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Photoinduced ion and ketyl radical formation in covalently linked benzophenone—dimethylaniline studied by time-resolved transient absorption spectroscopy with picosecond resolution
J. Photochern. Photobid. A: Chem., 70 (1993) 83-93
83
Photoinduced ion and ketyl radical formation in covalently linked benzophenone-dimethylaniline studied by time-resolved transient absorption spectroscopy with picosecond resolution G. Hiittmann,
W. Kiihnle
and H. Staerk
Abteihmg Spelctroskopie,Mar-Planck-Institut flir Biophysikaltiche Chemie, Postfach 2841, W-3460 Gijttingen (Gomany) (Received May 29, 1992; accepted September 10, 1992)
Abstract Time-resohred transient absorption (TRABS) experiments were carried out on a picosecond to nanosecond time scale in order to determine the mechanism of photoinduced hydrogen transfer between benzophenone (Bp) and dhnethylaniline (DMA) covalently linked by a methylene chain (Bp(CXL&DMA), abbreviated B3D. The results obtained with the polymethylene-linked system B3D in methylcyclohexane (MCH), acetonitrile (ACN) and isopropanol (i-PrOH) have been compared with earlier investigations on free (unlinked) systems. With B3D in ACN and i-PrOH triplet formation and electron transfer were observed within a time window of O-9 ns, but no proton transfer was detected. In the non-polar solvent MCH intramolecular hydrogen transfer takes place leading, within 10 ns, to the ketyl radical, but with no radical ion formation as an intermediate reaction. These results are different from investigations reported on free systems, where radical ion formation is regarded as a requirement for subsequent efficient ketyl radical formation. Inferences can be drawn from this
study which point to the greater importance of relative geometrical orientation reactions compared with electron transfer reactions.
1. Introduction
Benzophenone is a molecule of great spectroscopic interest, since it allows the study of two fundamental photophysical processes simultaneously. After excitation, electron transfer and proton or hydrogen transfer can be induced. The very high intersystem-crossing rate limits the lifetime of the first excited singlet state of benzophenone to only 10 ps [l] and leads to a triplet yield of close to unity. Electron transfer is possible from both the S, and T, states; the two states are separated by only 0.25 eV and, in polar solvents, lie energetically above the radical ion pair state [2]. With the choice of appropriate donors benzophenone provides the interesting opportunity, in principle, to observe radical ion pairs and exciplexes in different spin states [3-6]. In suitable solvents the diphenyl ketyl radical (BpH’) is also observed after excitation of benzophenone [7]. It is formed either through hydrogen transfer to the neutral benzophenone or by protonation of the radical anion. If the reaction mechanism is not yet firmly established we refer in the following to a hydrogen transfer. lOlO-6030/93/$6.00
in hydrogen or proton transfer
1.1.The early model. Model 1 (Scheme 1) In the presence of aliphatic or aromatic amines (AH) with good electron donor properties the ketyl formation reaction proceeds unusually rapidly. Cohen et al. [8] therefore suggested that the hydrogen transfer takes place in two steps via a charge transfer species, according to Scheme 1 (model 1). The individual transients of this photoreaction can be identified easily using their characteristic absorption bands. The first excited singlet state of benzophenone (‘BP*) absorbs in a broad spectral range from 400 to 600 nm with a maximum at 575 run [9]. The triplet state (‘BP*) shows a very characteristic band with a maximum at 530 nm. The band of the ketyl radical (BpH) is found slightly shifted to the red. Et can be differentiated well by its maximum at 550 nm and its band shape. Triplet and ketyl absorption bands are largely independent of the solvent used, whereas the absorption maximum of the benzophenone anion (BP-) depends strongly on the polarity of the solvent. ‘8;
+ AH Ef
3[E3p-
AH+] A
S;ti
+
;
Scheme I.
