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Para‐quinodimethane (pQDM) is a fundamental structural component in many π‐conjugated organic molecules and materials. The incorporation of phosphorus atom into π‐conjugated frameworks offers unique opportunities for controlling the properties of derived species. A phosphorus analogue of p‐quinodimethane (pQDM), (IPrC)2P4 [5, IPr=C{N(Ar)CH2}2; Ar=2,6‐iPr2C6H3] featuring a planar P4 ring, was readily accessible by KC8‐reduction of (IPrC)(PCl2)2 (2). Base‐mediated C−H functionalization of IPrCH2 (1) with PCl3 afforded 2. The formation of 5 was expected to occur through a dimerization of the transient 3H‐diphosphirene (IPrC)P2 (4), which was theoretically suggested to have an intermediate diradical character. Compound 5 underwent photo‐induced ring‐contraction reaction to form the singlet diradicaloid (IPrCP)2VI and white phosphorus (P4). The formation of and VI and P4 suggested the formal diphosphorus (P2) elimination from 5. Indeed, photolysis of a mixture of 1,3‐cyclohexadiene (CHD) and 5 led to the formation of P2‐entrapped product (CHD)2P2 (6). The compound 5 represents the first organophosphorus species that functions as a P2 source. Metal‐free P2 transfer: A phosphorus analogue of p‐quinodimethane [(IPrC)2P4] (5, IPr=1,3‐Bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene),was readily accessible through KC8‐reduction of (IPrC)(PCl2)2 (2). The P4 ring of 5 is perfectly planar and embedded with trans‐apical N‐heterocyclic vinylidene ligands. Compound 5 served as a metal‐free diphosphorus (P2) source, which was captured with 1,3‐cyclohexadiene (CHD), giving P2(CHD)2 (6). [ABSTRACT FROM AUTHOR]

To create a design guideline for efficient third-order nonlinear optical (NLO) molecules, the chain-length ( n) dependences of the diradical character y and the longitudinal second hyperpolarizability γ of quinoidal oligothiophenes (QTs), from monomers to octamers, involving thiophene- S, S-dioxide rings are investigated by using the density functional theory method. It turns out that the diradical character of the modified QTs is reduced as compared to those of the pristine QTs. By introducing an appropriate number of oxidized rings into the QT framework, intermediate y values can be achieved even in the systems with large values of n, in which the pristine QTs are predicted to have pure diradical character. Such intermediate diradical oligomers are shown to exhibit enhanced γ values as compared to the pristine QTs with the same value for n. From the calculation results, the introduction of the optimal number of thiophene-S,S-dioxide rings is predicted to be an efficient chemical modification for optimizing the third-order NLO properties of open-shell QTs through tuning the diradical characters. [ABSTRACT FROM AUTHOR]

To reveal new structure-property relationships in the nonlinear optical (NLO) properties of fullerenes that are associated with their open-shell character, we investigated the interplay between the diradical character ( yi) and second hyperpolarizability (longitudinal component, γzzzz) in several fullerenes, including C20, C26, C30, C36, C40, C42, C48, C60, and C70, by using the broken-symmetry density functional theory (DFT; LC-UBLYP ( μ=0.33)/6-31G*//UB3LYP/6-31G*). We found that the large differences between the geometry and topology of fullerenes have a significant effect on the diradical character of each fullerene. On the basis of their different diradical character, these fullerenes were categorized into three groups, that is, closed-shell ( yi=0), intermediate open-shell (0< yi<1), and almost pure open-shell compounds ( yi≅1), which originated from their diverse topological features, as explained by odd-electron-density and spin-density diagrams. For example, we found that closed-shell fullerenes include C20, C60, and C70, whereas fullerenes C26 and C36 and C30, C40, C42, and C48 are pure and intermediate open-shell compounds, respectively. Interestingly, the γzzzz enhancement ratios between C30/ C36 and C40/ C60 are 4.42 and 11.75, respectively, regardless of the smaller π-conjugation size in C30 and C40 than in C36 and C60. Larger γzzzz values were obtained for other fullerenes that had intermediate diradical character, in accordance with our previous valence configuration interaction (VCI) results for the two-site diradical model. The γzzzz density analysis shows that the large positive contributions originate from the large γzzzz density distributions on the right- and left-extended edges of the fullerenes, between which significant spin polarizations (related to their intermediate diradical character) appear within the spin-unrestricted DFT level of theory. [ABSTRACT FROM AUTHOR]