Fluorescence of Antiaromatic Systems:  An Experimental and Theoretical Study of 1,3,5-Tri-tert-butylpentalene

Abstract

The fluorescence spectrum of 1,3,5-tri-tert-butylpentalene (TTBP), the stable derivative of antiaromatic pentalene, has been measured in solution at room and low temperature in condensed phase. A good correspondence between the first allowed absorption band and the fluorescence spectrum is observed. The fluorescence quantum yield of TTBP 2 × 10-4 M in cyclohexane is estimated to be ≈2 × 10-3 (λex = 313 nm) at room temperature. At low temperature and exciting at 355 nm, the fluorescence spectrum of TTBP shows appreciable vibronic structure. The fluorescence data have been related to the electronic structure of the parent molecule. MCSCF/CAS calculations for the ground and lowest excited states of pentalene, up to S4, have been performed in D 2 h symmetry with 3-21G and 6-31G basis sets including polarization and diffuse orbitals. As a result of the strong interaction between the pairs of states S0/S1 and S2/S3, the lowest component of each pair distorts from D 2 h to lower symmetry, giving rise to new equilibrium geometries. According to ab initio calculations, the observed fluorescence is assigned to upper state, S3 → S0, emission.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Low energy absorption bands have been observed at 1.50 and 2.14 eV in the low temperature spectrum of the tert-butyl derivative and assigned to the two A g states previously mentioned [161]. Further, the derivative fluoresces from the lowest B u state at 3.17 eV (≈ 25550 cm −1 ) with quantum yield 0.002 and decay time 13 ps [29,160]. This band has been assigned as the emission counterpart of the 3.67 eV (≈ 29600 cm −1 ) absorption band (see Figure 17).…”

“…The ring geometry was determined by X-ray diffraction [159]. Low energy absorption bands have been observed at 1.50 and 2.14 eV in the low temperature spectrum of the tert-butyl derivative and assigned to the two A g states previously mentioned [161]. Further, the derivative fluoresces from the lowest B u state at 3.17 eV (≈ 25550 cm −1 ) with quantum yield 0.002 and decay time 13 ps [29,160].…”

“…In 1,3,5-tri-tert-butylpentalene the energy barrier was found to be ≈ 4 kcal/mol [162]. The calculated barrier height amounts to 9.7 kcal/mol [29] and 6.6 kcal/mol (including ZP E) [158]. Another structural rearrangement from D 2h to C 2v symmetry occurs in the second excited state due to the 2A g /1B 2u , i.e., S 2 /S 3 , vibronic coupling promoted by a b 2u vibration [29].…”

“…Simple arguments based on the second-order bond fixation in conjugated molecules [43,44] indicate that the symmetrical bond equalized D 2h structure corresponds to a maximum on the ground state energy surface and that the molecule distorts to C 2h symmetry with alternating single and double bonds. Bond alternation of the order of ≈ 0.15Å has been calculated by all ab initio methods for ground state C 2h pentalene [28,29,158] (see Table 8). …”

“…Cyclobutadiene has been considered the reference antiaromatic system [23,24] though recently criticism has been advanced to this view [25,26]. Going up with the ring size, while cyclooctatetraene is characterized by a non planar arrangement of the four π bonds [4,10,11], pentalene, where the ring is forced to planarity by crosslink [27,28], is equally well suited to model antiaromatic properties [29]. On the contrary, the more complex and formally antiaromatic structure of s-indacene peculiarly shows a bond equalized ring geometry [30,31].…”