Communication: Autodetachment versus internal conversion from the S1 state of the isolated GFP chromophore anion

Dynamics of Anions: From Bound to Unbound States and Everything In Between

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

Excited-state dynamics and fluorescence lifetime of cryogenically cooled green fluorescent protein chromophore anions

Time-resolved action spectroscopy together with a fs-pump probe scheme is used in an electrostatic ion-storage ring to address lifetimes of specific vibrational levels in electronically excited states. Here we specifically consider the excited-state lifetime of cryogenically cooled green fluorescent protein (GFP) chromophore anions which is systematically measured across the S0-S1 spectral region (450-482 nm). A long lifetime of 5.2 ± 0.3 ns is measured at the S0-S1 band origin. When exciting higher vibrational levels in S1, the lifetime changes dramatically. It decreases by more than two orders of magnitude in a narrow energy region ∼250 cm-1 (31 meV) above the 0-0 transition. This is attributed to the opening of internal conversion over an excited-state energy barrier. The applied experimental technique provides a new way to uncover even small energy barriers, which are crucial for excited-state dynamics.

Alkylated green fluorescent protein chromophores: dynamics in the gas phase and in aqueous solution

Fluorescent labelling of macromolecular samples, including using the green fluorescent protein (GFP), has revolutionised the field of bioimaging. The ongoing development of fluorescent proteins require a detailed understanding of the photophysics of the biochromophore, and how chemical derivatisation influences the excited state dynamics. Here, we investigate the photophysical properties associated with the S1 state of three alkylated derivatives of the chromophore in GFP, in the gas phase using time-resolved photoelectron imaging, and in water using femtosecond fluorescence upconversion. The gas-phase lifetimes (1.6-10 ps), which are associated with the intrinsic (environment independent) dynamics, are substantially longer than the lifetimes in water (0.06-3 ps), attributed to stabilisation of both twisted intermediate structures and conical intersection seams in the condensed phase. In the gas phase, alkylation on the 3 and 5 positions of the phenyl ring slows the dynamics due to inertial effects, while a 'pre-twist' of the methine bridge through alkylation on the 2 and 6 positions significantly shortens the excited state lifetimes. Formation of a minor, long-lived (≫ 40 ps) excited state population in the gas phase is attributed to intersystem crossing to a triplet state, accessed because of a T1/S1 degeneracy in the so-called P-trap potential energy minimum associated with torsion of the single-bond in the bridging unit connecting to the phenoxide ring. A small amount of intersystem crossing is supported through TD-DFT molecular dynamics trajectories and MS-CASPT2 calculations. No such intersystem crossing occurs in water at T = 300 K or in ethanol at T ≈ 77 K, due to a significantly altered potential energy surface and P-trap geometry.

High-Resolution Spectroscopy and Selective Photoresponse of Cryogenically Cooled Green Fluorescent Protein Chromophore Anions

By time-resolved action spectroscopy of cryogenically cooled molecular ions, we have achieved a remarkable vibrational resolution in the photoresponse of the deprotonated green fluorescent protein (GFP) chromophore, a key molecular unit in the bioimaging of living cells. We define four characteristic spectral regions of the S₀-S₁ band with competing electronic and nuclear decay channels. We determine the energy barrier toward internal conversion to be ∼250 cm^-1. This inhibits internal conversion and hence statistical fragmentation near the S₀-S₁ band origin, which is identified at 481.51 ± 0.15 nm (20768 ± 6 cm^-1). The origin is red-shifted by only 221 cm^-1 compared to that of wild-type GFP at 77 K. This, together with a striking agreement between the vibronic profiles of the protein and its chromophore, suggests their similar photophysics. In combination with theory, the data reveal the coexistence of mutually energy-borrowing mechanisms between nuclei and electrons mediated by specific vibrational modes.

