Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution

Nitroaromatic compounds are major constituents of the brown carbon aerosol particles in the troposphere that absorb near-ultraviolet (UV) and visible solar radiation and have a profound effect on the Earth’s climate. The primary sources of brown carbon include biomass burning, forest fires, and residential burning of biofuels, and an important secondary source is photochemistry in aqueous cloud and fog droplets. Nitrobenzene is the smallest nitroaromatic molecule and a model for the photochemical behavior of larger nitroaromatic compounds. Despite the obvious importance of its droplet photochemistry to the atmospheric environment, there have not been any detailed studies of the ultrafast photochemical dynamics of nitrobenzene in aqueous solution. Here, we combine femtosecond transient absorption spectroscopy, time-resolved infrared spectroscopy, and quantum chemistry calculations to investigate the primary steps following the near-UV (λ ≥ 340 nm) photoexcitation of aqueous nitrobenzene. To understand the role of the surrounding water molecules in the photochemical dynamics of nitrobenzene, we compare the results of these investigations with analogous measurements in solutions of methanol, acetonitrile, and cyclohexane. We find that vibrational energy transfer to the aqueous environment quenches internal excitation, and therefore, unlike the gas phase, we do not observe any evidence for formation of photoproducts on timescales up to 500 ns. We also find that hydrogen bonding between nitrobenzene and surrounding water molecules slows the S1/S0 internal conversion process.


