Following Bimolecular Excited-State Proton Transfer between Hydroxycoumarin and Imidazole Derivatives

The ultrafast dynamics of a bimolecular excited-state proton transfer (ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin (CouOH) and 1-methylimidazole (MI) base in aprotic chloroform-d1 solution were investigated using ultrafast transient infrared (TRIR) and transient absorption (TA) spectroscopies. The excited-state lifetime of the photoacid in solution is relatively short (52 ps), which at the millimolar photoacid and base concentrations used in our study precludes any diffusion-controlled bimolecular ESPT reactions. This allows the prompt ESPT reaction between hydrogen-bonded CouOH and MI molecules to be studied in isolation and the “contact” ESPT dynamics to be unambiguously determined. Our time-resolved studies reveal that ultrafast ESPT from the CouOH moiety to hydrogen-bonded MI molecules occurs within ∼1 ps, tracked by unequivocal spectroscopic signatures of CouO–* photoproducts that are formed in tandem with HMI+. Some of the ESPT photoproducts subsequently π-stack to form exciplexes on a ∼35 ps time scale, minimizing the attractive Coulombic forces between the oppositely charged aromatic molecules. For the concentrations of CouOH and MI used in our study (up to 8 mM), we saw no evidence for excited-state tautomerization of coumarin anions

Investigating the Role of the Organic Cation in Formamidinium Lead Iodide Perovskite Using Ultrafast Spectroscopy

Organic cation rotation in hybrid organic–inorganic lead halide perovskites has previously been associated with low charge recombination rates and (anti)­ferroelectric domain formation. Two-dimensional infrared spectroscopy (2DIR) was used to directly measure 470 ± 50 fs and 2.8 ± 0.5 ps time constants associated with the reorientation of formamidinium cations (FA⁺, NH₂CHNH₂⁺) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA⁺ agitates about an equilibrium position, with NH₂ groups pointing at opposite faces of the inorganic lattice cube, and undergoes 90° flips on picosecond time scales. Time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photogenerated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA⁺ antisymmetric stretching vibrational potential, resulting in an observed 5 cm^–1 shift. Our 2DIR results provide the first direct measurement of FA⁺ rotation inside thin perovskite films, and cast significant doubt on the presence of long-lived (anti)­ferroelectric domains, which the observed low charge recombination rates have been attributed to

2D-Raman-THz Spectroscopy with Single-Shot THz Detection

We present a 2D-Raman-THz setup with multichannel (single-shot) THz detection, utilizing two crossed echelons, in order to reduce the acquisition time of typical 2D-Raman-THz experiments from days to a few hours. This speed-up is obtained in combination with a high repetition rate (100 kHz) Yb-based femtosecond laser system and a correspondingly fast array detector. The wavelength of the Yb-laser (1030 nm) is advantageous, since it assures almost perfect phase matching in GaP for THz generation and detection, and since dispersion in the transmissive echelons is minimal. 2D-Raman-THz test measurements on liquid bromoform (CHBr3) are reported. An enhancement of ∼5.8 times in signal-to-noise ratio is obtained for single-shot detection when comparing to conventional step scanning measurements in the THz time-domain, corresponding to speed up of acquisition time of 34

Exploring ultraviolet photoinduced charge-transfer dynamics in a model dinucleotide of guanine and thymine

An understanding of the initial photoexcited states of DNA is essential to unravelling deleterious photoinduced chemical reactions and the intrinsic ultrafast photoprotection of the genetic code for all life. In our combined experimental and theoretical study, we have elucidated the primary non-radiative relaxation dynamics of a model nucleotide of guanine and thymine (2'-deoxyguanosine 3'-monophospate 5'-thymidine, d(GpT)) in buffered aqueous solution. Experimentally, we unequivocally demonstrate that the Franck-Condon excited states of d(GpT) are significantly delocalised across both nucleobases, and mediate d(G+pT–) exciplex product formation on an ultrafast (< 350 fs) timescale. Theoretical studies show that the nature of the vertical excited states are very dependent on the specific geometry of the dinucleotide, and dictate the degree of delocalised, charge-transfer or localised character. Our mechanism for prompt exciplex formation involves a rapid change in electronic structure and includes a diabatic surface crossing very close to the Franck-Condon region mediating fast d(G+pT–) formation. Exciplexes are quickly converted back to neutral ground state molecules on a ~10 ps timescale with a high quantum yield, ensuring the photostability of the nucleotide sequence

Crystallization and preliminary structure analysis of the blue laccase from the ligninolytic fungus Panus tigrinus

Blue laccase from the white-rot basidiomycete P. tigrinus, an enzyme involved in lignin biodegradation, has been crystallized. The crystals obtained give diffraction data at 1.4 Å, the best resolution to date for this class of enzymes, which may assist in further elucidation of the catalytic mechanism of multicopper oxidases