Action-Absorption Spectroscopy at the Band Origin of the Deprotonated Green Fluorescent Protein Chromophore In Vacuo

The application of action spectroscopy in connection with determination of the S₀ to S₁ band origin in the GFP anion model chromophore (deprotonated HBDI) is discussed. We specifically address the consequences of the lowest vibrational levels in S₁ being located behind a potential-energy barrier that inhibits internal conversion to the S₀ electronic ground state. Action spectroscopy based on consecutive absorption of two photons together with internal conversion will as a consequence reveal an apparent band origin that is significantly blue-shifted.

Complexation of Green and Red Kaede Fluorescent Protein Chromophores by a Zwitterion to Probe Electrostatic and Induction Field Effects

The photophysics of green fluorescent protein (GFP) and red Kaede fluorescent protein (rKFP) are defined by the intrinsic properties of the light-absorbing chromophore and its interaction with the protein binding pocket. This work deploys photodissociation action spectroscopy to probe the absorption profiles for a series of synthetic GFP and rKFP chromophores as the bare anions and as complexes with the betaine zwitterion, which is assumed as a model for dipole microsolvation. Electronic structure calculations and energy decomposition analysis using Symmetry-Adapted Perturbation Theory are used to characterize gas-phase structures and complex cohesion forces. The calculations reveal a preponderance for coordination of betaine to the phenoxide deprotonation site predominantly through electrostatic forces. Calculations using the STEOM-DLPNO-CCSD method are able to reproduce absolute and relative vertical excitation energies for the bare anions and anion-betaine complexes. On the other hand, treatment of the betaine molecule with a point-charge model, in which the charges are computed from some common electron density population analysis schemes, show that just electrostatic and point-charge induction interactions are unable to account for the betaine-induced spectral shift. The present methodology could be applied to investigate cluster forces and optical properties in other gas-phase ion-zwitterion complexes.

Internal conversion of the anionic GFP chromophore: in and out of the I-twisted S1/S0 conical intersection seam

The functional diversity of the green fluorescent protein (GFP) family is intimately connected to the interplay between competing photo-induced transformations of the chromophore motif, anionic p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI^-). Its ability to undergo Z/E-isomerization is of particular importance for super-resolution microscopy and emerging opportunities in optogenetics. Yet, key dynamical features of the underlying internal conversion process in the native HBDI^- chromophore remain largely elusive. We investigate the intrinsic excited-state behavior of isolated HBDI^- to resolve competing decay pathways and map out the factors governing efficiency and the stereochemical outcome of photoisomerization. Based on non-adiabatic dynamics simulations, we demonstrate that non-selective progress along the two bridge-torsional (i.e., phenolate, P, or imidazolinone, I) pathways accounts for the three decay constants reported experimentally, leading to competing ultrafast relaxation primarily along the I-twisted pathway and S₁ trapping along the P-torsion. The majority of the population (∼70%) is transferred to S₀ in the vicinity of two approximately enantiomeric minima on the I-twisted intersection seam (MECI-Is). Despite their sloped, reactant-biased topographies (suggesting low photoproduct yields), we find that decay through these intersections leads to products with a surprisingly high quantum yield of ∼30%. This demonstrates that E-isomer generation results at least in part from direct isomerization on the excited state. A photoisomerization committor analysis reveals a difference in intrinsic photoreactivity of the two MECI-Is and that the observed photoisomerization is the combined result of two effects: early, non-statistical dynamics around the less reactive intersection followed by later, near-statistical behavior around the more reactive MECI-I. Our work offers new insight into internal conversion of HBDI^- that both establishes the intrinsic properties of the chromophore and enlightens principles for the design of chromophore derivatives and protein variants with improved photoswitching properties.

