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J J Piet W Schuddeboom B R Wegewijs F C Grozema J M Warman 《Journal of the American Chemical Society》2001,123(22):5337-5347
The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results. 相似文献
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Catarina IV Ramos Flávio Figueira Marcelo D Polêto Francisco ML Amado Hugo Verli João PC Tomé M Graça PMS Neves 《Journal of mass spectrometry : JMS》2016,51(5):342-349
Electrospray mass spectrometry/mass spectrometry was used to investigate the gas‐phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso‐pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N‐fused hexaphyrin (28 π electrons). The gas‐phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius‐like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non‐aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius‐like aromatic systems to non‐aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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John M. Warman Wouter Schuddeboom Stephan A. Jonker Matthijs P. de Haas Michael N. Paddon-Row Klaas A. Zachariasse Jean-Pierre Launay 《Chemical physics letters》1993,210(4-6):397-404
Transient changes in the real and imaginary component of the complex permittivity of a flash-photolyzed solution can be monitored with nanosecond time resolution using the time-resolved microwave conductivity (TRMC) technique. Effects caused by rapid (picosecond), flip—flop switching between dipolar resonance structures in the S1 and T1 states of molecules with mirror symmetry have been observed. 相似文献