Accessing the Intrinsic Nature of Electronic Transitions from Gas-Phase Spectroscopy of Molecular Ion/Zwitterion Complexes.

A molecule's color is governed by the nature of its electronic transitions. Herein we show that the degree of charge transfer can be assessed by measuring the change in absorption induced by complexation with the betaine zwitterion. Our approach benefits from direct formation of complexes by electrospray of a mixture solution, followed by photodissociation action spectroscopy. We explored two ion groups: 1) No permanent dipole moment due to even charge delocalization (e.g. MnO4- ) and 2) Non-even charge distribution but where the charge according to resonance forms is either delocalized (e.g. oxyluciferin) or located at one site (e.g. m-nitrophenolate, mNP). The maximal shift for ions from (1) was <0.05 eV but as large as 0.3 eV and 0.2 eV for mNP and oxyluciferin. Hence our work supports the notion that oxyluciferin undergoes CT, and that the microenvironment can account for large variation in light emission from insects, ranging from green to red (shift of 0.3 eV).

A cylindrical quadrupole ion trap in combination with an electrospray ion source for gas-phase luminescence and absorption spectroscopy.

A relatively simple setup for collection and detection of light emitted from isolated photo-excited molecular ions has been constructed. It benefits from a high collection efficiency of photons, which is accomplished by using a cylindrical ion trap where one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The geometry permits nearly 10% of the emitted light to be collected and, after transmission losses, approximately 5% to be delivered to the entrance of a grating spectrometer equipped with a detector array. The high collection efficiency enables the use of pulsed tunable lasers with low repetition rates (e.g., 20 Hz) instead of continuous wave (cw) lasers or very high repetition rate (e.g., MHz) lasers that are typically used as light sources for gas-phase fluorescence experiments on molecular ions. A hole has been drilled in the cylinder electrode so that a light pulse can interact with the ion cloud in the center of the trap. Simulations indicate that these modifications to the trap do not significantly affect the storage capability and the overall shape of the ion cloud. The overlap between the ion cloud and the laser light is basically 100%, and experimentally >50% of negatively charged chromophore ions are routinely photodepleted. The performance of the setup is illustrated based on fluorescence spectra of several laser dyes, and the quality of these spectra is comparable to those reported by other groups. Finally, by replacing the optical system with a channeltron detector, we demonstrate that the setup can also be used for gas-phase action spectroscopy where either depletion or fragmentation is monitored to provide an indirect measurement on the absorption spectrum of the ion.

On the Exciton Coupling between Two Chlorophyll Pigments in the Absence of a Protein Environment: Intrinsic Effects Revealed by Theory and Experiment.

Exciton coupling between two or more chlorophyll (Chl) pigments is a key mechanism associated with the color tuning of photosynthetic proteins but it is difficult to disentangle this effect from shifts that are due to the protein microenvironment. Herein, we report the formation of the simplest coupled system, the Chl a dimer, tagged with a quaternary ammonium ion by electrospray ionization. Based on action spectroscopic studies in vacuo, the dimer complexes were found to absorb 50-70 meV to the red of the monomers under the same conditions. First-principles calculations predict shifts that somewhat depend on the relative orientation of the two Chl units, namely 50 and 30 meV for structures where the Chl rings are stacked and unstacked, respectively. Our work demonstrates that Chl association alone can produce a large portion of the color shift observed in photosynthetic macromolecular assemblies.

A PYP chromophore acts as a 'photoacid' in an isolated hydrogen bonded complex.

The light-induced response of a neutral Photoactive Yellow Protein chromophore in a hydrogen-bonded complex with a proton acceptor has been studied by dual-detection action absorption spectroscopy and density functional theory. We show that the chromophore is a 'photoacid' and that ultrafast excited-state proton transfer might be operative in an isolated complex.

Effect of a Single Water Molecule on the Electronic Absorption by o- and p-Nitrophenolate: A Shift to the Red or to the Blue?

Many photoactive biomolecules are anions and exhibit ππ* optical transitions but with a degree of charge transfer (CT) character determined by the local environment. The phenolate moiety is a common structural motif among biochromophores and luminophores, and nitrophenolates are good model systems because the nitro substituent allows for CT-like transitions. Here we report gas-phase absorption spectra of o- and p-nitrophenolate·H2O complexes to decipher the effect of just one H2O and compare them with ab initio calculations of vertical excitation energies. The experimental band maximum is at 3.01 and 3.00 eV for ortho and para isomers, respectively, and is red-shifted by 0.10 and 0.13 eV relative to the bare ions, respectively. These shifts indicate that the transition has become more CT-like because of localization of negative charge on the phenolate oxygen, i.e., diminished delocalization of the negative excess charge. However, the transition bears less CT than that of m-nitrophenolate·H2O because this complex absorbs further to the red (2.56 eV). Our work emphasizes the importance of local perturbations: one water causes a larger shift than experienced in bulk for para isomer and almost the full shift for ortho isomer. Predicting microenvironmental effects in the boundary between CT and non-CT with high accuracy is nontrivial. However, in agreement with experiment, our calculations show a competition between the effects of electronic delocalization and electrostatic interaction with the solvent molecule. As a result, the excitation energy of ortho and para isomers is less sensitive to hydration than that of the meta isomer because donor and acceptor orbitals are only weakly coupled in the meta isomer.