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2-]- ( = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O₂-M]^- (M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M^- or O₂^- "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M·O₂ van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O₂-glyoxal]^- spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

A photoelectron imaging study of the deprotonated GFP chromophore anion and RNA fluorescent tags

Green fluorescent protein (GFP), together with its family of variants, is the most widely used fluorescent protein for in vivo imaging. Numerous spectroscopic studies of the isolated GFP chromophore have been aimed at understanding the electronic properties of GFP. Here, we build on earlier work [A. V. Bochenkova, C. Mooney, M. A. Parkes, J. Woodhouse, L. Zhang, R. Lewin, J. M. Ward, H. Hailes, L. H. Andersen and H. H. Fielding, Chem. Sci., 2017, 8, 3154] investigating the impact of fluorine and methoxy substituents that have been employed to tune the electronic structure of the GFP chromophore for use as fluorescent RNA tags. We present photoelectron spectra following photoexcitation over a broad range of wavelengths (364-230 nm) together with photoelectron angular distributions following photoexcitation at 364 nm, which are interpreted with the aid of quantum chemistry calculations. The results support the earlier high-level quantum chemistry calculations that predicted how fluorine and methoxy substituents tune the electronic structure and we find evidence to suggest that the methoxy substituents enhance internal conversion, most likely from the 2ππ* state which has predominantly Feshbach resonance character, to the 1ππ* state.

Resolving the ultrafast dynamics of the anionic green fluorescent protein chromophore in water

The chromophore of the green fluorescent protein (GFP) is critical for probing environmental influences on fluorescent protein behavior. Using the aqueous system as a bridge between the unconfined vacuum system and a constricting protein scaffold, we investigate the steric and electronic effects of the environment on the photodynamical behavior of the chromophore. Specifically, we apply ab initio multiple spawning to simulate five picoseconds of nonadiabatic dynamics after photoexcitation, resolving the excited-state pathways responsible for internal conversion in the aqueous chromophore. We identify an ultrafast pathway that proceeds through a short-lived (sub-picosecond) imidazolinone-twisted (I-twisted) species and a slower (several picoseconds) channel that proceeds through a long-lived phenolate-twisted (P-twisted) intermediate. The molecule navigates the non-equilibrium energy landscape via an aborted hula-twist-like motion toward the one-bond-flip dominated conical intersection seams, as opposed to following the pure one-bond-flip paths proposed by the excited-state equilibrium picture. We interpret our simulations in the context of time-resolved fluorescence experiments, which use short- and long-time components to describe the fluorescence decay of the aqueous GFP chromophore. Our results suggest that the longer time component is caused by an energetically uphill approach to the P-twisted intersection seam rather than an excited-state barrier to reach the twisted intramolecular charge-transfer species. Irrespective of the location of the nonadiabatic population events, the twisted intersection seams are inefficient at facilitating isomerization in aqueous solution. The disordered and homogeneous nature of the aqueous solvent environment facilitates non-selective stabilization with respect to I- and P-twisted species, offering an important foundation for understanding the consequences of selective stabilization in heterogeneous and rigid protein environments.

Probing the Microsolvation Environment of the Green Fluorescent Protein Chromophore In Vacuo

We present vibrational and electronic photodissociation spectra of a model chromophore of the green fluorescent protein in complexes with up to two water molecules, prepared in a cryogenic ion trap at 160-180 K. We find the band origin of the singly hydrated chromophore at 20 985 cm^-1 (476.5 nm) and observe partially resolved vibrational signatures. While a single water molecule induces only a small shift of the S₁ electronic band of the chromophore, without significant change of the Franck-Condon envelope, the spectrum of the dihydrate shows significant broadening and a greater blue shift of the band edge. Comparison of the vibrational spectra with predicted infrared spectra from density functional theory indicates that water molecules can interact with the oxygen atom on the phenolate group or on the imidazole moiety, respectively.

Cryogenic Ion Spectroscopy of the Green Fluorescent Protein Chromophore in Vacuo

We present the spectrum of the S₁ ← S₀ transition of an anionic model for the chromophore of the green fluorescent protein in vacuo at cryogenic temperatures, showing previously unresolved vibrational features, and resolving the band origin at 20 930 cm^-1 (477.8 nm) with unprecedented accuracy. The vibrational spectrum establishes that the molecule is in the Z isomer at low temperature. At increased temperature, the S₁ ← S₀ band shifts to the red, which we tentatively attribute to emergent population of the E isomer.

