The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Time-resolved resonance Raman and computational investigation of the influence of 4-acetamido and 4-N-methylacetamido substituents on the chemistry of phenylnitrene

A time-resolved resonance Raman (TR(3)) and computational investigation of the photochemistry of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide in acetonitrile is presented. Photolysis of 4-acetamidophenyl azide appears to initially produce singlet 4-acetamidophenylnitrene which undergoes fast intersystem crossing (ISC) to form triplet 4-acetamidophenylnitrene. The latter species formally produces 4,4'-bisacetamidoazobenzene. RI-CC2/TZVP and TD-B3LYP/TZVP calculations predict the formation of the singlet nitrene from the photogenerated S(1) surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4'-bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR(3) experiments. In contrast, only one species can be observed in analogous TR(3) experiments after photolysis of 4-N-methylacetamidophenyl azide in acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the photochemistry of phenyl azides and the chemistry of the related nitrenes.     

瞬态吸收和共振拉曼光谱研究硝基对联苯氮宾、氮宾离子反应活性的影响

利用光解芳基叠氮化合物得到单重态氮宾,运用纳秒瞬态吸收光谱、瞬态共振拉曼光谱实验手段,辅以密度泛函理论(DFT)计算,研究了4'-硝基-4-联苯氮宾在乙腈和水溶液中的光化学反应中间体。实验结果表明,在非质子溶剂中, 4'-硝基-4-联苯氮宾发生系间窜越反应生成三线态氮宾;在质子溶剂中,单重态氮宾可被质子化产生氮宾离子。与4-联苯氮宾和氮宾离子相比,硝基对单重态氮宾系间窜越反应路径影响很小;降低了氮宾离子与水和叠氮阴离子的反应活性,却提高了其与鸟苷的反应活性。     

Early events in the photochemistry of aryl azides from femtosecond UV/Vis spectroscopy and quantum chemical calculations

The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide was studied by ultrafast spectroscopy. In every case, the singlet azide second excited states were observed by transient absorption spectroscopy and were found to have lifetimes of hundreds of femtoseconds. The decay of the S(2) states of the azides was accompanied by the growth of transient absorption of the corresponding singlet nitrenes. The intermediate S(1) state of the azides could not be observed due to its low instantaneous concentration resulting from fast fragmentation and nitrene formation. Quantum chemical calculations predict that the S(2) state of the azide is bound and that there is a much lower barrier toward arylnitrene formation from the S(1) state of the azide. Vibrational cooling of para-biphenylnitrene (11 ps) was experimentally observed. The lifetime of singlet ortho-biphenylnitrene was 16 ps in acetonitrile and was not affected by perdeuteration of the aryl ring. The lifetime of singlet 1-naphthylnitrene is 12 ps in acetonitrile at ambient temperature.     

A laser flash photolysis and quantum chemical study of the fluorinated derivatives of singlet phenylnitrene.

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k(OBS). The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where k(R) is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after assuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.     

Competition between alpha-cleavage and energy transfer in alpha-azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.     

Photochemistry of ortho, ortho' dialkyl phenyl azides

Phenyl azide, 2,6-diethylphenyl azide, 2,6-diisopropylphenyl azide, and 2,4,6-tri-tert-butylphenyl azide were studied by laser flash photolysis (LFP) methods. LFP (266 nm) of the azides in glassy 3-methylpentane at 77 K produces the transient UV-vis absorption spectra of the corresponding singlet nitrenes. At 77 K, the singlet nitrenes relax to the corresponding triplet nitrenes. The triplet nitrenes are persistent at 77 K and their spectra were recorded. The rate constants of singlet to triplet intersystem crossing were determined at this temperature. LFP of 2,4,6-tri-tert-butyl phenyl azide in pentane at ambient temperature again produces a singlet nitrene, which is too short-lived to detect by nanosecond spectroscopy under these conditions. Unlike the other azides, the first detectable intermediate produced upon LFP of 2,4,6-tri-tert-butyl phenyl azide at ambient temperature is the benzazirine (285 nm) which has a lifetime of 62 ns controlled by ring opening to a didehydroazepine. The results are interpreted with the aid of Density Functional Theoretical and Molecular Orbital Calculations.     

Photolysis of alpha-azidoacetophenones: direct detection of triplet alkyl nitrenes in solution

We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of alpha-azido acetophenone derivatives, 1. Alphazides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. (13)C and (15)N isotope labeling studies allowed us to characterize the C-N stretch of the nitrene intermediate at 1201 cm(-)(1).     

Ultrafast spectroscopic and matrix isolation studies of p-biphenylyl, o-biphenylyl, and 1-naphthylnitrenium cations

p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.     

Understanding the rate of spin-forbidden thermolysis of HN3 and CH3N3

The pyrolysis of the simplest azides HN(3) and CH(3)N(3) has been studied computationally. Nitrogen extrusion leads to the production of NH or CH(3)N. The azides have singlet ground states but the nitrenes CH(3)N and NH have triplet ground states. The competition between spin-allowed decomposition to the excited state singlet nitrenes and the spin-forbidden N(2) loss is explored using accurate electronic structure methods (CASSCF/cc-pVTZ and MR-AQCC/cc-pVTZ) as well as statistical rate theories. Nonadiabatic rate theories are used for the dissociation leading to the triplet nitrenes. For HN(3), (3)NH formation is predicted to dominate at low energy, and the calculated rate constant agrees very well with energy-resolved experimental measurements. Under thermal conditions, however, the singlet and triplet pathways are predicted to occur competitively, with the spin-allowed product increasingly favored at higher temperatures. For CH(3)N(3) thermolysis, spin-allowed dissociation to form (1)CH(3)N should largely dominate at all temperatures, with spin-forbidden formation of (3)CH(3)N almost negligible. Singlet methyl nitrene is very unstable and should rearrange to CH(2)NH immediately upon formation, and the latter species may lose H(2) competitively with vibrational cooling, depending on temperature and pressure.     