Gas-phase spectroscopy of a vinylheptafulvene chromophore.

The intrinsic spectral properties of the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch, free of solvent interactions, were investigated both experimentally and theoretically. A quaternary ammonium group was incorporated via an ethylene bridge to allow for the transfer of charged molecules to the gas phase by electrospray ionization, leaving the chromophore part itself neutral. Absorption by the two isomers was identified from ion dissociation (i.e., action spectroscopy) using a home-built sector instrument. Several fragment ions were observed, and dissociation occurred both at the charge tag side chain and at the chromophore unit itself. We measured an absorption band of VHF with a maximum at 430 ± 20 nm (2.9 eV ± 0.1 eV) but no band was discernible for the DHA in the visible region. This shows that little interconversion between the two isomers occurs during the electrospray and the subsequent trapping in an octopole for 25 ms; the latter is needed to produce ion bunches for spectroscopy where a pulsed laser system is used. For comparison, density functional theoretical calculations predicted lowest-energy vertical excitations of 3.33 eV to 3.48 eV for the DHA and 3.02 eV to 3.08 eV for the VHF (the ranges are based on the use of different functionals), which correspond to a maximal deviation between theory and experiment of 0.1 eV in the case of the VHF. The absorption by the bare ions is significantly blue-shifted compared to that by VHF in acetonitrile solution (2.64 eV), but similar within the experimental uncertainty to that by VHF in cyclohexane (2.78 eV); the transition, therefore has a significant charge-transfer character. Finally, we find that the absorption of two photons is needed to cause fragmentation of the VHF on the microsecond time scale, which indicates that prompt fragmentation from the electronically excited state or prior to the intramolecular vibrational redistribution of the excess energy plays no role. This is of particular importance for the use of the photosystem in advanced materials or molecular electronics where high photostability is required to allow for numerous isomerization cycles.

The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions.

We have performed gas-phase absorption spectroscopy in the Soret-band region of chlorophyll (Chl) a and b tagged by quaternary ammonium ions together with time-dependent density functional theory (TD-DFT) calculations. This band is the strongest in the visible region of metalloporphyrins and an important reporter on the microenvironment. The cationic charge tags were tetramethylammonium, tetrabutylammonium, and acetylcholine, and the dominant dissociation channel in all cases was breakage of the complex to give neutral Chl and the charge tag as determined by photoinduced dissociation mass spectroscopy. Two photons were required to induce fragmentation on the time scale of the experiment (microseconds). Action spectra were recorded where the yield of the tag as a function of excitation wavelength was sampled. These spectra are taken to represent the corresponding absorption spectra. In the case of Chl a we find that the tag hardly influences the band maximum which for all three tags is at 403 ± 5 nm. A smaller band with maximum at 365 ± 10 nm was also measured for all three complexes. The spectral quality is worse in the case of Chl b due to lower ion beam currents; however, there is clear evidence for the absorption being to the red of that of Chl a (most intense peak at 409 ± 5 nm) and also a more split band. Our results demonstrate that the change in the Soret-band spectrum when one peripheral substituent (CH3) is replaced by another (CHO) is an intrinsic effect. First principles TD-DFT calculations agree with our experiments, supporting the intrinsic nature of the difference between Chl a and b and also displaying minimal spectral changes when different charge tags are employed. The deviations between theory and experiment have allowed us to estimate that the Soret-band absorption maxima in vacuo for the neutral Chl a and Chl b should occur at 405 nm and 413 nm, respectively. Importantly, the Soret bands of the isolated species are significantly blueshifted compared to those of solvated Chl and Chl-proteins. The protein microenvironment is certainly not innocent of perturbing the electronic structure of Chls.

On the effect of a single solvent molecule on the charge-transfer band of a donor-acceptor anion.

Many biochromophore anions located within protein pockets display charge-transfer (CT) transitions that are perturbed by the nearby environment, such as water or amino acid residues. These anions often contain the phenolate moiety as the electron donor and an acceptor group that couples to the donor via a π-conjugated system. Here we show using action spectroscopy that single molecules of water, methanol, and acetonitrile cause blue shifts in the electronic transition energy of the bare m-nitrophenolate anion by 0.22, 0.22, and 0.12 eV, respectively (uncertainty of 0.05 eV). These shifts are similar to CC2-predicted ones and are in accordance with the weaker binding to the phenolate end of the ion by acetonitrile in comparison with water and methanol. The nitro acceptor group is almost decoupled from the phenolate donor, and this ion therefore represents a good model for CT excitations of an anion. We found that the shift caused by one acetonitrile molecule is almost half of that experienced in bulk acetonitrile solution, clearly emphasizing the important role played by the microenvironment. In protic solvents, the shifts are larger because of hydrogen bonds to the phenolate oxygen. Finally, but not least, we provide experimental data that serve to benchmark calculations of excited states of ion-solvent complexes.