On the Mechanism of Phenolate Photo-Oxidation in Aqueous Solution

The photo-oxidation dynamics following ultraviolet (257 nm) excitation of the phenolate anion in aqueous solution is studied using broadband (550-950 nm) transient absorption spectroscopy. A clear signature from electron ejection is observed on a sub-picosecond timescale, followed by cooling dynamics and the decay of the signal to a constant offset that is assigned to the hydrated electron. The dynamics are compared to the charge-transfer-to-solvent dynamics from iodide at the same excitation wavelength and are shown to be very similar to these. This is in stark contrast to a previous study on the phenolate anion excited at 266 nm, in which electron emission was observed over longer timescales. We account for the differences using a simple Marcus picture for electron emission in which the electron tunneling rate depends sensitively on the initial excitation energy. After electron emission, a contact pair is formed which undergoes geminate recombination and dissociation to form the free hydrated electron at rates that are slightly faster than those for the iodide system. Our results show that, although the underlying chemical physics of electron emission differs between iodide and phenolate, the observed dynamics can appear very similar.

Reversible Photoisomerization of the Isolated Green Fluorescent Protein Chromophore

Fluorescent proteins have revolutionized the visualization of biological processes, prompting efforts to understand and control their intrinsic photophysics. Here we investigate the photoisomerization of deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI^-), the chromophore in green fluorescent protein and in Dronpa protein, where it plays a role in switching between fluorescent and nonfluorescent states. In the present work, isolated HBDI^- molecules are switched between the Z and E forms in the gas phase in a tandem ion mobility mass spectrometer outfitted for selecting the initial and final isomers. Excitation of the S₁ ← S₀ transition provokes both Z → E and E → Z photoisomerization, with a maximum response for both processes at 480 nm. Photodetachment is a minor channel at low light intensity. At higher light intensities, absorption of several photons in the drift region drives photofragmentation, through channels involving CH₃ loss and concerted CO and CH₃CN loss, although isomerization remains the dominant process.

Origin of the Intrinsic Fluorescence of the Green Fluorescent Protein

Green fluorescent protein, GFP, has revolutionized biology, due to its use in bioimaging. It is widely accepted that the protein environment makes its chromophore fluoresce, whereas the fluorescence is completely lost when the native chromophore is taken out of GFP. By the use of a new femtosecond pump-probe scheme, based on time-resolved action spectroscopy, we demonstrate that the isolated deprotonated GFP chromophore can be trapped in the first excited state when cooled to 100 K. The trapping is shown to last for 1.2 ns, which is long enough to establish conditions for fluorescence and consistent with calculated trapping barriers in the electronically excited state. Thus, GFP fluorescence is traced back to an intrinsic chromophore property, and by improving excited-state trapping, protein interactions enhance the molecular fluorescence.

The Effect of Conjugation on the Competition between Internal Conversion and Electron Detachment: A Comparison between Green Fluorescent and Red Kaede Protein Chromophores

Kaede, an analogue of green fluorescent protein (GFP), is a green-to-red photoconvertible fluorescent protein used as an in vivo "optical highlighter" in bioimaging. The fluorescence quantum yield of the red Kaede protein is lower than that of GFP, suggesting that increasing the conjugation modifies the electronic relaxation pathway. Using a combination of anion photoelectron spectroscopy and electronic structure calculations, we find that the isolated red Kaede protein chromophore in the gas phase is deprotonated at the imidazole ring, unlike the GFP chromophore that is deprotonated at the phenol ring. We find evidence of an efficient electronic relaxation pathway from higher-lying electronically excited states to the S₁ state of the red Kaede chromophore that is not accessible in the GFP chromophore. Rapid autodetachment from high-lying vibrational states of S₁ is found to compete efficiently with internal conversion to the ground electronic state.

