
==== Front
J Phys Chem Lett
J Phys Chem Lett
jz
jpclcd
The Journal of Physical Chemistry Letters
1948-7185
American Chemical Society

39208262
10.1021/acs.jpclett.4c02154
Letter
Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution
https://orcid.org/0009-0007-3733-6818
Greene Hallam J. M. †
https://orcid.org/0000-0003-0716-1053
Ghosh Deborin †
Sazanovich Igor V. ‡
https://orcid.org/0000-0001-9036-2133
Phelps Ryan ‡
https://orcid.org/0000-0002-1705-473X
Curchod Basile F. E. †
https://orcid.org/0000-0001-5551-9609
Orr-Ewing Andrew J. *†
† School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
‡ Central Laser Facility, Research Complex at Harwell, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0QX, U.K.
* Email: a.orr-ewing@bristol.ac.uk.
29 08 2024
12 09 2024
15 36 91539159
22 07 2024
22 08 2024
20 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

H2020 European Research Council 10.13039/100010663 803718 Engineering and Physical Sciences Research Council 10.13039/501100000266 EP/V026690/1 document-id-old-9jz4c02154
document-id-new-14jz4c02154
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pmcBrown carbon (BrC) aerosols composed of organic particles dispersed in air absorb sunlight in the UV and visible regions at wavelengths >300 nm and thus have a significant impact on radiative forcing in the lower atmosphere.1,2 Nitroaromatic compounds are a major component of BrC, accounting for about half of the light absorption by these aerosols.2 A major class of nitroaromatic pollutants, nitrophenols are formed from biomass burning, alongside emissions from vehicles and release from industrial sources.2−5 Nitrophenols may also form as a result of secondary reactions between aromatic molecules, OH radicals and nitrogen oxides in the atmosphere.1,5 As well as contributing to BrC, ortho-substituted nitrophenols, such as ortho-nitrophenol (oNP - Figure 1a) have been identified as a source of nitrous acid (HONO) when photoexcited in the gas phase, albeit at low quantum yields.6,7

Figure 1 UV/vis absorption spectra of ortho-nitrophenol (oNP) and ortho-nitrophenolate (oNP–) in aqueous solution with inset structures: (a) ortho-nitrophenol in its nitro form, showing the intramolecular H-bond; (b) ortho-nitrophenolate, the anion formed by deprotonation; (c) the aci-nitro tautomer following proton transfer from the OH to the nitro group.

Studying the photochemical behavior of oNP in aqueous solution is of importance in understanding how it may behave in aqueous atmospheric aerosols, cloudwater and fog droplets exposed to sunlight. These droplets exhibit a wide range of pH,8 and thus the pH-dependence of the photochemistry should also be considered. oNP has a pKa of 7.2,9,10 and thus the dynamics of the anion (oNP– - Figure 1b) could influence the photochemistry of this species in solutions of higher pH. Both the neutral and anionic species have low quantum yields for photodegradation in aqueous solution (Φ = 5 × 10–6 and 2 × 10–6 respectively for irradiation at 365 nm),11 and no fluorescence or phosphorescence emission has been reported for oNP or its anion, indicating efficient nonradiative decay pathways. The ultrafast dynamics of photoexcited oNP in aqueous solution have previously been studied by Ernst et al. by femtosecond transient absorption (TA) spectroscopy, pumping at a wavelength of 350 nm and using 15 discrete probe wavelengths in the range 480–1100 nm, with time delays of up to 500 ps.12 The dynamics of the photoexcited anion were described by Michenfelder et al.13 and Bailey-Darland et al.10 Previous computational studies have also investigated the UV-induced dynamics of oNP in the gas and aqueous phase.14,15

In this work, TA spectroscopy using time delays seamlessly spanning femtosecond-to-microsecond time scales is used to investigate both the early and later time photochemistry of oNP, with white light continuum probes employed to better understand the evolution of the transient spectra. Time-resolved infrared spectroscopy further characterizes the states and species involved and the degree of parent-molecule recovery. Quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT), benchmarked with ADC(2), provide a framework for the interpretation of the experimental results. From these measurements and calculations, a comprehensive picture emerges for the relaxation pathways of UV-photoexcited aqueous oNP and their associated time scales.