@ 1993 - Elsevier Sequoia. All rights reserved
G. Hiittmann et al. I Ion and ketyl radical formation in bertzophenone-dimethylaniline
84
With the improved time resolution and sensitivity of transient absorption measurements in recent years it has been found that an increasing number of systems (benzophenone with different donors) exhibit radical ions as intermediate products preceding the protonation [9-151. Nevertheless, the exact mechanisms of electron and proton transfer of benzophenone are not sufficiently clear. Different models have been proposed which are based on contradictory experimental results. The starting point for our studies on benzophenone covalently bound to a donor molecule is the experience gained by others on free systems. The three most recent investigations and models derived from them are briefly summarized below. 1.2. Model 2 (Scheme 2) Simon and Peters [ll] have studied solutions of benzophenone (0.25 M) in acetonitrile (ACN) with the donors dimethylaniline (DMA) or diethylaniline (DEA) added at concentrations of 1 and 5 M. They observed that, immediately after excitation, the triplet absorption band at 525 nm, which decayed in 25 ps, was replaced by the absorption band of the ketyl radical. They noted a spectral shift of the Bp- absorption band from 720 to 690 nm which took place within 200 ps (with 1 M DEA or DMA as donor). The two different spectral positions of the Bp- band were assigned to the solvent-separated ion pair (SSIP) and contact ion pair (CIP) configurations. The interpretation of this set of measurements is summarized in Scheme 2. Electron transfer across a distance of about 7 8, leads to an SSIP which subsequently associates to a CIP. It was assumed that, in this tight complex, the proton is transferred from the donor to the acceptor (benzophenone). In ethanol, Simon and Peters [16] observed a Bpabsorption which shifted from 690 to 625 run. It was concluded that, in this protic solvent, weak ground state complexes (Bp-DMA), stabilized by hydrogen bonding, are present, and excitation leads directly to the CIP. Subsequent solvation and hydrogen-bridge binding of the keto oxygen to the OH group of the solvent shift the absorption maximum to 625 nm [17, 181. in
band in ACN, introduced in model 2, contradicts recent measurements by Devadoss and Fessenden [lo, 131 who observed a displacement of the Bpabsorption towards longer wavelengths, from 702 to 725 nm, within 50 ps; the concentration of the donors used (DEA and l+diazabicyclo[2.2.2]octane (DABCO)) was high (1 M). From this it can be concluded that, in contrast with model 2, a CIP-like state emerges after electron transfer which dissociates into an SSIP state in polar solvents. With all donors investigated these workers found indications for a CIP-like intermediate product prior to ketyl radical formation, in both polar and non-polar solvents. With the exception of triethylamine (TEA), which is not a good donor because of its high oxidation potential, the radical ion absorption could be followed spectroscopically, and Scheme 3 was established. After electron transfer the CIP is formed from which, in non-polar solvents, the BpK radical is generated with relatively high yield; however, in polar aprotic solvents the SSIP evolves from the CIP and predominantly recombines to the ground state. Only a small fraction is converted to the ketyl radical. However, in alcohols an ion pair (H-IP) bound by a hydrogen bridge is formed that can deactivate by recombination and also by ketyl radical formation. 1.4. Model
(Scheme 4)
Mataga and coworkers [9, 141 have communicated precise results of transient absorption measurements in the picosecond and femtosecond range with DMA and diphenylamine (DPA) as donors. By fitting the correct transient reference spectra, a detailed kinetic analysis of the absorption spectra was carried out which allowed the cal3B;
+ DEA m&$
.
+ DMA -
Bp- + DMA+SSIPjl2O”rn)
By-... LIMA+ W(690nm)
B;H
t DbA
-----+
Bp
alcohol
Bp-
alcohols: Bp ,..
Scheme 2.
DMA%Bp
DMA -
Bf..Otd ‘IP&W~~i
-
H.
“Bp+ DM& I4 - IP(620”rn,
Scheme 3.
+
DEA’
-%p-
+
.