■ INTRODUCTION
Nitroaromatic compounds are abundant in the Earth's atmosphere, with both natural and anthropogenic sources.They are formed in atmospheric photochemical reactions and by incomplete combustion of wood and fossil fuels. 1−13 The nitroaromatic chromophores contribute significantly to the absorption of solar near-UV and visible radiation by BrC aerosols 14 and hence to radiative forcing in the troposphere. 15itrobenzene is the smallest nitroaromatic molecule and is a model for the tropospherically important chromophores of nitrophenol, nitrocatechol, nitrosyringol, and nitroguaiacol compounds identified in BrC aerosols.−18 However, its photophysics and photochemistry are still not understood fully and, perhaps surprisingly, there have been relatively few investigations of photoinitiated processes of nitrobenzene in aqueous solutions, 19−21 despite water-containing aerosol droplets and particles being ubiquitous in the troposphere. 22he absorption spectrum of nitrobenzene has four distinct features in the UVB and UVC regions.In the gas phase, these are centered around 280, 240, 193, and 164 nm. 23In addition, a weak absorption feature is also observed in the UVA region around 345 nm. 24−29 The 345 nm absorption is attributed to a transition to the 1 (n A π*) state, mainly localized on the nitro group and hereafter referred to as S 1 .This transition is optically forbidden at the C 2v symmetry of nitrobenzene in its planar ground-state geometry; 29 however, the oscillator strength is f = 0.02 27 when the C 2v symmetry is broken by rotation around the C−N bond resulting in an average C 6 H 5 −NO 2 dihedral angle of around 13°. 29,30 The 280 nm absorption is attributed to transitions to the 1 (n B π*) and 1 (L b ππ*) states, 28 hereafter referred to as S 2 and S 3 , respectively. 29Changes in electron density upon excitation to S 2 are mainly localized on the nitro group and for S 3 are partly localized on the benzene ring with some charge transfer character from the benzene ring to the nitro group.The 240 nm absorption is attributed to a transition to the 1 (L a ππ*) state, which is a charge transfer state from the benzene ring to the nitro group and hereafter referred to as S 4 . 29This band dominates the absorption spectrum since the S 0 −S 4 oscillator strength is 2 orders of magnitude larger those of S 0 −S 1,2,3 . 29The 193 and 164 nm absorptions lie outside the scope of this work and are not considered further.In nheptane, methanol, and water solutions, absorptions have been observed around 340, 250−270, and 200 nm. 23The S 0 −S 4 absorption band shifts to a longer wavelength with increasing solvent polarity, 23 as expected for a transition to a chargetransfer state.
The photophysics and photochemistry following photoexcitation of nitrobenzene to its S 1 to S 4 states have been probed using a range of methods.In the gas phase, the photodissociation of nitrobenzene was first investigated by Galloway et al. 31 using UV (320−220 nm) and vacuum UV (125 nm) resonance-enhanced multiphoton ionization (REMPI) mass spectrometry.They determined branching ratios for elimination of NO 2 , NO, and O and found that NO 2 was the dominant photoproduct.They also determined that the NO 2 /NO branching ratio increased with increasing UV photodissociation energy, proposing that the production of both NO 2 and NO was through a nitrite intermediate at lower photodissociation energies but that direct C−N bond cleavage was possible at higher energies.The same trend was also reported in a more recent study by Lin et al. using multimass ion imaging. 32The mechanism for photodissociation was probed in detail by Hause et al. using state-selected direct current slice imaging. 33This work revealed two pathways following excitation at 266 nm.One involved an intramolecular rearrangement via an oxaziridine ring in the lowest energy triplet state and the other was attributed to a roaming reaction on the ground electronic state in which NO 2 and phenoxy products reorient until they isomerize into phenyl nitrite, before eventually decomposing to form NO. Recently, this picture has been challenged by computational CASPT2// CASSCF (14,11) investigations of the photodissociation pathways.This work suggested that the roaming pathway was only accessible following photodissociation at energies ≥5.13 eV (242 nm) and that photodissociation at lower energies occurred via formation of oxaziridine or epoxide rings. 34iussani and Worth have subsequently used the CASPT2// CASSCF method to show that the photodissociation yield can be modified by methyl substitution at the ortho position and by extending conjugation. 35Bejoy et al. have also shown that the nature of the ortho substituent can change the branching ratio between the different dissociation pathways, using velocity map imaging following photoexcitation at 266 nm. 36,37A recent computational study further investigated the roaming isomerization of nitrobenzene microsolvated by water molecules, with roaming observed despite hydrogenbond formation. 38he first time-resolved measurements for gas-phase nitrobenzene photodissociation were made by He et al. using ultrafast electron diffraction. 39Following 267 nm photoexcitation, they observed the formation of NO and phenoxyl radicals on an 8.8 ± 2.2 ps timescale and proposed a mechanism involving a repulsive triplet state following intramolecular rearrangement. 39In contrast to the observations of Galloway et al. 31 and Lin et al., 32 they found that NO was the dominant photoproduct.This discrepancy has since been rationalized by Giussani and Worth, who proposed that nitrobenzene undergoes isomerization in an excited state to produce NO directly and that the remaining photodissociation products are produced in the ground state. 29There have also Journal of the American Chemical Society been two femtosecond time-resolved photoelectron imaging (TRPEI) studies.Schalk et al. investigated the relaxation dynamics following photoexcitation at 200 nm to a higherlying 1 ππ* state. 40Timescales of 40 fs and ∼0.5 ps were reported and assigned to ultrafast relaxation from a higherlying 1 ππ* state and either internal conversion (IC) to the ground state or release of NO, respectively.More recently, Saalbach et al. reported timescales of ≤30 fs, 160−190 fs, and 90−160 ps, using 267 nm photoexcitation and TRPEI, 41 which they assigned to IC from S 3 /S 4 to S 1 , subsequent intersystem crossing (ISC) to the triplet manifold or competing IC to S 0 , and an additional ISC from the triplet manifold to S 0 , respectively.−47 Most recently, Hegazy et al. reported a time scale of 160 fs for ground state recovery, using 267 nm photoexcitation and mega-electron-volt ultrafast electron diffraction, and did not observe any evidence of photofragmentation within the first 5 ps. 48n solution, the photodynamics of nitrobenzene were first investigated by Hurley and Testa using electronic energy transfer (EET). 49Following photoexcitation in isopropyl alcohol at 336 nm, they observed a remarkably high triplet yield of > 0.6.The first time-resolved measurements were made by Yip et al. using picosecond transient absorption spectroscopy (TAS). 50Following photoexcitation at 355 nm in tetrahydrofuran (THF), they extracted dynamical timescales of <5 ps and 770 ± 90 ps, which they attributed to the growth and decay of the triplet-state population.A subsequent picosecond time-resolved transient grating study of nitrobenzene photoexcited at 320 nm in a range of organic solvents and aqueous sodium dodecyl sulfate solution, by Takezaki et al., gave values for the triplet quantum yield of ∼0.8 and time scales of ≤50 ps and ∼0.4−1 ns, which were assigned to the lifetimes of S 1 and T 1 , respectively. 51A more sophisticated time-resolved population grating study of nitrobenzene in ethanol refined the picosecond time scale to 6 ps and revealed an ultrafast time scale of 100 fs, which was assigned to vibrational relaxation within S 1 . 52A recent femtosecond TAS study by Crane et al. has reported multiple timescales for relaxation of 267 nm photoexcited nitrobenzene in hexane and isopropanol. 53All previously reported time constants and assignments are summarized in Table 1.
5][26][27][28][29]34 A recent comprehensive CASPT2//CASSCF (14,11) study has characterized the main decay paths taken following photoexcitation of the lowest S 1 and brightest S 4 excited states. 29The absence of radiative relaxation was explained by the presence of accessible conical intersections (CIs) and singlet−triplet crossing (STC) regions between the S 4 , S 1 , and T 1 3 (n A π*) and ground states, and the high triplet quantum yield was attributed to strong spin−orbit coupling (SOC) between the S 1 and T 2 3 (π O π*) states.Additional CIs and STC regions were also identified and proposed to account for the photoproducts observed in gasphase measurements.18,31−33,39,54−56 Motivated by the lack of detailed investigations of the photochemical dynamics of nitrobenzene in aqueous solution and the environmental importance of the near-UV photochemistry of nitroaromatic molecules, we have undertaken a combined femtosecond TAS and time-resolved infrared (TRIR) spectroscopy study of nitrobenzene in aqueous solution, supported by quantum chemistry calculations.We compare these results to analogous measurements undertaken in solutions of methanol (a protic polar organic solvent), acetonitrile (an aprotic polar organic solvent), and cyclohexane (a nonpolar organic solvent) to explore the consequences of different solvent−solute interactions.

■ RESULTS AND DISCUSSION
Steady-State Absorption Spectra.Steady-state UV− visible absorption spectra recorded in water, cyclohexane, acetonitrile, and methanol are presented in Figure 1 and are consistent with those reported previously. 23,24,41The absorption bands around 355 nm (weak), 280−340 nm (stronger), and 250−280 nm (strong) are attributed to S 0 −S 1 , S 0 −S 2 /S 3 , and S 0 −S 4 transitions, respectively.The spectrum of nitrobenzene in cyclohexane is similar to the gas-phase absorption spectrum, 23 as expected for a nonpolar solvent in which there are weak solute−solvent interactions.The maximum of the S 4 band is shifted considerably to longer wavelengths in water (267 nm) compared to cyclohexane (253 nm) as a result of its charge-transfer character.Although the S 1 shoulder is quite clearly resolved in cyclohexane, it is less distinguishable in water.This is most likely because it is masked by the redshifted S 4 band and presumably also the S 3 band, which has some charge-transfer character.Our femtosecond TAS measurements are recorded following photoexcitation to S 1 at 355 nm, and our femtosecond TRIR measurements are recorded following photoexcitation to S 1 at 340 nm and S 4 at 260 nm.
Transient Absorption Spectra.Transient absorption maps of nitrobenzene in water (15 mM), cyclohexane (104 mM), acetonitrile (68 mM), and methanol (102 mM), following photoexcitation at 355 nm, are presented in Figures S1−S4.All four maps have similar features.Figure 2 shows a selection of the transient absorption spectra of nitrobenzene in water at pump−probe delays in the range of 400 fs − 3 ns.Immediately on photoexcitation, two absorption features are observed at around 460 and 650 nm.Since 355 nm absorption populates S 1 , we attribute these features to S 1 excited-state absorptions (ESAs).During the first 10 ps, the broad structureless feature around 460 nm evolves rapidly to reveal a structured ESA and the long-wavelength edge of the S 1 ESA, at 485 nm, disappears.Until around 80 ps, the center wavelengths of the peaks in the structured ESA gradually shift to shorter wavelengths.At 80 ps, the average peak spacing is 859 cm −1 (Table S1).Concomitantly, the feature centered around 650 nm increases in intensity from 1.2 to 1.5 mOD during the first 10 ps, before decreasing in intensity.Nitrobenzene potential energy surfaces from Giussani and Worth 29 show that only T 1 and T 2 lie lower in energy than S 1 , hence the ingrowing structured band around 460 nm and ingrowing band around 650 nm must arise from triplet ESAs following ISC from S 1 .Our observations and assignment are in agreement with those made in the early picosecond TAS study by Yip et al. 50and are broadly similar to the recent femtosecond TAS study by Crane et al. 53 To extract accurate timescales for the relaxation pathways from these overlapping spectral features, it is necessary to decompose the spectra into relevant components.The process of spectral decomposition for the absorption feature centered at 460 nm is presented in Figure S12.The initial population is best represented as a basis function derived from the initial S 1 ESA.Representation of the in-growing structured triplet feature requires a sum of Lorentzian functions, whose individual Lorentzian components are allowed to shift and narrow to reflect the observed shift to a shorter wavelength and line narrowing.Lorentzian profiles were selected to reflect the dephasing that occurs in liquids. 57Shifting and broadening parameters for each transient absorption spectrum are presented in Figures S13−S16.To extract timescales, the scaled amplitude for the broad S 1 ESA, the amplitudes at the center of each Lorentzian component of the triplet ESA (peaks A, B, and C in Figure 2), and the growth and decay of the broad ESA centered around 650 nm, are plotted against time and fitted with exponential functions convoluted with Gaussian instrument response functions (Figure 3).
The S 1 decay has two timescales associated with it (0.82 and 11.8 ps, Table 2).The shorter of these is slower than the 100 fs timescale deduced from time-resolved population grating spectroscopy measurements following 320 nm photoexcitation of nitrobenzene in ethanol, 52 although we note that our corresponding timescale for 355 nm photoexcitation of nitrobenzene in methanol is around 440 fs (Table 2).The slower time scale was fit globally with the growth of peaks A, B, and C, and the feature centered around 650 nm.−53 We also obtained kinetic traces by plotting the integrated peak areas (Figure S11) instead of the peak heights.The time constants obtained (Table S2) are very similar to those obtained by plotting peak heights; however, there is more scatter and slightly poorer agreement between the rise times of peaks A, B, and C, which we attribute to the complexity of fitting data with overlapping ESAs from both singlet and triplet states.
Time-Resolved Infrared Spectra.Steady-state Fouriertransform infrared (FTIR) spectra of nitrobenzene in d 2 -water (16 mM), cyclohexane (24 mM), and d 3 -acetonitrile (16 mM), in the range of 1300−1600 cm −1 , are presented in Figure S17.Peaks around 1360 and 1530 cm −1 are assigned to symmetric and asymmetric NO stretches, respectively. 58   TRIR spectra, ground-state bleaches (GSBs) at around 1360 cm −1 are observed to decrease to the baseline, indicating complete recovery of the ground state.The baseline remains flat for delays as long as 500 ns, ruling out subsequent formation of photoproducts on the ground electronic state.This is in contrast with 266 nm gas-phase photodissociation measurements, in which NO 2 and NO were observed on time scales of 13 and 140 ns, respectively, 32 and polychromatic photolysis measurements, in which a range of aromatic intermediates were observed following irradiation of aqueous nitrobenzene with a 1 kW mercury lamp for 120 min, albeit with low quantum yields. 19inetic traces for the GSB recoveries and associated fits are presented in Figure 5.There are two timescales associated with the recovery of the ground state, one of which is a few tens of picoseconds and the other is around 1 ns (Table 3).The longer timescales are similar to those obtained from our TAS measurements that were assigned to ISC from the triplet manifold to S 0 (Table 2).However, the shorter timescales (17−30 ps) are up to almost 2 orders of magnitude longer than the TAS timescales that were assigned to S 1 −S 0 IC (0.4−0.8 ps).This difference can be attributed to the time taken to vibrationally cool in S 0 . 59Interestingly, the fast component of the ground-state recovery in d 2 -water (∼20 ps) is faster than that in cyclohexane or d 3 -acetonitrile (∼30 ps).This comparison is discussed in more detail below.
Computational Chemistry Calculations.To support our interpretation of the ESAs observed in our transient absorption measurements, CASPT2 (14,11) vertical excitation energies (VEEs) from some of the key geometries on the potential energy landscape 29 were calculated, both in the gas phase and in aqueous solution, using a polarizable continuum model (PCM).The full set of results and state characterizations are τ IRF is the instrument response function, τ IC is the timescale for S 1 /S 0 IC, τ ISC1 is the timescale for ISC from S 1 to the triplet manifold, and τ ISC2 is the timescale for ISC from the triplet manifold to S 0 .The numbers in parentheses are the relative amplitudes associated with the two competing S 1 relaxation processes.

