Systematic Studies on Photoluminescence of Oligo(arylene-ethynylene)s: Tunability of Excited States and Derivatization as Luminescent Labeling Probes for Proteins

• Published in: European Journal of Organic Chemistry 2006-07, Vol.2006 (14), p.3125-3139
• Main Authors: Zhi, Yong-Gang, Lai, Siu-Wai, Chan, Queenie K.-W., Law, Yuen-Chi, Tong, Glenna S.-M., Che, Chi-Ming
• Format: Article
• Language: English
• Subjects: Fluorescence; Oligo(phenylene-ethynylene); Protein-labeling; Solvatochromism; π-Conjugation
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.200600103

Functionalized oligo(phenylene‐ethynylene)s (OPEs) with different conjugation lengths, p‐X(C6H4C≡C)nSiMe3 (n = 1–4; X = NH2, NMe2, 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 –(C6H4C≡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 NH2‐ and NMe2‐substituted OPEs exhibited significant solvent dependence, whereas the parent OPEs (X = H) showed only minor shifts of the λmax 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(C6H4C≡C)nSiMe3 (n = 2, 3; X = NH2, NMe2, 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 NH2 and NMe2 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(C6H4C≡C)2SiH3 (X = NH2, 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(C6H4C≡C)nC6H4NHS, (n = 1, X = NH2, NMe2, SMe, OMe, OH, F; n = 2, X = NH2, NMe2) and p‐Me2NC6H4C≡C(C4H2S)C≡CC6H4NHS 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, 69451 Weinheim, Germany, 2006)