Electronic Energy Levels in all‐trans Long Linear Polyenes: The Case of the 3,20‐Di(tert‐butyl)‐2,2,21,21‐tetramethyl‐all‐trans‐3,5,7,9,11,13,15,17,19‐docosanonaen (ttbp9) Conforming to Kasha's Rule

Abstract

The absorption, fluorescence and fluorescence excitation spectra for 3,20-di(tert-butyl)-2,2,21,21-tetramethyl-all-trans-3,5,7,9,11,13,15,17,19-docosanonaen (ttbP9) in dilute solutions of 2-methylbutane were recorded at temperatures over the range 120-280 K. The high photostability of this nonaene allows us to assert that it exhibits a single fluorescence and that this can be unequivocally assigned to emission from its 1(1)B(u) excited state, it being the first excited electronic state. Available photophysical data for this polyene and the wealth of information reported for shorter all-trans polyenes allow us to conclude that if the first excited electronic state for the chromophore possessed 2(1)A(g) symmetry, then the energy of such a state might have been so close to that of the 1(1)B(u) state that: 1) the radiationless internal conversion mechanism would preclude the observation of the emission from the 1(1)B(u) state reported in this work and 2) the 2(1)A(g) state reached through internal conversion would be vibrationally coupled to 1(1)B(u) and would facilitate the detection of the emission from 2(1)A(g), which was not observed in any of the solvents used in this work. The spectroscopic and photochemical implications of these findings for other polyenes are discussed.

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.…”

The emission of α,ω-diphenylpolyenes: A model involving several molecular structures

“…Recently, Itoh [28] reported that the emission from the 1B u state in the compound may be a result of both direct excitation and delayed thermal activation. However, this model, involving the 2A g and 1B u states of the same molecular structure, is inconsistent with some photophysical evidence for these compounds [29][30][31][32][33][34][35].…”

“…(d) Finally, a dilute solution of a nonaene with four tertbutyl substituents at terminal positions [3,20-di(tertbutyl)-2,2,21,21-tetramethyl-all-trans-3,5,7,9,11,13, 15,17,19-docosanonaen] in 2-methylbutane was found to clearly exhibit the 0-0 component of its 1A g ! 1B u transition, which was shifted to a higher wavelength, with no change in the transition onset, as the temperature was slowly lowered [35]. These results were ascribed to the previous forms being highly flexible and adopting a planar structurewhich causes a red shift in the 0-0 component -in electronic transitions to more rigid planar forms.…”

“…modulate the relative stability of the two forms and hence their concentration. Despite the potential contribution of the molecular environment, the stability values of Table 4 suggest that, as chain-length increases, a point must be reached where production of the rotated form will be negligible, so only state 1B u , generated by absorption from the all-trans structure present in the ground state, will exist, as previously shown to occur in nonaene among linear polyenes [35]. The fact that both the emission from the 1B u state and that from corresponding C s forms produce excitation spectrum at room temperature ascribable to the 1A g !…”

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.…”