Designing Efficient and Ultralong Pure Organic Room‐Temperature Phosphorescent Materials by Structural Isomerism

Pure organic materials with ultralong room‐temperature phosphorescence (RTP) are attractive alternatives to inorganic phosphors. However, they generally show inefficient intersystem crossing (ISC) owing to weak spin–orbit coupling (SOC). A design principle based on the realization of small energy gap between the lowest singlet and triplet states (ΔE_ST) and pure ππ* configuration of the lowest triplet state (T₁) via structural isomerism was used to obtain efficient and ultralong RTP materials. The meta isomer of carbazole‐substituted methyl benzoate exhibits an ultralong lifetime of 795.0 ms with a quantum yield of 2.1 %. Study of the structure–property relationship shows that the varied steric and conjugation effects imposed by ester substituent at different positions are responsible for the small ΔE_ST and pure ππ* configuration of T₁.     

Photophysical Behavior of Angelicins and Thioangelicins: Semiempirical Calculations and Experimental Study

The decay processes of the lowest excited singlet and triplet states of five methylated angelicins (4,6,4′-trimethyl-angelicin, MA, and four methylated thioangelicins, MTA; see Scheme 1) were investigated in live solvents by stationary and pulsed fluorometric and flash photolytic techniques. In particular, the solvent effects on absorption, fluorescence, quantum yields of fluorescence (φ_F) and triplet formation (φ_T), lifetimes of fluorescence (τ_F) and the triplet state (τ_T) and the quantum yields of singlet oxygen production (φ_Δ) were investigated. Semiempirical (ZINDO/S-CI) calculations were carried out to obtain information (transition probabilities and nature) on the lowest excited singlet and triplet states. The quantum mechanical calculations and the solvent effect on the photophysical properties showed that the lowest excited singlet state (S¹) is a partially allowed π,π* state, while the close-lying S₂ state is n,π* in nature. The efficiencies of fluorescence, S₁→T₁ intersystem crossing (ISC) and S₁→ S₀ internal conversion (IC) strongly depend on the energy gap between S₁, and S₂ and are explained in terms of the so-called proximity effect. In fact, for MA in cyclohexane, only the S₁→ S₀ internal conversion is operative, while in acetonitrile and ethanol, where the n.π* state is shifted to higher energy, the efficiencies of fluorescence and ISC increase significantly. The energy gap between S₁ and S₂ increases in MTA, where the furanic oxygen is replaced by a sulfur atom. Consequently, the solvent effect on the photophysical parameters of MTA is less marked than for MA; e.g. fluorescence and triplet-triplet absorption are also detectable in the nonpolar cyclohexane. The lowest excited singlet state of molecular oxygen O₂(¹D_g) was produced efficiently in polar solvents by energy transfer from the T₁ state of MA and MTA.     

Unravelling the Reaction Mechanism for the Fast Photocyclisation of 2‐Benzoylpyridine in Aqueous Solvent by Time‐Resolved Spectroscopy and Density Functional Theory Calculations

A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR³ spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH⁺) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S₁) of 2‐BPy‐NH⁺ is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T₁) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH⁺) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions.     

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.     

Photophysical properties of some phenetidines in non-polar and hydrogen bonding solvents at room temperature and 77 K

The present study was undertaken to investigate the photophysical processes in o-, m- and p-phenetidines, when dissolved in nonpolar and hydrogen bonding solvents, in their ground state and excited electronic state S₁, both at 300 and 77 K. In the ground as well as in the S₁ state it is proposed that the o-phenetidine molecule possesses a structure in which NH₂ and OC₂H₅ groups are away from each other, both in nonpolar cyclohexane (CH) and H-bond acceptor solvent triethylamine (TEA). The formation of a transient or nonemissive charge transfer (CT) complex resulting from strong excited state hydrogen bonding interaction with TEA is found to be responsible for the observed fluorescence quenching of the proton donor phenetidines at 300 K. From the room as well as low (77 K) temperature electronic absorption and steady state fluorescence studies, it was deduced that nonplanarity in the structure of the molecules increases as one moves from aniline to the phenetidines. It is suggested that in the solvent stiffening temperature 77 K, triplet states of all the phenetidines (o-, m- and p-) acquire some nπ* character due to conformational changes, whereas ππ* character is retained in their S₁ state. This facilitates a larger intersystem crossing (ISC) rate in phenetidines relative to the situation in aniline where both S₁ and T₁ possess the same nπ* nature at 77 K due to its more planar structure. However, ISC efficiency in phenetidines at 77 K is found to be impeded, especially in the case of o- and m-isomers, in the presence of TEA as inferred from the lowering of φ_p/φ_⨍ values and the increment of τ_p. In p-phenetidine, rapid equilibrium between a triplet state hydrogen bonded species and free molecules during the triplet excited state lifetime is suggested.     