On the influence of water on the electronic structure of firefly oxyluciferin anions from absorption spectroscopy of bare and monohydrated ions in vacuo.

A complete understanding of the physics underlying the varied colors of firefly bioluminescence remains elusive because it is difficult to disentangle different enzyme-lumophore interactions. Experiments on isolated ions are useful to establish a proper reference when there are no microenvironmental perturbations. Here, we use action spectroscopy to compare the absorption by the firefly oxyluciferin lumophore isolated in vacuo and complexed with a single water molecule. While the process relevant to bioluminescence within the luciferase cavity is light emission, the absorption data presented here provide a unique insight into how the electronic states of oxyluciferin are altered by microenvironmental perturbations. For the bare ion we observe broad absorption with a maximum at 548 ± 10 nm, and addition of a water molecule is found to blue-shift the absorption by approximately 50 nm (0.23 eV). Test calculations at various levels of theory uniformly predict a blue-shift in absorption caused by a single water molecule, but are only qualitatively in agreement with experiment highlighting limitations in what can be expected from methods commonly used in studies on oxyluciferin. Combined molecular dynamics simulations and time-dependent density functional theory calculations closely reproduce the broad experimental peaks and also indicate that the preferred binding site for the water molecule is the phenolate oxygen of the anion. Predicting the effects of microenvironmental interactions on the electronic structure of the oxyluciferin anion with high accuracy is a nontrivial task for theory, and our experimental results therefore serve as important benchmarks for future calculations.

On the photoabsorption by permanganate ions in vacuo and the role of a single water molecule. New experimental benchmarks for electronic structure theory.

We report electronic spectra of mass-selected MnO4(-) and MnO4(-)⋅H2O using electronic photodissociation spectroscopy. Bare MnO4(-) fragments by formation of MnO3(-) and MnO2(-), while the hydrated complex predominantly decays by loss of the water molecule. The band in the visible spectral region shows a well-resolved vibrational progression consistent with the excitation of a Mn-O stretching mode. The presence of a single water molecule does not significantly perturb the spectrum of MnO4(-). Comparison with the UV/Vis absorption spectrum of permanganate in aqueous solution shows that complete hydration causes a small blueshift, while theoretical models including a dielectric medium have predicted a redshift. The experimental data can be used as benchmarks for electronic structure theory methods, which usually predict electronic spectra in the absence of a chemical environment.

Substitution effects on the absorption spectra of nitrophenolate isomers.

Charge-transfer excitations highly depend on the electronic coupling between the donor and acceptor groups. Nitrophenolates are simple examples of charge-transfer systems where the degree of coupling differs between ortho, meta and para isomers. Here we report the absorption spectra of the isolated anions in vacuo to avoid the complications of solvent effects. Gas-phase action spectroscopy was done with two different setups, an electrostatic ion storage ring and an accelerator mass spectrometer. The results are interpreted on the basis of CC2 quantum chemical calculations. We identified absorption maxima at 393, 532, and 399 nm for the para, meta, and ortho isomer, respectively, with the charge-transfer transition into the lowest excited singlet state. In the meta isomer, this π-π* transition is strongly redshifted and its oscillator strength reduced, which is related to the pronounced charge-transfer character, as a consequence of the topology of the conjugated π-system. Each isomer's different charge distribution in the ground state leads to a very different solvent shift, which in acetonitrile is bathochromic for the para and ortho, but hypsochromic for the meta isomer.

Electron-capture induced dissociation of doubly charged dipeptides: on the neutral losses and N-Cα bond cleavages.

Electron capture by doubly charged peptide cations leads to neutral losses in addition to N-C(α) bond cleavages that give c and z fragments. In this work we discuss the influence of amino acid sequence on hydrogen versus ammonia loss and the propensity for subsequent partial side-chain cleavage after ammonia loss to give w fragment ions. Experiments were done on two series of doubly protonated dipeptides, [XK+2H](2+) and [XR+2H](2+), where X is one of the twenty common amino acid residues, excluding aspartic acid (D), and K and R are lysine and arginine, respectively. While it was previously established that NH(3) is lost exclusively from the N-terminal ammonium group and not from side-chain ammonium groups, we find here that ammonia can be lost from guanidinium radicals as well. The ratio between H loss and NH(3) loss reveals some information on internal ionic hydrogen bonds and peptide conformation since proton sharing between the N-terminal ammonium group and a basic side chain decreases the probability for NH(3) loss due to a lower recombination energy and as a result reduced capture probability. The abundance of w ions was found to correlate with the reaction energy for their formation; highest yield was found for CK and lowest for AK and HK. The survival rate of charge-reduced species was higher for XR than for XK, which is likely linked to the formation of long-lived C(α) radicals in the latter case. The probability for N-C(α) bond cleavage is smaller on average for XR than for XK which indicates that hydrogen transfer from the ε-ammonium radical to the amide group triggers some of the cleavages, or is a result of the different distances between the amide group and the charges in XR and XK. Finally, our data support the previous concept that charge partitioning between c and z fragments can be explained by competition between the two fragments for the proton.