The TA spectra of oNP in aqueous solution presented in Figure 2 show four distinct features when pumped into the first bright state at its absorption maximum at 350 nm (see Figure 1). Band assignments are provided in SI Figure S1. A positive feature with a maximum at around 450 nm (i) is assigned to excited state absorption (ESA) from the singlet ππ* state. A negative feature in the range 600–700 nm (iii) is attributed to stimulated emission from the same state. A blue-shifting peak in the range 400–450 nm (ii), which overlaps with peak (i) in early time, is attributed to hot ground state absorption (HGSA) from vibrationally excited oNP in its S0 state.

Figure 2 Transient absorption spectra for ortho-nitrophenol photoexcited at 350 nm in aqueous solution (a) with 0.1 M HCl, and (b) at its intrinsic pH shown as contour plots; (c) Contour plot of the 100 ps −1 μs transient absorption spectra of oNP in aqueous solutions at its intrinsic pH, measured using electro-optical delays; (d,e) Kinetic traces of an integrated region from 400–600 nm, from the data shown in (a) and (b) respectively, fitted to a biexponential function (red) or higher order exponential (blue), both with a Gaussian convolution to account for the IRF; time constants are given in Table 1. The time delay Δt is plotted on a split linear-logarithmic y axis in (a,b,d,e) and a logarithmic axis in (c). Highlighted features show i – excited state absorption (ESA) from the first bright state, ii – hot ground state absorption (HGSA), iii – stimulated emission (SE) from the first bright state, iv – absorption from the deprotonated anion.

Data were collected for a solution at its intrinsic pH – denoted pH 7–and a solution containing 0.1 M HCl–denoted pH 1 (both values omit contributions from the weakly acidic oNP). A long-lived feature (iv) is visible only in late time in the spectra collected at pH 7. Further details of the behavior of iv can be seen in Figure 2c which shows the TA spectra measured for a similar solution using electro-optic delays (EOD) to extend the range of Δt beyond 1 μs. The peak has a center around 420 nm which matches that of the deprotonated oNP in its ground state (Figure 1), and this species does not accumulate in solution of low pH, thus the peak is attributed to the ortho-nitrophenolate anion (oNP– - Figure 1b). This assignment is supported by TRIR spectra reported below.

The delayed onset of absorption by oNP– indicates that it forms from the triplet state. The growth of the oNP– peak is multiple orders of magnitude slower than the decay of the first bright state, so deprotonation from this singlet state of oNP to form the S0 anion can be discounted. Deprotonation from an excited singlet state of oNP instead producing the S1 state of the anion can also be discounted because of the known sub-ps electronic deexcitation of the oNP– S1 state.10,13 The photoacid behavior of the triplet state is discussed in more detail below.

To investigate the kinetics, the overlapping ESA and HGSA features in early time TA spectra were fitted to a biexponential decay with a Gaussian convolution to account for the instrument response. The resulting time constants are reported in Table 1. The ∼400 fs time constant τ1 can be attributed to depopulation of the ππ* excited electronic state, and the ∼3 ps τ2 to vibrational cooling of the resulting internally hot molecules in the S0 state. The kinetics of these positive TA features in early time spectra are similar to those determined by Ernst et al., although the ∼3 ps component is here attributed to vibrational cooling of the hot ground state, rather than to decay of the singlet state by intersystem crossing (ISC).12 This HGSA assignment is confirmed by our TRIR measurements (see below). An alternative fitting using a higher-order exponential gives similar values for the depopulation of the ππ* excited electronic state τ1, and slightly faster hot ground state cooling τ2 on the order of 2 ps, which is similar to the 1.4 ± 0.3 ps measured for hot ground state cooling in the anion.13 This fitting also gives a weakly contributing third component, τ3, attributed to depopulation of S1 by ISC, which is an order of magnitude slower. As we show later, branching into the triplet states of oNP is only a minor pathway for relaxation.

