title: Experimental and theoretical study on the structure and electronic spectra of imiquimod and its synthetic intermediates.
abstract: Crystal structure of the imiquimod has been determined by single crystal X-ray analysis, imiquimod crystallizes in orthorhombic space group P2(1)2(1)2(1) and the molecules are linked along the c axis by the strong N-H ... N hydrogen bonds. A density functional theory (DFT) study on the electronic properties of imiquimod and its synthetic intermediates has been performed at B3LYP/6-31G* level, while taking solvent effects into account. Both the single configuration interaction (CIS) method and the time-dependent DFT (TDDFT) approaches have been used to calculate the electronic absorption spectra, and there is a good agreement between the calculated and experimental UV-visible absorption spectra. The fluorescence emission spectra of these three compounds in solution have also been measured, the relatively low fluorescence intensity is attributed to a chlorine-modulated heavy atom effect that enhances intersystem crossing between excited singlet and triplet states, and the relatively high fluorescence intensity of imiquimod results from an extended pi-conjugated system which enhances S(1)-->S(0) radiant transition.
authors.affiliation: College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing, PR China. zhaobo@njnu.edu.cn
authors.forename: Bo
authors.initials: B
authors.lastname: Zhao
authors.forename: Yu-Zhi
authors.initials: YZ
authors.lastname: Rong
authors.forename: Xiao-Hua
authors.initials: XH
authors.lastname: Huang
authors.forename: Jing-Shan
authors.initials: JS
authors.lastname: Shen
mesh_terms: D000276:Adjuvants, Immunologic / Q000138:chemical synthesis* / Q000494:pharmacology*; D000634:Aminoquinolines / Q000138:chemical synthesis* / Q000494:pharmacology*; D003460:Crystallization; D018360:Crystallography, X-Ray; D015195:Drug Design; D004583:Electrons; D006860:Hydrogen Bonding; D000077271:Imiquimod; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D009991:Oscillometry; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014466:Ultraviolet Rays
chemical_list: D000276:Adjuvants, Immunologic; D000634:Aminoquinolines; D012997:Solvents; D000077271:Imiquimod