Doubly linked chiral phenanthrene oligomers for homogeneously π-extended helicenes with large effective conjugation length

Abstract

Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices working on the nanometer scale. Herein, we report the synthesis of per-peri-perbenzo[5]- and [9]helicenes in addition to previously reported π-extended [7]helicene. The homogeneously π-extended helicenes can be regarded as helically fused oligo-phenanthrenes. The HOMO−LUMO gap decreased significantly from 2.14 to 1.15 eV with increasing helical length, suggesting the large effective conjugation length (ECL) of the π-extended helical framework. The large ECL of π-extended helicenes is attributed to the large orbital interactions between the phenanthrene subunits at the 9- and 10-positions, which form a polyene-like electronic structure. Based on the experimental results and DFT calculations, the ultrafast decay dynamics on the sub-picosecond timescale were attributed to the low-lying conical intersection.

Expanded [2,1][n]Carbohelicenes with 15- and 17-Benzene Rings

“…According to Porsev’s nomenclature with our slight modification of variables, concentrically expanded and extended carbohelicene and its analogues can be expressed as [ k , m ]­[ n ]­helicene, where k , m , and n represent the shaft radium, the width of the spiral ribbon, and the number of benzene rings, respectively (Figure a). There are several developments and mainstreams in helicene chemistry as shown in Figure a: (1) elongating the helicene in the helical direction with increasing the n number in classical all- ortho -fused [1,1]­[ n ]­helicenes (or simply [ n ]­helicene), (2) incorporating multiple helicene moieties to PAH for the synthesis of double, triple, quadruple, quintuple, and sextuple helicenes and those macrocycles, (3) embedding heteroatoms such as nitrogen and boron into helicene or substitution of benzene rings with nonhexagonal (hetero) aromatics such as furan, pyrrole, thiophene, and azulene, (4) extending the π-system in a peripheral direction (increasing m ) for the synthesis of laterally extended helicenes − as represented by hexa- cata -hexabenzo­[7]­helicene and peri -perbenzo­[9]­helicene, (5) increasing the size of helicene pore/diameter (increasing k ) by the alternation of linear and angular ring fusion of hexagonal/nonhexagonal (hetero)­aromatics, so-called expanded helicenes. − Among these, the development of expanded carbohelicenes has lagged due to the synthetic difficulty. They are expected to exhibit molecular encapsulation ability by taking advantage of their vacancies and novel electrical, optical, and chiroptical properties.…”

Chiral Open-[60]Fullerene Ligands with Giant Dissymmetry Factors

“…Nevertheless, the racemization often happens due to their configurational flexibility . Moreover, most of the helical molecules exhibit chiroptical activity only in the ultraviolet (UV) region . Thus, the design of rigid chiral molecules possessing broadband chiroptical activity is highly demanded.…”

Synthesis of a π‐Extended Double [9]Helicene

“…As observed for our previously reported pyrene‐fused double [7]helicene, [12] such redshifted emission might be attributed to the formation of intramolecular excimer [25] and/or intramolecular through‐space conjugation [26] . Notably, the absolute Φ of D9H was determined to be 18 %, which is significantly higher than that of H7H9 (6.3 %) and rare among NIR‐emissive carbohelicenes [10d, 27] . The increased Φ of D9H could be attributed to the faster radiative and slower nonradiative processes (Table S4).…”