Singlet Diradical Character from Experiment

Understanding the electronic structure of the singlet diradical state tackles the fundamentals of the chemical bond itself. The singlet diradical character is a key factor, which determines the chemical reactivity, the reaction products, as well as the chemical/physical (electronic, optical and magnetic) properties, although this quantity is defined in a purely theoretical manner. In this Letter, on the basis of the valence configuration interaction scheme, we present an explicit connection between the singlet diradical character and measured quantities obtained from one- and two-photon absorption spectra as well as from phosphorescence and electron spin resonance peaks. This enables us to estimate the singlet diradical character experimentally

Giant Enhancement of the Second Hyperpolarizabilities of Open-Shell Singlet Polyaromatic Diphenalenyl Diradicaloids by an External Electric Field and Donor–Acceptor Substitution

Switching on an external electric field (<i>F</i>) along the electron correlation direction produces a giant enhancement of the second hyperpolarizability γ in a polyaromatic diradicaloid having intermediate diradical character. This has been evidenced by carrying out spin-unrestricted density functional theory calculations with the LC-UBLYP long-range corrected exchange-correlation functional for the <i>s</i>-indaceno[1,2,3-<i>cd</i>;5,6,7-<i>c</i>′<i>d</i>′]diphenalene (IDPL) diradical compound in comparison to a closed-shell analogue of similar size composed of two pyrene moieties (PY2). For IDPL, the field-induced enhancement ratio is estimated to reach 4 orders of magnitude for an electric field of 0.0077 a.u., whereas it is less than a factor of 2 for PY2. Moreover, an enhancement is also observed by substituting both-end phenalenyl rings of IDPL with donor (NH₂)/acceptor (NO₂) groups, but this enhancement is limited to about 2 orders of magnitude. These enhancements are associated with a reduction of the diradical character (and therefore an improved thermal stability) as well as with the appearance of substantial type-I contributions to γ

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 (<b>8</b>) 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 (<b>12</b>), 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 (<b>3b</b>). 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 <b>8</b> has singlet diradical nature. Ab initio molecular orbital calculations of the ground state also suggested that <b>8</b> has an intermediate diradical character (<i>y</i>) of 0.685. Interconversion between diradical tetracation dimer <b>8</b> and closed-shell dication dimer <b>12</b> was achieved by oxidation/reduction in good yields and was accompanied by formation of monoradical trication dimer <b>13</b> as an intermediate. TPA measurements at near-infrared wavelengths revealed that diradical dimer <b>8</b> has large TPA cross sections (3600 GM at 1200 nm), while closed-shell <b>12</b> 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 <i>y</i> region (0 < <i>y</i> < 1)

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 (<b>8</b>) 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 (<b>12</b>), 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 (<b>3b</b>). 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 <b>8</b> has singlet diradical nature. Ab initio molecular orbital calculations of the ground state also suggested that <b>8</b> has an intermediate diradical character (<i>y</i>) of 0.685. Interconversion between diradical tetracation dimer <b>8</b> and closed-shell dication dimer <b>12</b> was achieved by oxidation/reduction in good yields and was accompanied by formation of monoradical trication dimer <b>13</b> as an intermediate. TPA measurements at near-infrared wavelengths revealed that diradical dimer <b>8</b> has large TPA cross sections (3600 GM at 1200 nm), while closed-shell <b>12</b> 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 <i>y</i> region (0 < <i>y</i> < 1)