Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and 1700 cm⁻¹ in room-temperature acetonitrile-d₃ solution. These bands and additional ones observed between 1300 and 1450 cm⁻¹ are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4O carbonyl exhibits substantial single-bond character, explaining the large (70 cm⁻¹) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed ¹nπ^* state as the triplet precursor.     

Insight into the drastically different triplet lifetimes of BODIPY obtained by optical/magnetic spectroscopy and theoretical computations

The triplet state lifetimes of organic chromophores are crucial for fundamental photochemistry studies as well as applications as photosensitizers in photocatalysis, photovoltaics, photodynamic therapy and photon upconversion. It is noteworthy that the triplet state lifetime of a chromophore can vary significantly for its analogues, while the exact reason was rarely studied. Herein with a few exemplars of typical BODIPY derivatives, which show triplet lifetimes varying up to 110-fold (1.4–160 μs), we found that for these derivatives with short triplet state lifetimes (ca. 1–3 μs), the electron spin polarization (ESP) pattern of the time-resolved electron paramagnetic resonance (TREPR) spectra of the triplet state is inverted at a longer delay time after laser pulse excitation, as a consequence of a strong anisotropy in the decay rates of the zero-field state sublevel of the triplet state. For the derivatives showing longer triplet state lifetimes (>50 μs), no such ESP inversion was observed. The observed fast decay of one sublevel is responsible for the short triplet state lifetime; theoretical computations indicate that it is due to a strong coupling between the T_z sublevel and the ground state mediated by the spin–orbit interaction. Another finding is that the heavy atom effect on the shortening of the triplet state lifetime is more significant for the T₁ states with lower energy. To the best of our knowledge, this is the first systematic study to rationalize the short triplet state lifetime of visible-light-harvesting organic chromophores. Our results are useful for fundamental photochemistry and the design of photosensitizers showing long-lived triplet states.

The electron spin polarization inversion and anisotropic decay of triplet substates explain the short triplet state lifetime of BODIPY derivatives.     

Distribution and conjugation of the π-electrons in the T1 state of some BNC-heterocycles

For the first time the lowest excited triplet state T₁ of BNC-heterocycles has been investigated. The phosphorescence and ODMR-spectroscopic measurements allow conclusions with respect to the distribution and conjugation of the π-electrons and thus the controversial question of the aromaticity of these compounds. Whereas T₁ in 2-methyl-1,3,2-benzodiazaborolidine may be described by a benzene-like ³B_1u state which is distorted along the ν₈ vibration the decay characteristics of the triplet state of 2-methyl-1,3,2-benzothiazaborolidine is dominated by the local C_2v symmetry around the heavy atom sulfur. In the case of 2-methyl-2-boradihydroperimidine we find a naphthalene-like ³A₁ state for T₁. The zero-field splitting confirms that the electronic triplet excitation is essentially localized in the benzene or naphthalene nucleus and the annellated heterochain participates only little in the π-conjugation.     

ChemInform Abstract: Structures,Energetics, and Vibrational Frequencies of the Silicon and Germanium Dichlorides and Dibromides and Their Dimers.

Ab initio Hartree-Fock calculations, performed to determine the geometries and energies of the ground singlet (1A1) and the first excited triplet (3B1) states of SiCl₂, SiBr₂, GeCl₂, and GeBr₂ and of the ground states of their dimers, show that the triplet state is not energetically accessible at the experimental temp. (600 °C) and therefore is not a possible model for the additional component in the electron diffraction radial distribution of GeBr₂.     

A theoretical comparison of the lowest-lying singlet and triplet states of H2CBe and of HCBeH

The low-lying singlet and triplet states of H₂CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H₂CBe has C_2v symmetry and is a triplet with one π electron (³ B₁). The results presented here suggest that the lowest-energy singlet structure is the (¹B₁) open-shell singlet, also with C_2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C_2v structure with two π electrons (¹A₁) is 15.9 kcal/mol higher than ³B₁. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (³Σ) and singlet (¹Δ) states. The triplet is more stable than the singlet (¹Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H₂ CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H₂ CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.     

