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Flash photolysis of transient radicals. Benzophenone ketyl radical
CHEMICAL
Volume 112, number 3
PHYSICS
LETTERS
7 December
1984
FLASH PHOTOLYSIS OF TRANSIENT RADICALS. BENZOPHENONE KETYL R-ADICAL V. NAGARAJAN
and Richard W. FESSENDEN
Radtition Laboratory and Deparrmenr of Chemisity. University of Norse Dame, Notre Dame, Indiana 16556, Received 29 June 1984; in final form 10 September
LiSA
1984
A number of Processes are found ro follow escitation of the benzophenone ketyl radical. The lowest excited state of theradicalabsorbs, fluorescesandreacts withsolvent. Rate constantsforreaction of thisstate with cyclohesaneand isopropan01 are 4 x lo7 and 7 x 10’ M-l s-l, respectively_ The lifetime of 5.1 ns found in a solution containing 1% cwlohesanc in acetonitrile at
room temperature is longer than that reported previously.
l_ Introduction Fluorescence
from the first excited state of benzo-
phenone ketyl radical (BPK) was first reported by Hodgson et al. [l]. Subsequently Mehnert et al. [2] measured the fluorescence lifetime to be 2.0 2 0.3 ns at room temperature using, in tandem, an electron pulse and a laser pulse. They also reported a break in the temporal absorption profile corresponding to bleaching of the ground state ketyl radical by the laser pulse. Since the “bleach” did not recover, they tentatively suggested a reaction of the type (Ph?COH)*
+ S + product
,
where S is some unknown scavenger_ Obi and Yamaguchi [3] determined the fluorescence lifetime at 77 K to be ~17 ns and ~20 ns in ethanol and EPA glasses, respectively. They ascribed the shortening of lifetime at room temperature to either diffusion-controlled bi-
cases lies above the first excited doublet. However, the time-resolved ESE results seem to indicate the possibility of populating Q. via ISC from D2, making BPK a good chaise for optically detecting the quarter in liquid solution. Except for the instance mentioned above [2], there has also not been any attempt to characterize the photochemistry of BPK
2. Experimental
molecular quenching or some kind of chemical reaction from the excited ketyl radical. Measurements of the rate of fluorescence quenching for a number of reactants have established the main reaction of the excited radical to be electron transfer [4] but high reactivity toward H atom donors has also been mentioned [5]. Using the transient electron spin echo (ESE) technique, Thumauer and Meisel [6] studied the photoreduction of benzophenone by various substrates. In order to explain a slowly growing ESE emission signal for BPK (time constant 02 w), They invoked intersystern crossing of the ketyl radical from its second ex0 OOg-2614/84/$ 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
cited doublet state (D2) to a quartet state (Qo) that lies energetically between the D, and the Dz states. The Q. state was then assumed to intersystem cross spin selectively to the ground state in appro-ximately 2 /z, giving rise to the slowly growing emissive signal. Except in one instance where quartet-doublet phosphorescence was observed 171, optical detection of the quartet state of organic radicals has not been reported. The main problem in such an observation would be populating the quartet state that in many
Absorption and initial fluorescence measurements were made with the apparatns described elsewhere [S]. The ketyl radicals were produced by photolysis of ~1 X 10m5 M solutions of benzophenone at 248 nm with light from a Tachisto 1 SO-XR excimer laser. The solvent was either cyclohexane or acetonitrile with an added hydrogen donor. Subsequent photolysis of BPK was with either 355 or 532 nm from a Quanta-Ray DCR-1 YAG laser (pulsewidth 6 ns). For detailed measurements of fluorescence decay, the initial radical B.V.
