Impact of Diradical Character on Two-Photon Absorption: Bis(acridine) Dimers Synthesized from an Allenic Precursor

The two-photon absorption (TPA) of a bis­(acridine) dimer
(8) having singlet diradical character in its ground
state
was found to be enhanced by more than 2 orders of magnitude as compared
to its closed-shell counterpart (12), which has the same
structural backbone and atom composition. The dimer, a tetracation
species consisting of two connected acridinium cation moieties with
high coplanarity, was obtained during our attempts to synthesize triplet
carbenes by double oxidation of an allenic precursor (3b). High conjugation over the two aromatic rings connected by dimerization
was revealed by X-ray analysis, and a small HOMO–LUMO gap was
found in the visible-near-infrared one-photon absorption spectrum
in solution and in the crystalline state, exhibiting that the ground
state of 8 has singlet diradical nature. Ab initio molecular
orbital calculations of the ground state also suggested that 8 has an intermediate diradical character (y) of 0.685. Interconversion between diradical tetracation dimer 8 and closed-shell dication dimer 12 was achieved
by oxidation/reduction in good yields and was accompanied by formation
of monoradical trication dimer 13 as an intermediate.
TPA measurements at near-infrared wavelengths revealed that diradical
dimer 8 has large TPA cross sections (3600 GM at 1200
nm), while closed-shell 12 has TPA cross sections of
<21 GM. This result represents a straightforward comparison between
the TPA activity of molecules with the same structural backbone and
atom composition but with different degree of the diradical character,
supporting the theoretical prediction that enhanced TPA intensity
can be observed in the intermediate y region (0 < y < 1).