Vibronic interaction and triplet state photophysics of phenylisatin and oxindole

Photophysical study of phenylisatin and oxindole triplet states have been made at room temperature and in different glasses at 77K. Qualitatively, in all respects the compounds have identical spectroscopic characteristics. Phosphorescence emission, excitation along with their polarization and lifetime suggest that a perturbation of the zero-point level of emitting state (³ππ*) by a close-lying triplet state (³nπ*) leads to a number of new spectral features. The experimental observations have been interpreted satisfactorily in terms of a switch (³ππ* state to ³nπ*) in the character of the lowest triplet states (T₁ and T₂) and also a similar switch in the character of the excited singlet states S₁ and S₂ for a change of glass matrix from MCH to ethanol. Invoking of first order and second order spin-orbit coupling explains the phosphorescence emission unambiguously.     

Theoretical Study on the Formation of H‐ and O‐Atoms,HONO, OH,NO, and NO2 from the Lowest Lying Singlet and Triplet States in Ortho‐Nitrophenol Photolysis

The photolysis of nitrophenols was proposed as a source of reactive radicals and NOx compounds in polluted air. The S₀ singlet ground state and T₁ first excited triplet state of nitrophenol were investigated to assess the energy dependence of the photofragmentation product distribution as a function of the reaction conditions, based on quantum chemical calculations at the G3SX//M06–2X/aug‐cc‐pVTZ level of theory combined with RRKM master equation calculations. On both potential energy surfaces, we find rapid isomerization with the aci‐nitrophenol isomer, as well as pathways forming NO, NO₂, OH, HONO, and H‐, and O‐atoms, extending earlier studies on the T₁ state and in agreement with available work on other nitroaromatics. We find that accessing the lowest photofragmentation channel from the S₀ ground state requires only 268 kJ/mol of activation energy, but at a pressure of 1 atm collisional energy loss dominates such that significant fragmentation only occurs at internal energies exceeding 550 kJ/mol, making this surface unimportant for atmospheric photolysis. Intersystem crossing to the T₁ triplet state leads more readily to fragmentation, with dissociation occurring at energies of ～450 kJ/mol above the singlet ground state even at 1 atm. The main product is found to be OH + nitrosophenoxy, followed by formation of hydroxyphenoxy + NO and phenyloxyl + HONO. The predictions are compared against available experimental data.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. II. Calculation of spin-orbit coupling

A calculation of the spin-orbit coupling in the lowest excited triplet state of tetramethyl-1,3-cyclobutanedithione (TMCBDT) has been performed. The results show the following. (1) In the TMCBDT crystal the ground singlet-to-lowest triplet transition moment is predicted to be exclusively ? c polarized, as observed. (2) The assignment of the lowest triplet state should be ³A_u as found earlier for the oxygen analog. TMCBD. (3) The two largest contributions (～ 60%) to the isolated-molecule T₁ → S₀ transition moment come from the two triplet-triplet transitions.

and

, both of which are polarized along the CS bonds. (4) The total contribution to the transition moment parallel to the CS bonds is 76% from the T_l ← T₁ transitions and 24% from the S_γ ← S₀ transitions. And, finally, (5) the calculated oscillator strength of 2 × 10^?4 for the largest T₁ ← S₀ component (along y) falls within the range of typical spin-allowed, singlet-singlet n-⁼ transitions.     

On the regioselectivity of the nucleophilic aromatic photosubstitution. The photoreaction of 4-nitroveratrole with n-hexylamine.

4-Nitroveratrole is photosubstituted with n-hexylamine giving rise to two isomeric anilines, N-hexyl-2-methoxy-5-nitroaniline and N-hexyl-2-methoxy-4-nitroaniline. Mechanistic evidence indicates that the first is produced in an SN₂Ar reaction through singlet and triplet excited states, whereas the second arises from a radical ion pair via electron transfer from the amine to a triplet excited state.     

Photoelectron spectroscopy of the trimethylenemethane negative ion

The photoelectron spectrum of the trimethylenemethane (TMM) negative ion is described. The electron affinity of TMM is found from the spectrum to be 0. 431±0.006 eV, and the energy difference between the [(X)\tilde]³ A￠₂\tilde X^3 A'_2 ³A′₂ and [(b)\tilde]¹ A₁\tilde b^1 A_1 ¹A₁ states of TMM is determined to be 16.1±0.2 kcal/mol. The energy difference between the lowest energy triplet and singlet states is estimated to be 13–16 kcal/mol. The enthalpy of formation of TMM is measured to be 70±3 kcal/mol, and the C-H bond enthalpy in 2-methylallyl radical is 90±2 kcal/mol. Previously unobserved vibrational frequencies of 425, 915, and 1310 cm⁻¹ are found for the triplet state of TMM, whereas a frequency of 325 cm⁻¹ is found for the singlet state. In addition, an overtone peak is observed for the triplet state at 1455 cm⁻¹, and both states contain peaks that are assigned to bands arising from excited vibrational levels of the ion.     

