On the Effect of a Single Solvent Molecule on the Charge-Transfer Band of a Donor–Acceptor Anion

Influence of the N atom and its position on electron photodetachment of deprotonated indole and azaindole

Dipole bound state and its vibrational structure observed in deprotonated 7-azaindole by recording the signal of 7-azaindolyl stable neutral radical.

Ion mobility action spectroscopy of flavin dianions reveals deprotomer-dependent photochemistry

Photo-induced proton transfer, deprotomer-dependent photochemistry, and intramolecular charge transfer in flavin anions are investigated using action spectroscopy.

The intrinsic optical properties and photochemistry of flavin adenine dinucleotide (FAD) dianions are investigated using a combination of tandem ion mobility spectrometry and action spectroscopy. Two principal isomers are observed, the more stable form being deprotonated on the isoalloxazine group and a phosphate (N-3,PO₄ deprotomer), and the other on the two phosphates (PO₄,PO₄ deprotomer). Ion mobility data and electronic action spectra suggest that photo-induced proton transfer occurs from the isoalloxazine group to a phosphate group, converting the PO₄,PO₄ deprotomer to the N-3,PO₄ deprotomer. Comparisons of the isomer selective action spectra of FAD dianions and flavin monoanions with solution spectra and gas-phase photodissociation action spectra suggests that solvation shifts the electronic absorption of the deprotonated isoalloxazine group to higher energy. This is interpreted as evidence for significant charge transfer in the lowest optical transition of deprotonated isoalloxazine. Overall, this work demonstrates that the site of deprotonation of flavin anions strongly affects their electronic absorptions and photochemistry.

Intrinsic photophysics of nitrophenolate ions studied by cryogenic ion spectroscopy

The intrinsic photophysics of nitrophenolate isomers (meta, para, and ortho) was studied at low temperature using photodissociation mass spectrometry in a cryogenic ion trap instrument.

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.

Spectroscopy of nitrophenolates in vacuo: effect of spacer, configuration, and microsolvation on the charge-transfer excitation energy

In a charge-transfer (CT) transition, electron density moves from one end of the molecule (donor) to the other end (acceptor). This type of transition is of paramount importance in nature, for example, in photosynthesis, and it governs the excitation of several protein biochromophores and luminophores such as the oxyluciferin anion that accounts for light emission from fireflies. Both transition energy and oscillator strength are linked to the coupling between the donor and acceptor groups: The weaker the coupling, the smaller the excitation energy. But a weak coupling necessarily also causes a low oscillator strength possibly preventing direct excitation (basically zero probability in the noncoupling case). The coupling is determined by the actual spacer between the two groups, and whether the spacer acts as an insulator or a conductor. However, it can be difficult or even impossible to distinguish the effect of the spacer from that of local solvent molecules that often cause large solvent shifts due to different ground-state and excited-state stabilization. This calls for gas-phase spectroscopy experiments where absorption by the isolated molecule is identified to unequivocally establish the intrinsic molecular properties with no perturbations from a microenvironment. From such insight, the effect of a protein microenvironment on the CT excited state can be deduced. In this Account, we review our results over the last 5 years from mass spectroscopy experiments using specially designed apparatus on several charged donor-acceptor ions that are based on the nitrophenolate moiety and π-extended derivatives, which are textbook examples of donor-acceptor chromophores. The phenolate oxygen is the donor, and the nitro group is the acceptor. The choice of this system is also based on the fact that phenolate is a common structural motif of biochromophores and luminophores, for example, it is a constituent of the oxyluciferin anion. A presentation of the setups used for gas-phase ion spectroscopy in Aarhus is given, and we address issues of whether double bonds or triple bonds best convey electronic coupling between the phenolate oxygen and the nitro group, the significance of separating the donor and acceptor spatially, the influence of cross-conjugation versus linear conjugation, and along this line ortho versus meta versus para configuration, and not least the effect of a single solvent molecule (water, methanol, or acetonitrile). From systematic studies, a clear picture has emerged that has been supported by high-level calculations of electronically excited states. Our work shows that CC2 coupled-cluster calculations of vertical excitation energies are within 0.2 eV of experimental band maxima, and importantly, that the theoretical method is excellent in predicting the relative order of excitation energies of a series of nitrophenolates. Finally, we discuss future challenges such as following the change in absorption as a function of the number of solvent molecules and when gradually approaching the bulk limit.

Gas-phase spectroscopy of protonated adenine, adenosine 5'-monophosphate and monohydrated ions

Microsolvation of chromophore ions commonly has large effects on their electronic structure and as a result on their optical absorption spectra. Here spectroscopy of protonated adenine (AdeH(+)) and its complex with one water molecule isolated in vacuo was done using a home-built mass spectrometer in combination with a tuneable pulsed laser system. Experiments also included the protonated adenosine 5'-monophosphate nucleotide (AMPH(+)). In the case of bare AdeH(+) ions, one-photon absorption leads to four dominant fragment ions corresponding to ammonium and ions formed after loss of either NH3, HCN, or NH2CN. The yields of these were measured as a function of the wavelength of the light from 210 nm to 300 nm, and they were combined to obtain the total photoinduced dissociation at each wavelength (i.e., action spectrum). A broad band between 230 nm and 290 nm and the tail of a band with maximum below 210 nm (high-energy band) are seen. In the case of AdeH(+)(H2O), the dominant dissociation channel after photoexcitation in the low-energy band was simply loss of H2O while photodissociation of protonated AMP revealed two dominant dissociation channels associated with the formation of either AdeH(+) or loss of H3PO4. The action spectra of AdeH(+), AdeH(+)(H2O), and AMPH(+) are almost identical in the 230-290 nm region, and they resemble the absorption spectrum of protonated adenine in aqueous solution recorded at low pH. Hence from our work it is firmly established that the lowest-energy transitions are independent of the surroundings.