Three-dimensional diabatic models for the ππ* → nπ* excited-state decay of uracil derivatives in solution

Uracil and its derivatives strongly absorb UV light but photodamage is hampered through effective non-adiabatic decay channels. As a first step towards a quantum dynamical (QD) study of the decay route of the photoexcited ππ* state to the underlying nπ* state, here we present our procedure to build a reliable reduced-dimensionality model of the decay process, and we discuss its theoretical foundation. We established the three most important nuclear coordinates for the decay process and we computed the S ₁ and S ₂ excited-state potential energy surfaces of Uracil and 5-fluoro-Uracil in acetonitrile and in water at TD-DFT level, describing the solvent in the frame of polarizable continuum model. Through a property-based diabatization we obtained the diabatic ππ* and nπ* states’ energies and coupling and we fitted them to analytical functions of the nuclear coordinates. We show how these diabatic models can be utilized for QD simulations of the ππ* → nπ* decay.     

Photophysical properties of the isomorphic emissive RNA nucleobase analogues and effect of water solution,ribose, and base pairing: A theoretical study

In the present work, a comprehensive theoretical investigation on the excited state properties of the isomorphic emissive RNA nucleobase analogues, namely ^tzA, ^tzG, ^tzC, and ^tzU, was performed. Vertical transition energies are determined with the time‐dependent density functional theory method at both the B3LYP and CAM‐B3LYP levels using the 6‐311++G(d,p) basis set. The nature of the low‐lying singlet excited states is discussed and the results are compared with the findings from experiment and those for thieno analogues and natural bases. In gas phase, it was found that the S₁ state is ππ* in nature for all the tz‐bases except for ^tzA, for which the S₁ state is predicted to be nπ* in nature with the ππ* state being the S₂. While in water solution, the S₁ state for all tz‐bases are predicted to be ππ* dominated by the configuration HOMO→LUMO. Compared with natural bases, the lowest ππ* states are about 0.85–1.22 eV red‐shifted. When compared with thieno analogues, it is interesting to note that the S₁ state (ππ*) transition energies of the two counterparts from the two alphabets are nearly equal due to the very little differences of their HOMO‐LUMO gaps. In addition, it was found that the hydration + PCM model can perfectly reproduce the photophysical properties of the tz‐bases since the calculated excitation maxima and fluorescence are in good agreement with the experimental data. The microenvironment effects of linking to ribose, base pairing, and further hydration of base pairs were also studied.     

Photochemical reaction mechanism of cyclobutanone: CASSCF study

The photochemical reaction channels of cyclobutanone have been studied at the CASSCF level with a 6‐31G* basis set. Starting from the n‐π* excited‐state (S₁) cyclobutanone, the three reactions can take place: decarbonylation (produce CO and cyclopropane or propylene), cycloelimination (produce ketene and ethylene), and ring expansion (produce oxacarbene). Our computation indicates that decarbonylation products CO and triplet trimethylene are formed on the triplet n‐π* excited state (T₁) in a stepwise way via a biradical intermediate after intersystem crossing (ISC) to T₁ from S₁. And, then, the triplet trimethylene undergoes a second ISC to the ground state (S₀) to produce the singlet trimethylene from which cyclopropane can be produced rapidly only overcoming a 1 to 2‐kcal/mol barrier while propylene can be formed as a secondary product. The cycloelimination products ketene and ethylene are formed on the S₀ in a concerted mechanism after internal conversion (IC) to S₀ from S₁ via a biradical conical intersection. The reaction channels corresponding to ring expansion on the S₀, T₁, and S₁ states have also been discussed, and the likeliest reaction path is that oxacarbene is formed on the ground state following S₁/S₀ internal conversion. The surface topology of cyclobutanone on the S₁ surface is characterized by a transition state separating the minimum from the S₁/S₀ conical intersection, which is consistent with the previous computations and can explain the wavelength dependence of the fluorescence emission yield. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been stud-ied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S₂ state with a ππ^* character from S₀state. The time evolution of the parent ion sig-nals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo-electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S₂ state. In brief, the ultrafast component is a population transfer from S₂ to S₁ through the S₂/S₁ conical intersections, the slow component is attributed to simultaneous IC from the S₂ state and the higher vibrational levels of S₁ state to S₀ state, which involves the coupling of S₂/S₀ and S₁/S₀ conical intersections. Additionally, the observed ultrafast S₂→S₁ transition occurs only with an 18% branching ratio.     