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VII. Complete assignment of the 00, 81, 6171 and 41 anticrossing spectra

In paper VI of this series, we have made the statistical analysis of the singlet-triplet coupling matrix elements for ten N_S = 0 singlet vibrational levels of glyoxal, without determining the triplet quantum numbers. In this paper we present the complete assignment (triplet rotational quantum numbers and vibrational symmetry) of each anticrossing observed from four singlet vibrational levels: 0⁰, 8¹, 6¹7¹ and 4¹ (which give respectively 36, 76, 155 and 145 anticrossings by 0 to 7.5 T scans). Therefore we determine the zero-field energy origin of most of the triplet vibrational levels which are located within 7 cm⁻¹ of the four singlet levels studied. Two kinds of selection rules are found: the first stemming from direct vibronic spin-orbit interactions and the second from indirect ones (involving an intermediate triplet state). About half of the anticrossings are due to direct vibronic spin-orbit interactions, the mean value of their matrix elements is more than ten times larger than indirect matrix elements and thus are dominant in ϱ〈V_st〉. In conclusion, we confirm that ISC in glyoxal is governed by direct vibronic spin-orbit interactions.     

Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer1B2u and 3B1u spectra

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C₁₀H₈; 1-DC₁₀H₇; 2-DC₁₀H₇; 1,4-D₂C₁₀H₆; 1,4,5-D₃C₁₀H₅; 1,4,5,8-D₄C₁₀H₄; 1,2,4,5,8-D₅C₁₀H₃; a β-D₄C₁₀H₄ and a β₂-D₆C₁₀H₂ as guests in C₁₀D₈ host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest—host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 ± 20 (b/a), which is in poor agreement with the transition octupole—transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm^?1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm^?1) for the 0—0 transition of the I-DC₁₀H₇ guest in C₁₀D₈ host. The order of the α and β substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest Π-Π^* singlet and triplet states of naphthalene.     

Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study

Photochemical activation by triplet photosensitizers is highly expedient for a green focus society. In this work, we have theoretically probed excited state characteristics of thioxanthone and its derivatives for their triplet harvesting efficiency using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Absorption and triplet energies corroborate well with the available experimental data. Our results predict that both the S₁ and T₁ states are π-π^* in nature, which renders a high oscillator strength for S₀ to S₁ transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S₁ (¹π-π^*) and high energy ³n- π^* state. Apart from that, there is both radiative and non-radiative channel from S₁ to S₀, which competes with the ISC channel and reduces the triplet harvesting efficiency. For thioxanthones with −OMe (Me=Methyl) or −F substitution at 2 or 2’ positions, the ISC channel is not energetically feasible, causing sluggish intersystem crossing quantum yield (Φ_ISC). For unsubstituted thioxanthone and for isopropyl substitution at 2’ position, the S₁-T₁ gap is slightly positive ( ), rendering a lower triplet harvesting efficiency. For systems with −OMe or −F substitution at 3 or 3’ position of thioxanthone, because of buried π state and high energy π^* state, the S₁-³nπ^* gap becomes negative. This leads to a high Φ_ISC (>0.9), which is key to being an effective photocatalyst.     

Nonempirical Investigation of Equilibrium Geometry and Electronic Structure of Kynurenine and 3-Hydroxykynurenine Molecule

By Hartree-Fock-Roothaan method with complete geometry optimization in the basis 6-31G* ab initio calculations of equilibrium geometry and electronic structure were performed for kynurenine C₁₀H₁₂N₂O₃ and 3-hydroxykynurenine C₁₀H₁₂N₂O₃ molecules in the singlet ground state and the first triplet excited state. The molecules in the triplet state can react at the oxygen of the carbonyl group adjacent to the aromatic ring by quite different pathway compared to the molecules in the ground singlet state.     

