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, NHCHNH) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA agitates about an equilibrium position, with NH 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 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.

The polymorphic nature of G‐quadruplex (G4) DNA structures points to a range of potential applications in nanodevices and an opportunity to control G4 in biological settings. Light is an attractive means for the regulation of oligonucleotide structure as it can be delivered with high spatiotemporal precision. However, surprisingly little attention has been devoted towards the development of ligands for G4 that allow photoregulation of G4 folding. We report a novel G4‐binding chemotype derived from stiff‐stilbene. Surprisingly however, whilst the ligand induces high stabilization in the potassium form of human telomeric DNA, it causes the unfolding of the same G4 sequence in sodium buffer. This effect can be reversed on demand by irradiation with 400 nm light through deactivation of the ligand by photo‐oxidation. By fuelling the system with the photolabile ligand, the conformation of G4 DNA was switched five times.

The significance of removal of atmospheric ammonia and amines by reaction with Criegee intermediates is assessed by kinetic studies.

The electronic structure of glycan functionalised carbon nano-dots is greatly affected by the choice of carbohydrate.

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 ultrafast ESPT from the CouOH moiety to hydrogen bonded MI molecules occurs within ~1 ps, tracked by unequivocal spectroscopic signatures of CouO-* photoproducts which are formed in tandem with HMI +. Some of the ESPT photoproducts subsequently p-stack to form exciplexes on a ~35 ps timescale, 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.

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 0 -deoxyguanosine 3 0 -monophosphate 5 0 -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 À ) exciplex product formation on an ultrafast (o350 fs) timescale. Theoretical studies show that the nature of the vertical excited states is 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 À ) formation. Exciplexes are quickly converted back to neutral ground state molecules on a B10 ps timescale with a high quantum yield, ensuring the photostability of the nucleotide sequence.

Scheme 1. Synthesis of the pyridinium stiff-stilbenes (E)-1 and (Z)-1.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.

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