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CHEhlICAL PHYSICS LETTERS
Volume 112. number 3
production was with 266 nm light from the YAG laser and excitation of the radical was at 337 m-n with a PRA LN-1000 nitrogen laser (800 ps pulsewidth). A Tektronix 7912 AD digitizer with 7A29 vertical amplifier was used to record the time prome of the fluorescence_ The digitizer was triggered directly from the light pulse by means of a vacuum photodiode. The total system response (excitation pulse, photomultiplier, and digirizer) corresponded to a time constant of about 1.8 ns for laser light scattered into the photomultiplier. Benzophenone and 4,4’-dichlorobenzophenone (both Aldrich Gold Label) were recrystallized from ethanol. Cyclohexane (Fisher), isopropanol (Fisher), acetomtrile (Aldrich Gold Label and B&J High Purity Solvent with
7 December 1984
tion with a lifetime of a few ps_ Absence of a new absorption when BPK was excited at 355 nm could be due to a poor extinction coefficient for the quartetquartet transition and hence population of Qo cannot be ruled out based on this result alone. A rapidly decaying fluorescence with a lifetime shorter than our experimental time resolution was observed when BPK was excited either at 35.5 or at 532 nm. The spectrum of this fluorescence agrees well with those which have been published [ 1,9]_ In the absence of 248 nm light, the 355 nm excitation gave rise to a small increase in absorption, but no fluorescence_ At short time scales, a quickly decaying absorption followed by the permanent bleach was seen at around 350 nm. Since the exciting and monitoring wavelengths were very close for 355 nm excitation, scattered light posed a serious problem at this laser wavelength. Hence only 532 nm light was used to excite BPKfor observation of the short-lived absorption signal. The time profile of this absorption essentially follows the exciting
3. Results and discussion
laser pulse and may match that of fluorescence (fig. 2). The absorption spectrum corresponding to this short-
The ketyl radical was excited at both its UV (Do + D3) and visible (Do + D1) bands using the 355 and 5i2 nm outputs of the laser, respectively_ On the mi-
lived state is shown in fig. 3. This spectrum is tentatively assigned to a D1 + D, transition. The permanent bleach must result from a chemical change in the
crosecond time scale, a permanent bleach in the ketyl radical absorption is seen corresponding to excitation
D, state because it follows excitation in the visible band. It should be noted that the maximum of this
in cyclohexane at either wavelength at room temperature (fig. 1). This observation is in agreement with that of hlehnert et al. [2]. The bleach amplitude was found to be linearly dependent upon the laser intensity. A photobleaching spectrum revealed no new absorp-
spectrum may not be the true peak. The absorption due to BPK rises rather steeply starting at around 350 nm and hence the bleaching in BPK absorption might cancel the increase in D, absorption beginning at this wavelength. Because of the very short lifetime of the D, state, it could not be confidently determined, for wavelengths shorter than 340 nm, whether the bleach
Time. ps
Fig. 1. Production and excitation of benzophenone ketyl radical (BPK) in cyclohesane. The bleach in absorption was caused by the excitation of BP6 at 532 nm and was observed at 335 nm. No change in absorption was seen when BP triplet alone was excited.
208
signal showed a fast initial change corresponding to D, absorption followed by a slower change. It is probable that the state responsible for the shortlived absorption is that which also fluoresces_ An experiment at low temperature where the fluorescence lifetime is larger should allow verification of this suggestion. BPK in acidic ethanol glass at 77 K was prepared by radiolysis [lo] and was excited at 337 MI. A fluorescence spectrum identical with the room temperature spectrum was obtained. The fluorescence lifetime was found to be ~23 ns, slightly longer than the published value [3]. When the absorption was monitored close to the laser wavelength, light scattered by the bubbling N2 saturated the detector_ Band pass fd-
Volume 112, number 3
CHEMICAL PHYSICS LETTERS
7 December 1984
I
I
Fig. 2. Room temperature(left) and 77 K (right) absorption (open circles) and emission (closed circles) signals caused by escitation of BPK at 532 nm. The absorption and emission s?@~aIswere monitored at 350 and 580 nm for the left portion in qclohesane and at 348 and 570 nm for the right portion respectively_The decays of the xoom temperature s&x& are limited by the laser pulse width and the instrument rise time. ters were used to keep this scattered light from reachAn instantaneous rise in absorption followed by a decay with a time constant of *24 ns was observed at 348 nm (a wavelength for which a ing the detector.