Triplet excited states of nitronaphthalenes. III. 1,4-dinitronaphthalene

Nanosecond flash photolysis of 1,4-dinitronaphthalene (1,4-DNO₂N) in aerated and deaerated solvents shows a transient species with absorption maximum at 545 nm. The maximum of the transient absorption is independent of solvent polarity and its lifetime seems to be a function of the hydrogen donor efficiency of the solvent. The transient absorption is attributed to the lowest excited triplet state of 1,4-DNO₂N. The reactivity of this state for hydrogen abstraction from tributyl tin hydride (Bu₃SnH), K_q = 3.8 × 10⁸M^?1 sec, is almost equal to that of nitrobezene triplet state which has been characterized as an n → π* state. Based on spectroscopic and kinetic evidence obtained in the present work, the triplet state of 1,4-DNO₂N behaves as an n → π* state in nonpolar solvents, while in polar solvents the state is predominantly n → π* with a small amount of intramolecular charge transfer character.     

Ab initio calculation of the lowest singlet and triplet states in CH2, CHF,CF2, and CHCH3

The lowest singlet and triplet states of the radicals CH₂, CHF, CF₂, and CHCH₃ have been investigated both in SCF and IEPA approximation (“independent electron pair approach” to account for electron correlation). The SCF calculations yield triplet ground states for CH₂, CHF, and CHCH₃, and a singlet ground state for CF₂. Electron correlation stabilizes the singlet state by about 14 kcal/mole with respect to the triplet for all four radicals leading to a singlet ground state also for CHF. The final triplet-singlet energy separations are 10, 6, ?11, ?47 kcal/mole for CH₂, CHCH₃, CHF, CF₂, respectively. Values for equilibrium bond angles, ionization potentials and bond energies are also given.     

Exploring the Reaction Paths on the Potential Energy Surfaces of the S1 and T1 States in Methylenecyclopropane

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T₁) state and the lowest excited singlet (S₁) state, as well as that of the ground state (S₀), were explored by using the nudged elastic band method at the MRMP2//MCSCF/6‐31++G(d,p) and DFT(B3LYP)/6‐31++G(d,p) levels of theory. After vertical excitation of 1, three transition states on the PES of the lowest triplet state and one transition state on the S₁ PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S₁ state produced by vertical excitation at the S₀ energy minimum in 1. Fast transition is predicted to occur from the T₁ state or from the S₁ state to the S₀ state due to strong spin‐orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S₀ PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol^?1 were obtained for C migration reaction (backward reaction), 1,2‐H migration reaction to cyclobutene 3, and 1,3‐H migration reaction to bicyclopropane 4, respectively, started at 2. The introduction of phenyl groups makes the energy barriers smaller due to the π conjugation between the carbene center and phenyl groups.     

Effects of temperature on the triplet behavior of the styrylphenanthrene isomers

The properties of the triplet state of five styrylphenanthrene (StPh) trans isomers were studied in 2-methyltetrahydrofuran (MTHF) as a function of temperature. At room temperature the T-T absorption was observed only for 4- and 9-StPh, while under these conditions 1-, 2-, and 3-StPh have too low a quantum yield of triplet formation (Φ_T <0.02); their T-T absorption spectra were obtained at low temperature. Φ_T of 1- and 2-StPh increases more than tenfold on going from 293 to 77 K, and the triplet lifetime (τ_T) increases by four orders of magnitude and approaches values of 5–40 ms at 77 K. The change in τ_T is explained in terms of an equilibrium between trans and perpendicular (perp) conformations of the lowest triplet state in fluid solution and temperature and viscosity effects on the trans → perp rotation. Evidence is presented for the existence of two conformeric trans triplet states of 3-StPh at 77 K. Semi-empirical calculations were performed to obtain the energy of the triplet state, the wavelengths of several T-T absorption maxima (λ_T), and the oscillator strength. The calculated λ_T values coincide with those measured in n-hexane.     

Synthesen mit Nitrilen. 34. Mitteilung. Phenylhydrazone des Dicyanmethylenindandions: Photolyse von 2-Dicyanmethylen-1,3-indandion-monophenylhydrazon in Methylenchlorid

Photolysis of 2-dicyanomethylene-1, 3-indandione-monophenylhydrazone 1 during several days yielded 3-(N-chlorimino)-5-oxo-2-phenyl-2.3-dihydro-5H-indeno[1.2-c]pyridazine-4-carbonitrile 2 . Isomerization to 3-imino-5-oxo-2-phenyl-2.3-dihydro-5H-indeno[1.2-c]pyridazine-4-carbonitrile 4 was also observed. Radical formation from the first triplet state leads to chlorine abstraction from the solvent and to formation of 2 . The same reaction occurs from the first triplet state of 4 .