Journal of the American Chemical Society
presented in Tables S4 and S5, and the results relevant to interpretation of the TAS measurements are presented in Table 4. Nitrobenzene has a large dipole moment in the ground state, with a calculated value of 4.32 D that compares well with the experimental value of 4.22 D. 60 The excited singlet states have similar dipole moments and are stabilized relative to S 0 in an aqueous environment.
Our TAS measurements (Figure 2) revealed absorption bands centered around 460 and 650 nm immediately on 355 nm photoexcitation to S 1 .Our assignment of these to S 1 ESAs is consistent with the calculated VEEs from the S 1 1 (n A π*) minimum which has bright S 1 −S 5 and S 1 −S 3 transitions at 466 and 656 nm, respectively.The structured absorption band around 460 nm and the band that grows at around 650 nm were attributed to triplet ESAs.From the relaxation pathway proposed by Giussani and Worth, 29 the system should cross from the 1 (n A π*) state to a region of the triplet manifold where the minimum energy structure has 3 (π O π min *) character.The relevant VEEs at the CASPT2 level from this triplet state, optimized in the gas phase and water PCM at the CASSCF level, are listed in Table 4.This state is labeled T 2 (π O π*) as it is the T 2 state at the Franck−Condon point, although T 1 and T 2 are nearly degenerated when including the water PCM (Table S5).The triplet ESAs are consistent with the T 2 −T 6 and T 2 −T 5 transitions at 445 and 566 nm.
To inform our interpretation of the slower IC time constant observed in our TAS measurements for nitrobenzene in water compared to cyclohexane, acetonitrile, or methanol, and reported by Crane et al. for hexane and isopropanol, 53 as well as of the faster ground-state recovery observed in our TRIR measurements for nitrobenzene in d 2 -water compared to cyclohexane or d 3 -acetonitrile (Table 3), ground-state microsolvation calculations were undertaken (Figure 6).Since none of the solvents are aromatic, the dominant intermolecular interactions are those between the polar nitro group and the solvent.The shortest NO−H intermolecular distances, solvent dipole moments, polarizabilities, and dielectric constants are reported in Table 5.The shortest NO−H intermolecular distances for cyclohexane and acetonitrile microsolvation are around 2.4−2.5 Å, which are indicative of minimal solute− solvent interactions and consistent with the aprotic nature of these solvents.For water microsolvation, the shortest NO−H intermolecular distance is considerably shorter, around 1.9 Å, and there is a hydrogen-bonding network between the NO 2 group and the immediate water molecules (Figure 6a).Although also protic, the shortest NO−H intermolecular distance in the methanol microsolvated cluster is similar to that in cyclohexane and acetonitrile, and there is no evidence of a hydrogen-bonding network with the NO 2 group of nitrobenzene, presumably because the higher polarizability of methanol favors dispersion interactions.The insights that emerge from these computational outcomes for the influence of solvent−solute interactions on nitrobenzene photochemistry are discussed below.
Vibrational Structure in the Triplet ESA Band.The structured absorption band spanning 450−400 nm in the TAS data (Figure 2) is attributed to vibronic transitions associated with the triplet T 2 −T 6 ESA (Table 4).The peaks shift to VEEs were calculated in the gas phase and in aqueous solution (PCM), using structures optimized at the CASSCF level.shorter wavelengths and narrow with increasing time delay, which can be attributed to vibrational cooling in the triplet manifold, 61,62 and the band narrowing is dependent on the solvent (Figures S13−S16).The vibrational structure decays uniformly with a time constant of ∼800 ps (Table 2); hence, the structure most likely arises from a vibrational progression in the upper T 6 state.The average peak spacing in all solvents at a time delay of 80 ps is around 850 cm −1 .Infrared absorption spectroscopy studies of nitrobenzene in CCl 4 and the gas phase, and MP2(full)/aug-cc-pVTZ calculations, suggest that ground-state nitrobenzene has an ONO bending vibration with A 1 symmetry at 850 cm −1 . 58,63It seems likely, therefore, that this is the vibrational mode responsible for the structure in the T 2 −T 6 ESA.This interpretation is consistent with the recent 266 nm TAS study of nitrobenzene in isopropanol and hexane reported by Crane et al., although we note that their analysis procedure did not allow them to retrieve the band shifting and narrowing evident in our spectra. 53 1 /S 0 Internal Conversion.Following photoexcitation to S 1 at 355 nm, our TAS data reveal S 1 relaxation times of 0.82 ps in water and <0.6 ps in cyclohexane, acetonitrile, and methanol (Table 2), and our TRIR data (following photoexcitation to S 1 at 340 nm) reveal that ground-state recovery from direct S 1 /S 0 IC is 20−30% (Table 3).This IC fraction is consistent with a computational study for gaseous nitrobenzene proposing that 20% of the S 1 population relaxed back to the ground state via IC 27 and is supported by the identification of an available conical intersection between S 1 and S 0 lying 0.54 eV above the S 1 minimum.29 Following photoexcitation to S 4 at 260 nm, our TRIR data suggests that >30% of the excited-state population returns to the ground state via IC from S 1 .We can gain some insights into these excitation-wavelength-dependent IC fractions by considering the energies of the key geometries along the relaxation pathways following S 1 and S 4 photoexcitation.29 In both cases, the crossing point on the S 1 /S 0 IC seam lies higher in energy than the S 1 /T 2 STC point; however, the difference in barrier heights between the IC and ISC pathways is slightly lower following photoexcitation to S 4 compared to that following photoexcitation to S 1 (0.25 and 0.30 eV, respectively) and could explain why the IC/ISC branching ratio is higher following photoexcitation to S 4 rather than S 1 .
