The excited state dynamics of carbonyl carotenoids is very complex because of the coupling of single- and doubly excited states and the possible involvement of intramolecular charge-transfer (ICT) states. In this contribution we employ ultrafast infrared spectroscopy and theoretical computations to investigate the relaxation dynamics of trans-8'-apo-β-carotenal occurring on the picosecond time scale, after excitation in the S2 state. In a (slightly) polar solvent like chloroform, one-dimensional (T1D-IR) and two-dimensional (T2D-IR) transient infrared spectroscopy reveal spectral components with characteristic frequencies and lifetimes that are not observed in nonpolar solvents (cyclohexane). Combining experimental evidence with an analysis of CASPT2//CASSCF ground and excited state minima and energy profiles, complemented with TDDFT calculations in gas phase and in solvent, we propose a photochemical decay mechanism for this system where only the bright single-excited 1Bu(+) and the dark double-excited 2Ag(-) states are involved. Specifically, the initially populated 1Bu(+) relaxes toward 2Ag(-) in 200 fs. In a nonpolar solvent 2Ag(-) decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1Bu(+) state which then quickly relaxes to the GS (18 ps in chloroform). The 1Bu(+) state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2Ag(-) → 1Bu(+) population transfer is evidenced by a cross peak on the T2D-IR map revealing that the motions along the same stretching of the conjugated chain on the 2Ag(-) and 1Bu(+) states are coupled.

The solvation dynamics of methyl acetate in heavy water are analyzed by means of two-dimensional infrared spectroscopy, in conjunction with Car-Parrinello molecular dynamics simulations. The C horizontal lineO stretching infrared band of methyl acetate in water splits into a doublet as a consequence of the hydrogen bond interaction with the solvent, which leads to the equilibrium between two solvated species, consisting of one methyl acetate molecule bonded to one and two water molecules. The structure and dynamics of the water molecules bound to methyl acetate are characterized by means of experiments and simulations, allowing an accurate description of the kinetics of the exchange process and the lifetime of the hydrogen bond.

Recent theoretical and experimental efforts have shown the remarkable and counter-intuitive role of noise in enhancing the transport efficiency of complex systems. Here, we realize simple, scalable, and controllable optical fiber cavity networks that allow us to analyze the performance of transport networks for different conditions of interference, dephasing and disorder. In particular, we experimentally demonstrate that the transport efficiency reaches a maximum when varying the external dephasing noise, i.e. a bell-like shape behavior that had been predicted only theoretically. These optical platforms are very promising simulators of quantum transport phenomena, and could be used, in particular, to design and test optimal topologies of artificial light-harvesting structures for future solar energy technologies. The transmission of energy through interacting systems plays a crucial role in many fields of physics, chemistry, and biology. In particular, the study and a full understanding of the mechanisms driving the energy transport may open interesting perspectives both to improve the process of transferring quantum or classical information across complex networks, and to explain the high efficiency of the excitation transfer through a network of chromophores in photosynthetic systems. Indeed, recently, several experiments on light-harvesting complexes have suggested a possible correlation between the remarkable transport efficiency of these systems and the presence of long-lived quantum effects, observed also at room temperature [1][2][3][4][5][6].Stimulated by these results, a large theoretical effort has been undertaken to study transport mechanisms through a network of chromophores or, more in general, through a complex network, bringing to evidence the active role of noise in energy transport. In fact, it is usually accepted that the uncontrollable interaction of a transmission network with an external noisy environment negatively affects the transport efficiency by reducing the coherence of the system [7]. However, the noise has also been found to play a positive role in assisting the transport of energy [8][9][10][11][12][13] and information [14], and loss-induced optical transparency has been observed in waveguide systems [15]. In certain circumstances, the presence of noise can lead to the inhibition of destructive interference and to the opening of additional pathways for excitation transfer, with a consequent increase of the transport efficiency [10]. This phenomenon has generally gone under the name of noise-assisted transport (NAT). In this context, the role of geometry has been theoretically analyzed in terms of structure optimization [16], in the case of disordered systems [17], and also for the proposal of design principles for biomimetic structures [18]. Therefore, the possibility to experimentally reproduce NAT effects in purely optical networks with controllable parameters and topology, would allow one to verify the predictions of NAT models in simple test systems. Moreover, the investigation...

Vibrational dynamics of liquid formamide is studied in the spectral region of the amide I mode by means of linear and two-dimensional infrared spectroscopies. The two-dimensional spectrum has a complex structure to be connected to the partially excitonic nature of the vibrational states. The measurements performed on a 1:10 (12)C:(13)C formamide isotopic mixture allow separating the broadening contribution due to the inhomogeneous frequency distribution of the local oscillators from that of excitonic origin. A model based on the Kubo picture of the line broadening is used, together with the dynamic information obtained from a molecular dynamics simulation, to fit the spectra of the (12)C formamide impurity in the isotopic mixture. The relevant dynamical information, such as the amplitude of the frequency fluctuations, lifetime of the second vibrational excited state, and anharmonicity, is thus recovered. By appropriately combining the outcomes of experiments and molecular dynamics simulation, we demonstrate that motional narrowing determines the line shape of the amide I resonance to a large extent. The same analysis provides an estimate of the transition dipole moment of formamide, which results in good agreement with an ab initio calculation. The calculated frequency fluctuation correlation time is found to be comparable to the hydrogen-bond lifetime, which defines the basic structural relaxation rate of the networked liquid.

We describe the theory, some experimental details and the data analysis procedures of two‐dimensional infrared (2D‐IR) spectroscopy. A brief description of an application of the technique to the study of a dipeptide in solution is also reported. Like multi‐dimensional NMR spectroscopy, 2D‐IR can provide additional pieces of information hidden in the inhomogenously broadened bands observed in linear IR spectra. In addition, the presence of off‐diagonal peaks allows a direct estimate of the couplings between vibrational modes. By means of this technique, making use of ultrashort mid‐infrared pulses, exploration of the structure and of the dynamics of molecular systems in the condensed phase and on very short time scales becomes now achievable. The effects of solvent dynamics on Glycine‐L‐Alanine‐Methylamide by 2D‐IR are discussed. Copyright © 2013 John Wiley & Sons, Ltd.