band pass filter was available). The agreement between
fluorescence and absorption the origin D, state. tion have radical [I
lifetimes confirms that of the short-lived absorption is indeed the A similar fluorescence and transient absorprecently been reported for diphenylmethyl 1 f_ The D1 absorption spectrilm at 77 K
WAVELENGTHhd Fig_
3. D1-t Dn absorption spectra of BPK (closed circIes) and
DCl,L%PK(open circles) in cyclohcsane, normalized TOthe same height. Inset: absorption spectra of BPK (solid fine) and
DCIBPK (dotted line) with UV peaks normalized to the same hejght to show the shift. In rea3ity, however, the shift is even kuger because the visible bands axe of comparable strength.
could not be recorded because of the lack of a sufficient number of band pass fflters in the 340-370 nm region and the OD at 532 nm was too small to produce any significant conccentration of excited BPK After the fast initial decay, a very flat residual absorption remained. A spectrum taken at this flat region showed it to be the triplet absorption. Some BPK and benzophenone must be simuitaneously excited by the 337 nm light. A steady state absorption spectrum taken after the photolysis showed an increase in BPK absorption from the one taken before photolysis. This increase must be due to the triplets formed upon excitation at 337 nm. (Formation of triplets could not be avoided, since a further decrease in the concentration of benzophenone would have required a longer radiolysis time which has associated with it problems in replying the liquid Nz_) The region of 300-650 nm was scanned and for the entire range only a step increase in absorption that subsequently stayed flat for at least 10 ius could be seen_ Even if Q. did not absorb in this region, a bleach signal lasting a few JIZG and superimposed on the triplet absorption would have been observed if Q. had been formed. Except for the unlikely event that Qo has an absorption spectrum identical with that of Do, it has to be concluded that Qo is not populated to any significant degree in the excitation of BPK and that D2 + D, internal conversion is a vea efficient process. Similar results were obtained for 4,4’-dichlorobenzo209
Voiunre 112. number 3
phenone ketyl radical (DCIBPK) upon excitation at room temperature. The D, + D,, absorption spectrum for DCIBPK is given in fig. 3. It is seen that the spectrum is red-shifted compared to that of BPK. This shift parallels that in the spectrum of ground state DClBPK. The reported reactivity of BPK towards H atom donors [5] suggests that the permanent bleach seen in fig. 1 could be the result of the excited radical reacting with the solvent_ In order to investigate this possibility, measurements of fluorescence lifetime were made using the SO0 ps pulse of the PRA laser for excitation as described in section 2. The apparatus response function was determined by scattering laser light into the monochromator and the observed fluorescence curves were analysed by deconvolution. No fluorescence was observed in the absence of the first laser pulse. Contrary to earlier reports [4,5] which used a one-color-two-photon method and no H atom donor, the lifetime in acetonitrile with 1% (u/u) cyclohexane as hydrogen donor was found to be 5.1 + 0.1 ns. The previously reported value of 2.3 ns was subsequently improved to 3.5 ns [4]. Although the excited state is very reactive toward good hydrogen donors or water (see below), it would take more than 0.1% impurity in the acetonitrile to cause such a shortening in liferime. The lifetime in neat cyclohexane was found to be 2.5 + 0.2 ns. Data for various concentrations (l-10%) of isopropyl alcohol in acetonitrile were also obtained_ A plot of pseudo first-order rate constant against conten tration gave a straight line with a slope corresponding to a rate constant of 5 X 10s M- 1 s-t for reaction of the excited radical with isopropanol. Such an analysis was not pratical for cyclohexane in acetonitrile because the reaction is slower and cyclohexane is miscible with acetonitrile only up to 13%. Based on the lifetime at 100% cyclohexane and taking the natural emission