The S 1 /S 0 IC timescales also merit discussion.IC is slower for aqueous nitrobenzene than for nitrobenzene in cyclohexane, acetonitrile, or methanol (Table 2).This could be the result of the barrier to the S 1 /S 0 conical intersection being higher in water because the 1 (n A π*) state is destabilized by the protic solvent.Consequently, excitation with a 355 nm photon would leave less excess internal energy in the nitrobenzene S 1 molecules when they are photoexcited in water compared to other solvents.Our CASPT2//CASSCF(14,10) calculations confirm this.They show that the minimum energy conical intersection is lower in aqueous solution compared to the gas phase by approximately 0.1 eV but that the S 1 minimum is lowered much further, by 0.5 eV.Such changes to the S 1 potential energy surface in aqueous solution could result in it taking longer to reach the S 1 /S 0 IC from the Franck−Condon region following photoexcitation at 355 nm.Moreover, TAS measurements following photoexcitation at 340 nm reveal a faster S 1 /S 0 IC timescale of 0.72 ps (Figures S23−S25), compared with 0.82 ps following photoexcitation at 355 nm (Table S3).It is also possible that, since relaxation through the S 1 /S 0 conical intersection is characterized by activity in the ONO bending mode, 27 IC could be dynamically hampered by the hydrogen-bonding network between the NO 2 group and the immediate water molecules described above (Figure 6 and Table 5).After internal conversion to S 0 , vibrational relaxation is faster in water, which is likely a result of stronger coupling between vibrationally hot nitrobenzene molecules to the bath of water molecules (Figure 6 and Table 5).
Regardless of the precise mechanism controlling the rate of S 1 /S 0 internal conversion in aqueous solution, our observation of separate timescales for S 1 population relaxation via the S 1 /S 0 conical intersection and the S 1 /T 2 STC shows that these nonadiabatic processes are dynamically separated on the S 1 PES.For example, after photoexcitation to the S 1 state, there may be a bifurcation of the excited-state dynamics toward these two different crossing points or dynamics that approach the region of the S 1 /S 0 IC seam before the S 1 /T 2 STC region.These dynamics must occur in competition with the quenching of excess internal energy in the S 1 state by coupling to the solvent bath.
S 1 (n A π*)/T 2 (π O π*) and T 1 (n A π*)/S 0 Intersystem Crossing.Our 355 nm TAS data reveal the timescale of ISC from S 1 to the triplet manifold to be 11.8 ps for aqueous nitrobenzene and to be largely independent of the solvent (Table 2).We observe similar dynamics following 340 nm photoexcitation (9.9 ps; Table S3).This timescale is slightly longer than those determined in the recent femtosecond TAS study by Crane et al., 53 at an excitation wavelength of 267 nm which places greater internal energy into the excited-state nitrobenzene.Our more detailed spectral decomposition of the overlapping ESA bands in the TA spectra of nitrobenzene may also influence the determined timescales.
Our TAS and TRIR data reveal the timescale for subsequent ISC from the triplet manifold back to S 0 to be around 800− 900 ps, again largely independent of the solvent (Tables 2 and  3).Again, the 355 and 340 nm TAS measurements give similar timescales (Table S3).This determination is consistent with the conclusions drawn from transient grating measurements 51 and supported by computational calculations of the relaxation pathway 29 that show that efficient ISC occurs from S 1 to T 2 (π O π*) due to the large spin−orbit coupling constant (68 cm −1 ) and the STC being located <0.3 eV higher than the S 1 minimum.The T 2 (π O π*) and T 1 (n A π*) states are nearly degenerate (Table S5), so while we attribute the triplet ESA to excitation from the T 2 (π O π*) state, the IC between the two triplet states is likely to be efficient.
Subsequent T 1 (n A π*)/S 0 ISC to repopulate the ground state has been proposed to occur either through an out-of-plane deformation of the NO 2 group (i.e., distortion along the ONCO out-of-plane improper dihedral angle) 27 or a planar C 2v geometry. 29In the former case, because the STC is 0.15 eV higher than the T 1 minimum, the geometry must deform from being planar at the T 1 minimum to an ONCO dihedral angle of around 115°(an out-of-plane dihedral angle of around 65°).At this STC, the spin−orbit coupling was calculated to be 75 cm −1 . 27In the second case, the STC is 0.42 eV higher than the T 1 minimum and the spin−orbit coupling is also 75 cm −1 . 29If ISC to the ground state requires large structural deformation and some degree of reactivation to reach the STC energy, the relaxation timescale might be expected to depend on the solvent.Our microsolvation calculations indicate that aqueous nitrobenzene might have a longer ISC time constant than in other solvents because the NO 2 out-of-plane Journal of the American Chemical Society deformation would disrupt the solute−solvent hydrogenbonding network; however, this solvent dependence is not seen.Therefore, our measurements argue against ISC via NO 2 out-of-plane deformation, and we instead propose that the population returns to the ground state driven by the coupling from the STC characterized by a planar geometry, as suggested by Giussani and Worth. 29Takezaki et al. found the timescale for ISC to S 0 to increase with decreasing temperature; this is consistent with our interpretation of our experimental observations that the crossing occurs from the T 1 minimum. 51

