Molecular photoswitching, light induced reversible color/fluorescence modulation, has mostly been realized in organic molecules via E/Z isomerization of azobenzenes and stilbenes and ring opening/closing reactions of spiropyrans and diarylethenes. We report here new fluorescent molecular photoswitches based on triphenylamine (TPA)–imidazole derivatives, N-phenyl-N-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)benzenamine (NTPB) and N-phenyl-N-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)benzenamine (NPPB), that exhibited light induced reversible fluorescence switching via conformational change from a twisted molecular structure to more planar. NTPB and NPPB in CHCl₃ showed red shift of absorption and fluorescence upon UV light irradiation whereas white light exposure reversed both absorption as well as fluorescence. The role of the TPA–imidazole twisted molecular structure in photoswitching was established based on structure property, computational and photophysical studies. The isobestic point observed in time dependent fluorescence change under UV light irradiation clearly demonstrated the presence of two different conformational isomers. Interestingly, polymorphism and torsion angle (τ) dependent fluorescence of NTPB and NPPB in the solid state also supported the role of the twisted molecular structure of TPA–imidazole in fluorescence switching/tuning. Interestingly, NTPB showed fluorescence photoswitching in the solid state also whereas rigid phenanthrene based NPPB did not show fluorescence photoswitching. Thus the present studies provide structural insight for designing a new type of fluorescent organic molecular photoswitches based on conformational modulation that could be of potential interest in optoelectronic devices.

中文翻译：

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咪唑衍生物的异常荧光光开关：分子构象和扭曲角控制的有机固态荧光的作用†

分子光开关，是光诱导的可逆颜色/荧光调节，主要通过偶氮苯和对苯乙烯的E / Z异构化以及螺吡喃和二芳烃的开环/闭环反应在有机分子中实现。我们在此报告基于三苯胺（TPA）-咪唑衍生物，N-苯基-N-（4-（1,4,5-三苯基-1 H-咪唑-2-基）苯基）苯胺（NTPB）的新型荧光分子光电开关和N-苯基-N-（4-（1-苯基-1 H-菲并[9,10 - d ]咪唑-2-基）苯基）苯胺（NPPB），通过光表现出可逆的荧光转换从扭曲的分子结构到更平坦的构象变化。CHCl _3中的NTPB和NPPB在紫外光照射下显示出吸收和荧光的红移，而暴露在白光下则使吸收和荧光都发生了逆转。基于结构特性，计算和光物理研究，确定了TPA-咪唑扭曲的分子结构在光开关中的作用。在紫外线照射下随时间变化的荧光变化中观察到的等渗点清楚地表明存在两种不同的构象异构体。有趣的是，多态性和扭转角（τ）固态的NTPB和NPPB依赖性荧光也支持TPA-咪唑扭曲的分子结构在荧光转换/调谐中的作用。有趣的是，NTPB在固态时也显示荧光光开关，而基于刚性菲的NPPB却不显示荧光光开关。因此，本研究为基于构象调制的新型荧光有机分子光电开关的设计提供了结构上的见解，这在光电器件中可能具有潜在的意义。