The theoretical properties of antiaromatic systems and the experimental studies on two stable antiaromatic derivatives of pentalene and s-indacene are reviewed. Taking as prototype system cyclobu- tadiene, two points of particular interest are (i) the multiplicity (singlet or triplet) of the lowest state in square geometry and (ii) the distortion from square to a structure of lower symmetry. Theoretical results on pentalene indicate that this system distorts from D_2h to the bond alternating C_2h symmetry, in close similarity with the square —> rectangular distortion of cyclobutadiene.

The vibrational spectra of 1.3.5-tri-t-butyl pentalene (TTBP) and 1.3.5.7-tetra-t-butyl-s-indacene (TTBI) are discussed in terms of normal modes mostly localized on the antiaromatic ring or on the substituents. The experimental results are in excellent agreement with ab initio density functional calculations on the two derivatives.

The lowest excited states of pentalene and s-indacene show decreased bond alternation, according to semiempirical MO calculations. In addition, a large energy gap is found between S₁ and S₂. The second factor favors the observation of the S₂ —> S₀ fluorescence. Picosecond transient absorption experiments on TTBI suggest that direct S₂→ S₀ internal conversion is responsible for non radiative decay processes from S₂. The large geometry change between S₀ and the lowest allowed states of TTBI has been probed by resonance Raman and absorption techniques at low temperature.