Rapid relaxation pathway of the excited state of linear merocyanines in solutions

Excited‐state relaxation of linear merocyanine dyes in solution is investigated using time‐resolved spectroscopy techniques and quantum chemical calculations. The merocyanine L‐Mero4 and phenyl‐substituted P‐L‐Mero4 have a S‐trans and S‐cis structure, respectively, consisting of indole moiety as the donor, indandione as the acceptor, and the tetramethine as the bridge. The time‐correlated single‐photon counting (TCSPC) picosecond measurements after excitation at wavelength 515 nm to the ππ* state yield emission curves with a short component τ₁ in the range of 27–160 ps and a second component τ₂ of 200–780 ps for L‐Mero4. In P‐L‐Mero4, τ₁ lies in the range of 18–150 ps and τ₂ 220–520 ps. The subfemtosecond transient absorption measurements yield a short component around 0.4–1.4 ps, and the second/third components are similar to those in the TCPSC measurements. The analysis of the experimental data demonstrates that the ground state recovery exhibits a biexponential rise and rapidly indicates that the conversion back to the electronic ground state provides a fast, nonradiative pathway. Quantum chemical calculations on the electronic structures and their dependence on the molecular confirmation are performed. We identify the excited states and the relaxation path along the twist of the center double bonds in tetramethine that might be the nonradiative pathway. The C=C double bond is weakened in the ππ* state. The phenyl substitution in the conjugated double bond weakens this C=C bond, lowers the isomerization barrier, increases the nonradiative rate, and reduces the emission quantum yield. In polar solvents, the energy of the perpendicular conformer along the trans–cis isomerization path is increased to achieve less coupling to the ground state surface. Because of the small barrier to the trans form, these two conformers establish an equilibrium condition. The trans form, which lies at a lower energy, gains more population and thus has a higher emission yield.     

Fluorescence Quenching of Benzaldehyde in Water by Hydrogen Atom Abstraction

We computed the mechanism of fluorescence quenching of benzaldehyde in water through relaxed potential energy surface scans. Time‐dependent density functional theory calculations along the protonation coordinate from water to benzaldehyde reveal that photoexcitation to the bright ππ* (S₃) state is immediately followed by ultrafast decay to the nπ* (S₁) state. Evolving along this state, benzaldehyde (BA) abstracts a hydrogen atom, resulting in a BAH^. and OH^. radical pair. Benzaldehyde does not act as photobase in water, but abstracts a hydrogen atom from a nearby solvent molecule. The system finally decays back to the ground state by non‐radiative decay and an electron transfers back to the OH^. radical. Proton transfer from BAH⁺ to OH^? restores the initial situation, BA in water.     

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

Transient absorption spectroscopy is used to study the excited‐state dynamics of Co₃(dpa)₄(NCS)₂, where dpa is the ligand di(2‐pyridyl)amido. The ππ*, charge‐transfer, and d–d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low‐lying metal‐centered d–d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand‐centered ππ* and the charge‐transfer states to this metal‐centered d‐d state within 100 fs. The excited molecule then crosses to a second d–d state within the ligand‐field manifold, with a time coefficient of 0.6–1.4 ps. Because the ground‐state bleaching band recovers with a time coefficient of 10–23 ps, we propose that an excited molecule crosses from the low‐lying d–d state either directly within the same spin system or with spin crossing via the state ²B to the ground state ²A₂ (symmetry group C₄). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.     

Solvent-Dependent Stabilization of a Charge Transfer State is the Key to Ultrafast Triplet State Formation in an Epigenetic DNA Nucleoside

2’-Deoxy-5-formylcytidine (5fdCyd), a naturally occurring nucleoside found in mammalian DNA and mitochondrial RNA, exhibits important epigenetic functionality in biological processes. Because it efficiently generates triplet excited states, it is an endogenous photosensitizer capable of damaging DNA, but the intersystem crossing (ISC) mechanism responsible for ultrafast triplet state generation is poorly understood. In this study, time-resolved mid-IR spectroscopy and quantum mechanical calculations reveal the distinct ultrafast ISC mechanisms of 5fdCyd in water versus acetonitrile. Our experiment indicates that in water, ISC to triplet states occurs within 1 ps after 285 nm excitation. PCM-TD-DFT computations suggest that this ultrafast ISC is mediated by a singlet state with significant cytosine-to-formyl charge-transfer (CT) character. In contrast, ISC in acetonitrile proceeds via a dark ¹nπ* state with a lifetime of ∼3 ps. CT-induced ISC is not favored in acetonitrile because reaching the minimum of the gateway CT state is hampered by intramolecular hydrogen bonding, which enforces planarity between the aldehyde group and the aromatic group. Our study provides a comprehensive picture of the non-radiative decay of 5fdCyd in solution and new insights into the factors governing ISC in biomolecules. We propose that the intramolecular CT state observed here is a key to the excited-state dynamics of epigenetic nucleosides with modified exocyclic functional groups, paving the way to study their effects in DNA strands.     