lifetime to be 20 ns, the rate constant for hydrogen abstraction from cyclohexane is found to be about 4 X 1O7 hi- 1 s- 1_These rate constants are much higher than those for the ground state carbon-centered radicals. For example, the reaction of metyl radical with isopropanol has a rate constant of about 3 X 1 O3 M-l s-l 1121. The lifetime of 5.1 ns for the excited radical in acetonitrik may also involve some type of reaction because the lifetime at low temperature is ~20 ns. This reaction is more evident in the case of DCIBPK where the lifetime in acetonitrile (with 5% cyclohexane) is Zess than that in pure cyclohexane 210
7 December 1964
CHEMICAL PHYSICS LETTERS
(2.7 versus 3.9 ns). It is possible that ionization of the OH proton occurs and that the anion form of BPK does not fluoresce_ The dichloro derivative should be more acidic and therefore, undergo deprotonation more rapidly. To determine the effect of added water, small amouts (0.1-l -0%) of water were added to a solution of bezophenone in 1% cyclohexane-acetonitrile. The fluorescence lifetime decreased with larger amounts of water. The corresponding rate constant is linear in water concentration and is described by an apparent rate constant of 7.6 X 1O8 IM- 1 so- I_ Further experiments are needed to identify the reaction. While the reaction of the excited radical with the hydrogen donor provides a reason for the permanent loss ofabsorption by removing BPK and producing the corresponding alcohol that absorbs below 300 nm, there must be some other pathway as well. For example, excited BPKin acetonitrile with 1%cyclohexane decays almost totally by whatever reaction occurs in neat acetonitrile and this system does show a significant permanent bleach in the UV. Some other product must be fomied from D, but is not clear at this time what that product is_
4. Conclusion BPKin cyclohexane has been excited to the D, and D2 states. The results are summarized in the diagram of fig. 4; a number of processes are evident. Excitation
0”
-7
-3eV
-
Prcduc!
Pk. 4. Scheme showinS the various pbotoprocesses occurring in BPK. Product refers mainly to the product of H atom abstraction from the hydrogen donor. However, there many be other products formed as well (see test)_
Volume
112, number
3
CHEMICAL
PHYSICS
of the radical at either 532 or 355 nm leads to the same behaviour suggesting rapid relaxation from D, to D1. D1 + DI1 absorption with an apparent peak at 350 nm has been observed_ The state D, is at =5 eV above the radical ground state and so contains a very Large excessofener,y+ The D, state fluoresces but also reacts rapidly with hydrogen donors and appears to undergo some type of reaction in acetonitrile as well. Further study of the reactions of D1 appears to be worthwhile. No evidence was found for a state of lifetime --J, ~_lswhich could be identified with the Q,, state proposed by Thurnauer and Meisel [6].
LETTERS
The authors are indebted to Dr. PK. Das for many helpful discussions. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2604 from the Notre Dame Radiation Laboratory.
December 1984
References [l] [2] [3] [4]
[S] [6] [71
Acknowledgement
7
[S] [9]
[ 10) [ 111 j12]
B-W_ Hodgson, J.P. Keene, E.J. Land and A. J. Swallow, 1. Chem Phys. 63 (1975) 3671. R. hiehnert, 0. Brede and W_ Helmstreit, 2. Chem. 15 (1975) 448. K. Obi and H. Yamagucbi, Chem. Phys. Letters 54 (1978) 448. H. Baumann, C. Merckel, H.J. Tiipe, A. Graness, J. I(leinschmidt, 1-R. Gould and N.J. Turro, Chem. Phys. Letters 103 (1984) 497. H. Baumann, K.P. Schumacher, H.J. Timpe and V. Rehak. Chem. Phys. Letters 89 (1982) 315. MC. Thurnauer and D. Meisel, Chem. Phys. Letters 92 (1982) 343. C.l.hf_ Bru_%an. R.P.H. Rettschinck and G-J. Hoytink. Chem. Phys. Letters S (1971) 263. V. Nagarajan and R.W. Fessenden, submitted for publication. M-R. Topp, Chcm. Phys. Letters 39 (1976) 423; K. Razi Naqvi and Up. Wild, Chcm. Phys. Letters 41 (1976) 570. hl. Hoshino, S. Arai, ht. Imamura, K:. Ikehara and Y. Hama, J. Phys. Chem. 84 (19SO) 2576. A. Bromberg, K.H. Schmidt and D. hleisel, J. Am. Chem. Sot. 106 (1984) 3056. J.K.Thomas, J. Phys. Chem. 71 (1967) 1919.
