Synthesis of New Two-Photon Absorbing Fluorene Derivatives via Cu-Mediated Ullmann Condensations

Publisher Website
Google Scholar
Abstract
The Ullmann amination reaction was utilized to provide access to a number of fluorene analogues from common intermediates, via facile functionalization at positions 2, 7, and 9 of the fluorene ring. Through variation of amine or iodofluorene derivative, analogues bearing substitutents with varying electron-donating and electron-withdrawing ability, e.g., diphenylamino, bis-(4-methoxyphenyl)amine, nitro, and benzothiazole, were synthesized in good yield. The novel fluorene derivatives were fully characterized, including absorption and emission spectra. Didecylation at the 9-position afforded remarkably soluble derivatives. Target compounds 4, 5, and 9 are potentially useful as fluorophores in two-photon fluorescence microscopy. Their UV−vis spectra display desirable absorption in the range of interest suitable for two-photon excitation by near-IR femtosecond lasers. Preliminary measurements of two-photon absorption indicate the derivatives exhibit high two-photon absorptivity, affirming their potential as two-photon fluorophores. For example, using a 1210 nm femtosecond pump beam, diphenylaminobenzothiazolylfluorene 4 exhibited nondegenerate two-photon absorption, with two-photon absorptivity (δ) of ca. 820 × 10-50 cm4 s photon-1 molecule-1 at the femtosecond white light continuum probe wavelength of 615 nm.