Dual fluorescence and ultrafast intramolecular charge transfer with 6-N,N-dialkylaminopurines. A two-state model

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ'(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ'(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ'(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ? k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ～ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ? ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0).     

Nonadiabatic dynamics of uracil: population split among different decay mechanisms

Nonadiabatic dynamics simulations performed at the state-averaged CASSCF method are reported for uracil. Supporting calculations on stationary points and minima on the crossing seams have been performed at the MR-CISD and CASPT2 levels. The dominant mechanism is characterized by relaxation into the S(2) minimum of ππ* character followed by the relaxation to the S(1) minimum of nπ* character. This mechanism contributes to the slower relaxation with a decay constant larger than 1.5 ps, in good agreement with the long time constants experimentally observed. A minor fraction of trajectories decay to the ground state with a time constant of about 0.7 ps, which should be compared to the experimentally observed short constant. The major part of trajectories decaying with this time constant follows the ππ* channel and hops to the ground state via an ethylenic conical intersection. A contribution of the relaxation proceeding via a ring-opening conical intersection was also observed. The existence of these two latter channels together with a reduced long time constant is responsible for a significantly shorter lifetime of uracil compared to that of thymine.     

UV‐Dissipation Mechanisms in the Eumelanin Building Block DHICA

The UV‐dissipative mechanisms of the eumelanin building block 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) and the 4,7‐dideutero derivative (DHICA‐d₂) in buffered H₂O or D₂O have been characterized by using ultrafast time‐resolved fluorescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluorescence. The band peaking at λ=378 nm is caused by emission from the excited initial geometry. The second band around λ=450 nm is owed to a complex formed between the mono‐anion and specific buffer components. In the absence of complex formation, the mono‐anion solely decays non‐radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around λ=427 nm with a short lifetime of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub‐ps excited‐state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future studies will unravel whether this also occurs in larger building blocks and ESIPT is a built‐in photoprotective mechanism in epidermal eumelanin.     

Coupled electron and proton transfer processes in 4-dimethylamino-2-hydroxy-benzaldehyde

TDDFT calculations, picosecond transient absorption, and time-resolved fluorescence studies of 4-dimethylamino-2-hydroxy-benzaldehyde (DMAHBA) have been carried out to study the electron and proton transfer processes in polar (acetonitrile) and nonpolar (n-hexane) solvents. In n-hexane, the transient absorption (TA) as well as the fluorescence originate from the ππ* state of the keto form (with the carbonyl group in the benzaldehyde ring), which is produced by an intramolecular proton transfer from the initially excited ππ* state of the enol form (OH group in the ring). The decay rate of TA and fluorescence are essentially identical in n-hexane. In acetonitrile, on the other hand, the TA exhibits features that can be assigned to the highly polar twisted intramolecular charge transfer (TICT) states of enol forms, as evidenced by the similarity of the absorption to the TICT-state absorption spectra of the closely related 4-dimethylaminobenzaldehyde (DMABA). As expected, the decay rate of the TICT-state of DMAHBA is different from the fluorescence lifetime of the ππ* state of the keto form. The occurrence of the proton and electron transfers in acetonitrile is in good agreement with the predictions of the TDDFT calculations. The very short-lived (～1 ps) fluorescence from the ππ* state of the enol form has been observed at about 380 nm in n-hexane and at about 400 nm in acetonitrile.     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

Proximity effect in spin sublevel dynamics: Possible influence of a low-lying3 A2 (nπ*) state: On singlet → triplet intersystem crossing of Pyrimidine

Comparison of spin sublevel population rates in pyrimidine and 5-methylpyrimidine suggests that the vibronic perturbation of ³B₂ and/or ³A₁ ππ* states by a higher-lying ³A₂nπ* state plays an important role in S₁ → T₁ intersystem crossing of pyrimidine. The result also implies that the n₊π*—n_?π* splitting in pyrimidine is significantly smaller than the corresponding gap in pyrazine. The appearance of the 0.0 band in the τ_x spectrum indicates that the triplets state symmetry is lower tha C_2v.     

Excited-state lifetime of adenine near the first electronic band origin

The excited-state lifetime of supersonically cooled adenine was measured in the gas phase by femtosecond pump-probe transient ionization as a function of excitation energy between 36?100 and 37?500cm(-1). The excited-state lifetime of adenine is ～2ps around the 0-0 band of the (1)L(b) ππ(?) state (36?105cm(-1)). The lifetime drops to ～1ps when adenine is excited to the (1)L(a) ππ(?) state with the pump energy at 36?800cm(-1) and above. The excited-state lifetimes of (1)L(a) and (1)L(b) ππ(?) states are differentiated in accordance with previous frequency-resolved and computational studies.