Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhopal, 462 066, Madhya Pradesh, India.
Chemphyschem. 2019 Dec 3;20(23):3221-3227. doi: 10.1002/cphc.201900845. Epub 2019 Nov 5.
In this work, we propose a new analysis of the time resolved emission spectra of a photo-acid, HA, pyranine (8-hydroxypyrene-1,3,6-trisulphonic acid, HPTS) based on time resolved area normalized emission spectra (TRANES). Presence of an isoemissive point in TRANES confirms the presence of two emissive species (HA and A ) inside the system in bulk water and inside a co-polymer hydrogel [F127, (PEO) -(PPO) -(PEO) ]. We show that following electronic excitation, the local pH around HPTS, is much lower than the bulk pH presumably because of ejection of proton from the photo-acid in the excited state. With increase in time, the local pH increases and reaches the bulk value. We further, demonstrate that the excited state pKa of HPTS may be estimated from the emission intensities of HA and A at long time. The time constant for time evolution of pH is ∼630 ps in water, ∼1300 ps in F127 gel and ∼4700 ps in CTAB micelle. The location and local viscosity sensed by the probe is ascertained using fluorescence correlation spectroscopy (FCS) and fluorescence anisotropy decay. The different values of the local viscosity reported by these two methods are reconciled.
在这项工作中，我们基于时间分辨面积归一化发射光谱（TRANES）对光酸 HA 及其荧光探针吡喃（8-羟基芘-1,3,6-三磺酸，HPTS）的时间分辨发射光谱进行了新的分析。TRANES 中存在等发射点证实了在本体水和共聚水凝胶[F127，（PEO）-（PPO）-（PEO）]中体系内存在两种发射物种（HA 和 A）。我们表明，在电子激发后，HPTS 周围的局部 pH 值远低于本体 pH 值，这可能是因为光酸在激发态下质子的逐出。随着时间的增加，局部 pH 值增加并达到本体值。我们进一步证明，可以从 HA 和 A 的发射强度在长时间处估计 HPTS 的激发态 pKa。在水中的 pH 值时间演化的时间常数约为 630 ps，在 F127 凝胶中约为 1300 ps，在 CTAB 胶束中约为 4700 ps。通过荧光相关光谱（FCS）和荧光各向异性衰减来确定探针感知的位置和局部粘度。这两种方法报告的局部粘度的不同值是一致的。
