Photoelectron spectroscopyPD-PE spectra of four phenolic nitrate complexes HA·NO3−, HA = phenol (a), three positional isomers of phenolcarboxylic acid (PCA), n-PCA (n = 2–4) (c–e), along with benzoic acid (BA, b) were obtained at the cryogenic temperature of 20 K using the 157 nm photons (Fig. 1) (see spectral data in Supplementary Data 1). The spectrum of phenol·NO3− consists of three well-defined bands spanning from 4.5 to 7.5 eV. The experimental adiabatic and vertical detachment energy (ADE and VDE) are estimated from the signal threshold and the maximum of the first band to be 4.5 and 5.0 eV, respectively. Substituting phenol with n-PCA (n = 2–4) increases ADE/VDE to 4.9/5.5, 5.1/5.6, and 4.9/5.3 eV, respectively (Table 1), and the corresponding spectra all feature three resolved broad bands. Among four phenolic nitrate clusters, 3-PCA·NO3− possesses the highest ADE/VDE. Especially interesting is the rising edge of the first band in the 2-PCA·NO3− spectrum that exhibits a particularly slower slope compared to phenol or the other two phenolic acids. For comparison, the spectrum of nitrate benzoic acid complex is composed of a low-intensity first band ranging from 5.0 to 6.0 eV, followed by a second intense band from 6.0 to 7.5 eV with the estimated ADE/VDE of 5.1/5.5 eV.Fig. 1: Photoelectron spectroscopy.Cryogenic (20 K) 157 nm PE spectra of HA·NO3− with HA = phenol (a), BA (b), and n -PCA (n = 2–4) (c–e).Table 1 Experimental and calculated energeticsStructure optimization and spectral assignmentsTo unravel potential PT events upon electron detachment, both anionic and neutral complexes that contribute to the measured PE spectra were optimized at the M06-2X/aug-cc-PVTZ level. Figure 2 shows the optimized structures for each low-lying HA·NO3− anion with their relative energies within 0.3 eV (see atomic coordinates in Supplementary Data 2). The corresponding neutral complex exhibits an overall similar structural scaffold compared to the anion except for the intermolecular O−H···O hydrogen bond (HB) lengths. To better visualize possible PT motion triggered by electron detachment oxidation, the bond length between the H to the hydroxyl or carboxyl oxygen of HA (labeled as H···A, r1) and between the H to the nitrate oxygen (represented as H···O, r2) are displayed in black for anions and red for neutrals. Structure optimizations indicate a mildly longer r2 in phenol···ONO2− (1.61 Å) than in BA···ONO2− (1.55 Å). This implies a stronger interaction for benzoic acid than phenol when binding to NO3−, a fact consistent with the experimentally observed VDEs (5.5 vs. 5.0 eV) and calculated BEs (1.22 vs. 0.98 eV). Especially noteworthy is the pair of r1/r2 changing from 1.01/1.61 Å in the anion to 1.63/1.00 Å in the neutral for the phenol nitrate complex, indicating the proton initially bound to phenol relocated to the nitrate side upon electron detachment. In contrast, such a proton relocation is not observed in the benzoic acid nitrate case.Fig. 2: Calculated structures.M06-2X/aug-cc-PVTZ optimized structures of HA·NO3− (HA = phenol, BA, and n-PCA (n = 2–4)) with key bond lengths (in Å) along the intermolecular O−H···O HBs listed (black for anions and red for neutrals). DLPNO−CCSD(T)/ma-def2-TZVP//M06-2X/aug-cc-PVTZ relative energies (in eV) are given under each isomer.Given each PCA molecule possesses two interactive groups of −OH and −COOH, to which the nitrate anion can bind, multiple isomers are expected and subsequently identified (Fig. 2). The isomers with NO3− solely interacting with −OH (iso A of 2- and 4- and iso B of 3-PCA·NO3−) are hereinafter referred to as hydroxyl isomers and those with −COOH (iso B of 2- and 4- and iso C of 3-PCA·NO3−) are referred to as carboxyl isomers. The hydroxyl isomer is found to be more stable than the respective carboxyl isomer for all three PCAs (Fig. 2), in accordance with the calculated BEs for different types of isomers (Table 1). The complex of 3-PCA·NO3− exhibits an additional isomer A wherein NO3− binds to both the −OH and −COOH sites. The