The first intramolecular charge transfer transition based on 2-ureido-4[1H]-pyrimidinone binding module was reported. Keywordsintramolecular charge transfer, 2-ureido-4[ lH]-pyrimidinone binding module, stockes shift, w30).Due to the strength, directionality, and specificity, mays of multiple hydrogen bonds are useful buildin blocks for the reliable assembly of complex structures. Since 1998, self-complementary quadruply hydrogen-bonded homodimers have received increasing attention. Particularly the 2-ureido-4[ IH]-pyrimidinone AADD binding module developed by Meijer etc. has shown extensive applications in assembling supramolecular oligomers and poly~ners.~ However, there are relatively few examples4 given on their spectroscopic properties, especially on their native photophysical characteristics. Here we reported for the first time that the intramolecular charge transfer state was involved in this attractive 2-ureido-4[ lH]-pyrimidmone binding module, which significantly depended on the substitutents of pyrimidinone. Unlike pyrimidinone 2 with the NJV-dimethylarninophenyl connected via a methylene at 5-position (Chart l), compound 1 with 6-substituted NJV-dimethy laminophenyl group exhibited a red-shift of the lowest absorption band and broad as well as featureless fluorescence in dilute solution at room temperature. Steady-state and time-resolved fluorescence measurements in various polar solvents revealed that the lowest excited state of compound 1 was derived from the intramolecular charge transfer (ICT) state.Compound 1 was synthesized by treatment of ethyl acetate in THF with 2 equiv. of LDA in -78 'C for 5 min before addition of acyl chloride, as shown in Scheme 1. The reaction mixture was stirred for 15 min to give the Pketo esters, which reacted with gunidinium carbonate i n refluxing absolute ethanol and then with butylisocyanate in refluxing pyridine to afford compound Is in good yield. Compound 2 was prepared by condensation of 4-(dimethy1amino)benzaldehyde and ethyl acetoacetate. After hydrogenation, the intermedi- ate reacted with gunidinium carbonate and butylisocyanate respectively, following the same procedures as described in the syntheses of compound 1. 'H NMR spectra revealed that compound 1 and 2 existed as assemblies 1.1 and 2.2 in'CDCl3. The large downfield shift for N-H protons provided direct evidence for the involvement of strong hydrogen bonding. Their AADD hydrogen-bonding motif was determined by 2D-NOESY spectra. No other binding modes were observed. Dilution of the solutions of the compounds in CDCl3 to 1 X m~l * d r n -~ did not lead to observable dissociation, giving a lowest estimation of binding constant of 1 X lo7 dm3*m01, which was in good agreement with the value for a similar ~ompound.~The absorption spectra of compound 1 and 2 were taken in dichloromethane and presented in Figure la. Both compounds exhibited intense vibronic-structured absorption bands at A < 300 nm with extinction coefficients on the order of lo4 dm3*rnol-'=cm-'. Moderately intense low-energy absorption ba...