2-Naphthyl(carbomethoxy)carbene revisited: combination of ultrafast UV-vis and infrared spectroscopic study

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN2CO2CH3) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (tau = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene ((1)NpCCO2CH3). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by approximately 50% of their original intensity in cyclohexane in 3 ns. It is concluded that (1)NpCCO2CH3 has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only (1)NpCCO2CH3 is formed within 50 ps of the laser pulse rather than a singlet-triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.     

Influence of solvent on carbene intersystem crossing rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.     

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.     

Ultrafast study of p-biphenylyldiazomethane and p-biphenylylcarbene

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.     

Solvent dependence of the 2-naphthyl(carbomethoxy)carbene singlet-triplet energy gap

The solvent dependence of the 2-naphthyl(carbomethoxy)carbene (2) singlet-triplet energy gap has been examined by time-resolved infrared (TRIR) and computational methods. The ground state of 2 changes from the triplet state in hexane to the singlet state in acetonitrile. Preferential stabilization of the singlet carbene is the result of its increased dipole moment in polar solvents. Variable-temperature TRIR experiments provide measurements of the enthalpic and entropic differences between (1)2 and (3)2 and suggest that solvent and geometry effects on the entropy of singlet and triplet carbenes can offset differences arising from spin multiplicity. B3LYP calculations using the polarizable continuum solvation model (PCM) reproduce the general trends in enthalpic differences seen experimentally.     

Ultrafast study of p-biphenylyldiazoethane. The chemistry of the diazo excited state and the relaxed carbene

Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.     

Computational and experimental studies of the effect of substituents on the singlet-triplet energy gap in phenyl(carbomethoxy)carbene

The effect of aromatic substitution on the singlet-triplet energy gap in substituted phenyl(carbomethoxy)carbene (X-Ph-C-CO(2)CH(3), PCC) has been explored by time-resolved infrared (TRIR) spectroscopy and gas-phase computational methods. The ground state of para-substituted PCC is calculated to change from the triplet state in p-NO(2)-PCC (Delta G(ST) = 6.1 kcal/mol) to the singlet state in p-NH(2)-PCC (Delta G(ST) = -2.8 kcal/mol). The absence of solvent perturbation in the TRIR spectra of p-N(CH(3))(2)-PCC (which should have electronic properties similar to p-NH(2)-PCC) and parent PCC is consistent with their ground states lying > +/-2 kcal/mol from the next available electronic state, in line with the computational results. The observation of solvent perturbation in the TRIR spectra of p-OCH(3)-PCC and p-CH(3)-PCC implies that their ground states lie < +/-1 kcal/mol from their next available electronic state. This is in agreement with our computational results, which predict a gas-phase Delta G(ST) of -0.8 and 1.6 kcal/mol for p-OCH(3)-PCC and p-CH(3)-PCC as compared to Delta G(ST) values of -3.9 and -1.3 kcal/mol from polarizable continuum model (PCM) calculations with acetonitrile as a solvent. Gas-phase computational results for the meta- and ortho-substituted PCC species are also presented, along with selected linear free energy (LFE) relationships for the para and meta species.     

Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O? H insertions are assumed to be characteristic of singlet states, whereas C? H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen‐bonded singlet ground state complex that was synthesized in low‐temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of O? H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.     

Carbene Formation or Reduction of the Diazo Functional Group? An Unexpected Solvent-Dependent Reactivity of Cyclic Diazo Imides

The control of the reactivity of diazo compounds is commonly achieved by the choice of a suitable catalyst, e.g. via stabilization of singlet carbenes or radical intermediates. Herein, we report on the light-promoted reactivity of cyclic diazo imides with thiols, where the choice of solvent results in two fundamentally different reaction pathways. In dichloromethane (DCM), a carbene is formed initially and engages in a cascade C−H functionalization/thiolation reaction to deliver indane-fused pyrrolidines in good to excellent yields. When switching to acetonitrile solvent, the carbene pathway is shut down and an unusual reduction of the diazo compound occurs under otherwise identical reaction conditions, where the aryl thiol acts as reductant. A combined set of experimental and computational studies was carried out to obtain mechanistic understanding and to support that indane formation proceeds via the insertion of a triplet carbene, while the reduction of diazo imides proceeds via an electron transfer process.     

Study of concerted and sequential photochemical Wolff Rearrangement by femtosecond UV-vis and IR spectroscopy

Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-vis and IR transient absorption spectroscopy. For BpCN2COCH3 in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either 1BpCN2COCH3*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene. The slow time constant of the ketene formation dynamics obtained by ultrafast IR (700 ps) spectroscopy agrees with the relaxed carbene decay of 800 +/- 100 ps obtained by UV-vis absorption spectroscopy.     

The dynamics of carbene solvation: an ultrafast study of p-biphenylyltrifluoromethylcarbene

Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse. Similar effects are observed upon ultrafast photolysis of 2-fluorenyltrifluoromethyl diazomethane (FlCN2CF3) and therefore cannot be associated with torsional motion around the two phenyl rings of the biphenyl compound. Instead, the effect is attributed to the dynamics of solvation of the singlet carbene. The time constant of solvation in normal alcohols lengthens with solvent viscosity in a linear manner. Furthermore, the time constants of the red shift in methanol-O-d (16 ps), ethanol-O-d (26 ps), 2-propanol-OD (40 ps), and 2,2,2-trifluoroethanol-O-d (14 ps) are longer than those recorded in methanol (9.6 ps, KIE = 1.7), ethanol (14.3 ps, KIE = 1.8), 2-propanol (28 ps, KIE = 1.4), and 2,2,2-trifluoroethanol (4.4 ps, KIE = 3.2), which indicates that the solvent reorganization involves formation of hydrogen bonds. The kinetic data are consistent with motion of the solvent to achieve a specific interaction with the carbene, with the creation of a new hydrogen bond. The solvated carbene reacts with the solvent over tens, hundreds, and thousands of ps, depending upon the solvent.     