formation of this isomer can be ascribed to the suitable separation (meta-) between the hydroxyl and carboxyl groups. In contrast, the para-distributed two groups in 4-PCA result in an excessive site spacing that makes simultaneously binding to nitrate not feasible, while the ortho-spaced −OH and −COOH groups in 2-PCA prefer to form an intramolecular HB between themselves that prevents them both interacting with NO3−.By comparing the A···H (r1) vs. H···O (r2) bond lengths from the anion to the neutral, it becomes evident that upon electron detachment, the hydroxyl proton transfers to the nitrate oxygen for all hydroxyl isomers. However, in the case of the carboxyl isomers, except for iso B of 2-PCA·NO3−, the proton remains where it was. Isomer B of 2-PCA·NO3− features an intramolecular HB between the ortho-positioned −OH and −COOH groups as well as an intermolecular HB between the −COOH and NO3−. Formation of such an HB-bridge-like configuration enables an exceptional PT propagation from hydroxyl to carboxyl and to nitrate, a scenario closely resembling the concept of electron-coupled (EC) double PT previously proposed in studying the Sir1694 BLUF photoreceptor39,40. The formation of this intramolecular HB also greatly reduces the energy gap between the hydroxyl and carboxyl isomers to 0.07 eV, in comparison to the REs of 0.15 and 0.28 eV for NO3− complexed with 3- and 4-PCA, respectively.To aid in spectral assignments and identify which isomers contribute to the experiments, we simulated the density of states (DOS) spectrum of each low-lying isomer, calculated its theoretical VDE, and compared them with the corresponding experimental spectrum and data in Fig. 3 and Table 1. The simulated DOS spectra of phenol nitrate and benzoic acid nitrate based on their most stable structures reasonably reproduce all major spectral bands, and their calculated VDEs agree well with the experimental values (Table 1). For 2-PCA·NO3−, the iso B DOS spectrum effectively fills up the small gap at ca. 5.8 eV between the first two bands in iso A. Including iso B noticeably improves the simulated spectrum in comparison to the experiment, suggesting both isomers coexist, consistent with the fact that they are nearly iso-energetic and their calculated VDEs both match well with the experimental VDEs. In the case of 3-PCA·NO3−, the spectral plateau observed from 6.5 to 7.2 eV appears to be contributed by iso A and B, according to their respective calculated DOS spectral profiles. The modest RE = 0.12 eV for iso B further hints at its coexistence and contribution to the observed spectrum. Conversely, for 4-PCA·NO3−, the hydroxyl iso A is 0.28 eV more stable than the carboxyl iso B with the calculated VDE of iso A (5.16 eV) in good agreement compared to the experimental VDE (5.3 eV) while the VDE of iso B (5.85 eV) considerably too high. All major spectral bands are reproduced by the iso A DOS spectrum, indicating the exclusive presence of iso A.Fig. 3: Simulated spectra.Comparison of experimentally measured (red) and simulated spectra (black for total DOS, while orange and blue for partial DOS derived mainly from HA and NO3−, respectively) of HA·NO3− complexes (HA = phenol (a), BA (b) and n-PCA (n = 2–4) (c–e)). The DOS spectrum was generated by convoluting the calculated stick spectrum of high-lying occupied molecular orbitals using Gaussian functions of 0.3 eV full width at half-maximum for each stick while shifting the HOMO level to the calculated VDE. The isomers shown in bold contribute to the experiments.Figure 4 examines the highest occupied molecular orbital (HOMO) of each structure of HA·NO3−. For all hydroxyl isomers, the HOMO is predominantly localized on HA, implying that electron detachment occurs from ionizing HA. In contrast, electron detachment principally takes place from NO3− for the carboxyl isomers except for iso B of 2-PCA·NO3−, in which an intramolecular HB O−H···OCOH is formed. Now it becomes apparent that, for an anion structure, as long as its HOMO is prominently distributed on HA, an