Mapping the UV Photophysics of Platinum Metal Complexes Bound to Nucleobases: Laser Spectroscopy of Isolated Uracil·Pt(CN)₄^2– and Uracil·Pt(CN)₆^2– Complexes

We report the first UV laser spectroscopic study of isolated gas-phase complexes of platinum metal complex anions bound to a nucleobase as model systems for exploring at the molecular level the key photophysical processes involved in photodynamic therapy. Spectra of the Pt^IV(CN)₆^2–·Ur and Pt^II(CN)₄^2–·Ur complexes were acquired across the 220–320 nm range using mass-selective photodepletion and photofragment action spectroscopy. The spectra of both complexes reveal prominent UV absorption bands (λ_max = 4.90 and 4.70 eV) that we assign primarily to excitation of the Ur π–π* localized chromophore. Distinctive UV photofragmentation products are observed for the complexes, with Pt^IV(CN)₆^2–·Ur photoexcitation resulting in complex fission, while Pt^II(CN)₄^2–·Ur photoexcitation initiates a nucleobase proton-transfer reaction across 4.4–5.2 eV and electron detachment above 5.2 eV. The observed photofragments are consistent with ultrafast decay of a Ur localized excited state back to the electronic ground state followed by intramolecular vibrational relaxation and ergodic complex fragmentation

Electron Detachment as a Probe of Intrinsic Nucleobase Dynamics in Dianion-Nucleobase Clusters: Photoelectron Spectroscopy of the Platinum II Cyanide Dianion Bound to Uracil, Thymine, Cytosine, and Adenine

We report the first low-temperature photoelectron spectra of isolated gas-phase complexes of the platinum II cyanide dianion bound to nucleobases. These systems are models for understanding platinum-complex photodynamic therapies, and a knowledge of the intrinsic photodetachment properties is crucial for characterizing their broader photophysical properties. Well-resolved, distinct peaks are observed in the spectra, consistent with complexes where the Pt­(CN)₄^2– moiety is largely intact. Adiabatic electron detachment energies for the dianion-nucleobase complexes are measured to be 2.39–2.46 eV. The magnitudes of the repulsive Coulomb barriers of the complexes are estimated to be between 1.9 and 2.1 eV, values that are lower than for the bare Pt­(CN)₄^2– dianion as a result of charge solvation by the nucleobases. In addition to the resolved spectral features, broad featureless bands indicative of delayed electron detachment are observed in the 193 nm photoelectron spectra of the four dianion-nucleobase complexes and also in the 266 nm spectra of the Pt­(CN)₄^2–·thymine and Pt­(CN)₄^2–·adenine complexes. The selective excitation of these features in the 266 nm spectra is attributed to one-photon excitation of [Pt­(CN)₄^2–·thymine]* and [Pt­(CN)₄^2–·adenine]* long-lived excited states that can effectively couple to the electron detachment continuum, producing strong electron detachment signals. We attribute the delayed electron detachment bands observed here for Pt­(CN)₄^2–·thymine and Pt­(CN)₄^2–·adenine but not for Pt­(CN)₄^2–·uracil and Pt­(CN)₄^2–·cytosine to fundamental differences in the individual nucleobase photophysics following 266 nm excitation. This indicates that the Pt­(CN)₄^2– dianion in the clusters can be viewed as a “dynamic tag” which has the propensity to emit electrons when the attached nucleobase displays a long-lived excited state

Performance of M06, M06-2X, and M06-HF Density Functionals for Conformationally Flexible Anionic Clusters: M06 Functionals Perform Better than B3LYP for a Model System with Dispersion and Ionic Hydrogen-Bonding Interactions

We present a comparative assessment of the performance of the M06 suite of density functionals (M06, M06-2X, and M06-HF) against an MP2 benchmark for calculating the relative energies and geometric structures of the Cl^–·arginine and Br^–·arginine halide ion–amino acid clusters. Additional results are presented for the popular B3LYP density functional. The Cl^–·arginine and Br^–·arginine complexes are important prototypes for the phenomenon of anion-induced zwitterion formation. Results are presented for the canonical (noncharge separated) and zwitterionic (charge separated) tautomers of the clusters, as well as the numerous conformational isomers of the clusters. We find that all of the M06 functions perform well in terms of predicting the general trends in the conformer relative energies and identifying the global minimum conformer. This is in contrast to the B3LYP functional, which performed significantly less well for the canonical tautomers of the clusters where dispersion interactions contribute more significantly to the conformer energetics. We find that the M06 functional gave the lowest mean unsigned error for the relative energies of the canonical conformers (2.10 and 2.36 kJ/mol for Br^–·arginine and Cl^–·arginine), while M06-2X gave the lowest mean unsigned error for the zwitterionic conformers (0.85 and 1.23 kJ/mol for Br^–·arginine and Cl^–·arginine), thus providing insight into the types of physical systems where each of these functionals should perform best

Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π–π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. <i>J. Phys. Chem. C</i> <b>2009</b>, <i>113</i>, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed