Systematic studies on photoluminescence of oligo(arylene-ethynylene)s: Tunability of excited states and derivatization as luminescent labeling probes for proteins

Abstract

Functionalized oligo(phenylene-ethynylene)s (OPEs) with different conjugation lengths, p-X(C 6H 4C≡C) nSiMe 3 (n = 1-4; X = NH 2, NMe 2, H) were synthesized by Sonogashira coupling of (phenylene-ethynylene)s and 1-iodo-4-(trimethylsilylethynyl)benzene, followed by desilylation of the p-substituted (trimethylsilylethynyl)benzenes with potassium hydroxide. The photoluminescent properties for the OPE series with different chain lengths and their solvatochromic responses were examined. The absorption maxima were red-shifted with increasing numbers of -(C 6H 4C≡C)- units (n), and a linear plot of the absorption energy maxima vs. 1/n was obtained for each series. The emission spectra in dichloromethane showed a broad and structureless band, the energies of which (in wavenumbers) also fit linearly with 1/n. Both the absorption and emission wavelength maxima of the NH 2- and NMe 2-substituted OPEs exhibited significant solvent dependence, whereas the parent OPEs (X = H) showed only minor shifts of the λ max values in different solvents. Substituent effects upon the photoluminescent characteristics of the OPEs and the tunability of the excited states were examined with the p-X(C 6H 4C≡C) nSiMe 3 (n = 2, 3; X = NH 2, NMe 2, H, SMe, OMe, OH, and F) series. The H- and F-substituted counterparts exhibited high-energy vibronically structured emissions attributed to the 3(ππ*) excited states of the (arylene-ethynylene) backbone. For compounds bearing NH 2 and NMe 2 groups, a broad red-shifted emission with a remarkable Stokes shift from the respective absorption maximum was observed, which can be assigned to an n → π* transition. The n → π* assignment was supported by MO calculations on the model compounds p-X(C 6H 4C≡C) 2SiH 3 (X = NH 2, H). Functionalization of the oligo(arylene-ethynylene)s with the N-hydroxysuccinimidyl (NHS) moiety enabled covalent attachment of the fluorophore to HSA protein molecules. A series of fluorescent labels, namely p-X(C 6H 4C≡C) n-C 6H 4NHS, (n = 1, X = NH 2, NMe 2, SMe, OMe, OH, F; n = 2, X = NH 2, NMe 2) and p-Me 2NC 6H 4C≡C(C 4H 2S)-C≡CC 6H 4NHS were synthesized, and their conjugates with HSA (human serum albumin) were characterized by MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, and gel electrophoresis. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.