BP H+DEA
lIEA+) ====-
ACN: ‘8;
in
3 (Scheme 3) The observed blue. shift of the Bp-
1.3. Model
DEA’
+ DEA
G. Htittmann Ed aL I ion and ketyl radical formalion in beruoplzetwne-dimethyianiIine
culation of the rate constants and yields for the observed (or conjectured) reactions. The important result of this investigation is the distinction between three different types of radical ion pairs formed either by electron transfer in the ,excited singlet or tripiet state or by excitation of a charge transfer complex in the ground state. The evaluation of the measurements carried out with the donor DMA revealed that the ketyl radical is not formed directly but has a radical ion pair as an intermediate species. AI1 three types of radical ions exhibit different rates for the protonation and hence different ketyl radical yields (4) (see Scheme 4). By exciting the charge transfer complex in the ground state (which is formed to some extent between benzophenone and DMA at high concentrations) a very short-lived singlet radical ion pair is formed so that proton transfer cannot be accomplished during its lifetime (85 ps). Moreover, at high DMA concentration (1 M), radical ions are formed directly from the singlet state of benzophenone or, after intersystem crossing, from the triplet state. Since the multiplicity does not change with the electron transfer, the generated radical ion pairs must be in the corresponding overall spin state. Yet, protonation of the triplet radical ion pair is 7.5 times faster than protonation of the singlet radical ion pair. As in the study associated with model 2 (Simon and Peters [ll]), a shift of the Bp- band to shorter wavelengths is observed with 1 M DMA in the solvent ACN. It is explained as a superposition of the radical ion absorption (with a maximum at 740 nm), which originates from the charge transfer complex, with the bands of the radical ion species absorbing at shorter wavelengths (690-720 nm). The fast decay of the band of the excited charge transfer complex at 740 nm, which dominates the spectrum at early times, appears as a shift of the whole transient absorption. The conflicting shifts of the Bp- band, reported in the measurements associated with models 2 and 4 on the one hand and model 3 on the other,
‘(B~:..DMA+)
‘EL
+ DMA
\
“6;
Bp...DMA-
Scheme 4.
6,~ +DMA
+ DMA-fBpY
.OMA+)
019
85
may be explained by traces of water in the solvent ACN, since the groups associated with models 2 and 4 used the solvent without further purification, whereas the group associated with model 3 used dry ACN. It has been observed that water forms hydrogen bonds with benzophenone that lead, depending on the amount of water, to a blue shift of the Bp- absorption [13, 191. Measurements with the donor DPA revealed that electron transfer and ketyl formation must be competing processes originating from a nondetectable encounter complex, in contrast with ketyl formation via the radical ion pair in the case of DMA [14]. Charge transfer complexes in the ground state have also been observed in this example. In order to explain the different ketyl radical yields, the three observed types of radical ions and the different reaction mechanisms with DMA and DPA, it has been assumed that geometrical factors (relative orientation of donor and acceptor) strongly influence the proton and hydrogen transfer. 1.5. Experimental approach with a covaletily linked donor+zcceptor system Almost all measurements of benzophenone and aniline (donor) have been carried out on nonlinked systems in different polar and non-polar solvents where diffusion of the reacting pair precedes the actual reaction. If it is intended to resolve fast processes, high donor concentrations must be used to achieve a small mean distance between benzophenone and donor. In the measurements cited above the amine concentration was between 0.1 and 5 M. However, a concentration of 5 M DEA in ACN means that there are five quencher molecules to four solvent molecules [lo]. In this case the concept of an SSIP becomes questionable, and it is not guaranteed that only two molecules participate in the reaction. It is also conceivable that electron and proton transfer take place with two different amine molecules at a time. A further complication is the formation of ground state complexes which is favoured at high donor concentrations [3, 9, 16, 201. As in earlier studies of electron transfer reactions from the singlet state [21-251, it is possible to use bridged systems in investigations with benzophenone in order to keep donor and acceptor at a certain distance from each other. Only a few reports are available of studies on alkyl-linked benzophenone-DMA [3, 12, 17, 26, 271 and these are of qualitative character.
G. Hiimnann et al. 1 Ion and ketyr mdical fornation in &nzopIwnone-dimethylaniline
86
If bridged compounds show a different behaviour from free donor-acceptor systems, new inferences on the mechanism of the photoreaction of benzophenone may possibly be drawn. To clarify this question, time-resolved transient absorption measurements were carried out on systems containing DMA and benzophenone covalently linked by a 1,Zpropylene chain (see Scheme 5). The propylene chain was used since it allows a diffusional (though restricted) passage of the reactants and a very close proximity of benzophenone and DMA, as has been shown with pyrene (or anthracene) as acceptor, which facilitates efficient exciplex formation [21, 23, 241. 2. Experimental
details
2. I. Materials
Benzophenone (Bp) was obtained from J. T. Baker. Benzophenone-(CH,),-DMA ([4-[3-[4-(dimethylamino)phenyl]propyl]phenyl]phenyl-methanone, Reg. No. 73060-15-8, abbreviated here as l33D) was synthesized in this laboratory and purified by high performance liquid chromatography (HPLC). The mean distance between benzophenone and DMA in this compound corresponds to the mean distance of a non-linked system at 1 M quencher concentration. At a concentration of about 3 X lop3 M of B3D, intermolecular effects need not be considered within the time window (maximum, 8.4 ns) of this study. Methylcyclohexane (MCH) from Baker was purified by column chromatography and dried with a molecular sieve (MS, 4 A). Isopropanol (i-PrOH) (Uvasol from Merck) was dried (MS, 3 A) and distilled. ACN (Uvasol from Merck) was dried with a molecular sieve (3 A) on a vacuum line before the samples were degassed in the usual way (six freeze-pump-thaw cycles). 2.2. Instrumental To obtain transient absorption spectra, a doublebeam spectrometer was constructed (the apparatus is shown schematicalIy in Fig. 1) combined with a picosecond Nd:YAG laser. The Nd:YAG laser oscillator is both passively and actively mode
Scheme
5.
