ABSTRACT
Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.
ABSTRACT
Deciphering the exact electronic and geometric changes of photoexcited molecules is an important task not only to understand the fundamental atomistic mechanisms but also to rationally design molecular properties and functions. Here, we present a combined experimental and theoretical study of the twisted intramolecular charge transfer (TICT) process in hemithioindigo photoswitches. Using ultrafast transient IR spectroscopy as the main analytical method, a detailed understanding of the extent and direction of charge transfer within the excited molecule is obtained. At the same time, the geometrical distortion is monitored directly via changes of indicative vibrational modes over the time course of the photoreaction. These high-resolution data deliver a detailed molecular movie of the TICT process in this important class of chromophores with picosecond time resolution.
ABSTRACT
The hydrogen bond network accounts for many of the extraordinary physical properties of liquid water and ice. Its vibrational dynamics are quite complex in their entirety but can be accessed in detail by investigating small groups of only a few water molecules. Here, aqueous salt hydrates turned out to be an exceptional model system for water molecules arranged in well-defined geometrical structures that can be accessed by means of femtosecond spectroscopy of the OH stretching vibration. In this study, we find striking resemblance between the vibrational properties of three water molecules connected via strong hydrogen bonds in the trihydrate of LiNO3 and those of ordinary ice Ih. As in ice, the vibrations of the hydrate water molecules show ultrafast excited state dynamics that are strongly accelerated when proceeding from deuterated to neat H2O samples. The latter is analyzed by means of an additional relaxation channel that is due to Fermi resonance between the OH stretching vibration and the bend overtone accompanied by delocalization of the vibration over neighboring water molecules in the H2O species. Moreover, in the hydrate and ice samples severe spectral broadening is examined when comparing fundamental and excited state absorption bands. Here, proton delocalization along the strong hydrogen bonds is given as a possible underlying mechanism.
ABSTRACT
Vibrational excitations provoked by coupling effects during charge transport through single molecules are intrinsic energy dissipation phenomena, in close analogy to electron-phonon coupling in solids. One fundamental challenge in molecular electronics is the quantitative determination of charge-vibrational (electron-phonon) coupling for single-molecule junctions. The ability to record electron-phonon coupling phenomena at the single-molecule level is a key prerequisite to fully rationalize and optimize charge-transport efficiencies for specific molecular configurations and currents. Here we exemplarily determine the pertaining coupling characteristics for a current-carrying chemically well-defined molecule by synchronous vibrational and current-voltage spectroscopy. These metal-molecule-metal junction insights are complemented by time-resolved infrared spectroscopy to assess the intramolecular vibrational relaxation dynamics. By measuring and analyzing the steady-state vibrational distribution during transient charge transport in a bis-phenylethynyl-anthracene derivative using anti-Stokes Raman scattering, we find â¼0.5 vibrational excitations per elementary charge passing through the metal-molecule-metal junction, by means of a rate model ansatz and quantum-chemical calculations.