Table 1 Time Constants Derived from Integrated Regions of the TA Dataa

Data set	pH 7	pH 1	 	pH 7 EOD	
 	Integration region (nm)	400–600 nm	400–600 nm	 	400–475 nm	
 	Time window fitted	0.1–1000 ps	0.1–1000 ps	 	0.1–4000 ns	
Biexponential Fitting	τ1	410 ± 10 fs	400 ± 20 fs	τ4	1.5 ± 0.5 ns	
τ2	3.0 ± 0.1 ps	3.4 ± 0.2 ps	τ5	390 ± 50 ns	
 	
 	Time window fitted	0.1–3750 ps	0.1–3750 ps	 	 	
Higher-Order Fitting	τ1	380 ± 10 fs	310 ± 20 fs	 	 	
τ2	2.0 ± 0.2 ps	1.6 ± 0.1 ps	 	 	
τ3	18 ± 5 ps	23 ± 5 ps	 	 	
τ4	1.5 nsb	1.5 nsb	 	 	
a Error margins are statistical errors of the fits.

b Constrained to value determined by fitting of EOD data.

The stimulated emission evident in Figure 2a,b at wavelengths >600 nm was also observed by Ernst et al. and found to increase in intensity with probe wavelength. In the region between 540–700 nm it is overlapped by the positive ESA band. The 0.3 ± 0.1 ps time constant for decay of this stimulated emission reported by Ernst et al.12 is consistent with our measurements, and supports the proposed subpicosecond decay of the first bright state population.

The kinetics of the late time features are more challenging to extract from the TA spectra, given the weak signals, but suggest that the growth of the oNP– anion occurs over time scales on the order of τ4 ≈ 1.5 ns with subsequent decay over τ5 ≈ 400 ns (Table 1 – fitting shown in SI Figure S6). Time constants of >100 ps, > 500 ps and 900 ± 50 ps, attributed to the decay of the triplet state, have been found in 2-propanol, n-hexane, and benzene, respectively.12,16 These values suggest that the triplet state could be sufficiently long-lived in aqueous solution to have a decay pathway to oNP– via deprotonation with a time constant of ∼1.5 ns. Whether there is a competitive decay route in water allowing direct ISC from T1 back to S0 oNP is hard to determine from the data. In the TA spectra of aqueous solutions of the related nitrobenzene, T1 ESA bands are seen around 700 nm and <450 nm.17 Ernst et al. reported triplet ESA at around 480 nm in 2-propanol and n-hexane, but found negligible signal in water.12 Similarly, we are unable to attribute any features of the TA spectra to the triplet state.

Time-resolved infrared spectra shown in Figure 3 further support the band assignments in the TA spectra. Negative features in the TRIR spectrum of oNP in D2O, excited at 350 nm (Figure 3a–f) are ground state bleaches (GSBs) and correspond to the position of absorption peaks in the FTIR spectrum. A series of positive features (a′-f′), which emerge within the first few ps and shift to higher wavenumber with increasing time delay, are identified as HGSA peaks. Each is anharmonically shifted from an associated GSB feature a–f at slightly higher wavenumber and with matching decay kinetics. A positive feature in the late time (A) matches a major peak in the FTIR spectrum of the anion and is thus attributed to that species, in accord with our anion band assignment from TA spectra (band iv in Figure 2b). A second peak (T) which decays more quickly than A, is tentatively attributed to absorption from the oNP triplet state.

Figure 3 Time-resolved infrared spectra of a solution of oNP in D2O, pumped at 350 nm. Data are shown as a contour plot. The FTIR spectra of oNP and oNP– in acidic and basic D2O respectively are provided for comparison. Features a–f are peaks in the FTIR spectrum of the neutral species and appear as ground state bleaches (GSBs) in the TRIR spectra; a′–f′ are HGSA peaks located to the low wavenumber side of GSB features because of anharmonicity; A is a major peak in the anion FTIR which appears in the late time spectra. T is a weak absorption feature in the TRIR attributed to the triplet.