C3H4: Theoretical study of structures and stabilities of isomers

The various isomers including stable structures, carbenes, and diradicals on the C₃H₄ surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR ) spectra. The π electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH₂ (DOUBLE BOND) CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. © 1996 John Wiley & Sons, Inc.     

Concerted Wolff rearrangement in two simple acyclic diazocarbonyl compounds

The photochemistry of two simple acyclic diazo carbonyl compounds, azibenzil and diazoacetone, were studied using the tools of ultrafast time-resolved spectroscopy. In the former case, UV-vis detection allows observation of an absorption band of singlet benzoylphenylcarbene, decaying with a 740 ± 150 ps time-constant in acetonitrile. IR detection shows that the ketene product of Wolff rearrangement (～2100 cm(-1)) is formed by two parallel pathways: a stepwise mechanism with carbene intermediacy with a slow rise time-constant of 660 ± 100 ps, and directly in the diazo excited state as confirmed by the immediate formation of an IR band of a nascent hot ketene species. Photolysis (270 nm) of diazoacetone in chloroform leads mainly to the ketene species through a concerted process, consistent with the predominance of the syn conformation in the diazoacetone electronic ground state and a zero quantum yield of the internal conversion process.     

Photodissociation of bromobenzene in solution

The photodissociation of bromobenzene in solution was investigated with ultrafast transient absorption spectroscopy following excitation at 266 nm. Ab initio calculations of lower singlet and triplet states were performed in order to guide the interpretations. The main feature of the kinetics measured between 300 and 930 nm in acetonitrile is a 9±1 ps decay, which we mainly assign to predissociation. Similar decays were observed in hexane, dichloromethane and tetrachloromethane at 400 and 800 nm. Other features in acetonitrile, such as complicated short-time dynamics between 420 and 620 nm and a long-lived component, might indicate the involvement of lower triplet states.     

Excited state dynamics of Michler's ketone: a laser flash photolysis study

Steady state absorption and fluorescence as well as the time resolved absorption studies in the pico and subpicosecond time domain have been performed to characterize the excited singlet and triplet states of Michler's ketone (MK). The nature of the lowest excited singlet (S₁) and triplet (T₁) states depends on the polarity of the solvent - in nonpolar solvents they have either pure nπ * character or mixed character of nπ * and ππ * states but in more polar solvents the states have CT character. Concentration dependence of the shapes of the fluorescence as well the excited singlet and triplet absorption spectra provide the evidence for the association of the MK molecules in the ground state.     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

High resolution probe of spin-orbit coupling and the singlet-triplet gap in chlorocarbene

Among the most important of chemical intermediates are the carbenes, characterized by a divalent carbon that generates low-lying biradical (triplet) and spin-paired (singlet) configurations with unique chemical reactivities. The "holy grail" of carbene chemistry has been determining the singlet-triplet gap and intersystem crossing rates. We report here the first high resolution spectra of singlet-triplet transitions in a prototypical singlet carbene, CHCl, which probe in detail the triplet state structure and spin-orbit coupling with the ground singlet state. Our spectra reveal a pronounced vibrational state dependence of the triplet state spin-spin splitting parameter, which we show is a sensitive probe of spin-orbit coupling with nearby singlet states. The parameters derived from our spectra, including a precise determination of the singlet-triplet energy gap, are in excellent agreement with recent ab initio calculations.     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

A comparison of acetyl- and methoxycarbonylnitrenes by computational methods and a laser flash photolysis study of benzoylnitrene

Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand the chemical and reactivity differences between acetylnitrene (CH(3)C(=O)N) and methoxycarbonylnitrene (CH(3)OC(=O)N) and related compounds. CBS-QB3 theory alone correctly predicts that acetylnitrene has a singlet ground state. We agree with previous studies that there is a substantial N-O interaction in singlet acetylnitrene and find a corresponding but weaker interaction in methoxycarbonylnitrene. Methoxycarbonylnitrene has a triplet ground state because the oxygen atom stabilizes the triplet state of the carbonyl nitrene more than the corresponding singlet state. The oxygen atom also stabilizes the transition state of the Curtius rearrangement and accelerates the isomerization of methoxycarbonylnitrene relative to acetylnitrene. Acetyl azide is calculated to decompose by concerted migration of the methyl group along with nitrogen extrusion; the free energy of activation for this concerted process is only 27 kcal/mol, and a free nitrene is not produced upon pyrolysis of acetyl azide. Methoxycarbonyl azide, on the other hand, does have a preference for stepwise Curtius rearrangement via the free nitrene. The bimolecular reactions of acetylnitrene and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to have enthalpic barriers that are near zero and free energy barriers that are controlled by entropy. These predictions were tested by laser flash photolysis studies of benzoyl azide. The absolute bimolecular reaction rate constants of benzoylnitrene were measured with the following substrates: acetonitrile (k = 3.4 x 10(5) M(-1) (s-1)), methanol (6.5 x 10(6) M(-1) s(-1)), water (4.0 x 10(6) M(-1) s(-1)), cyclohexane (1.8 x 10(5) M(-1) s(-1)), and several representative alkenes. The activation energy for the reaction of benzoylnitrene with 1-hexene is -0.06 +/- 0.001 kcal/mol. The activation energy for the decay of benzoylnitrene in pentane is -3.20 +/- 0.02 kcal/mol. The latter results indicate that the rates of reactions of benzoylnitrene are controlled by entropic factors in a manner reminiscent of singlet carbene processes.     

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.