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A: Kinetics, Dynamics, Photochemistry, and Excited States
Enhanced Excited-State Proton Transfer via a Mixed Methanol-Water Molecular Bridge of 1-Naphthol-3,6-Disulfonate in Methanol-Water Mixtures Oren Gajst, Luís Pinto da Silva, Joaquim C.G. Esteves da Silva, and Dan Huppert J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10374 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018
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The Journal of Physical Chemistry
Enhanced excited-state proton transfer via a mixed methanol-water molecular bridge of 1-naphthol-3,6-disulfonate in methanol-water mixtures Oren Gajst,a Luís Pinto da Silva,b,c Joaquim C.G. Esteves da Silvac,d,* and Dan Hupperta a Raymond
and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
b
Chemistry Research Unit (CIQUP), Department of Chemistry and Biochemistry,
Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. E-mail:
[email protected] c
LACOMEPHI, GreenUP, Department of Geosciences, Environment and Territorial
Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. d
Chemistry Research Unit (CIQUP), Department of Geosciences, Environment and Territorial Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. E-mail:
[email protected] *Corresponding author: Joaquim C.G. Esteves da Silva E-mail:
[email protected] Phone: + 351 220402569
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Abstract Steady-state and time-resolved fluorescence techniques were used to study the excited-state proton transfer (ESPT) from an irreversible photoacid, 1-naphthol-3,6disulfonate (1NP36DS), to methanol-water mixtures. We found that at χwater=0.3 the ESPT rate constant is higher by a factor of ten that in neat methanol. TD-DFT calculations show that a mixed molecular bridge of 2 methanol molecules and one water molecule enables the ESPT from the 1-OH to the 3-sulfonate. The RO-(S1) state is stable by -2.5 kcal/mol in comparison to the ROH(S1) state. We compare the ESPT rate constant of a reversible photoacid, 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS), in the same methanol-water mixtures. At χwater≈0.3 the ESPT rate constant of HPTS increased by only 15%. We explain the large difference of the ESPT rate of 1NP36DS by the formation of a water bridge or a mixed methanol-water bridge from 1-OH to one of the sulfonates and the absence of such a bridge in HPTS. The water or mixed methanol-water bridge of 1NP36DS enhances the ESPT rate in methanol-water mixtures of low water mole ratio.
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The Journal of Physical Chemistry
Introduction Photoacids are weak organic acids in their ground electronic state, with pKa in the range 5-10, whereas in their first excited electronic state they are stronger acids by 713 pKa units. They are investigated by employing several steady-state (timeintegrated) and time-resolved techniques in the range of 10-8-10-13 seconds. In their excited electronic state, the weak photoacids have values of pKa* in the range of 0-3.3. In the strongest photoacids, the pKa* is in the range of -8 to -2. In the past five decades, time-resolved fluorescence, transient UV-Visible and transient mid infra-red techniques
have
been
used
to
study
the
properties
of
photoacids.1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 There are two kinds of photoacids - reversible and irreversible. In the case of reversible photoacids, after their proton is transferred to the solvent, the proton's geminate-recombination (GR) with the excited deprotonated form RO-* leads to reforming of the excited-state protonated form, ROH*, of the photoacid.
RO * H ROH *
1
The excited-state proton transfer (ESPT) can repeat itself as long as the RO-* and ROH* forms are in their excited-state. For the reversible photoacid, the time-resolved population of ROH*(t) has a long-lived tail because of the GR process which decays non-exponentially.19,20 The long-time asymptotic decay rate of the ROH*(t) tail, when compensated for the excited-state lifetime is given below
P (t ) I
ROH F
(t ) exp(
t
F
)~
K eq* exp( V (a )) (4 Dt )3/ 2
2
where D is approximately the proton-diffusion coefficient, Keq* is the excited-state equilibrium constant, and τF is usually the excited-state lifetime of the RO- form of the photoacid. The proton geminate-recombination (GR) of irreversible photoacids leads to formation of the ground-state ROH(g)
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RO * H ROH ( g )
3
The GR process of irreversible photoacids terminates the photoacidic cycle. Nevertheless, it affects the population of the excited-state of the deprotonated form RO-*(t). The probability of the GR process is high at short time after the ESPT process, since the RO-*---H+ couple are at close proximity and the Coulomb attraction is large. The RO-*(t) decay time has a concave shape at short times after excitation and later the decay is exponential because of the finite exponential radiative decay. Recently we studied the ESPT process of 1-naphthol-5-sulfonate (1N5S) in rich methanol-water mixtures.21 We found that the ESPT rate is greater than the radiative rate in rich methanol mixtures of H 2O
