The photophysics of all-trans polyenes from ttbP5, a nonphotolabile pentaene

Abstract

The all-trans pentaene, 3,12-di(tert-butyl)-2,2,13,13-tetramethyl-3,5,7,9,11-tetradecapentaene (ttbP5) fluoresces in two different regions of the visible spectrum. It produces an extremely weak emission in the gas phase that can also be detected in the condensed phase; such an emission exhibits a negligible Stokes shift with respect to the 1Ag-->1Bu absorption transition and can in principle be assigned to the 1Bu-->1Ag emission of the compound. ttbP5 also exhibits a second fluorescence emission at approximately 520 nm in both the gas phase and the condensed phase. The emission in the condensed phase increases in strength and structure, with no change in spectral position, as the solvent viscosity increases by effect of the solution temperature being lowered. The spectral behavior of this pentaene (ttbP5) is different enough from that reported [J. Catalan et al., J. Chem. Phys. 128, 104504 (2008)] for its tetraene counterpart (ttbP4) to warrant a separate analysis in order to facilitate a better understanding of the way the photophysics of these polyenes changes as their chain is lengthened.

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

“…those for a 7 × 10 −6 M solution measured in ClB at the same temperature range. As can be seen, the spectral behavior of β‐carotene in the two solvents was as expected for a polyene: that is, lowering the temperature significantly increases the vibronic structure and the intensity of the absorption band. The increased spectral intensity was partly a result of the volume contraction and increase in density undergone by the solvent as the temperature was lowered.…”

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.…”

Evidence of the Torsion of a Polyene Chain in a Strongly Hindered Molecular Environment: The Ttbp4 Crystal

“…Most of the research done has focused on the study of diphenylpolyenes [18][19][20]. However, it is worth noting the novel photophysical insights achieved through the study of tetratertbutyl polyenes (ttbPX, being X the number of double bonds of the polyene chain) [21][22][23][24][25]. The last are non photolabile compounds [24,25] with a planar molecular structure that in contrast with diphenylpolyenes [26,27], have the twisting of their terminal groups hindered by the tert-butyl groups (see Scheme 1).…”