evident PT will take place upon electron detachment. Inversely, no PT occurs for those structures with their HOMOs being localized on the NO3− moiety. The above findings highlight that electron detachment oxidation of the anion complex itself cannot ensure PT occurs. Rather its occurrence is critically dependent on the oxidation site within the proton donor and acceptor pair, which in turn is inherently related to the specific HB binding motif between donor and acceptor in the anionic charge state.Fig. 4: Highest occupied molecular orbitals.3D visualization of the highest occupied molecular orbital (HOMO) for the optimized HA·NO3− (HA = phenol, BA and n-PCA (n = 2–4)) structures calculated at the M06-2X/aug-cc-PVTZ level with the contour value of 0.02(a0)−3/2.Note that DOS simulations are based on the generalized Koopman’s theorem41, suggesting that the ground electronic state in a DOS spectrum reflects the HOMO information of the anion. Upon deconvoluting the total DOS into the HA and NO3− contributions (Fig. 3), we readily found that there exists a close correlation between the origin of the first band and the occurrence of PT: for every isomer that exhibits PT behavior, its first peak is attributed to ionizing the HA moiety, while for those without PT character, the first bands are derived from detaching the NO3− part, consistent with the above HOMO findings.To provide further evidence that distinguishes a PT from a non-PT event, we conducted theoretical ADE calculations based on the optimized anion and neutral structures for each isomer. Evidently, for all structures identified undergoing PT, their calculated ADEs are markedly lower than the experimental values by 0.65–1.18 eV, a disparity substantially bigger than the one for those without PT, e.g., 0.35 eV for [BA·NO3−] (Table 1). This is because PT from anionic to neutral species constitutes a substantial structural transformation, rendering Franck–Condon factors for transitions from the vibrational ground state of the anion to that of the neutral complexes negligible. Essentially, the experimentally derived ADE value only represents the upper limit for the true ADE, and a much smaller and further reduced theoretical ADE than the experimental value represents another barometer for the occurrence of PT.Supplementary Table 1 summarizes the NPA charge distributions of each anion and neutral complex [HA·NO3]−/0 among three components (A, NO3, and H interacting with both), the latter being computed at fixed anion (unrelaxed) and optimized structures, respectively. The NPA charge variations of each component upon vertical electron detachment agree with the HOMO analysis (Fig. 4), that is, the aromatic moiety A contributes to the electron detachment for the isomers with PT characteristic, in contrast, photodetached electrons overwhelmingly originate from the NO3− anion for the other structures, in which no PT occurs. Comparing NPA charge distributions between non-optimized and optimized neutral complexes exhibits additional charge redistribution brought by the PT process. The NPA analysis of Iso B for 4-PCA·NO3− is an exception due to the nearly degenerate orbitals of the HOMO and HOMO-1, where the electron mainly localizes on the NO3− moiety for HOMO but on PCA moiety for HOMO-1 (see Supplementary Fig. 1).PD-PES is a direct spectroscopic reporter for PCETOne longstanding issue is how we know when PCET occurs. Is there a simple method that can distinguish coupled vs. sequential electron–proton transfer? Since electron photodetachment (equivalent to a half ET reaction in the absence of an electron acceptor) is an ultrafast process completing in femtosecond time scale, the outgoing electrons will not carry any PT-related structural information if the PT follows sequentially but will encode important structural changes for a concerted electron–proton transfer. To demonstrate this, we compare the photoelectron spectra of phenol/2-PCA·NO3−, which show the PTs from phenols to nitrate upon electron detachment with the photoionization