Fluorescence of the benzophenone ketyl radical: solvent effects

Journal ofPhotochemistry

and Photobiology,

A: Chemistry, 47 (1989)

FLUORESCENCE OF THE BENZOPHENONE RADICAL: SOLVENT EFFECTS P. YANKOV,

ZH.

NICKOLOV

155 - 165

155

KETYL

and V. ZHELYASKOV

Faculty of Physics, Sofia University, 5 A. Ivanou Bul., BG-1126

Sofia (Btdgaria)

I. PETKOV Faculty of Chemistry, Sofia University, 1 A. Ivanov Bul., BG-1126 (Received

June 24, 1988;

in revised form November

Sofia (Bulgaria)

25,198s)

Summary

The fluorescence of the benzophenone ketyl radical (BKR) was studied in different solvents by a double-pulse excitation technique using two synchronized lasers. Fluorescence quantum yields of the BKR were determined employing a new technique suitable for measurement of shortilived transients with low level emission. By analysing the fluorescence spectral shape and the values of the quantum yields in the different solvents it can be concluded that charge transfer complexes are formed in the excited doublet state of BKR, with the molecules of the solvents exibiting electrondonor properties.

1. Introduction The fluorescence from the first excited state of the benzophenone ketyl radical (BKR) has been known since 1975 [l]. It has been investigated using nitrogen laser excitation of benzophenone in the presence of hydrogen donors and the emission in the 570 - 590 nm region with a lifetime of approximately 2 ns [2] has been attributed to the doublet-doublet transition of the radical [ 3 - 63. This fluorescence is produced either by absorption of a photon by the radical within its lifetime or by energy transfer from a benzophenone triplet to the radical in the doublet ground state 143. However, if a pump-probe technique is employed (the pump laser pulse performing the photoinduced BKR formation), then after a suitable delay time the probe pulse will produce pure photoinduced fluorescence of BKR

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156

The purpose of this paper is to investigate the dependence of the fluorescence of BKR in different solvents using a double pulse excitation technique with two synchronized lasers and optical multichannel detection. 2. Experimental details 2.1. Materials and set-up Benzophenone was obtained from Merck and was used without further purification. The solvents used were fluorescence grade commercial products; they were used without purification. They were checked to show that no fluorescence occurred at the two excitation wavelengths (355 nm and 532 nm). The following solvents were used in our investigations: isooctane (IO), 1,2dichloroethane (1,2-DH), 1,Pdioxane (1,4-DO), chloroform (HF), ethyl acetate (EtAc), tetrahydrofuran (THF), isopropanol (i-PrOH), ethanol (EtOH), methanol (MeOH), dimethylformamide (DMFA), pyridine (Py) and dimethylsulphoxide (DMSO). A diagram of the experimental set-up is shown in Fig. 1. The third and second harmonics (hi = 355 nm and h2 = 532 nm respectively) of two synchronized Nd:YAG lasers were used for excitation. The first excitation pulse with an energy of 7.5 mJ at hi produced the ketyl radicals (pump pulse). The second pulse at h2 after a variable time delay was used to excite the fluorescence of BKR (probe pulse). Its energy was regulated from zero to 5 mJ. The duration of the pump pulse was 4.5 ns and that of the probe pulse was 5 ns. Both pulses were synchronized with a jitter of +-3 ns for time delays of 30 ns to 10 /.LSand 22% for time delays of 10 ,USto 10 s. The pump and probe pulses coincided spatially in the sample cell with solutions of 4 X lop3 M initial concentration of benzophenone for all solvents. The fluorescence spectra of the BKRs were recorded by a gated

Fig. 1. Experimental set-up: 1, 2, Nd:YAG lasers; 3, synchronization unit; 4, time delay monitor; 5, 6, pulse energy monitors; 7, high voltage pulser; 8, quartz prism; 9, sample; 10, lens condenser; 11, 12, 13, gated optical multichannel analyser; 14, computer; 15, printer; BS, beam splitters; M, mirrors.