Role of conformation and electronic structure in the chemistry of ground and excited state o-pyrazolylphenylnitrenes

The chemistry of 2-(1-pyrazolyl)- (2a) and 2-[1-(3,5-dimethylpyrazolyl]phenylnitrene (2b) has been studied in EtOH solution at room temperature, in EtOH glasses at 90 K, and in Ar matrices at 12 K. These nitrenes were chosen as suitable models for clarifying the mechanism of intramolecular reactions because attack at the pyrazole ring can occur according to different modes and the asymmetry of the substituent gives rise to different conformations. Detailed DFT and CASSCF/CASPT2 studies on the conformation and decay paths of both spin states of the nitrenes have been carried out. Ring expansion to dehydroazepines (via benzoazirines) is calculated to be competitive in both nitrenes, but in the dimethyl derivative, 2b, attack onto the N lone pair (which is made more nucleophilic by the methyl groups) is favored. The higher barriers (by 4-8 kcal/mol) in singlet 2a cause 60-70% of this nitrene to decay by intersystem crossing to the triplet. Thus, the seemingly straightforward formation of benzo-fused heterocycles through intramolecular attack of the pyrazoline N lone pair by the singlet phenylnitrene can only overcome ring expansion and intermolecular reactions under favorable circumstances. The comparatively persistent triplet nitrenes are characterized in matrices, and the yields of photocyclization products (mainly pyrazolo[1,5-a]benzimidazole (7) from 2a and 5,6-dihydropyrazolo[1,5-a]quinoxaline (8) from 2b) are shown to depend on the preferred conformation of the starting azide and nitrene.     

Ultrafast study of 9-diazofluorene: direct observation of the first two singlet states of fluorenylidene

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.     

Nitrenes and the Decomposition of Carbonylazides

The decomposition of organic carbonylazides can lead to the formation of nitrenes. Ethoxycarbonylnitrene is formed in the photolytic and thermal decomposition of ethyl azidoformate and by α-elimination from N-(p-nitrobenzenesulfonyloxy)urethan. Both of the possible electronic states of this nitrene take part in intermolecular reactions. Pure singlet nitrene is formed by α-elimination from the urethan and on thermal decomposition of ethyl azidoformate, but changes so rapidly into the triplet form that the reactions of both forms are observed. Singlet ethoxycarbonylnitrene undergoes selective and stereospecific insertion into C? H bonds and adds stereospecifically to olefins. Triplet ethoxycarbonylnitrene, however, does not undergo insertion into C? H bonds, and adds to olefins with complete loss of the geometric configuration. By following quantitatively the stereospecificity of the addition reaction and by selective interception of the triplet and singlet forms of the nitrene, it can be shown that the photolysis of ethyl azidoformate leads directly to nitrene of which one third is in the triplet state. In the decomposition of aryl- and alkylcarbonylazides (acid azides), the removal of nitrogen is accompanied by a synchronous rearrangement to isocyanates (Curtius rearrangement). In this system, nitrenes are obtained only by photolysis. They add to double bonds and undergo very selective insertion into C? H bonds, but do not rearrange at a measurable rate to isocyanates. The photolytic Curtius rearrangement is also a concerted reaction.     

Solution phase isomerization of vibrationally excited singlet nitrenes to vibrationally excited 1,2-didehydroazepine

Photolysis of phenyl and o-biphenylyl azide (at 270 nm) releases vibrationally excited singlet nitrene which isomerizes to the corresponding hot 1,2-didehydroazepine at a rate competitive with thermal relaxation. Using ultrafast vibrational spectroscopy we observe the formation of vibrationally excited 1,2-4,6-azacycloheptatetraene (1,2-didehydroazepine) in picoseconds following photolysis of phenyl azide in chloroform and o-biphenylyl azide in acetonitrile at ambient temperature.     

Early events in the photochemistry of 2-naphthyl azide from femtosecond UV/Vis spectroscopy and quantum chemical calculations: direct observation of a very short-lived singlet nitrene

Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.     

Ultrafast spectroscopy and computational study of the photochemistry of diphenylphosphoryl azide: direct spectroscopic observation of a singlet phosphorylnitrene

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ～ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ～ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.     

Direct observation of a sulfonyl azide excited state and its decay processes by ultrafast time-resolved IR spectroscopy

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO(2)N(3)) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO(2)N(3) with 330 nm light promotes 2-NpSO(2)N(3) to its S(1) state. The S(1) excited state has a prominent azide vibrational band. This is the first direct observation of the S(1) state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S(1) state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO(2)) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO(2)N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl(4)) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO(2)N). Internal conversion of the S(1) excited state to the ground state S(0) is an efficient deactivation process. Intersystem crossing of the S(1) excited state to the azide triplet state contributes only modestly to deactivation of the S(1) state of 2-NpSO(2)N(3).     

Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands

Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N₂ elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N₂ loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d⁸-configured Pt^II ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N₂ loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N₂-releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts.     

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.     

Bonding and structure of copper nitrenes

Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes, a bona fide structurally characterized example of a terminal copper nitrene has, to our knowledge, not been reported. In anticipation of such a report, terminal copper nitrenes are studied from a computational perspective. The nitrene complexes studied here are of the form (beta-diketiminate)Cu(NPh). Density functional theory (DFT), complete active space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet (S = 1) is the ground state, CASSCF calculations indicate that a singlet (S = 0) is the ground state, with only a small energy gap between the singlet and triplet. Moreover, the ground-state (open-shell) singlet copper nitrene is found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen, contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene bond.