Resolution of Conformer-Specific All-trans-1,6-diphenyl-1,3,5-hexatriene UV Absorption Spectra

Abstract

All-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) exists in solution as a mixture of s-trans,s-trans and s-cis,s-trans conformers. The latter is higher in energy, and its contribution increases with increasing temperature. ttt-DPH UV absorption spectra broaden with increasing temperature and undergo blue shifts with decreasing polarizability. We describe here the resolution of two spectrothermal matrices of ttt-DPH UV absorption spectra into two conformer-specific components. The first matrix consists of DPH spectra measured in n-dodecane in the 283 to 374 K T range and the second of ttt-DPH absorption spectra measured in the even numbered n-alkanes (n-C(8)-n-C(16)) at temperatures selected to achieve isopolarizability (284-372 K). Principal component analysis (PCA) treatments showed that reasonable two-component systems are attained by compensation for T-induced broadening and shifting in the pure conformer spectra. The self-modeling (SM) method used to resolve the n-C(12) matrix is successfully tested on a simulated matrix closely mimicking ttt-DPH experimental spectra in n-C(12). Compensation for nonlinear effects yields robust two-component matrices from the experimental spectra. Their resolution into pure component spectra is based on the application of the Lawton and Sylvestre (LS) non-negativity criterion at the spectral onset to define the spectrum of the low energy s-trans-conformer and the optimum linearity van't Hoff (vH) plot criterion to find the spectrum of the higher-energy s-cis-conformer. Resolved spectra are somewhat sensitive to the choice of the spectral region in which the LS criterion is applied. The surprising result is that both resolutions lead to the conclusion that the molar fraction of the s-cis-conformer equals, or even exceeds, the molar fraction of the s-trans-conformer as the highest T's employed in our study are approached.

On Saltiel's isopolarizability approach and its applicability to diphenylpolyenes

“…Consequently, the thermochromism of these compounds should also be linearly related to the polarizability of the medium. This conclusion challenges the assignation by Saltiel et al [2] of the thermochromism of DPH in n-dodecane to the polarizability of the medium because they found a linear relationship between the position of the first absorption band for DPH and the polarizability function ˛ of the medium at temperatures from 10 to 100 • C. If the assumption of these authors is correct, then, because our polarizability values SP cannot account for the whole thermochromic effects observed, they must be erroneous and confirms our conclusion that some conformational changes in the coplanar arrangement of the diphenylpolyenes must exist [8,9]. As a result, our conclusion would be in fact, in Saltiel's words [2], 'without merit'.…”

“…All polyenes were in their most stable (all-trans) form and no formation of any other isomers during the cooling process was found. This results in considerably simplified analyses because of the absence of stereo isomers potentially formed at temperatures above room temperature according to Saltiel et al [1,2].…”

“…Consequently, Saltiel's isopolarizability approach, which was established from polyene solutions at variable temperatures, is, in his own words, 'without merit' [2]. Clearly, however, it might hold for any other compounds undergoing no significant structural change with temperature, which is certainly not the case for polyenes [8,9].…”

“…As a result, on excitation diphenylpolyenes should undergo structural changes leading to differences not only in the area under their first absorption band but also in its energy. These contributions gave rise to the following comment in a recent letter by Saltiel et al [2]: 'A recent criticism of our isopolarizability approach 17 is shown to be without merit. ' In 2008, our group [10] studied the solvatochromism of a wide range of tetra-tert-butylpolyenes including ttbP3, ttbP4, ttbP5, ttbP6, ttbP7, ttbP8, and ttbP9, and established solvatochromic equations to account for their UV/vis spectral behavior.…”

“…The isopolarizability approach of Saltiel et al [1,2] relies on the assumption that the bathochromic effect found in the solvatochromism and thermochromism of polyenes merely echoes the polarizability of the solvent. This assumption has been accepted by authors such as Hauser et al [3] and Sklar et al [4], and also in two reviews on the behavior of polyenes by Hudson et al [5] and Saltiel et al [6].…”