TRIR spectra also give an indication of the yields of the decay pathways.17,18 An exponential fit of integrated intensities for regions covering the two largest GSB and HGSA features in early time TRIR data gives decay time constants in the order of ∼3 ps (see SI section S4) which are comparable to the HGSA decay seen in the TA spectra. The majority of the GSB recovers within the first 20 ps, indicating that a route involving rapid decay of the first bright singlet state into S0 followed by vibrational relaxation is the dominant decay pathway. Fitting of the GSB recovery in the TRIR data suggests that this pathway accounts for 94% of excited species (see SI Figure S9 and Table S2). Quantum yields of the triplet state, and consequently the oNP– anion, are therefore low. There is some evidence of bleach recovery in the range 100 ps −1 ns implying that there may be a pathway allowing direct ISC between T1 and S0 although this feature could also be due to overlapping absorption from the anion.

To better understand the competing relaxation pathways of oNP, LR-TDDFT within the Tamm-Dancoff approximation (TDA) and with the ωB97XD functional and aug-cc-pVDZ basis set was used to find critical geometries in the first excited electronic state. The outcomes are shown in Figure 4. The ground state minimum adopts the nitro form (Figure 1a). Excited state calculations found a minimum on the adiabatic S1 surface with nπ* character and a geometry representative of the S1/S0 intersection seam (IS) with ππ* character (Figure 4a). The intersection seam is accessed by excited state intramolecular proton transfer (ESIPT) from the OH group to the nitro group, forming the aci tautomer (Figure 1c), and torsion of the nitro group, as previously described by Ernst et al., among others.12,15,19,20 The computed geometry changes needed to access the intersection seam have recently been supported by gas-phase ultrafast electron diffraction data,20 and decay via this intersection has been shown to slow when rotation is inhibited by a more viscous solvent.12 The S1(nπ*) minimum is in the nitro form and corresponds to a reduction in the ∠ONO bond angle in the nitro group. This minimum has not been previously described because prior theory on oNP has concentrated on the region of the adiabatic S1 state with ππ* character.

Figure 4 A linear interpolation in internal coordinates using 10 intermediate steps between geometries of the S1(nπ*) minimum, ground state minimum, and a geometry representative of the S1(ππ*)/S0 intersection seam. Energies of the ground and excited states relative to the S0 minimum (a) and the S1-Tn spin–orbit coupling (SOC) at each geometry (b) are calculated using (LR-TD)DFT/TDA/ωB97X-D3/ZORA-def2-TZVP. Continuous lines show the adiabatic singlet surfaces in a, point markers denote the character of the excited singlet states. A bold black arrow indicates the dominant relaxation pathway via the conical intersection, a thinner arrow the minor pathway proceeding to the S1(nπ*) minimum. All results shown are from gas-phase calculations.

The behavior of the potential energy surfaces was investigated along linear interpolation in internal coordinates (LIIC) pathways from the Franck–Condon (FC) geometry to the S1(ππ*)/S0 IS geometry (Figure 4a–rightwards) and to the S1(nπ*) minimum (Figure 4a–leftwards), with energies calculated using (LR-TD)DFT/TDA at the ωB97X-D3/ZORA-def2-TZVP level of theory in the gas phase. This level of theory was benchmarked with ADC(2) (see SI Figure S3). While care is needed when describing the S1/S0 intersection seam with LR-TDDFT,21,22 the pathway to the S1(ππ*)/S0 IS is consistent with findings using high-level wave function-based methods. The adiabatic S1 and S2 surfaces are of similar energy around the FC region, which can be interpreted as a crossing of the bright ππ* and dark nπ* diabatic surfaces. In this calculation, the first bright state is formally S2, but the ordering of the lowest bright and dark states is sensitive to the level of theory, basis set and solvation model used (see SI), hence why the first bright state is described as S1 in other work.12,15,19 Our calculated gas-phase LIICs suggest there is no energy barrier to either the S1(nπ*) minimum or the S1(ππ*)/S0 IS. A small energy barrier to the S1(ππ*)/S0 IS is apparent when solvation is modeled using a water CPCM (SI Figure S2). The influence of solvation on the potential energy surfaces is discussed below.