Resonances of the anthracenyl anion probed by frequency-resolved photoelectron imaging of collision-induced dissociated anthracene carboxylic acid

The use of CID and photoelectron spectroscopy of organic carboxylic acid anions is discussed as a route to studying the dynamics of resonances in polyaromatic hydrocarbon (PAH) anions.

Resonances in polyaromatic hydrocarbon (PAH) anions are key intermediates in a number of processes such as electron transfer in organic electronics and electron attachment in the interstellar medium. Here we present a frequency- and angle-resolved photoelectron imaging study of the 9-anthracenyl anion generated through collision induced dissociation (CID) of its electrosprayed deprotonated anthracene carboxylic acid anion. We show that a number of π* resonances are active in the first 2.5 eV above the threshold. The photoelectron spectra and angular distributions revealed that nuclear dynamics compete with autodetachment for one of the resonances, while higher-lying resonances were dominated by prompt autodetachment. Based on electronic structure calculations, these observations were accounted for on the basis of the expected autodetachment rates of the resonances. Virtually no ground state recovery was observed, suggesting that the smallest deprotonated PAH that leads to ground state recovery is the tetracenyl anion, for which clear thermionic emission has been observed. The use of CID and photodissociation of organic carboxylic acid anions is discussed as a route to studying the dynamics of resonances in larger PAH anions.

ortho and para chromophores of green fluorescent protein: controlling electron emission and internal conversion

Green fluorescent protein (GFP) continues to play an important role in the biological and biochemical sciences as an efficient fluorescent probe and is also known to undergo light-induced redox transformations. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate how the phenoxide moiety controls the competition between electron emission and internal conversion in the isolated GFP chromophore anion, following photoexcitation with ultraviolet light in the range 400-230 nm. We find that moving the phenoxide group from the para position to the ortho position enhances internal conversion back to the ground electronic state but that adding an additional OH group to the para chromophore, at the ortho position, impedes internal conversion. Guided by quantum chemistry calculations, we interpret these observations in terms of torsions around the C-C-C bridge being enhanced by electrostatic repulsions or impeded by the formation of a hydrogen-bonded seven-membered ring. We also find that moving the phenoxide group from the para position to the ortho position reduces the energy required for detachment processes, whereas adding an additional OH group to the para chromophore at the ortho position increases the energy required for detachment processes. These results have potential applications in tuning light-induced redox processes of this biologically and technologically important fluorescent protein.

Decoupling Electronic versus Nuclear Photoresponse of Isolated Green Fluorescent Protein Chromophores Using Short Laser Pulses

The photophysics of a deprotonated model chromophore for the green fluorescent protein is studied by femtosecond laser pulses in an electrostatic ion-storage ring. The laser-pulse duration is much shorter than the time for internal conversion, and, hence, contributions from sequential multiphoton absorption, typically encountered with ns-laser pulses, are avoided. Following single-photon excitation, the action-absorption maximum is shown to be shifted within the S_{0} to S_{1} band from its origin at about 490 to 450 nm, which is explained by the different photophysics involved in the detected action.

Excited-State Proton-Transfer-Induced Trapping Enhances the Fluorescence Emission of a Locked GFP Chromophore

