Basicity of Excited 3-Styrylpyridines
601
Excited State eactivity of Aza Aromatics. I. Basicity of 3-Styrylpyridines in the First Excited Singlet State G . Favaro, U. Mazzucato,* and F. Masetti insritufo di Chimica Fisica, University of Perugia, 1-06100 Perugia, /fa/y
(Received August 8, 1972)
Putilication costs assisted by the Consiglio Nationale deila Ricerche (Roma)
The basicities of the three isomeric trans-styrylpyridines in the first excited singlet state have been calculated from their absorption and fluorescence spectra. For 3-styrylpyridine and some of its 4’-substituted derivatives the pK* has also been directly measured by fluorimetric titration. The basicity increases strongly on excitation, with the exception of the 4’-N02 derivative which becomes a stronger acid in the excited state. The substituent effect on the basicity of 3-styrylpyridine is larger in the excited than in the ground-state molecule. Kinetic parameters for the protolytic equilibration in the excited state have been evaluated from fluorescence-pH curves and fluorescence lifetimes.
Introduction
rate band onsets for these measurements. That calculated Calculations of the basicities of styrylpyridines (StP’s) pK* values are reliable is in any case confirmed by the very good agreement with those fluorimetrically measured. by applying the Fbrster cycle1 to the absorption spectra Absorption spectra were taken with both single- and showed that 2- and 4-StP become much stronger bases in double-beam grating Optica CF4 spectrophotometers. For the excited singlet ~ t a t e , ~The J pK* of 3-StP, roughly fluorescence spectra a CGA DC-3000/1 spectrophotofluorievaluated from absorption maxima, seemed close to the meter was used. Some corrected spectra were run on a ground stat e pK. However, molecular orbital calculations Perkin-Elmer MPF-3 fluorescence spectrophotometer with indicated a noticeable migration of charge toward the hetan accessory for spectral correction which uses Rhodamine eroatom for all three isomers on excitation.2 The pK* B as a quantum counter. Fluorescence-pH curves were values have now been recalculated using the more reliable obtained exciting the compounds at the isosbestic point method of averaging the absorption and fluorescence between the absorption bands of the free base and its confrequencies“ and they indicate a strong increase of basicijugate acid. ty for the excited 3-StP also. This result has been conAll measurements were made at room temperature in firmed directly by measurements of fluorescence intensity water containing 10% (v/v) ethanol. Solutions were freshas a function of pH (obviously restricted to the fluorescent ly prepared and kept in the dark before use to avoid trans isomer. which attains a partial equilibrium with the trans-cis photoisomerization. Britton buffers were used in proton during the lifetime of the fluorescent state). The the pH interval 2 to 12; NaOH solutions were used for investigation has been extended to include some 4’-substihigher pH’s. tuted 3-StP’s. The data obtained, together with fluoresA Sargent PXB pH meter with a glass electrode was cence lifetimes, are important in the interpretation of the employed. High pH’s were corrected for the alkaline error. photochemical behavior of these compound^.^ M . The Solute concentration was of the order of 2 X ionic strength was kept constant at 0.01 for the measureExperimental Seetion ments of ground-state equilibria and at 1 nn the region of The styrylpyridines were prepared by standard procethe excited state pK*. In the latter case, Na2S04 was duress6Ground-state pK values were determined by both used as added salt because of its very low quenching absorption and fluorescence spectrometry. As the precipower on the luminescence. Under these conditions the sion of the absorption measurements cannot be high for emission intensity was proportional to concentration and 3-StP’s, due to the small changes of the spectra with P H , ~ the fluorescence-pH curves showed a regular trend. the dissociation constants were also measured by fluoApparent pK* values (K* is the dissociation constant of rimetry, where the spectral shift due to protonation is the styrylpyridinium acid) were corrected ( -0.3) to the much larger.8 thermodynamic values in Table I1 by the approximate forExcited state ionization constants were directly determula for the activity coefficients (see e.g., reftl). mined by fluorimetry, when partial establishment of the See e.g., A. Weiier, Progr. React. Kinet., 1, ‘187 (1961), and referexcited state equilibrium was detected in a plot of the relences cited therein. ative fluorescence intensity as a function of pH. Values in P. Bortolus, G. Cauzzo, U. Mazzucato, and S . Galiazzo, 2. Phys. Chem. (Frankfurt am Main). 51, 264 (1966). the tables are’ means of three independent experiments J. C. Doty, J. L. R. Williams, and P. J. Grisdale, Can. J Chem.. 47, with a mean deviation of 0.05. All pK* values were also 2355 (1969) calculated by the Forster cycle using the average of the (a) E. L. Wehry and L. B. Rogers, Spectrochim. Acta. 21, 1976 (1965); (b) J. Amer. Chem. Soc., 87,4234 (‘1965). absorption and fluorescence maximum frequencies (if deG. Bartocci, P. Bortoius, and U. Mazzucato, J. Phys. Chem.. 77, tectable, the long-wavelength shoulder of the absorption 605.(1973). spectra was used). Uncertainty from frequency determinaG. Galiazzo, Gazz. Chim. /tal,. 95, 1322 (1965). G. Cauzzo, G. Galiazzo, U. Mazzucato, and M. Mongiat, Tetrahetion can introduce errors of 0.3-0.4 pK units. The low soldron, 22, 589 (1966). ubility of these compounds did not allow us to take abG. Beggiato, G. Favaro, and U. Mazzucato, J . bfeferocyci. Chem.. sorption spectra at high concentrations and so to get accu7,583 (1970). The Journal 0: Physical Chemistry, Vol. 77, No. 5, 1973
G . Favaro, U. Mazzucato, and F. Masetti
602
TABLE I: Assurried 0-0 Frequencies ( l o 3cm- I ) and Basicity of trans-Styrylpyridines in !he Ground and Excited States in 90/10 (v/v) Water-Ethanol at 25" Compd
UblP
2-StP 3-StP 4-StP
29.22 29.45 29.57
VStPH
+
26.22 25.87 26.02
PK
pK* (calcd)
4.98O 4.76 5.73a
11.4 12.4 13.3
a From ref 7 .
TABLE I I : Assurned 0-0 Frequencies (lo3cm-"), Basicity, and Percentage of Excited Molecules in Acidic Form at pH -8 for Some trans-4'-X-3-Styrylpyridines in 90/10 (v/v) Water-Ethanol at Room Temperature _l_l
x H
PStP
ubtPll
29.45 125.87
CH3 28.50 QCH3 27.1'7
24.82 23.32
CI Br
25.68 25.45
I NO2
29.25 29.10 28.35 23.07
24.80
24.42
PK
PK* (calcd) PK* (fluor)
%stpH+*
4.76
12.4
12.3
17
4.77 4.73 4.74 4.74 4.73 4.44
12.6 12.9 12.4 12.5 12.3 1.6
12.5 13.0 12.3 12.4
34 20 9 6.5
