The identification of protonation sites in electrosprayed molecules remains a challenge in contemporary physical science. We present the first demonstration that low-resolution, UV laser photodissociation spectroscopy can be applied in situ to identify the protomers of para-aminobenzoic acid (PABA) formed via electrospray from a single solution. Electronic absorption spectra are recorded via photodepletion and photofragmentation for PABA electrosprayed from solutions of water and acetonitrile. Using this approach, two protomers can be straightforwardly identified, with only the carboxylic acid protomer being produced on electrospray from water while the amine-protonated isomer dominates upon electrospray from acetonitrile. High-level SORCI and MRCI calculations are presented to provide insight into the origin of the distinctive electronic spectra displayed by the protomers. Our results are in excellent agreement with previous PABA studies conducted using established techniques, and demonstrate that UV photodissociation spectroscopy of electrosprayed ions has potential as a new diagnostic tool for identifying protomeric species.

Isolated molecular clusters of adenine, cytosine, thymine and uracil bound to hexachloroplatinate, PtCl₆ ²⁻, have been studied using laser electronic photodissociation spectroscopy to investigate photoactivation of a platinum complex in the vicinity of a nucleobase. These metal complex–nucleobase clusters represent model systems for identifying the fundamental photochemical processes occurring in photodynamic platinum drug therapies that target DNA. This is the first study to explore the specific role of a strongly photoactive platinum compound in the aggregate complex. Each of the clusters studied displays a broadly similar absorption spectra, with a strong λ_max ∼ 4.6 eV absorption band and a subsequent increase in the absorption intensity towards higher spectral-energy. The absorption bands are traced to ligand-to-metal-charge-transfer excitations on the PtCl₆ ²⁻ moiety within the cluster, and result in Cl⁻·nucleobase and PtCl₅ ⁻ as primary photofragments. These results demonstrate how selective photoexcitation can drive distinctive photodecay channels for a model photo-pharmaceutical. In addition, cluster absorption due to excitation of nucleobase-centred chromophores is observed in the region around 5 eV. For the uracil cluster, photofragments consistent with ultrafast decay of the excited state and vibrational predissociation on the ground-state surface are observed. However, this decay channel becomes successively weaker on going from thymine to cytosine to adenine, due to differential coupling of the excited states to the electron detachment continuum. These effects demonstrate the distinctive photophysical characteristics of the different nucleobases, and are discussed in the context of the recently recorded photoelectron spectra of theses clusters.

Flavin chromophores play key roles in a wide range of photoactive proteins, but key questions exist in relation to their fundamental spectroscopic and photochemical properties. In this work, we report the first gas-phase spectroscopy study of protonated alloxazine (AL∙H⁺), a model flavin chromophore. Laser photodissociation is employed across a wide range (2.34–5.64 eV) to obtain the electronic spectrum and characterize the photofragmentation pathways. By comparison to TDDFT quantum chemical calculations, the spectrum is assigned to two AL∙H⁺ protomers; an N5 (dominant) and O4 (minor) form. The protomers have distinctly different spectral profiles in the region above 4.8 eV due to the presence of a strong electronic transition for the O4 protomer corresponding to an electron-density shift from the benzene to uracil moiety. AL∙H⁺ photoexcitation leads to fragmentation via loss of HCN and HNCO (along with small molecules such as CO₂ and H₂O), but the photofragmentation patterns differ dramatically from those observed upon collision excitation of the ground electronic state. This reveals that fragmentation is occurring during the excited state lifetime. Finally, our results show that the N5 protomer is associated primarily with HNCO loss while the O4 protomer is associated with HCN loss, indicating that the ring-opening dynamics are dependent on the location of protonation in the ground-state molecule.

Photofragment action spectroscopy and femtosecond time-resolved photoelectron imaging are utilized to probe the dissociation channels in iodide-uracil (I⁻ ⋅ U) binary clusters upon photoexcitation. The photofragment action spectra show strong I⁻ and weak [U—H]⁻ ion signal upon photoexcitation. The action spectra show two bands for I⁻ and [U—H]⁻ production peaking around 4.0 and 4.8 eV. Time-resolved experiments measured the rate of I⁻ production resulting from excitation of the two bands. At 4.03 eV and 4.72 eV, the photoelectron signal from I⁻ exhibits rise times of 86 ± 7 ps and 36 ± 3 ps, respectively. Electronic structure calculations indicate that the lower energy band, which encompasses the vertical detachment energy (4.11 eV) of I⁻U, corresponds to excitation of a dipole-bound state of the complex, while the higher energy band is primarily a π–π^∗ excitation on the uracil moiety. Although the nature of the two excited states is very different, the long lifetimes for I⁻ production suggest that this channel results from internal conversion to the I⁻ ⋅ U ground state followed by evaporation of I⁻. This hypothesis was tested by comparing the dissociation rates to Rice-Ramsperger-Kassel-Marcus calculations.