New contributions to the photophysical model for all-trans-polyenesfrom ttbP4, a nonphotolabile octatetraene

Abstract

All-trans-octatetraene 3,10-di(tert-butyl)-2,2,11,11-tetramethyl-3,5,7,9-dodecatetraene emits fluorescence in three different regions of the visible spectrum. Thus, it produces an extremely weak emission in the gas phase that can hardly be detected in the condensed phase; such an emission exhibits a negligible Stokes shift with respect to the 1A(g)-->1B(u) absorption transition and can, in principle, be assigned to the 1B(u)-->1A(g) emission for the compound. A second, structureless fluorescence emission, centered in the region of 525 nm, is observed in the gas phase and at somewhat higher wavelengths in the condensed phase [viz., 570 nm in 2-methylbutane (2MB) and 550 nm in squalane (SQ)]. While detectable, this emission increases significantly, with no change in spectral position, as the solution temperature is lowered; also, it is abruptly replaced by a new, strongly blueshifted emission at approximately 490 nm in 2MB and 455 mm in SQ when the viscosity of the medium exceeds a given level. The fact that the two fluorescence emissions considerably depart from the expected behavior for a 1B(u)-->1A(g) emission in an all-trans-polyene, and that one disappears while the other simultaneously appears as the medium becomes more rigid, suggests that the two emissions are produced by two different molecular structures and that the rigidity of the medium switches their production from the originally excited all-trans 1B(u) form. The observed spectral behavior is consistent with a recently proposed model [J. Catalan, Chem. Phys. 335, 69 (2007)] in which the 1B(u) excited state of octatetraene can give two distinct molecular conformers as a result of twisting about different C-C single bonds.

On the Photophysics of Polyenes. 1. Bathochromic Shifts in Their 1Ag → 1Bu Electronic Transitions Caused by the Polarizability of the Medium

“…Interestingly, the solvatochromically calculated 0−0 component for ttbP3 was 34 072 ± 66 cm −1 , which is quite consistent with the measured value in the gas phase at 353 K (see Figure ): 34100 cm −1 . The corresponding value for ttbP4, 30976 ± 51 cm −1 , is also consistent with that in the gas phase at 353 K: 31104 cm −1 …”

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.…”