spectrum of isolated neutral phenol42 in Fig. 5a. The front edge of the first band in the phenol·NO3− spectrum, derived from ionizing the phenol moiety, is observed to rise with a substantially slower pace compared to the isolated neutral phenol with the |ADE−VDE| of the former appreciably larger than that of the latter. Note that the |ADE−VDE| of phenol represents the λox of PhOH → [PhOH]•+, an oxidation process also identified in photodetaching [PhOH·NO3−] to [PhOH•+·NO3−]. A larger |ADE−VDE| of the complex signifies additional energy spreading trigged by the PT from [PhOH]•+ to the nearby NO3− that must occur in an ultrafast time scale comparable to the outgoing electron motion. This spectroscopic observation, as well as those for n-PCA·NO3− (n = 3, 4), i.e., larger |ADE−VDE| values or slower rising edges than the isolated PhOH (see Supplementary Fig. 2), clearly signals that the outgoing electrons are coupled with the PT motion on the neutral surface. The much slower slope for 2-PCA·NO3− is due to that it consists of two processes, i.e., PCET from iso A and electron-coupled double PT from iso B. Isomers A and B have different calculated ADE/VDE (4.26/5.48 eV for isomer A and 4.09/5.55 eV for isomer B, Table 1). The contribution of isomer B certainly will widen the overall observed |ADE−VDE| in the spectrum of this complex and explain this complex has the slowest rising edge compared to n-PCA·NO3‒ (n = 3, 4) and phenol·NO3‒. In contrast, photodetaching [BA·NO3−] leads to [BA·NO3•], in which no proton/hydrogen atom relocation is involved. It is illuminating to see in this scenario that the rising edge of the first band of BA·NO3− is very similar to that of isolated nitrate anion43 in Fig. 5b. This is because the first band of nitrate benzoic acid complex is derived from the NO3− moiety and there is no PT occurring in the neutral surface after electron removal. Considering the suboptimal spectral quality of the BA·NO3− spectrum, we proceeded to compare the spectra of NO3− and CH3COOH·NO3−, detailed in Supplementary Fig. 3, which reaches the same conclusion. Therefore, the first band of a photoelectron spectrum of a hydrogen-bonded cluster contains valuable information that provides a direct signature to identify if a PCET reaction takes place.Fig. 5: Spectra comparison.Comparison of photoelectron spectra of HA·NO3− (HA = phenol and 2-PCA) with photoionization spectrum of isolated neutral phenol (a), and photoelectron spectra of NO3− and BA·NO3− (b). All spectra, plotted with the EBE axis spanning in a 3 eV range, are shifted in EBE to ensure the same ADE position in the overlay plots for better comparisons. The spectra of phenol and NO3− are adapted from refs. 42,43 (with permission from Elsevier Copyright 2024 for ref. 42, with permission from AIP Publishing LLC. Copyright 2024 for ref. 43), respectively.To offer further insights into the PT and ET processes, a two-dimensional (2D) potential energy surface scan was performed and depicted in Fig. 6 for (a) phenol·NO3−, (b) BA·NO3−, and (c) iso A and (d) iso B of 2-PCA·NO3−, respectively. Only the intermolecular O−H···O HBs (r1 and r2) were scanned while the rest of the molecules were frozen. For each species, the minimums exhibited on the 2D potential energy surface represent potential structural arrangements, with the r1 vs. r2 value at each minimum directly denoting the proton locations. For all anionic species, one sole minimum on the 2D potential energy surfaces with r1 < r2 means that the proton binds to the A group. In the cases of neutral [phenol·NO3]· and iso A and iso B of [2-PCA·NO3], there also exists only one minimum where r1 > r2, corresponding to the PT neutral species attributed to electron detachment from the aromatic segment. Considering the sole minimum on the 2D potential energy surfaces, the PT reactions occurred in the transitions from anionic to neutral species for [phenol·NO3]−/• and iso A and iso B of [2-PCA·NO3]−/• are indeed induced by the excess electron loss and the related ET and PT processes are coupled