■ CONCLUSIONS
We report detailed femtosecond to nanosecond TAS and TRIR spectroscopy studies of UV-photoexcited nitrobenzene in aqueous solution, supported by quantum chemistry calculations, and compare the results with analogous measurements for nitrobenzene in cyclohexane, acetonitrile, and methanol.The relaxation pathways and our spectroscopic observations are summarized in Figure 7.
−53 The key differences are that, unlike in the gas phase, we do not observe any evidence for the formation of photoproducts on timescales up to 500 ns because vibrational energy transfer to the solvent rapidly quenches internal excitation in the S 1 or triplet states, and the hydrogen-bonding interactions between the nitrobenzene NO 2 group and surrounding water molecules slow the S 1 /S 0 internal conversion process.
Immediately on photoexcitation to S 1 , transient absorption bands are observed around 460 and 650 nm, which we attribute to S 1 excited-state absorptions.The rapid decay of the 460 nm ESA band profile within the first 10 ps reveals a structured absorption band that is assigned to the T 2 −T 6 ESA with a prominent vibrational progression in T 6 .Decomposition of the transient absorption spectra reveals a biexponential decay of the S 1 ESAs, attributed to S 1 /S 0 IC and S 1 /T 2 ISC (followed by rapid T 2 /T 1 IC) on timescales of 0.82 and 11.8 ps, respectively.S 1 /S 0 IC is slower in aqueous solution compared to cyclohexane, acetonitrile, and methanol solutions of nitrobenzene, where it occurs on a timescale <0.6 ps.The 11.8 ps timescale for the S 1 decay matches the growth of the structured ESA band attributed to the population of T 2 .During the next 80 ps, the peak positions in the structured absorption band shift to shorter wavelengths and narrow, as a result of vibrational relaxation in the triplet manifold, before the band intensity decays on a timescale of 804 ps, attributed to T 1 /S 0 ISC.Complementary TRIR measurements reveal full groundstate recovery occurring on two timescales, 17.1 and 900 ps, attributed to vibrational relaxation in S 0 following IC from S 1 and ISC from T 1 , respectively.The IC/ISC branching ratio is found to be around 0.2, following photoexcitation to S 1 .Additional TRIR measurements following 260 nm photoexcitation to S 4 revealed an increase in the branching ratio.This difference is rationalized in terms of the relative barrier heights of the S 1 /S 0 conical intersections and S 1 /T 2 singlet− triplet crossings on the different relaxation pathways.
Nitrobenzene and nitroaromatic molecules containing the nitrobenzene molecular motif play important roles in the atmosphere, both as gaseous molecules and as condensed constituents of aerosol particles.However, their photochemical dynamics in aqueous solutions differ significantly from those in the gas phase.The efficient relaxation pathways of aqueous nitrobenzene photoexcited at UVA and UVB wavelengths suggest that it will be less rapidly photodegraded in aerosol particles than in the gas phase in a lower atmosphere.A detailed understanding of the type presented here for the ultrafast electronic relaxation pathways following near-UV photoexcitation of nitrobenzene in aqueous environments is crucially important for the development of accurate models of nitroaromatic photochemistry in aqueous droplets and organic (e.g., BrC) aerosol particles present in the troposphere.
Femtosecond TAS experiments have been described previously. 64riefly, femtosecond laser pulses were generated from a regenerative amplifier seeded by a Ti:sapphire oscillator (Coherent Astrella-HE-USP).For UV excitation, tunable laser pulses were generated using an optical parametric amplifier (OPA; Coherent OPerA Solo).Transient absorption spectra were recorded using a commercial transient absorption spectrometer (Ultrafast Systems HELIOS Fire) following photoexcitation at 355 nm (3.49eV) with pulse energies of 500 nJ at the sample.The broadband probe beam was created via white-light generation by focusing a small portion of the 800 nm fundamental beam into a calcium fluoride plate to give a probing range of 350−650 nm.The relative polarizations of the pump and probe beam were set at the magic angle of 54.7°.Nitrobenzene solutions were flowed continuously at 10 mL min −1 through a Harrick flow cell using a liquid diaphragm pump (KNF, SIMDOS 02).The concentrations and path lengths were selected to give an absorbance of around 0.3 at 355 nm: 15 mM water (1000 μm), 104 mM cyclohexane (250 μm), 68 mM acetonitrile (250 μm), and 102 mM methanol (250 μm).NMR studies indicate negligible aggregation in 15 mM aqueous nitrobenzene.The instrument response functions were determined by fitting them to solvent-only spectra.
Time-Resolved Infrared Spectroscopy.TRIR measurements of UV-photoexcited solutions of nitrobenzene were carried out at the LIFEtime Facility at the STFC Rutherford Appleton Laboratory.−69 In brief, UV excitation laser pulses and mid-IR probe pulses were generated by a synchronized pair of Yb-KGW amplifiers (Light Conversion, PHAROS 100 kHz, 15 W, 260 fs output pulses and PHAROS 100 kHz, 6 W, 180 fs output pulses) seeded by a