211
Volume 112, number 3
PHYSICS
LETTERS
7 December
1984
FLASH PHOTOLYSIS OF TRANSIENT RADICALS. BENZOPHENONE KETYL R-ADICAL V. NAGARAJAN
and Richard W. FESSENDEN
Radtition Laboratory and Deparrmenr of Chemisity. University of Norse Dame, Notre Dame, Indiana 16556, Received 29 June 1984; in final form 10 September
LiSA
1984
A number of Processes are found ro follow escitation of the benzophenone ketyl radical. The lowest excited state of theradicalabsorbs, fluorescesandreacts withsolvent. Rate constantsforreaction of thisstate with cyclohesaneand isopropan01 are 4 x lo7 and 7 x 10’ M-l s-l, respectively_ The lifetime of 5.1 ns found in a solution containing 1% cwlohesanc in acetonitrile at
room temperature is longer than that reported previously.
l_ Introduction Fluorescence
from the first excited state of benzo-
phenone ketyl radical (BPK) was first reported by Hodgson et al. [l]. Subsequently Mehnert et al. [2] measured the fluorescence lifetime to be 2.0 2 0.3 ns at room temperature using, in tandem, an electron pulse and a laser pulse. They also reported a break in the temporal absorption profile corresponding to bleaching of the ground state ketyl radical by the laser pulse. Since the “bleach” did not recover, they tentatively suggested a reaction of the type (Ph?COH)*
+ S + product
,
where S is some unknown scavenger_ Obi and Yamaguchi [3] determined the fluorescence lifetime at 77 K to be ~17 ns and ~20 ns in ethanol and EPA glasses, respectively. They ascribed the shortening of lifetime at room temperature to either diffusion-controlled bi-
cases lies above the first excited doublet. However, the time-resolved ESE results seem to indicate the possibility of populating Q. via ISC from D2, making BPK a good chaise for optically detecting the quarter in liquid solution. Except for the instance mentioned above [2], there has also not been any attempt to characterize the photochemistry of BPK
2. Experimental
molecular quenching or some kind of chemical reaction from the excited ketyl radical. Measurements of the rate of fluorescence quenching for a number of reactants have established the main reaction of the excited radical to be electron transfer [4] but high reactivity toward H atom donors has also been mentioned [5]. Using the transient electron spin echo (ESE) technique, Thumauer and Meisel [6] studied the photoreduction of benzophenone by various substrates. In order to explain a slowly growing ESE emission signal for BPK (time constant 02 w), They invoked intersystern crossing of the ketyl radical from its second ex0 OOg-2614/84/$ 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
cited doublet state (D2) to a quartet state (Qo) that lies energetically between the D, and the Dz states. The Q. state was then assumed to intersystem cross spin selectively to the ground state in appro-ximately 2 /z, giving rise to the slowly growing emissive signal. Except in one instance where quartet-doublet phosphorescence was observed 171, optical detection of the quartet state of organic radicals has not been reported. The main problem in such an observation would be populating the quartet state that in many
Absorption and initial fluorescence measurements were made with the apparatns described elsewhere [S]. The ketyl radicals were produced by photolysis of ~1 X 10m5 M solutions of benzophenone at 248 nm with light from a Tachisto 1 SO-XR excimer laser. The solvent was either cyclohexane or acetonitrile with an added hydrogen donor. Subsequent photolysis of BPK was with either 355 or 532 nm from a Quanta-Ray DCR-1 YAG laser (pulsewidth 6 ns). For detailed measurements of fluorescence decay, the initial radical B.V.
