Wenska Grazyna, Koput Jacek, Pedzinski Tomasz, Marciniak Bronislaw, Karolczak Jerzy, Golankiewicz Bozenna
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
J Phys Chem A. 2006 Sep 28;110(38):11025-33. doi: 10.1021/jp062680j.
The spectral and photophysical behavior of four fluorescent 9-oxo-imidazo[1,2-a]purine derivatives containing pyridyl, pyridylphenyl, phenyl, and biphenylyl substituents at the C(6) position of the tricyclic skeleton is described. The studies were performed in several aprotic and protic organic solvents using absorption spectroscopy as well as steady-state and time-resolved fluorescence spectroscopy. The results are also presented of TDDFT calculations on singlet-singlet excitation energies and oscillator strengths for two models of 9-oxo-imidazo[1,2-a]purine, with phenyl or pyridyl substituents, both in the gas phase and in methanol solution. While the derivatives with aryl substituents did not show any significant dependence of their static and dynamic fluorescence properties on the nature of the solvent, the compounds containing a pyridine residue exhibited a remarkable reduction of their fluorescence quantum yields and lifetimes in the alcoholic solutions. The solute-solvent hydrogen-bonding interaction in the first excited singlet state is responsible for the fast radiationless decay rates determined for pyridyl- and pyridylphenyl-substituted compounds in protic solvents. The results of experimental and theoretical studies show that the hydrogen of the alcohols' hydroxyl group and the nitrogen atom of the pyridine moiety are involved in the interaction. The fluorescence-quenching experiments performed for the pyridyl-substituted 9-oxo-imidazo[1,2-a]purine derivative using trifluoroethanol, methanol, and butanol as quenchers revealed that the quenching efficiencies, expressed by the Stern-Volmer quenching constants, correlate with the H-bond donating abilities of the alcohols. The quenching is a dynamic process, and the H-bonded complex formed is nonfluorescent. The experimentally determined and the calculated values of the dipole moment change associated with the electronic excitation indicate that the excited S(1) states of all of the molecules studied in this work have an intramolecular charge-transfer character and that electronic charge is transferred to the C(6) substituent upon excitation. Thus, the ability of the pyridyl substituent nitrogen atom to act as an H-bond acceptor in the excited S(1) state is enhanced. The 6-pyridyl-9-oxo-imidazo[1,2-a]purine presents a novel fluorophore, which, besides its medical applications, may be useful as a sensor of hydroxyl groups in microorganized systems.
描述了三种含吡啶基、吡啶基苯基、苯基和联苯基取代基的荧光9-氧代咪唑并[1,2-a]嘌呤衍生物在三环骨架C(6)位置的光谱和光物理行为。研究在几种非质子和质子有机溶剂中进行，采用吸收光谱以及稳态和时间分辨荧光光谱。还给出了对两种9-氧代咪唑并[1,2-a]嘌呤模型（含苯基或吡啶基取代基）在气相和甲醇溶液中的单重态-单重态激发能和振子强度的TDDFT计算结果。虽然含芳基取代基的衍生物的静态和动态荧光性质对溶剂性质没有任何显著依赖性，但含吡啶残基的化合物在醇溶液中其荧光量子产率和寿命显著降低。在第一激发单重态中的溶质-溶剂氢键相互作用是质子溶剂中吡啶基和吡啶基苯基取代化合物快速无辐射衰减率的原因。实验和理论研究结果表明，醇羟基的氢和吡啶部分的氮原子参与了相互作用。使用三氟乙醇、甲醇和丁醇作为猝灭剂对吡啶基取代的9-氧代咪唑并[1,2-a]嘌呤衍生物进行的荧光猝灭实验表明，由斯特恩-沃尔默猝灭常数表示的猝灭效率与醇的氢键供体能力相关。猝灭是一个动态过程，形成的氢键复合物是非荧光的。与电子激发相关的实验测定和计算的偶极矩变化值表明，本工作中研究的所有分子的激发S(1)态具有分子内电荷转移特征，并且激发时电子电荷转移到C(6)取代基上。因此，吡啶基取代基氮原子在激发S(1)态中作为氢键受体的能力增强。6-吡啶基-9-氧代咪唑并[1,2-a]嘌呤呈现出一种新型荧光团，除了其医学应用外，还可作为微生物系统中羟基的传感器。
