Tetraphenylethylene is a prototypical example of a molecule displaying aggregation-induced emission. Despite many studies on the optical properties of TPE and its derivatives, the origin of the non-emissive behavior in the gas phase or in dilute solutions has yet to be unravelled. Here, we identify the ultrafast deactivation mechanisms responsible for the fluorescence quenching in isolated TPE.

Trajectory surface hopping (TSH) methods have been widely used to study photoinduced nonadiabatic processes. In the present study, nonadiabatic dynamics simulations with the widely used Tully's fewest switches surface hopping (FSSH) algorithm and a Landau-Zener-type TSH (LZSH) algorithm have been performed for the internal conversion dynamics of pyrazine. The accuracy of the two TSH algorithms has been critically evaluated by a direct comparison with exact quantum dynamics calculations for a model of pyrazine. The model comprises the three lowest excited electronic states (B 3u (nπ *), A 1u (nπ *), and B 2u (ππ *)) and the nine most relevant vibrational degrees of freedom. Considering photoexcitation to the diabatic B 2u (ππ *) state, we examined the time-dependent diabatic and adiabatic electronic population dynamics. It is found that the diabatic populations obtained with both TSH methods are in good agreement with the exact quantum results. Fast population oscillations between the B 3u (nπ *) and A 1u (nπ *) states, which reflect nonadiabatic electronic transitions driven by coherent dynamics in the normal mode Q 8a , are qualitatively reproduced by both TSH methods. In addition to the model study, the TSH methods have been interfaced with the second-order algebraic diagrammatic construction ab initio electronic-structure method to perform full-dimensional onthe-fly nonadiabatic dynamics simulations for pyrazine. It is found that the electronic population dynamics obtained with the LZSH method is in excellent agreement with that obtained by the FSSH method using a local diabatization algorithm. Moreover, the electronic populations of the full-dimensional on-the-fly calculations are in excellent agreement with the populations of the three-state nine-mode model, which confirms that the internal conversion dynamics of pyrazine is accurately represented by this reduced-dimensional model on the time scale under consideration (200 fs). The original FSSH method, in which the electronic wave function is propagated in the adiabatic representation, yields less accurate results. The oscillations in the populations of the diabatic B 3u (nπ *) and A 1u (nπ *) states driven by the mode Q 8a are also observed in the full-dimensional dynamics simulations.

Amyloids have unique structural, chemical and optical properties. Although much theoretical effort has been directed toward understanding amyloid nucleation, the understanding of their optical properties has remained rather limited.In particular, the photophysical mechanisms leading to near UV excitation and characteristic blue-green luminescence in amyloid systems devoid of aromatic amino acids have not been resolved.We use ab initio static calculations and nonadiabatic dynamics simulations to study the excited electronic states of model amyloid-like peptides. We show that their photophysics is essentially governed by the multitude of nπ * states with excitation localized on the amide groups. The strong stabilization of the nπ * states with respect to the amide group deplanarization and the concomitant increase of the oscillator strength makes excitation in the near UV possible. With respect to emission, our dynamics simulations revealed that the amyloid cross β arrangement effectively hinders the nonradiative relaxation channels usually operative in proteins. Finally, we show that after relaxation of the photoexcited peptides toward the minimum of the different nπ * states, fluorescence takes place in the visible (green) part of the electromagnetic spectrum.

The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial ππ* surface, most importantly the one involving a transfer of electronic excitation to an nπ* surface, induced by distortions of the backbone peptide bond.

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail.

The excitation wavelength dependent photodynamics of pyrrole are investigated by nonadiabatic trajectory-surface-hopping dynamics simulations based on time dependent density functional theory (TDDFT) and the algebraic diagrammatic construction method to the second order (ADC(2)). The ADC(2) results confirm that the N-H bond dissociation occurring upon excitation at the origin of the first excited state, S1(πσ*), is driven by tunnelling [Roberts et al., Faraday Discuss., 2013, 163, 95] as a barrier of ΔE = 1780 cm(-1) traps the population in a quasi-bound minimum. Upon excitation to S1(πσ*) in the wavelength range of 236-240 nm, direct dissociation of the N-H bond takes place with a time constant of 28 fs. The computed time constant is in very good agreement with recently reported measurements. Excitation to the lowest B2 state in the 198-202 nm range returns a time constant for N-H fission of 48 fs at the B3LYP/def2-TZVPD level, in perfect agreement with the experiment [Roberts et al. Faraday Discuss., 2013, 163, 95]. For the same wavelength range the ADC(2)/aug-cc-pVDZ decay constant is more than three times longer than the experimentally reported one. The accuracy of the B3LYP/def2-TZVPD dynamics is checked against reference complete-active-space second-order perturbation theory (CASPT2) calculations and explained in terms of correct topography of the ππ* surface and the lack of mixing between the ππ* and the 3px Rydberg states which occurs in the ADC(2) method.

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The intramolecular hydrogen bond in the enol-acethylacetone (ACAC) is investigated by performing reduced-dimensional quantum calculations. To analyze the shared proton vibrations, two sets of coordinates were employed: normal mode coordinates describing the motion in the vicinity of the most stable configuration, and internal coordinates accounting for the double minimum proton motion. It is proved that the extreme broadness of the OH-stretch band in ACAC is a consequence of the coexistence of two enol-ACAC structures: the global minimum and the transition state for rotation of the distal methyl group. Further, a ground-state tunneling splitting of 116 cm(-1) is found, and it is shown that the inclusion of the kinematic coupling is mandatory when treating large-amplitude proton motion. In the OH-stretch direction a splitting of 853 cm(-1) was predicted.