We studied the luminescence properties of indole in poly (vinyl alcohol) (PVA) film. The indole molecules are effectively immobilized in this polymer film and display both fluorescence and phosphorescence emission at room temperature. We noticed that the phosphorescence of indole in PVA film can be effectively excited at a longer wavelength than its typical singlet to triplet population route involving intersystem crossing. The maximum of the phosphorescence excitation is about 410 nm which corresponds to the energy of indole's triplet state. Interestingly, the phosphorescence anisotropy excited with the longer wavelength (405 nm) is positive and reaches a value of about 0.25 in contrast to the phosphorescence anisotropy excited within the indole singlet absorption spectrum (290 nm), which is negative. Very different temperature dependences have been observed for fluorescence and phosphorescence of indole in PVA film. While fluorescence depends minimally, the phosphorescence decreases with temperature dramatically. The fluorescence lifetime was measured to be a single component 4.78 ns while the intensity weighted average phosphorescence lifetime with 290 nm and 405 nm excitations were 6.57 and 5.62 ms, respectively. We believe that the possibility of the excitation of indole phosphorescence in the blue region of visible light and its high anisotropy opens a new avenue for future protein studies.

中文翻译：

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关于吲哚直接三重态激发的可能性。

我们研究了聚乙烯醇（PVA）膜中吲哚的发光特性。吲哚分子有效地固定在该聚合物膜中，并在室温下显示荧光和磷光发射。我们注意到，PVA膜中吲哚的磷光可以比其涉及系统间穿越的典型单线态到三线态总体路线更长的波长有效地激发。磷光激发的最大值约为410nm，其对应于吲哚的三重态的能量。有趣的是，与在吲哚单重态吸收光谱（290nm）内激发的磷光各向异性（负）相反，在更长的波长（405nm）下激发的磷光各向异性为正并达到约0.25的值。对于PVA膜中的吲哚的荧光和磷光，已经观察到非常不同的温度依赖性。虽然荧光的依赖性最小，但是磷光随温度急剧降低。测得的荧光寿命为单一组分4.78 ns，而290 nm和405 nm激发的强度加权平均磷光寿命分别为6.57 ms和5.62 ms。我们认为，在可见光的蓝色区域激发吲哚磷光及其高各向异性的可能性为将来的蛋白质研究开辟了一条新途径。78 ns，而290 nm和405 nm激发的强度加权平均磷光寿命分别为6.57 ms和5.62 ms。我们认为，在可见光的蓝色区域激发吲哚磷光及其高各向异性的可能性为将来的蛋白质研究开辟了一条新途径。78 ns，而290 nm和405 nm激发的强度加权平均磷光寿命分别为6.57 ms和5.62 ms。我们认为，在可见光的蓝色区域激发吲哚磷光及其高各向异性的可能性为将来的蛋白质研究开辟了一条新途径。