On the use of β‐carotene as a probe for solvent polarizability

Abstract

The solvatochromism of β-carotene confirms its high sensitivity not only to the polarizability of the medium, but is also contaminated by additional solute/solvent interactions due to its dipolarity and acidity, as well as to changes in its molecular structure in some solvents. A thermochromic analysis of β-carotene dissolved in 2-methylbutane and 1-chlorobutane (ClB) revealed the influence of the solvent dipolarity on its UV/Vis-spectroscopy behavior in these solvents. Applying Abe's method to the solvent-induced shift of the first Vis absorption band of β-carotene in ClB revealed that the electronic excitation substantially increases its polarizability and its dipole moment. Other experimental evidence also confirms that β-carotene is not a suitable polarizability probe of the medium.

On the photo‐activation of the S0 → S1 transition in polyenes

“…Let us first present the results achieved in our laboratory for both DPH and DPO dissolved in EPA at a concentration of 1.5 × 10 −5 M by direct immersion in liquid nitrogen at 77 K as we did for DPH dissolved in n‐hexane and for DPO dissolved in n‐octane …”

On the photophysics of a polyene dissolved in n‐alkanes when the temperature drops from 293 to 77 K

“…This information together with other evidence provided in recent years by our laboratory [ 1,2,11–16 ] allows us to draw the following conclusions in order to propose an adequate model to correctly interpret the photophysics of polyene compounds: The position of the first intense peak in the excitation spectrum of the DPO in the six n ‐alkanes used represents a bathochromism that varies linearly with the temperature from 293 to the melting point temperature of the corresponding alkane. This behavior is red‐shifted in parallel fashion as the alkane chain is lengthened, as a result of the alkane's increasing polarizability on lengthening of its chain.The reported increase of the bathochromism with the decrease of the temperature undoubtedly also includes a structural contribution because the phenyl groups in the diphenyl polyenes in liquid solvents present such a small torsion barrier what makes their two phenyl groups are continuously rotating out of the coplanar situation.…”

“…It is important to note that under these conditions in the spectra at 77 K, the presence of an electronic transition whose 0–0 component is around 447 nm is detected, that is to say in the same position where the 0–0 component is almost invariably detected of the emission of the compound. [ 1,2,13 ] c)In the entire analyzed temperature range, between 293 and 77 K, the corresponding emission spectra practically maintain not only their position but also their spectral envelope, with their origin always around 450 nm. This fact is very relevant spectroscopically because it indicates that both the state which generates the fluorescence and the state that receives the emission keep its molecular structure virtually unchanged under such disparate experimental conditions. d)According to the theoretical calculations [ 19 ] a polyene molecule on absorption of radiation, it does so by exciting an electron located in a double bond that delocalizes along its π*‐system tending to average the double and single bonds of the polyenic chain of the compound, see Table IV of reference 19.…”

“…See for example the different spectral behavior shown by the DPO dissolved in n ‐octane and in 2,2,4‐trimethylpentane. [ 2 ]…”

“…Analyzing experimental evidence obtained in our laboratory with 1,6‐diphenyl‐1,3,5‐hexatriene dissolved in different hexanes ( n ‐hexane, 2‐methylpentane, and 3‐methylpentane) [ 1 ] and with 1,8‐diphenyl‐1,3,5,7‐octatetraene (DPO) dissolved in different octanes ( n ‐octane, cyclooctane, and 2,2,4‐trimethylpentane) [ 2 ] demonstrates that the photophysics of these diphenylpolyenes does not rest on the presence of an underlying phantom electronic state, but on conformational changes that cause these compounds to exhibit their peculiar photophysics. Our observations also point to the singular role played by linear alkanes, n ‐hexane and n ‐octane, especially at temperatures close to their melting points.…”