These computational data, alongside our TA and TRIR measurements, show that upon excitation at 350 nm into the first bright state, over 90% of molecules rapidly return to the ground state via a conical intersection (CI), accessed by proton transfer and NO2 torsion, with a time constant of ∼400 fs. These nonadiabatic dynamics produce vibrationally hot molecules in the electronic ground state which relax by vibrational energy transfer to the solvent with a time constant of ∼3 ps. The pathway proceeding from the FC region along the S1(ππ*) surface to the IS in Figure 4 is consistent with this major decay mechanism, but does not account for species entering the triplet manifold.

To investigate the possible formation of the triplet state of oNP, the spin–orbit coupling (SOC) magnitude between the first adiabatic singlet state S1 and the low-lying triplet states was calculated at each geometry as an indication of the probability of ISC (Figure 4b). Intersystem crossing from the region of the S1 surface of ππ* character is unlikely given the small SOC, larger singlet–triplet energy gaps, and the rapid decay of S1 population via the CI. A low likelihood of ISC from this surface was also identified by Nunes et al.20 A more plausible explanation is that ISC occurs from the nπ* region of the S1 surface. Significant SOC between S1 and T2 is calculated at the S1(nπ*) minimum where it has a magnitude of over 40 cm–1. T2 is of ππ* character at this geometry, and thus the significant SOC is in accordance with the El-Sayed rule.23,24 There is also a small energy gap of 0.2 eV between the two states at this geometry, indicating favorable conditions for ISC from internally excited S1 molecules. Favorable conditions for ISC from the S1(nπ*) state can also be explained by the fact that radiative decay back to S0 is symmetry forbidden, allowing time for ISC to occur. We cautiously attribute a weakly contributing signal observed in the TA spectra which decays with a time constant, τ3 ≈ 20 ps (Table 1), to absorption from the S1(nπ*) state which decays via this ISC. Assuming that ISC occurs primarily from geometries around this S1(nπ*) minimum, a bifurcation must first occur near the FC region on the ππ* state, leading to branching into nπ* and ππ* regions of the S1 surface.

Our experimental data show that, from the triplet state, oNP molecules deprotonate to form the anion–a process with a time constant in the order of 1.5 ns. This deprotonation must first make the anion in a triplet spin state, and ISC from T1 to S0 will contribute to the rate of formation of the oNP– anion in its ground state. Reprotonation of the ground-state (S0) anion occurs on a time scale of ∼400 ns in aqueous solution at its intrinsic pH – but this time scale reduces significantly upon the addition of acid. In 0.1 M HCl, the reprotonation of the anion occurs on a time scale similar to, or faster than, its formation, which is why no features attributable to the anion are observable in the TA and TRIR data at pH 1.

Quantum chemical calculations help explain the proposed behavior of the triplet state. Following ISC into T2(ππ*) at geometries near the S1(nπ*) minimum, rapid internal conversion to T1(ππ*) is assumed to occur. A geometry optimization of this T1 state was performed using ωB97XD/aug-cc-pVDZ, first with LR-TDDFT/TDA and then using unrestricted DFT (UDFT) for the lowest energy triplet state. A LIIC from the FC region to the T1 minimum energy geometry was then performed using both LR-TDDFT/TDA and UDFT calculations (SI Figure S5). The resulting data suggest that the minimum energy geometry of the triplet state is the aci form (Figure 1c), as found by Ernst et al., among others,7,12,25 suggesting that ESIPT also occurs in the triplet state. Consequently, deprotonation from the triplet state will be from the protonated nitro group of the aci form, instead of the OH group found in the nitro form. Although calculating values for the SOC to the ground state by the chosen method is prone to error, the T1 - S0 SOC at the T1 minimum was found by LR-TDDFT/TDA (ωB97X-D3/ZORA-def2-TZVP) to be only 0.8 cm–1, accounting for the slow ISC to S0 that allows a competing decay pathway via deprotonation.

