The vibrational and electronic spectra of the stable pentalene derivative 1,3,5-tri-tert-butylpentalene (TTBP) are reported, and their properties are discussed on the basis of QCW-PI MO semiempirical calculations, which allow geometry optimization and normal mode analysis for the ground and lowest excited states. The infrared and Raman spectra have intensities mostly arising from modes of pentalene origin. The observed frequencies are compared with those obtained from vibrational calculations on the parent molecule and on several tert-butyl derivatives. The nn* electronic transitions of TTBP in the visible and UV regions are satisfactorily predicted by our calculations, including the interaction between singly excited configurations. Optimization procedures show that bond altemation, a distinctive feature of antiaromatics, is reduced in the excited states and that ground and excited state potential surfaces have minima displaced one with respect to the other. Low-temperature absorption spectra in the S2 and S3 regions are interpreted in terms of FranckCondon vibronic transitions whose strength depends directly on the structural change upon excitation. The theoretical results on SO -. S2 and SO -S3 band profiles, with consideration of the normal mode rotation in the excited state, that is, the Duschinsky effect, are in fair agreement with experiment.

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.

The Comment on "Fluorescence of Antiaromatic Systems: an Experimental and Theoretical Study of 1,3,5-Tri-tert-butylpentalene" reports on energy surface crossings between electronic states of pentalene and their role in promoting nonradiative decay. Specifically, it is shown that (i) the S 1 /S 0 crossing and the S 1 minimum have almost identical geometries and comparable energies while (ii) the S 2 /S 1 crossing is at a much higher energy, ≈40 kcal/mol, than the S 2 minimum and corresponds to a distorted structure with respect to the latter. It is therefrom implied that nonradiative decay dominates over radiative emission in S 1 while no simple deactivation mechanism can be envisaged for S 2 , thus favoring fluorescence from this state.We appreciate the point of view emphasizing surface crossings as the key factor for nonradiative decay in pentalene and we regret our ignorance of the paper dealing with the subject. 1 On the other hand, we were aware of the qualitative basis of our 2,3 and previous 4 understanding on S n f S 0 (n g 2) emission in pentalene and indacene derivatives, TTBP and TTBI, respectively. It is definitely positive that the experimental investigation about this antiaromatic property is supported by a more quantitative model of nonradiative decay from S n .The second point (ii) of the Comment deserves more attention, being possible a direct comparison with experimental data. It is in fact suggested, on the basis of the S 2 /S 1 surface crossing, the assignment of the weak room temperature fluorescence of TTBP when excited at 380 nm to the S 2 f S 0 transition. There are several indications contrary to this hypothesis. First, no experimental evidence of the S 2 (A g ) state was obtained in the energy region 27000-20000 cm -1 , below the allowed S 0 f S 3 transition, though extensive and careful investigations were carried out on glassy samples of TTBP at 77 K and with different concentrations. 3 Second, highly accurate CASPT2 calculations have been reported on vertical S 0 f S n , transition energies, locating the S 0 f S 1 and S 0 f S 2 values of pentalene at 1.69 and 2.91 eV, respectively. 5 The red shift of the transition energies following the butyl substitution has been estimated semiempirically to be 0.3 eV for the former and 0.47 eV for the latter. 5 The final values, 1.39 eV (≈11200 cm -l ) and 2.44 eV (≈19700 cm -1 ), are in good agreement with the broad maxima of the visible absorption spectrum at 15 K, 12100, and 17300 cm -l , shown in Figure 1. The relative intensities of the two bands are also consistent with calculated oscillator strengths. 5 Third, the room temperature S 0 f S 3 absorption band has non vanishing molar extinction at 380 nm, as seen in Figure 3 of ref 3. These results are a strong evidence that the S 2 state should correspond to the 17300 cm -1 band (≈578 nm) of Figure 1 and that the weak fluorescence observed with λ exc ) 380 nm at room temperature is due to the S 3 state. Finally, this interpretation is consistent with preliminary results 6 from transient femtosec...

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