Interplay between the Diradical Character and Third‐Order Nonlinear Optical Properties in Fullerene Systems. Issue 5 (18th December 2012)

• Record Type: Journal Article
• Authors: Muhammad, Shabbir; Fukuda, Kotaro; Minami, Takuya; Kishi, Ryohei; Shigeta, Yasuteru; Nakano, Masayoshi
• Is Part Of: Chemistry. Volume 19:Issue 5(2013)
• Page Start: 1677
• Page End: 1685
• Publication Date: 2012-12-18
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201203463
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>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 (<italic>y<sub>i</sub></italic>) and second hyperpolarizability (longitudinal component, <italic>γ<sub>zzzz</sub></italic>) in several fullerenes, including <bold>C<sub>20</sub></bold>, <bold>C<sub>26</sub></bold>, <bold>C<sub>30</sub></bold>, <bold>C<sub>36</sub></bold>, <bold>C<sub>40</sub></bold>, <bold>C<sub>42</sub></bold>, <bold>C<sub>48</sub></bold>, <bold>C<sub>60</sub></bold>, and <bold>C<sub>70</sub></bold>, by using the broken‐symmetry density functional theory (DFT; LC‐UBLYP (<italic>μ</italic>=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 (<italic>y<sub>i</sub></italic>=0), intermediate open‐shell (0<<italic>y<sub>i</sub></italic><1), and almost pure open‐shell compounds (<italic>y<sub>i</sub></italic>≅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 <bold>C<sub>20</sub></bold>, <bold>C<sub>60</sub></bold>, and <bold>C<sub>70</sub></bold>, whereas fullerenes <bold>C<sub>26</sub></bold> and <bold>C<sub>36</sub></bold> and <bold>C<sub>30</sub></bold>, <bold>C<sub>40</sub></bold>, <bold>C<sub>42</sub></bold>, and <bold>C<sub>48</sub></bold> are pure and intermediate open‐shell compounds, respectively. Interestingly, the <italic>γ</italic><sub>zzzz</sub> enhancement ratios between <bold>C<sub>30</sub></bold>/<bold>C<sub>36</sub></bold> and <bold>C<sub>40</sub></bold>/<bold>C<sub>60</sub></bold> are 4.42 and 11.75, respectively, regardless of the smaller π‐conjugation size in <bold>C<sub>30</sub></bold> and <bold>C<sub>40</sub></bold> than in <bold>C<sub>36</sub></bold> and <bold>C<sub>60</sub></bold>. Larger <italic>γ<sub>zzzz</sub></italic> 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 <italic>γ<sub>zzzz</sub></italic> density analysis shows that the large positive contributions originate from the large <italic>γ<sub>zzzz</sub></italic> 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.</p> </abstract>