rather than sequential. In [phenol·NO3]− and iso A of [2-PCA·NO3]−, the NO3− interacts with the phenolic group in both cases. Examining their corresponding neutral surfaces allows for a direct comparison of their respective 2D potential energy surfaces (Fig. 6a and c), indicating that the formation of the intramolecular HB in iso A can significantly expedite the PCET process by shifting the neutral minimum further downhill. For the anion-to-neutral transition of iso B, the variation in the 2D potential energy surface minimum also demonstrates the PT occurrence. Additional 2D neutral potential energy surfaces were constructed for iso B by explicitly including the intramolecular HB coordinates (see Supplementary Fig. 4). The results indicate the double PT along intra- and inter-molecular HB occurs concurrently, all coupled with the electron detachment.Fig. 6: 2D Potential energy surfaces.Two-dimensional potential energy surfaces for anionic and corresponding neutral complexes of [phenol·NO3]−/• (a), [BA·NO3]−/• (b), iso A (c), and iso B of [2-PCA·NO3]−/• (d). The anionic structures are treated as the starting scan point for both the anions and neutrals. All distances are given in Å and energies in eV.In contrast, two potential wells are identified for neutral [BA·NO3]•, representing the structure of [PhCOOH·NO3]• for r1 < r2 and [PhCOO·HNO3]• for r1 > r2, respectively. It is important to note that the r1 > r2 structure does not correspond to a PT event but rather to a hydrogen atom transfer (HAT) in the neutral species, because the electron is detached from the negatively charged NO3− segment. The HAT event going from the r1 < r2 minimum to the r1 > r2 well needs to overcome a 0.18 eV barrier and is 0.11 eV thermodynamically unfavored (see Supplementary Fig. 5). Given electron photodetachment is a vertical process, the initial neutral species adopts the anion structure in the vicinity of the r1 < r2 minimum, suggesting there will be no HAT upon electron photodetachment.The Lewis structures of phenol and 2-PCA, along with their resonant radical structures after the loss of one electron and one proton, are illustrated in Supplementary Fig. 6, providing additional confirmation concerning the significance of phenolic compounds in PCET reactions. The mechanism of PCET can be delineated as follows: electron detachment oxidation on the phenol site produces a charge-separated state (ArOH•+)·NO3−, which, in turn, facilitates the PT from the phenol radical cation to the nitrate anion to complete charge neutralization. This is a highly exothermic process with minimum or no barrier such that the PT motion is coupled with the electron outgoing motion. The existence of a charge-separated state has been elucidated in the previous theoretical studies of the phenoxyl-phenol system28 and the phenol oxidation in aqueous solutions11. The PCET process shares similarities with excited-state intramolecular proton transfer (ESIPT)44,45 and electron-induced proton transfer (EIPT)17 involving charge-separated states. In all these processes, the proton receptor is negatively charged, while the donor is positively charged, which prompts automatic PT towards balancing the charge distribution. Thus, the underlying mechanism outlined here provides an overarching principle governing the coupled electron and proton motions.Homolytic bond dissociation enthalpies (BDEs) of proton donor and acceptor are important thermodynamic parameters and have been applied to theoretically predict organic systems implicating PCET46. In order to interpret the PCET phenomena in this study from an alternative standpoint, the BDEs of H−ONO2, PhO−H, and PhCOO−H are calculated and compared in Supplementary Table 2. The BDE of H−ONO2 (4.70 eV) notably surpasses that of PhO−H (4.16 eV), yet marginally falls short in comparison to that of PhCOO−H (4.90 eV). The BDE analyses provide an alternative rationale that all PCET events observed in this study occur between NO3 and phenol, rather than BA in the final detached neutral charge state.