The chemical locking of the central single bond in core chromophores of green fluorescent proteins (GFPs) influences their excited-state behavior in a distinct manner. Experimentally, it significantly enhances the fluorescence quantum yield of GFP chromophores with an ortho-hydroxyl group, while it has almost no effect on the photophysics of GFP chromophores with a para-hydroxyl group. To unravel the underlying physical reasons for this different behavior, we report static electronic structure calculations and nonadiabatic dynamics simulations on excited-state intramolecular proton transfer, cis–trans isomerization, and excited-state deactivation in a locked ortho-substituted GFP model chromophore (o-LHBI). On the basis of our previous and present results, we find that the S₁ keto species is responsible for the fluorescence emission of the unlocked o-HBI and the locked o-LHBI species. Chemical locking does not change the parts of the S₁ and S₀ potential energy surfaces relevant to enol–keto tautomerization; hence, in both chromophores, there is an ultrafast excited-state intramolecular proton transfer that takes only 35 fs on average. However, the locking effectively hinders the S₁ keto species from approaching the keto S₁/S₀ conical intersections so that most of trajectories are trapped in the S₁ keto region for the entire 2 ps simulation time. Therefore, the fluorescence quantum yield of o-LHBI is enhanced compared with that of unlocked o-HBI, in which the S₁ excited-state decay is efficient and ultrafast. In the case of the para-substituted GFP model chromophores p-HBI and p-LHBI, chemical locking hardly affects their efficient excited-state deactivation via cis–trans isomerization; thus, the fluorescence quantum yields in these chromophores remain very low. The insights gained from the present work may help to guide the design of new GFP chromophores with improved fluorescence emission and brightness.

Anion resonances and above-threshold dynamics of coenzyme Q0

Temporary radical anions (resonances) of isolated co enzyme Q0 (CQ0) and their associated above-threshold dynamics have been studied using frequency-, angle-, and time-resolved photoelectron imaging (FAT-PI). Experimental energetics and dynamics are supported with ab initio calculations. All results support that CQ0 exhibits similar resonances and energetics compared with the smaller para-benzoquinone subunit, which is commonly considered as a prototype electrophore for larger biological para-quinone species. However, the above-threshold dynamics in CQ0 relative to para-benzoquinone show significantly enhanced prompt detachment compared with internal conversion, particularly around the photoexcitation energy of 3.10 eV. The change in dynamics can be attributed to a combination of an increase in the shape character of the optically-accessible resonance at this energy, a decrease in the autodetachment lifetime due to the higher density of states in the neutral, and a decrease in the probability that the wavepacket formed in the Franck-Condon window can access the local conical intersection in CQ0 over the timescale of autodetachment. Overall, this study serves as a clear example in understanding the trends in spectroscopy and dynamics in relating a simple prototypical para-quinone electrophore to a more complex biochemical species.

How far can a single hydrogen bond tune the spectral properties of the GFP chromophore?

Photoabsorption of the hydrogen-bonded HBDI·HBDI⁻ dimer, simultaneously resembling the two states of the Green Fluorescent Protein chromophore, is measured in vacuum.

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Resonances in deprotonated tetracene decay predominantly to the anion ground state.

Resonantly Enhanced Multiphoton Ionization Spectrum of the Neutral Green Fluorescent Protein Chromophore

The photophysics of the green fluorescent protein is governed by the electronic structure of the chromophore at the heart of its β-barrel protein structure. We present the first two-color, resonance-enhanced, multiphoton ionization spectrum of the isolated neutral chromophore in vacuo with supporting electronic structure calculations. We find the absorption maximum to be 3.65 ± 0.05 eV (340 ± 5 nm), which is blue-shifted by 0.5 eV (55 nm) from the absorption maximum of the protein in its neutral form. Our results show that interactions between the chromophore and the protein have a significant influence on the electronic structure of the neutral chromophore during photoabsorption and provide a benchmark for the rational design of novel chromophores as fluorescent markers or photomanipulators.

Competition between photodetachment and autodetachment of the 2(1)ππ* state of the green fluorescent protein chromophore anion

Using a combination of photoelectron spectroscopy measurements and quantum chemistry calculations, we have identified competing electron emission processes that contribute to the 350-315 nm photoelectron spectra of the deprotonated green fluorescent protein chromophore anion, p-hydroxybenzylidene-2,3-dimethylimidazolinone. As well as direct electron detachment from S0, we observe resonant excitation of the 2(1)ππ* state of the anion followed by autodetachment. The experimental photoelectron spectra are found to be significantly broader than photoelectron spectrum calculated using the Franck-Condon method and we attribute this to rapid (∼10 fs) vibrational decoherence, or intramolecular vibrational energy redistribution, within the neutral radical.