207
CHEhlICAL PHYSICS LETTERS
Volume 112. number 3
production was with 266 nm light from the YAG laser and excitation of the radical was at 337 m-n with a PRA LN-1000 nitrogen laser (800 ps pulsewidth). A Tektronix 7912 AD digitizer with 7A29 vertical amplifier was used to record the time prome of the fluorescence_ The digitizer was triggered directly from the light pulse by means of a vacuum photodiode. The total system response (excitation pulse, photomultiplier, and digirizer) corresponded to a time constant of about 1.8 ns for laser light scattered into the photomultiplier. Benzophenone and 4,4’-dichlorobenzophenone (both Aldrich Gold Label) were recrystallized from ethanol. Cyclohexane (Fisher), isopropanol (Fisher), acetomtrile (Aldrich Gold Label and B&J High Purity Solvent with
7 December 1984
tion with a lifetime of a few ps_ Absence of a new absorption when BPK was excited at 355 nm could be due to a poor extinction coefficient for the quartetquartet transition and hence population of Qo cannot be ruled out based on this result alone. A rapidly decaying fluorescence with a lifetime shorter than our experimental time resolution was observed when BPK was excited either at 35.5 or at 532 nm. The spectrum of this fluorescence agrees well with those which have been published [ 1,9]_ In the absence of 248 nm light, the 355 nm excitation gave rise to a small increase in absorption, but no fluorescence_ At short time scales, a quickly decaying absorption followed by the permanent bleach was seen at around 350 nm. Since the exciting and monitoring wavelengths were very close for 355 nm excitation, scattered light posed a serious problem at this laser wavelength. Hence only 532 nm light was used to excite BPKfor observation of the short-lived absorption signal. The time profile of this absorption essentially follows the exciting
3. Results and discussion
laser pulse and may match that of fluorescence (fig. 2). The absorption spectrum corresponding to this short-
The ketyl radical was excited at both its UV (Do + D3) and visible (Do + D1) bands using the 355 and 5i2 nm outputs of the laser, respectively_ On the mi-
lived state is shown in fig. 3. This spectrum is tentatively assigned to a D1 + D, transition. The permanent bleach must result from a chemical change in the
crosecond time scale, a permanent bleach in the ketyl radical absorption is seen corresponding to excitation
D, state because it follows excitation in the visible band. It should be noted that the maximum of this
in cyclohexane at either wavelength at room temperature (fig. 1). This observation is in agreement with that of hlehnert et al. [2]. The bleach amplitude was found to be linearly dependent upon the laser intensity. A photobleaching spectrum revealed no new absorp-
spectrum may not be the true peak. The absorption due to BPK rises rather steeply starting at around 350 nm and hence the bleaching in BPK absorption might cancel the increase in D, absorption beginning at this wavelength. Because of the very short lifetime of the D, state, it could not be confidently determined, for wavelengths shorter than 340 nm, whether the bleach
Time. ps
Fig. 1. Production and excitation of benzophenone ketyl radical (BPK) in cyclohesane. The bleach in absorption was caused by the excitation of BP6 at 532 nm and was observed at 335 nm. No change in absorption was seen when BP triplet alone was excited.
