RESUMO
The intrinsic relationship between helicenes and circulenes is of fundamental interest and importance in molecular engineering. Herein, electrophilic borylation of phenanthroline-derived aza[5]helicenes is presented, resulting in the incorporation of a boryl unit into two termini of helicenes to afford quasi-[7]circulenes. Their bowl-shaped structures were determined by X-ray diffraction. UV-vis absorption and fluorescence spectroscopy, as well as electrochemical measurements and DFT calculations, gave insight into their electronic properties. Variable-temperature NMR studies and DFT calculations revealed bowl-to-bowl inversion at room temperature and bowl-to-helix equilibria at elevated temperature, highlighting the important role of B â N bond strength in tuning their dynamic properties.
RESUMO
The present study details the experimental and theoretical characterization of the photophysical properties of 14 examples of 2-(phenylamino)-1,10-phenanthrolines (1). The absorption spectra of 1 are substituent-dependent but in a general manner present absorption bands at wavelengths of ~230; ~300; ~335 and a shoulder at ~380 nm. Electron-donating groups (EDG) and electron-withdrawing groups (EWG), respectively, result in bathochromic and hypsochromic shifts. Compounds 1 are highly luminescent, in contrast to phenanthroline, and emit in the region between 350 and 500 nm with substituent-dependent λmax emission. The emission spectra show a redshift for EDG (4-OMe 62 nm; 4-Me 19 nm) and a blueshift for EWG (4-CN 41 nm; 4-CF3 38 nm) relative to the emission of the unsubstituted parent compound 1a. Plotting the λ max EM against Hammett σ+ constants gave an excellent linear correlation demonstrating the electron-deficient nature of the excited state and how the substituents (de)stabilize S1 . Theoretical calculations revealed a HOMO-LUMO π-π* electronic transition to S1 which in combination with difference (S1 -S0 ) in electron density maps revealed charge-transfer character. Strongly electron-withdrawing substituents switch off the charge transfer to give rise to a local excitation.
