Direct Spectroscopic Evidence of the Mechanism behind the Phase Transition of [2,2]‐Paracyclophane

Abstract

[2,2]-Paracyclophane undergoes phase transitions at 45 and 60 K. Based on simultaneous Raman spectroscopy and inelastic neutron scattering experiments (12-70 K), it was shown that a twisting motion of the ethylene bridge perpendicular to the plane of the aromatic rings drives the phase transition. The low-temperature (<45 K) and high-temperature (>60 K) conformers only differ by this twisting motion, which freezes out below 45 K and is thermally averaged above 60 K. Between 45 and 60 K, the system gains energy until the phase transition is complete.

Protonation-Induced Sign Inversion of the Cotton Effects of Pyridinophanes. A Combined Experimental and Theoretical Study

“…Parent [2.2]paracyclophane is known to be polymorphic, and the D 2h (twisted) and D 2 (centrosymmetric) structures are obtainable at different temperatures divided by the phase transition. 23 Generally speaking, such conformational variations should affect the spectral properties, which is indeed the case with some substituted cyclophanes. 24 However, the calculated CD spectra of these conformers were found almost identical for both 1 and 1-H + (data not shown), since such a subtle conformational change does not significantly affect the overall electronic nature.…”

“…Our calculation at the DFT-D3­(BJ)-TPSS/def2-TZVP level , on 1 and 1-H + afforded a pair of twisted structures ( skew + and skew –, or s + and s –, which differ in the torsional angle of the linker, Chart ) with quite comparable energies (Table ). Parent [2.2]­paracyclophane is known to be polymorphic, and the D 2 h (twisted) and D 2 (centrosymmetric) structures are obtainable at different temperatures divided by the phase transition . Generally speaking, such conformational variations should affect the spectral properties, which is indeed the case with some substituted cyclophanes .…”

Coherent Intramolecular Excimer Formation in Solid [2,2]‐Paracyclophane: Time‐Resolved Springing of a Molecular “Trap”

“…XRD measurements on single crystals of 2PC, predict a D 2h molecular point group at room temperature, resulting from thermal population of vibrational states lying above the 0.2 kcal/mol barrier of the S 0 double well potential. [22] This was verified by Raman and inelastic neutron scattering experiments on single crystals which show a phase transition near where the low temperature D 2 symmetry where 2PC is locked into one of the equivalent twisted minima, transforms to D 2h above 50 K. [23] However, results from room temperature MCD and absorption measurements on 2PC in liquid solutions suggest that solvation could slightly break its equilibrium D 2h symmetry by slight ring warping around 'z' in either S 0 or S 1 . [10,24] The extensive vibronic structure in the absorption of 2PC predicts that large displacive forces are switched on in S 1 upon photoexcitation.…”

“…To test this conjecture, pump -probe investigations were performed on an isotropic crystal film of 2PC where pump and probe are confined to be polarized in the x-z plane (Figure 1c). Given the D 2h symmetry of 2PC, [22,23] GS absorption is vibronically allowed and polarized in the x-y plane. [13,14] Assuming the strongly modulated absorption is 'z' polarized, aligning pump and probe polarizations to coincide with 'x' in one half of the molecules should significantly reduce the depth of absorption modula- tion.…”

Experimental charge-density studies: data reduction and model quality: the more the better?

“…In a charge-density investigation on [2,2]-paracyclophane (1), we compared four data sets that we had collected in-house and at the Advanced Photon Source (APS) at two different temperatures (Wolf et al, 2014;Wolf, Jørgensen et al, 2015;Wolf, Lock et al, 2015). In Table 1 some quality indicators are listed.…”