157

optical multichannel analyser (OSA-500) interfaced to a computer. The vidicon was gated with 40 ns strobe pulses from a high voltage pulse generator synchronized with the probe pulse. All recorded spectra were corrected for the spectral transmission of the receiving system (vidicon + polychromator). 2.2. Method for the determination of the low fluorescence quantum yields of short-lived transients Because of the high absorption coefficients at Xi (e&T1 state benzok: 3700 mole1 1 cm-’ [7]), the phenone) = 7650 mol-’ 1 cm-‘, e&BKR) concentration of benzophenone triplets and ketyl radicals in the excited volume is not constant along the direction of excitation. In this case the normal practice of measuring unknown fluorescence quantum yields by comparison with a standard of known quantum yield [8,9] cannot be applied because of the gradients in the optical density of the standard and the unknown. Therefore the condition of measuring optically dilute solutions will not be preserved. In this work we have developed a new method for measuring fluorescence quantum yields of transients. Only light from a given portion of the cell defined by the entrance slit width and the focusing lens condenser enters the polychromator of the optical multichannel analyser. The decrease in the spontaneous Raman scattering signal at a suitable line of the solvent due to the absorption of the solute is measured. This result is then compared with the same measurement carried out under the same experimental conditions with a fluorescence quantum yield standard. It is important to note that the Raman signal should be recorded simultaneously with the fluorescence of the unknown or the standard. This condition is satisfied by employing an optical multichannel analyser for spectrum detection. Another important point is that when the Raman signal of the solvent falls in the fluorescence band only measurements of species with low quantum yields are possible with this method. If the fluorescence quantum yield of the unknown is 4, and that of the standard is &

=- Nfl N abs

(1)

=- Nl

N&s

where Nfl and Nh are the numbers of fluorescence photons for the unknown and the standard, and Nabs and N& are the numbers of absorbed photons at the excitation wavelength for the unknown and the standard. Let us introduce a factor y

NR

y=No,

(2)

158

where NR and NORare the numbers of photons of the Raman scattering signal from the solvent, with and without the solute, passing through the entrance slit of the polychromator and detected by the vidicon of the multichannel analyser. Given that %

=

&Wexc ----Nabs)

%i = &Nexc

(3)

where AR is the efficiency coefficient of the Raman scattering at the corresponding line of the solvent, N,,, is the total number of photons of the excitation and Nabs is the total number of photons absorbed by the transient in the excitation volume in the cell which is projected on the input slit of the polychromator. Then N

‘=‘-F

exe

(4)

and

@u = &=

Nfl Nexctl -Y)

Nh N&,(1 - 7’)

(5)

where y and y’ refer to the unknown and the standard respectively, and N exe = N:,, because of the stability of the energy of the probe pulses = kO.O25E,, where E, is the probe pulse energy). Finally WE,

(6) where n, and n, are the refractive indices of the solvents, used in the conventional sense [9], for the standard and the unknown. The number of fluorescence photons in both cases can be calculated after integration of the spectrally corrected fluorescence bands from the multichannel analyser normalized to the excitation energy at hz. The accuracy of the method depends mainly on the precision of the measurement of y (see eqn. (2)). If, as in our case, the Raman line of the solvent is superimposed on the fluorescence band, then the shape of that band should be derived from excitation with another wavelength. In Fig. 2 we present the normalized fluorescence spectrum of BKR in 1,4-DO when excited with the pump pulse (A,) only. Here the excitation at hi is used both to produce the radicab and to induce their fluorescence. The Raman peak is not seen in Fig. 2(a) because it does not fall in the region of BKR fluorescence at this excitation wavelength. In Fig. 2(b) the same spectrum is shown with the Raman signal superimposed on it (pump pulse (A,) + probe pulse (A,)). The difference spectrum between 2(b) and 2(a) leading to the Raman spectrum of the solvent in the presence of the transient is shown in

159

500

540

580

620

660

700

740

h . NM Fig. 2. Fluorescence and Raman spectra of BKR in 1,4-DO: (a) normalized fluorescence spectrum, excitation at hi; (b) normalized fluorescence spectrum and Raman spectrum of the solvent, excitation at &; (c) difference spectrum between (b) and (a) giving NR; (d) normalized Raman spectrum of the solvent, excited at X2, giving I&.

Fig. 2(c). The Raman spectrum of the pure solvent (excitation at X,) is shown in Fig. 2(d). Nfl can easily be calculated from the spectrum at hl, whereas IVR and pa are calculated from the Raman line at about 633 nm (Figs. 2(c) and 2(d)). Our estimation shows that the total error in the determination of the factor y is about *3% for our experimental set-up and methods of spectra processing. The problem in the accuracy of the determination of #, is connected with the accuracy of determination of the standard quantum yield &. Most fluorescence standards have quantum yields higher than 0.5 and therefore are not suitable for measurement of species with very low quantum yields such as BKR. As a standard we used the dye 2-(4-dimethylaminostyril)-3ethylbenzothiazolium iodide in ethanol because of its comparatively low quantum yield, which allows proper registration of the Raman line of the solvent. Its quantum yield was determined by the conventional technique [9] in comparison with that of Rhodamine 101 in ethanol (@, = 0.019, accurate to within 5%). Taking into account the accuracies in the determination of @,, y and N,, we estimate that the fluorescence quantum yields of BKR in the various solvents are measured with relative errors of *13%.