Journal of the American Chemical Society
common Yb:KGW ultrafast oscillator and pumping three OPAs (Light Conversion, ORPHEUS).The output of one OPA was converted to the UV region using second harmonic and sum frequency generation, whereas two OPAs were fitted with difference frequency generation (DFG) units to produce mid-IR probe pulses of bandwidth ∼200 cm −1 and partially overlapping spectral ranges.A 12 ns optical delay stage controlled the timing between the UV excitation and mid-IR probe pulses, prior to their spatial overlap at the center of a Harrick cell containing the sample.A peristaltic pump continuously circulated sample solutions through the Harrick cell, which was fitted with CaF 2 windows separated by 100 μm spacers.The transmitted mid-IR probe pulses were dispersed onto two separate 128-element MCT detector arrays (Infrared Associates).UV excitation energies were 80 nJ/pulse at 260 nm and 800 nJ/pulse at 340 nm with a focal spot size of 150 μm.TRIR spectra were obtained by comparison of measured absorbances with and without the UV excitation pulses.
Computational Chemistry Calculations.For the microsolvation calculations, nitrobenzene was optimized on the S 0 surface using the CAM-B3LYP 70 /Def2-TZVP 71 method employing empirical dispersion with Becke−Johnson damping (GD3BJ). 72The analysis of the vibrational frequencies confirmed this geometry as a minimum.Molecules of each solvent system were positioned by hand around the nitrobenzene molecule to maximize interactions with the NO 2 group, and the resulting microsolvated system was reoptimized at the CAM-B3LYP/Def2-SVP level of theory.The analysis of the vibrational frequencies again confirmed the resulting microsolvated systems as minima.These calculations were performed using the Gaussian 16 program. 73ollowing our earlier work, 29 all excited-state calculations on the various gas-phase nitrobenzene structures were performed at the CASPT2 level, with the same CAS (14,11) active space as before, which includes the π-orbitals on the ring and the lone pairs on the nitro-oxygen atoms. 29The basis set was the atomic natural orbital (ANO) of S-type double-ζ with polarization (ANO-S-VDZP). 74The geometries were taken from our earlier work 29 and calculations were performed using the OpenMolcas program 2023. 75The CASSCF wave functions were state averaged over 6 singlets and 5 triplets, except for the triplet excitations from the T 1 (n A π*) and T 2 (π O π*) minima which included 7 triplet states.Solvation effects were included using a water PCM, with the equilibrium charges from the ground state used for nonequilibrium PCM calculations of the excited states.Structure optimizations were all at the CASSCF level.
Transient absorption heatmaps, spectral profiles, and kinetics for nitrobenzene in cyclohexane, acetonitrile, and methanol; spectral decomposition process and associated center, width, and amplitude parameters for the spectral decomposition of all transient absorption spectra; time-resolved infrared spectra, kinetics, and spectral decomposition of cyclohexane and d 3 -acetonitrile spectra; and computational details (PDF)

