Excited-state proton transfer from pyranine to acetate in gamma-cyclodextrin and hydroxypropyl gamma-cyclodextrin.
Excited-state proton transfer (ESPT) from pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) to acetate has been studied by picosecond and femtosecond emission spectroscopy in gamma-cyclodextrin (gamma-CD) and 2-hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) cavities. In both the CDs, ESPT from HPTS to acetate is found to be very much slower (90 and 200 ps) than that in bulk water (0.15 and 6 ps). From molecular modeling, it is shown that in the cyclodextrin cavity the acetate is separated from the OH group of HPTS by water bridges. As a result, proton transfer in the cavity requires rearrangement of the hydrogen-bond network involving the cyclodextrin. This is responsible for the marked slowdown of ESPT. ESPT of HPTS in substituted gamma-CD is found to be slower than that in the unsubstituted one. This is attributed to the hydroxypropyl groups, which prevent close approach of acetate to HPTS.
Mondal
Sudip Kumar
SK
Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India.
Sahu
Kalyanasis
K
Ghosh
Subhadip
S
Sen
Pratik
P
Bhattacharyya
Kankan
K
D000085:Acetates / Q000737:chemistry*; D001190:Arylsulfonates / Q000737:chemistry*; D006860:Hydrogen Bonding; D015394:Molecular Structure; D011522:Protons; D013052:Spectrometry, X-Ray Emission; D047408:gamma-Cyclodextrins / Q000737:chemistry*
D000085:Acetates; D001190:Arylsulfonates; D011522:Protons; D047408:gamma-Cyclodextrins; C479659:hydroxypropyl-gamma-cyclodextrin; C005047:pyranine