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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution
A Ratio-Analysis Method to the Dynamics of Excited State Proton Transfer: Pyranine in Water and Micelles Kalyanasis Sahu, Nilanjana Nandi, Suman Dolai, and Avisek Bera J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b04271 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018
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The Journal of Physical Chemistry
A Ratio-Analysis Method to the Dynamics of Excited State Proton Transfer: Pyranine in Water and Micelles
Kalyanasis Sahu*, Nilanjana Nandi, Suman Dolai, and Avisek Bera
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India *E-mail:
[email protected] 1 ACS Paragon Plus Environment
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Abstract Emission spectrum of a fluorophore undergoing excited state proton transfer (ESPT) often exhibits two distinct bands each representing emissions from protonated and deprotonated forms. The relative contribution of the two bands, best represented by an emission intensity ratio (R) (intensity maximum of the protonated band / intensity maximum of the deprotonated band), is an important parameter which usually denotes feasibility or promptness of the ESPT process. However, the use of ratio is only limited to the interpretation of steady-state fluorescence spectra. Here, for the first time, we exploit the time-dependence of the ratio (R(t)), calculated from time-resolved emission spectra (TRES) at different times, to analyze ESPT dynamics. TRES at different times were fitted with a sum of two lognormal-functions representing each peaks and then, the peak intensity ratio, R(t) was calculated and further fitted with an analytical function. Recently, a time-resolved areanormalized emission spectra (TRANES)-based analysis was presented where the decay of protonated emission or the rise of deprotonated emission intensity conveniently accounts for the ESPT dynamics. We show that these two methods are equivalent but the new method provides more insights on the nature of the ESPT process.
1. Introduction Proton transfer is a ubiquitous elementary process occurring frequently in both chemical and biological systems.1-5 In a typical spectroscopic investigation, a proton transfer event is usually initiated upon electronic excitation a fluorophore (also called photoacid) whose acidity enhances dramatically in the electronic excite state. The fluorophore subsequently discharges a proton to the neighboring solvent (or other acceptor).6-8 The phenomenon is termed as excited state proton transfer (ESPT). 2 ACS Paragon Plus Environment
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8-Hydroxypyrene−1, 3, 6−trisulfonate (HPTS or pyranine, scheme 1) is a well-known photoacid with a ground state pK of 7.2-7.79-10 and a very low excited state pK ∗ of 0.51.5.10-11 Thus, in neutral or slightly acidic water, HPTS shows an absorption maximum at 403 nm characteristics of the protonated form. However, it displays a strong emission band centered at ∼512 nm and a very weak emission band at ∼440 nm characteristics of the deprotonated and the protonated forms, respectively.12-13 The very strong deprotonated emission compared to the protonated emission arises from the fact that the protonated form promptly converts into the deprotonated form within its excited state lifetime. The relative contribution of the two forms to the emission spectrum or the ratio (R) of the emission intensity of the two forms (protonated/deprotonated) is an indicator of the feasibility or promptness of the ESPT process. The ratio is very sensitive to the nature and dynamics of hydrogen bonding network of water molecules around the fluorophore. This makes HPTS a highly suitable probe to study nature of water entrapped inside confined media like cyclodextrin,14-15membrane,16-17 protein,18-19 micelle,20-21 and reverse micelle.22-26
Scheme 1. Chemical structure of 8-hydroxypyrene−1, 3, 6−trisulfonate (HPTS). A very low value of R usually denotes very fast ESPT compared to the excited state lifetime of HPTS and conversely, a high R-value indicates that the ESPT is significantly 3 ACS Paragon Plus Environment
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retarded. In some cases, a photoacid may distribute in multiple environments and some subpopulations may be completely inaccessible to water and thus, cannot undergo ESPT. These fractions can only contribute to protonated emission band which results in a high value of R. We found that at reverse micellar interface there may be some regions completely dehydrated and the relative amount of the de-hydrated and hydrated fraction vary with w0 ([water]/[surfactant]).25-26 A systematic variation of the ratio is quite common when an organic solvent is mixed with water.27-28 Very recently, we showed that the intensity ratio can be used as an indicator of alcohol content in an aqueous mixture of alcohol and also to tract the solubility of an alcohol inside water pool of a reverse micelle.29 Many other studies used the relative ratio to show the retardation of ESPT. However, the ratio concept has only applied to the steady-state emission spectra. Here, we represent a quantitative treatment of ESPT dynamics based on the intensity ratio (R) calculated from the time dependent emission spectrum (TRES). In most ESPT studies, one usually records fluorescence decays at two specific emission wavelengths representing protonated or deprotonated emissions and analyze them with respect to a specific ESPT model. The fluorescence decay measured at a specific wavelength not only displays the ESPT dynamics (interconversion between protonated and deprotonated forms) but also are affected by the depopulation dynamics. Single wavelength measurement meant for one form may also contain contamination from the other form, if there is a finite spectral overlap between the two bands.23 In addition, rearrangement of solvent (solvation dynamics) may cause time-dependent spectral relaxation of the emission bands, which may cause error in correct modelling of the fluorescence decay components. Time-resolved area-normalized emission spectra (TRANES) is a very convenient and popular way to illustrate the spectral changes occurred in the excited state.23,
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30-33
The
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presence of an isoemissive point indicates the presence of only two excited species and was demonstrated for ESPT of 2-napthol in water by Periasamy and co-workers.30-31 Very recently, we showed that intensity decay of the protonated band of TRANES or the rise of the deprotonated TRANES intensity can directly give ESPT times scales.34 In a subsequent work, Ghosh and coworkers also demonstrated a similar procedure independently and also discussed the effect of solvation dynamics on the appearance of TRANES pattern.35 Here, we propose another important observable, the ratio, R of the emission intensity of two forms as a quantitative measure of the ESPT dynamics and derive an analytical expression to fit the time dependence of the ratio.
2.
Experimental Section 8-Hydroxypyrene-1,3,6-trisulfonate (HPTS, pyranine) was purchased from Sigma-
Aldrich Chemicals. We used water (resistivity 18.2 MΩ cm) from a Millipore system. Absorption and emission spectra were recorded in a Perkin-Elmer Lamda-750 spectrophotometer and Fluorolog (Jobin-Yvon) spectrofluorometer, respectively. We measure fluorescence transients in a time-correlated single photon counting (TCSPC) setup using a picosecond laser diode DeltaDiode 375 (Horiba Instruments) with an excitation wavelength of 375 nm. The fluorescence transients were recorded by keeping the analyzer at the magic angle (55°) with respect to the polarizer. The FWHM of the set up was