3. Results and discussion Because of the rapid singlet-triplet conversion [lo], the triplet state is populated during the laser pulse (rpump = 4.5 ns). As our investigations

160

were directed towards studying the dependence of the BKR fluorescence on the nature of the solvent we had to be certain that there were no other absorbing species during the probe pulse with the exception of the radicals. This can be fulfilled in two ways. Firstly, the absorption by the T1 state of benzophenone, which is in the same spectral region as that of BKR, should be negligibly small. According to ref. 7 the triplet lifetime rT is 0.2 - 3 ~.ts depending on the solvent. Taking into account the values of the absorption coefficients for the triplet (ET = 7650 mol-’ 1 cm-l) and for BKR (Ed = 3700 mole1 1 cm-l [7] ), we can describe the processes according to the diagram in Fig. 3 with the simplified rate equations

W’l -=--

PI

dt

TT

d[Rl

-=h,[T] dt

(7) -k,[R]

where [T] is the concentration of benzophenone molecules in the T1 state, [R] is the concentration of BKR in the D,, state, TT = (kTs + k&l is the benzophenone triplet lifetime, kH is the rate constant for hydrogen transfer and k, is the rate constant for photoproduct formation. We have estimated from the solution of the rate equations (eqn. (7)) that if the delay time rd between both pulses is of the order of (3 - 4) rr the absorption of the probe pulse by the T1 state of benzophenone will be at least 10 times smaller than the absorption by the ground state of BKR. The dependence of the fluorescence intensity of BKR on the delay time rd for some of the solvents is shown in Fig. 4. The rate constant kR of depopulation of the ground state of the radical, which is proportional to the decrease in the fluorescence intensity, is much slower .than the triplet decay rate constant 7r-l [ 7,111. The values of the measured decay constants kR of BKR are given in Table 1.

&Cc= 355nm

benzophenone ketyf radical

benzophenone

Fig. 3. Simplified

excited

state energy diagram.

photoreaction products

161

Fig. 4. Dependence of the fluorescence intensity at constant energy of the probe pulse (AZ) on the delay time between pump and probe pulses for various solvents: 0, IO; X, i-F’rOH; A, MeOH;

TABLE Decay Number

q, EtOH.

1 rate constants and fluorescence quantum yields of BKR in various solvents Solvent

Decay rate constant k,

Fluorescence quantum yield @

Dielectric

1.5 1.9 1.3 4.9 1.2 3.7 3.3 1.9 1.5 3.5 (1 (2 -

1.94 2.18 2.21 4.6 6.01 7.58 19.25 24.55 32.7 36 46.7

constant

ea

(s-l) 1 2 3 4 5 6 7 8 9 10 11

12

IO 1,2-DH 1,4-DO HF EtAc THF i-PrOH EtOH MeOH DMFA DMSO PY

3.5 7.6

x 106 x lo5

8.5 x 105 1.7 x 106 4.8 2.2 3.5

x 106 x 106 x 106

x x x x x x x x x x 4) 5)

10-s 10-S 10-4 10-6 10-s 10-4 10-4 10-4 10-4 10-s x lo- -5 x lo- -4

12.3

aData taken from ref. 12.

Secondly, as new species are formed (mostly pinacol [lo, 131) no fluorescence should occur from these. This was prevented by substituting the solutions in the cell with fresh solutions during the experiment. A spectral shift in the fluorescence spectra of BKR was observed in our experiments on increasing the polarity of the solvents. According to the general theory [14] this may be due either to general effects, which depend on the difference between the electronic and orientational polar-

162

izability, or to specific solute-solvent interactions. To analyse the influence of different solvents on the fluorescence properties of fluorophores Lippert’s equation is usually employed [15]. In Fig. 5 the Stokes shift for BKR is plotted vs. Lippert’s expression for general solvent effects. Solvents 1, 2, 4, 5, 6, 7 and 10 lie on a straight line; this indicates that they should obey general solute-solvent interaction theory. Solvents 3, 8 and 9 do not show a linear dependence. In these solvents specific short-range interactions probably take place. In order to draw conclusions about the nature of the specific interactions which are closely connected with the excited state solute-solvent interaction let us consider the values of the fluorescence quantum yields measured by the method described in the previous section (Table 1). It can be seen that the fluorescence quantum yields of BKR in solvents 3, 6, 7,8 and 9 are an order of magnitude or more higher than those for the other solvents. Such an increase can be connected with the formation of charge transfer (CT) complexes [ 161. While the existence of such complexes is difficult to establish from absorption spectra due to their short-lived nature, fluorescence characteristics are suitable for this purpose. A second feature proving the existence of CT complexes can be the appearance of a new band in the fluorescence spectrum [ 141. However, from the shape of the fluorescence spectra obtained in our work it is difficult to see any indication of a new band. Nevertheless, a band may be present, but may not be observed due to strong mutual overlapping with the fluorescence spectrum of BKR in solvents where complexes are not formed. Recently a new technique has been developed [17] for the analysis of complex spectral bands consisting of intrinsically overlapping components.

