理的理论研究

用B3LYP, MP2和CASSCF方法, 采用cc-pVDZ和6-31++G**基组, 研究了硫代乙酰胺在基态和最低三态上消除硫化氢以及其它光解离反应, 并考虑了单个溶剂分子参与反应对质子迁移反应的影响, 得到了消除硫化氢反应的反应机理, 计算结果可以很好地解释实验结果. 进而用CASSCF方法计算了第一激发单态上的各驻点, 以及各交叉点. 计算结果表明, 在S1和T1态上发生除分子内转动以外的化学反应的可能性比较小, 当分子被激发到S2态上时, 将通过S2/S1交叉点到S1态, 在S1态上的分子有两条途径去活化, 通过S1/S0交叉点到热基态, 通过S1/T1交叉点系间窜越到T1态. 因而得出CH3CSNH2发生光解离反应的可能性不大. 基于此, 可将硫代酰胺结构引入蛋白或多肽中, 有望在不破坏分子整体结构的情况下对其进行光化学研究.     

Photostability versus photodegradation in the excited-state intramolecular proton transfer of nitro enamines: competing reaction paths and conical intersections

The phototautomerization mechanism of a model nitro enamine (NEA) chromophore (incorporated in the structure of a highly photolabile pesticide, tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) has been studied using complete active space self-consistent field reaction path computations. The optically accessible 1pipi* excited state of NEA involves separation of charge and correlates diabatically with the ground state of the tautomerized acinitro imine (ANI) form. For optimum photostabilization, the 1pipi* state of NEA should be S1: in this case, the tautomer would be efficiently formed via a diabatic intramolecular proton-transfer pathway passing through an S1/S0 conical intersection, followed by a facile thermal back proton-transfer reaction. However, in NEA itself the lowest excited states correspond to nitro group 1npi* states, and there are additional surface crossings that provide a mechanism for populating the 1npi* manifold. The above results indicate that the high photolability observed for the pesticide [Kleier, D.; Holden, I.; Casida, J. E.; Ruzo, L. O. J. Agric. Food Chem. 1985, 33, 998-1000] has to be ascribed to photochemistry originating on the 1npi* manifold of states, populated indirectly from the 1pipi* state.     

Ultrafast relaxation dynamics of the excited states of Michler's thione

Ultrafast relaxation dynamics of the S2 and S1 states of 4,4'-bis(N,N-dimethylamino)thiobenzophenone (Michler's thione, MT) have been investigated in different kinds of solvents, using steady-state absorption and emission as well as femtosecond transient absorption and fluorescence up-conversion spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation to the S2 state of MT, reveal weak fluorescence from the S2 state (phi F approximately 10(-3) in nonpolar and 10(-4) in polar solvents) but much weaker fluorescence from the S1 state. Yield of fluorescence from the S2 state is reduced in polar solvents because of reduced energy gap between the S2 and S1 states, Delta E(S2-S1), as well as interaction with the solvent molecules. Occurrence of S2-fluorescence in polar solvents, despite small energy gap, suggests that symmetry allowed S2(1A1) --> S0 (1A1) radiative and symmetry forbidden S2(1A1) --> S1 (1A2) nonradiative transitions are the factors responsible for the S2 fluorescence in MT. Lifetime of the S2 state is shorter (varying in the range 0.28-3.5 ps in different solvents) than that predicted from the Delta E(S2-S1) value and this can be attributed to its flexible molecular structure, which promotes an efficient intramolecular radiationless deactivation pathways. The lifetime of the S1 state (approximately 1.9-6.5 ps) is also very short because of small energy difference between the S1 and T1 states (Delta E(S1-T1) approximately 300 cm(-1)) in cyclohexane and hydrogen-bonding interaction as well as the presence of the isoenergetic T1(pipi*) state to enhance the rate of the intersystem crossing process from the S1(npi*) state in protic solvents.     

Photochemical Reduction of 2-Bromo-4,4-dimethyl-2-cyclohexenone

The photochemical behaviour of 2-Bromo-4,4-dimethyl-2-cyclohexenone (1) was studied in 2-propanol and cyclohexane. In both solvents (n-π*)-excitation followed by intersystem crossing leads to population of a low-lying triplet (T₁) state, which can be quenched by 1,3-cyclohexadiene but does not undergo chemical transformation efficiently. (π-π*)-Excitation affords 4,4-dimethyl-2-cyclohexenone ( 2 ) as the only product. While in 2-propanol 2 is formed in 60% from the S₂-state and in 40% from the T₂-state, in cyclohexane reduction occurs exclusively from this upper triplet state. The T₂-state can also be populated via energy transfer using acetone or benzene as sensitizer. The mechanistic dissimilarities for the reduction of excited 1 in either 2-propanol or cyclohexane are discussed.