Figure 1 .
Figure 1.UV−visible spectra of nitrobenzene in water (15 mM, purple), cyclohexane (104 mM, red), acetonitrile (68 mM, green), and methanol (102 mM, blue).Inset: expanded view (×10) of the UV−visible spectra in the 300−400 nm range.The dashed vertical lines mark the wavelengths employed in this work.Data have been normalized to the maximum of the band centered around 250−280 nm.
Figure 4 shows TRIR spectra of nitrobenzene in d 2 -water at different pump− probe delays after photoexcitation at 340 and 260 nm.Corresponding TRIR spectra of nitrobenzene in cyclohexane and d 3 -acetonitrile are presented in FiguresS18 and S19.In the

Figure 2 .
Figure 2. Transient absorption spectra of 15 mM nitrobenzene in water at specified pump−probe delays following photoexcitation at 355 nm.Letters A−E label the peaks in the structured triplet ESA (see text).

Figure 3 .
Figure 3. Kinetic traces and associated kinetic fits of the transient absorption spectrum of 15 mM nitrobenzene in water after photoexcitation at 355 nm.Peaks A, B, and C are assigned to the triplet vibrational structure illustrated in Figure 2. S 1 represents the decay of the ESA from the pump-excited S 1 state.The broad feature centered around 650 nm is derived from the basis function fitting in the 598−650 nm window.Data have been normalized to the maximum ΔA.

Figure 4 .
Figure 4. TRIR spectra of 16 mM nitrobenzene in d 2 -water at specified pump−probe delays following photoexcitation at 340 nm (left) and 260 nm (right) for the feature centered around 1360 cm −1 , which is a NO symmetric stretch.58

Figure 5 .
Figure 5. Kinetic traces and associated kinetic fits of the TRIR spectra of 16 mM nitrobenzene in d 2 -water after photoexcitation at 340 (red) and 260 nm (blue).Ground-state recovery signals are monitored around 1360 cm −1 .Data have been normalized to the maximum ΔA.

Figure 7 .
Figure 7. Schematic representation of the relaxation processes following photoexcitation at 355 nm in our TAS experiments (left) and at 340 and 260 nm in our TRIR experiments (right).Note that T 1 and T 2 are nearly degenerate.

Table 2 .
Timescales (in ps) Obtained from Fits to the Kinetic Traces (Figures 3 and S8−S10) Following Nitrobenzene Photoexcitation at 355 nm a

Table 3 .
Time Constants (in ps) Obtained from Global Fits to the TRIR Spectra Following Photoexcitation at 340 and 260 nm (Figures 4, S18, and S19) a VR (S 0 ) is the time scale for vibrational relaxation in S 0 following S 1 / S 0 IC, and τ ISC2 is the time scale for ISC from the triplet manifold to S 0 .A VR and A ISC2 are the relative amplitudes associated with these ground-state recovery processes.
a τ

Table 5
a Solvated bond lengths were calculated at the CAM-B3LYP/Def2-TZVP GD3BJ level of theory.