Effect of ring torsion on intramolecular vibrational redistribution dynamics of 1,1′-binaphthyl and 2,2′-binaphthyl

Abstract

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1'-binaphthyl (1,1'-BN) and 2,2'-BN. The dispersed fluorescence spectra of 1,1'-BN in the origin region of S(1)-S(0) were well resolved, which presented 25-27 cm(-1) gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1'-BN is similar to that of naphthalene. In contrast, the spectra of 2,2'-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm(-1) for 1,1'-BN and 658 cm(-1) for 2,2'-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1'-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2'-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into "strong" and "weak" torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.

Simulation of Vibronic Spectra of Flexible Systems: Hybrid DVR-Harmonic Approaches

“…9PA belongs to a class of molecules characterized by two aromatic rings connected by a formally single bond, whose spectroscopic properties are strongly related to the torsion about this bond. For this reason, high-resolution electronic spectroscopies, both for absorption and for emission processes, have been widely used to characterize the torsional potential for ground and excited electronic states 111,112. This is the case also for 9PA, which has been recorded at high-resolution over the energy range associated t to vibrational transitions involving the torsional motion 113.…”

“…9PA belongs to a class of molecules characterized by two aromatic rings connected by a formally single bond, whose spectroscopic properties are strongly related to the torsion about this bond. For this reason, high-resolution electronic spectroscopies, both for absorption and for emission processes, have been widely used to characterize the torsional potential for ground and excited electronic states. , This is the case also for 9PA, which has been recorded at high resolution over the energy range associated to vibrational transitions involving the torsional motion . From a computational point of view, while a wide range of theoretical approaches have been used to model the potential energy surface along the torsional angle for those systems, − characterization of the vibrational and vibronic properties at the anharmonic level has been scarce.…”

Determining Equilibrium Fluctuations Using Temperature-Dependent 2D-IR

“…Simultaneously, we shed light on the validity of previous descriptions of tricarbonyl(η 4 -diene)iron complexes. In these piano stool systems, there are two possible sources of vibrational coupling that 2D-IR can differentiate (Chart ), (I) quantum mechanical IVR (possibly coupled to the wagging motion of the ligands) , and (II) chemical exchange via a turnstile motion.…”

Electron Dynamics and IR Peak Coalescence in Bridged Mixed Valence Dimers Studied by Ultrafast 2D-IR Spectroscopy

“…Due to the time scale and the lack of correlation to the FTIR line shape, we attribute the additional, faster time constant to an IVR process. Enhanced rates of IVR have previously been correlated with the presence of geometric fluctuations, potentially explaining why the biexponential decay is only seen in pyrazine bridged mixed valence samples.…”