Combined Experimental and Theoretical Study on Circular Dichroism and Circularly Polarized Luminescence of Configurationally Robust D₃-Symmetric Triple Pentahelicene

Abstract

Pentahelicene (PH) exhibits the largest absorption ( g) and luminescence ( g) dissymmetry factors among the helicene family but is configurationally and (photo)chemically labile, encumbering its application to chiroptical materials. To bypass the pitfalls, three PH units are merged in a single molecule to build D-symmetric triple pentahelicene, hexabenzotriphenylene (HBT), which attains indeed the configurational and (photo)chemical robustness through equilibrium with a C-symmetric conformer that interrupts the racemization and photocyclization. UV-vis, circular dichroism (CD), and circularly polarized luminescence (CPL) spectral examinations reveal the significantly larger g and g values for HBT than for any of configurationally robust single [ n]helicene ( n ≥ 6) and C-symmetric triple pentahelicene, trinaphthotriphenylene (TNT). Theoretical calculations precisely reproduce the main features of the experimental CD and CPL spectra of PH, HBT, and TNT, and the relevant electric and magnetic transition moments and their mutual angles well rationalize the relative CD and CPL intensities of all the single and triple pentahelicenes.

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

“…Planar and helical chiralities, as found in helicenes, were highlighted to promote important dissymmetry factors because of the larger magnetic transition dipole moments induced by aromatic distortion. Hence, important efforts were made to rationalize this observation with the calculation of and and the determination of the θ angle (Sato et al, 2017 ; Ito et al, 2018 ; Tanaka et al, 2018b , c ). The outstanding | g lum | value of 0.152 reported for the single-wall carbon nanotubes (Dhbaibi et al, 2019 ) cyclo-2,8-chrysenylene by Isobe and Sato was attributed to the unusually important magnetic transition dipole moment originating from a cyclic aromatic structure which allows and facilitates a magnetic field-induced current loop (Sato et al, 2017 ).…”

Chiral Benzo[b]silole‐Fused 9,9′‐Spirobi[fluorene]: Synthesis, Chiroptical Properties, and Transformation to π‐Extended Polycyclic Arene

“…The angle between μ and m is 86.6° and gives the cos θ term as small as 0.06. Such orientation of μ and m would be one of the reasons for the relatively small g lum value of 1 [12] …”

“…Such orientation of μ and m would be one of the reasons for the relatively small g lum value of 1. [12] In summary, we have synthesized the benzo[b]silole-fused SBF 1 as the first example of a silole-containing spiro πconjugated compound without a spiro silicon center. Comparison with its model compound indicates that the spiroconjugation does not work efficiently.…”

Propeller‐Shaped Semi‐fused Porphyrin Trimers: Molecular‐Symmetry‐Dependent Chiroptical Response

“…Helicenes have long been regarded as one of the most promising platforms with superb chiroptical properties owing to their helically conjugated molecular orbitals ( 1 in Figure ) . However, the C 2 ‐symmetry of helicenes does not promise parallel orientation of their electric and magnetic transition moments that is favorable for large chiroptical response, and often leads to weak absorption and emission intensities due to partially forbidden electronic transitions . To overcome these drawbacks and realize improved chiroptical properties, control of the electronic transitions and molecular symmetry are of the prime importance in the design of helicene derivatives with large chiroptical response.…”

“…To overcome these drawbacks and realize improved chiroptical properties, control of the electronic transitions and molecular symmetry are of the prime importance in the design of helicene derivatives with large chiroptical response. Along this line, a wide range of peripherally functionalized helicenes and fused multi‐helicenes have been explored so far. Further, various achiral chromophores and fluorophores have been finely integrated into chiral systems with covalent and non‐covalent interactions…”