208
signal showed a fast initial change corresponding to D, absorption followed by a slower change. It is probable that the state responsible for the shortlived absorption is that which also fluoresces_ An experiment at low temperature where the fluorescence lifetime is larger should allow verification of this suggestion. BPK in acidic ethanol glass at 77 K was prepared by radiolysis [lo] and was excited at 337 MI. A fluorescence spectrum identical with the room temperature spectrum was obtained. The fluorescence lifetime was found to be ~23 ns, slightly longer than the published value [3]. When the absorption was monitored close to the laser wavelength, light scattered by the bubbling N2 saturated the detector_ Band pass fd-
Volume 112, number 3
CHEMICAL PHYSICS LETTERS
7 December 1984
I
I
Fig. 2. Room temperature(left) and 77 K (right) absorption (open circles) and emission (closed circles) signals caused by escitation of BPK at 532 nm. The absorption and emission s?@~aIswere monitored at 350 and 580 nm for the left portion in qclohesane and at 348 and 570 nm for the right portion respectively_The decays of the xoom temperature s&x& are limited by the laser pulse width and the instrument rise time. ters were used to keep this scattered light from reachAn instantaneous rise in absorption followed by a decay with a time constant of *24 ns was observed at 348 nm (a wavelength for which a ing the detector.
band pass filter was available). The agreement between
fluorescence and absorption the origin D, state. tion have radical [I
lifetimes confirms that of the short-lived absorption is indeed the A similar fluorescence and transient absorprecently been reported for diphenylmethyl 1 f_ The D1 absorption spectrilm at 77 K
WAVELENGTHhd Fig_
3. D1-t Dn absorption spectra of BPK (closed circIes) and
DCl,L%PK(open circles) in cyclohcsane, normalized TOthe same height. Inset: absorption spectra of BPK (solid fine) and
DCIBPK (dotted line) with UV peaks normalized to the same hejght to show the shift. In rea3ity, however, the shift is even kuger because the visible bands axe of comparable strength.
could not be recorded because of the lack of a sufficient number of band pass fflters in the 340-370 nm region and the OD at 532 nm was too small to produce any significant conccentration of excited BPK After the fast initial decay, a very flat residual absorption remained. A spectrum taken at this flat region showed it to be the triplet absorption. Some BPK and benzophenone must be simuitaneously excited by the 337 nm light. A steady state absorption spectrum taken after the photolysis showed an increase in BPK absorption from the one taken before photolysis. This increase must be due to the triplets formed upon excitation at 337 nm. (Formation of triplets could not be avoided, since a further decrease in the concentration of benzophenone would have required a longer radiolysis time which has associated with it problems in replying the liquid Nz_) The region of 300-650 nm was scanned and for the entire range only a step increase in absorption that subsequently stayed flat for at least 10 ius could be seen_ Even if Q. did not absorb in this region, a bleach signal lasting a few JIZG and superimposed on the triplet absorption would have been observed if Q. had been formed. Except for the unlikely event that Qo has an absorption spectrum identical with that of Do, it has to be concluded that Qo is not populated to any significant degree in the excitation of BPK and that D2 + D, internal conversion is a vea efficient process. Similar results were obtained for 4,4’-dichlorobenzo209
Voiunre 112. number 3
phenone ketyl radical (DCIBPK) upon excitation at room temperature. The D, + D,, absorption spectrum for DCIBPK is given in fig. 3. It is seen that the spectrum is red-shifted compared to that of BPK. This shift parallels that in the spectrum of ground state DClBPK. The reported reactivity of BPK towards H atom donors [5] suggests that the permanent bleach seen in fig. 1 could be the result of the excited radical reacting with the solvent_ In order to investigate this possibility, measurements of fluorescence lifetime were made using the SO0 ps pulse of the PRA laser for excitation as described in section 2. The apparatus response function was determined by scattering laser light into the monochromator and the observed fluorescence curves were