1.0

1

I

0.1

I

0.2

--I

0.3 Af

Fig. 5. Lippert’s equation diagram. Af = (E - 1)/(2~ + 1) - (n* - 1)/(2n2 + 1) us. (J, - 3f) (Stokes shift). Pa is the peak absorption wavenumber, Pf is the peak fluorescence wavenumber (see Table 1 for the numbers of the solvents).

163

It is known as the Fourier deconvolution method and has been successfully applied to the decomposition of the OH Raman band of liquid water [18] and to the fluorescence band of chlorophyll “a” [19]. In general, the Fourier deconvolution technique comprises a set of mathematical operations which are carried out on the Fourier transform of the complex spectrum in order to resolve it into components. In practice, the Fourier transform of the complex spectrum is divided by the Fourier transform of a trial function with a shape similar to that of the overlapping components. Then an inverse Fourier transformation is performed on the result. By varying the width of the trial function different extents of spectral resolution can be reached. We used a deconvolution procedure based on a lorentzian-type profile and applied this technique to the fluorescence spectra of BKR in different solvents. The results were quite encouraging. The deconvoluted fluorescence spectra of BKR in IO and EtOH (solvents 1 and 8) are shown in Fig. 6. It can be clearly seen that a new band appears around 622 nm in EtOH in addition to the bands in non-polar solvents (560 - 570 nm). This is true for the spectra of BKR in solvents 3, 6, 7 and 9; thus complete correspondence is

700

74

;\ .NM

500

580 A .NM

Fig. 6. Normalized and deconvoluted spectra of: A, BKR in IO; B, BKR in EtOlS The negative lobes in the deconvoluted spectra are due to over deconvolution and/or discrepancies between the real subcomponent shapes and the shape of the trial function.

164

Fig. 7. Structural

scheme

of BKR

in its D1 state.

observed between the fluorescence quantum yield increase and the appearance of a new band in the spectrum. Thus the probability of CT complex formation between the excited state of BKR and the molecules of solvents 1,4-DO, THF, i-PrOH, EtOH and MeOH is quite high. In our opinion the reason for the formation of these complexes lies in the proton donor nature of BKR [6,12]. Assuming a quasi-planar structure of BKR with respect to one of the phenyl rings [20] (Fig. 7) the formation of a CT complex in the excited state will further stabilize it and this could be a good reason for the increased fluorescence quantum yields. In the alcohols the fluorescence quantum yields of the CT complexes decrease with an increase in the dielectric constant (see Table 1). A similar behaviour has been observed in the emission spectra of excited complexes of aromatic hydrocarbons with amines in solvents with increasing polarity [ 131. We have assumed that the quenching of the fluorescence of the excited complex for the three alcohols is due to formation of radical-ion pairs ( [BKR*]- + (solvent)+). Information is also included in Table 1 on the fluorescence of BKR in pyridine and dimethylsulphoxide. The fluorescence quantum yields in these solvents were not determined with the desire-d accuracy because they were not of fluorescence grade purity. Therefore the values of r#~ can complement our assumptions only qualitatively. Pyridine is an electron donor and its behaviour is similar to that of THF and 1,4-DO; the estimated quantum yield is high (( 2 - 5) X 10e4). DMSO is similar in its electron donor properties to DMFA, but its dielectric constant is higher; therefore $I = (1 - 4) X lo-‘.

4. Conclusions This investigation of the fluorescence of the benzophenone ketyl radical in different solvents shows that a charge transfer complex is formed in its first doublet excited state with solvent molecules having electron donor properties. In this way the proton donor behaviour of the radical is emphasized rather than the expected high hydrogen abstraction affinity. The application of Fourier deconvolution techniques to the analysis of the fluorescence spectra of BKR, together with the proposed novel method of

165

measuring fluorescence quantum yields of short-lived transients, has proved an interesting alternative for studying weakly fluorescent excited complexes.

Acknowledgments This work was performed as part of Problem 261 sponsored by the Bulgarian Ministry of Culture, Science and Education. The authors wish to thank Dr. Delligeorgiev for supplying the dye 2-( 4-dimethylaminostyril)-3ethylbenzothiazolium iodide.

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