Considering the effects of the solvent, when calculations include implicit solvation by water using a CPCM, the nπ* states of oNP increase in energy due to stabilization of the electron density in nonbonding orbitals by solvent–solute interactions (SI Figures S1 and S2). The first bright state becomes formally S1, and access to the conical intersection therefore requires no change in the adiabatic surface. Implicit solvation by water moves the crossing of the lowest energy ππ* and nπ* diabatic surfaces further from the FC region, which is expected to result in lower branching onto the nπ* surface, and hence a lower triplet quantum yield. These effects are expected to become more pronounced in calculations that explicitly treat hydrogen bonding in a protic solvent.

From the TRIR data (Figure 3) it is evident that there is only a small quantum yield for the triplet state and thence the anion. This relaxation pathway is unfavorable because of low branching onto the nπ* surface in the FC region. The low quantum yield into the triplet state explains the absence of discernible contributions from this state to the TA spectra, and the low intensity of the oNP– anion absorption band in the TA spectra, despite its relatively high molar absorption coefficient. The reported ESA from the triplet state of oNP photoexcited in other solvents may be indicative of a higher triplet quantum yield because of more favorable branching to the nπ* region of the S1 state. Recent work has shown how solvent effects can significantly influence the decay pathways of nitroaromatic compounds.26 The TRIR data also show a complete ground state bleach recovery on <1 μs time scales. This efficient regeneration of parent oNP in its S0 state is consistent with the very low quantum yields of phototransformation (5 × 10–6) reported previously for oNP in aqueous solution.11

The ultrafast photochemistry of near-UV excited oNP is dominated by an efficient relaxation pathway through a S1(ππ*)/S0 conical intersection accessed by intramolecular proton transfer and NO2 torsion which occurs on a 400 fs time scale, producing vibrationally excited molecules in the ground electronic state. Fewer than 10% of molecules instead enter the triplet manifold, most likely from the S1 state in a region of nπ* character following a branching near the FC region. Proton transfer to form the aci tautomer occurs on T1, after which decay of the triplet state is facilitated by deprotonation on the ns time scale. Aqueous oNP therefore has at least two nonradiative relaxation routes after excitation at 350 nm. These two pathways ensure a low quantum yield for photodegradation, making this species potentially environmentally persistent when dissolved in aqueous atmospheric aerosols.

Experimental and Computational Methods

Ultrafast laser spectroscopy was conducted at the University of Bristol, and at the Central Laser Facility of the STFC Rutherford Appleton Laboratory using the LIFEtime instrument. Details of the laser setup at the University of Bristol are given in the SI section S6, whereas the LIFEtime setup is described elsewhere.18,27

For TA spectroscopy, aqueous solutions of oNP were prepared at an optical density of 0.4–0.5 at the pump wavelength and flowed continuously through cells with CaF2 windows with a 250-μm path length. Reservoirs of 100 cm3 were used, in case of photodegradation. Aqueous HCl was added to relevant samples at a concentration of 0.1 M. The pump pulse was set at 350 nm, the spot size and pulse energy at the sample to ∼150 μm fwhm and 0.5 μJ in the LIFEtime setup and ∼300 μm fwhm and 1 μJ in the setup at Bristol. In data collected at Bristol a white light continuum (WLC) was generated by focusing the 800 nm fundamental onto a CaF2 window. A long pass filter (λ > 360 nm) was used after the sample to remove pump scatter. For experimental measurements at LIFEtime, a WLC was generated by focusing the frequency doubled output of a Yb:KGW laser on sapphire to produce a continuum in the range 380–500 nm.18