analysed by deconvolution. No fluorescence was observed in the absence of the first laser pulse. Contrary to earlier reports [4,5] which used a one-color-two-photon method and no H atom donor, the lifetime in acetonitrile with 1% (u/u) cyclohexane as hydrogen donor was found to be 5.1 + 0.1 ns. The previously reported value of 2.3 ns was subsequently improved to 3.5 ns [4]. Although the excited state is very reactive toward good hydrogen donors or water (see below), it would take more than 0.1% impurity in the acetonitrile to cause such a shortening in liferime. The lifetime in neat cyclohexane was found to be 2.5 + 0.2 ns. Data for various concentrations (l-10%) of isopropyl alcohol in acetonitrile were also obtained_ A plot of pseudo first-order rate constant against conten tration gave a straight line with a slope corresponding to a rate constant of 5 X 10s M- 1 s-t for reaction of the excited radical with isopropanol. Such an analysis was not pratical for cyclohexane in acetonitrile because the reaction is slower and cyclohexane is miscible with acetonitrile only up to 13%. Based on the lifetime at 100% cyclohexane and taking the natural emission lifetime to be 20 ns, the rate constant for hydrogen abstraction from cyclohexane is found to be about 4 X 1O7 hi- 1 s- 1_These rate constants are much higher than those for the ground state carbon-centered radicals. For example, the reaction of metyl radical with isopropanol has a rate constant of about 3 X 1 O3 M-l s-l 1121. The lifetime of 5.1 ns for the excited radical in acetonitrik may also involve some type of reaction because the lifetime at low temperature is ~20 ns. This reaction is more evident in the case of DCIBPK where the lifetime in acetonitrile (with 5% cyclohexane) is Zess than that in pure cyclohexane 210
7 December 1964
CHEMICAL PHYSICS LETTERS
(2.7 versus 3.9 ns). It is possible that ionization of the OH proton occurs and that the anion form of BPK does not fluoresce_ The dichloro derivative should be more acidic and therefore, undergo deprotonation more rapidly. To determine the effect of added water, small amouts (0.1-l -0%) of water were added to a solution of bezophenone in 1% cyclohexane-acetonitrile. The fluorescence lifetime decreased with larger amounts of water. The corresponding rate constant is linear in water concentration and is described by an apparent rate constant of 7.6 X 1O8 IM- 1 so- I_ Further experiments are needed to identify the reaction. While the reaction of the excited radical with the hydrogen donor provides a reason for the permanent loss ofabsorption by removing BPK and producing the corresponding alcohol that absorbs below 300 nm, there must be some other pathway as well. For example, excited BPKin acetonitrile with 1%cyclohexane decays almost totally by whatever reaction occurs in neat acetonitrile and this system does show a significant permanent bleach in the UV. Some other product must be fomied from D, but is not clear at this time what that product is_
4. Conclusion BPKin cyclohexane has been excited to the D, and D2 states. The results are summarized in the diagram of fig. 4; a number of processes are evident. Excitation
0”
-7
-3eV
-
Prcduc!
Pk. 4. Scheme showinS the various pbotoprocesses occurring in BPK. Product refers mainly to the product of H atom abstraction from the hydrogen donor. However, there many be other products formed as well (see test)_
Volume
112, number
3
CHEMICAL
PHYSICS
of the radical at either 532 or 355 nm leads to the same behaviour suggesting rapid relaxation from D, to D1. D1 + DI1 absorption with an apparent peak at 350 nm has been observed_ The state D, is at =5 eV above the radical ground state and so contains a very Large excessofener,y+ The D, state fluoresces but also reacts rapidly with hydrogen donors and appears to undergo some type of reaction in acetonitrile as well. Further study of the reactions of D1 appears to be worthwhile. No evidence was found for a state of lifetime --J, ~_lswhich could be identified with the Q,, state proposed by Thurnauer and Meisel [6].
LETTERS
The authors are indebted to Dr. PK. Das for many helpful discussions. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2604 from the Notre Dame Radiation Laboratory.
December 1984
References [l] [2] [3] [4]
[S] [6] [71
Acknowledgement
7
[S] [9]
[ 10) [ 111 j12]
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