11278 Chem. Commun., 2013, 49, 11278--11280 This journal is c The Royal Society of Chemistry 2013 Cite this: Chem. Commun., 2013, 49, 11278 Photophysical studies and submicron ring formation of morpholino U-nucleoside monomers† Sibasish Paul, a Santanu Jana, b Jhuma Bhadra a and Surajit Sinha* a Synthesis, photophysical properties and submicron ring formation of functionalized uracil morpholino monomers have been reported. A series of characterization techniques indicated that the rings are formed by the inter-molecular hydrogen bonding of the uracil nucleus having a trityl-protected morpholino moiety. This is the smallest nucleoside unit known to date for submicron size ring formation. During the past decades, organic materials have attracted wide- spread attention due to their exceptional optical and electronic properties, though most of the studies are focused on macro- molecules. 1–3 There has been notable interest in nucleosides where the base unit has been modified (especially uracil nucleo- sides) to study the fluorescence properties. 4,5 Interests are also growing for the use of nucleobase pairs from the field of materials research and nanosciences. 6–9 Topologically important structures like Borromean rings, catenanes, and other shapes or patterns have already been built either by chemical synthesis 10,11 or using DNA. 12–14 Formation of DNA minicircles was reported by several groups 15–18 including the controlled synthesis of minicircles through the introduction of a customized sequence into the template. 18 Formation of protein nanorings with diameters ranging from 8 to 20 nm has been described by the Wagner group. 19 Small organic molecules containing both hydrogen bonding and hydrophobic parts are able to self-assemble to form objects of different morphology. 20–22 Uracil functionalized single-walled carbon nanotubes were also known to self-assemble with a diameter of 50–70 nm to form regular nanorings. 23 DNA-mediated self-assembly of single-walled carbon nanotubes into nanorings using DNA hybridization has also been studied. 24 Morpholino nucleoside-containing oligomers are now gaining interest due to their biological activity, which were initially introduced by Summerton and Weller as steric-blocking antisense reagents. 25 More recently, these have also found growing applications in nanotechnology 26 and surface hybridization. 27,28 Though the photo- physical properties of nucleoside monomers are known in the literature, 5 the same is not reported in the case of morpholino monomers. As a part of our project toward the synthesis of morpholino monomers 29,30 and morpholino oligomers, 31 herein we report the synthesis, photophysical studies and self-assembly into submicron rings of functionalized uracil morpholino monomers which, to the best of our knowledge, is the first report . In addition, we reveal some insight into the mechanism of the formation of rings. It is also important to mention that submicron ring formation using such a small molecular scaffold has hardly been described before. Nucleoside monomers 3, 4, 5 and 6 were synthesized from 1 through Sonogashira coupling with appropriate aryl alkynes according to our reported procedure in good to excellent yields (Scheme 1). 29 The UV-Vis absorption spectra of compounds 3 to 6 were measured in chloroform (Fig. 1a). The absorption band of all the compounds falls in between 300 and 400 nm, which belongs to the gap between the HOMO and LUMO of all compounds. In the case of compound 4 the HOMO–LUMO gap decreases due to the presence of an electron accepting group whereas compounds having an electron donating group show a higher band gap. The photoluminescence (PL) spectra are shown in Fig. 1b. Compounds 3, 5 and 6 show very bright blue luminescence (digital pictures are shown in the inset of Fig. 1b, the excitation wavelength is 365 nm), Scheme 1 Synthesis of 5-substituted uracil morpholino monomers. a Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. E-mail: ocss5@iacs.res.in; Fax: +91 33 2473 2805; Tel: +91 33 2473 4971 b Department of Material Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc45082a Received 6th July 2013, Accepted 10th October 2013 DOI: 10.1039/c3cc45082a www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 11 October 2013. Downloaded by Indian Association for the Cultivation of Science on 13/06/2015 12:00:59. View Article Online View Journal | View Issue