For TRIR measurements, saturated solutions of oNP in D2O were prepared by sonication of an excess of the solid in D2O, followed by filtration. A small amount (∼10%) of excess D2O was added to prevent recrystallization. Concentrated DCl was added to relevant samples to create concentrations of 0.1 M. Solutions of volume ∼15 cm3 were flowed continuously through cells with CaF2 windows with a 100-μm path length. Fourier transform infrared (FTIR) spectra of oNP and its deprotonated anion were measured using near-saturated solutions held in similar cells with added DCl or NaOD, respectively. Pump characteristics were comparable with those used for TA at each facility. In data collected at Bristol a single IR probe pulse in the range 1450–1650 cm–1 was used. In data collected at LIFEtime, two synchronized overlapping IR probe pulses in the range 1280–1460 cm–1 and 1450–1700 cm–1 were used. All TA and TRIR data were processed and analyzed using KOALA software.28

Ground state DFT geometry optimization was conducted using the ωB97XD functional,29 and aug-cc-pVDZ basis set,30,31 using Gaussian 16 (RevA.03).32 LR-TDDFT geometry optimizations of excited states were conducted using the same functional and basis set. The Tamm-Dancoff approximation was used in all LR-TDDFT calculations.33 Frequency calculations were performed for all minima to confirm their nature. The aug-cc-pVDZ basis set was chosen to match that used by Ernst et al.,12 with ωB97XD chosen in place of B3LYP due to its inclusion of nonbonding interactions, and long-range correction.29 The geometry representative of the S1/S0 intersection seam was chosen as the geometry with the lowest S1 energy and closest S1/S0 approach (0.006 eV) in geometry optimization cycles of the S1(ππ*) state. Key geometries are provided as xyz files.

The geometries forming the linear interpolations in internal coordinates were obtained using Entos Envision.34 Ten intermediate geometries were used for the paths connecting S0 to S1(nπ*) or the S1(ππ*)/S0 IS. LR-TDDFT/TDA was used to calculate excitation energies, and singlet–triplet SOC,35 of the key and intermediate geometries using the ωB97X-D3 functional,36 and ZORA-def2-TZVP basis set,37,38 conducted in ORCA 5.0.3.39,40 The ZORA-def2-TZVP basis set was chosen as a suitable alternative to aug-cc-pVDZ using zeroth order regular approximation (ZORA) to the Dirac equation for scalar relativistic effects.41 ADC(2) calculations,42 conducted in ORCA using the aug-cc-pVDZ basis set, provided a comparison of the behavior of excited singlet states derived from a wave function-based method. Natural transition orbitals (NTOs)43 were calculated in ORCA. Visualization of surfaces and structures was performed in Avogadro,44,45 or IQMol.46

Unrestricted DFT calculations conducted in Gaussian were used to find minimum energy geometries of the lowest triplet state using ωB97XD/aug-cc-pVDZ. Unrestricted DFT, used to find the energy of the lowest triplet state, was performed in ORCA using ωB97X-D3/ZORA-def2-TZVP.

Data Availability Statement

Data are available at the University of Bristol data repository, data.bris, at https://doi.org/10.5523/bris.1z3j033v7744y2nax4q9q0w3r5.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.4c02154.Absorption band assignments, additional LIICs, NTOs, kinetic traces of transient absorption spectra, additional TRIR spectra, kinetics and fitting, investigation of isotope effects by TA spectroscopy, and a description of the laser setup at the University of Bristol (PDF)

Key geometries (ZIP)

Supplementary Material

jz4c02154_si_001.pdf

jz4c02154_si_002.zip

The authors declare no competing financial interest.

Acknowledgments

This work was funded by the EPSRC through Programme Grant EP/V026690/1. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 803718, project SINDAM). We acknowledge the Central Laser Facility at the STFC Rutherford Appleton Laboratory for time on the ULTRA LIFEtime facility, provided under application 24130009-1, and the assistance of Marta Szynkiewicz. Calculations were carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol - http://www.bristol.ac.uk/acrc/.
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