locked, having a repetition rate of 0.3 s-l. One pulse is selected with an opto-electronic pulse slicer, amplified and split with a beam splitter. One part of the 1064 nm pulse is polarization rotated 90” with a A/2 plate, delayed 2 ns and reflected back in the original direction. The two pulses polarized perpendicular to each other are further amplified and then split with a polarizing beam splitter. The vertically polarized pulse is frequency tripled with KDP I/II crystals yielding 0.5 al at 354 nm and is used as the excitation pulse (pump pulse). The horizontally polarized pulse (probe pulse), 10-20 mJ at 1064 nm, is provided for continuum generation in the doublebeam spectrometer (see Fig. 1). The continuum is obtained by focusing the laser pulse into a 5 cm cell containing a 1:l mixture of D20-H,O. The continuum is split into two parts by a half mirror. The probe beam is focused onto the sample cell. The reference beam penetrates the sample at a non-excited area. The use of field lenses (Fl,, 2) is important. However, chromatic and spatial group velocity dispersion [28-301 leads to a travelling time difference in the wavelength range 700 nm (arrives first) to 470 nm (arrives last) of approximately 6 ps between the centre of the continuum generator and the sample cuvette [31]. A doublediode array (DDA) at the output of the polychromator is used as the detector in the spectrometer whose layout is shown schematically in Fig. 1 [31]. Signal averaging is carried out by integration over 7 pixels of the DDA corresponding to 5 nm. 2.3. Data evahation Data acquisition is accomplished with a personal computer equipped with a multi-I/O card and an IEEE bus. The transient absorption spectrum at a delay time I is calculated, in principle, from the signals of the probe beam and the reference beam (subscripts P and R) with and without excitation (superscript 0) of the sample. The absorbance A is A@#)=-log
$$ R
x $#) f
P
(1)
In practice, however, corrections have to be made. Generally, four different pairs of signals must be measured separately in order to obtain a correct absorption spectrum at a tied delay time. (1) The dark signals of the DDA are read without excitation of the sample and with probing beams off, yielding the background “spectra” U,(h) and UR(h) for probe and reference channel. (2) The signals of both detector arrays are read with the excitation beam off but probing and
G. Hiittmann et al. I Ion and J&y1 miical I !
Sample
Pump
fomhbn iv benwphenvne-dhethylan.%e
Pulse
iI
Probe
87
Pulse
Cuvette
CP200
Fig. 1. Double-beam spectrometer for time-resolved (picosecond to nanosecond) broad-band transient absorption measurements: BI, aperture (0.5 mm); 6, aperture (4 mm); B,, aperture (1x2 nun’); B,, aperture; Bs, aperture (5 mm); B, aperture; CP 200, supercorrected holographic grating spectrograph (Jobin Yvon); DDA, double-diode array (Spectroscopy Instruments); F,, filter (KG3, 2 mm or dielectric mirror); Fz, edge filter and various attenuation filters; Fs, filter (UG2, 1 mm); FL,,, FL, plano-convex field lenses (f- 60 mm, d-21 mm); LI, plaoo-uxwex lens (f=lOOO nun); L,, L,, acbromat cf==60 nun, d=18 mm); L,, achromat (f-80 mm, d - 18 mm); L,, achromat (f= 100 mm, d = 18 mm); L6, achromat (f= 120 mm, d =34 mm); L,, achromat (f= 80 mm, d = 34 mm); L, piano-convex quartz lens (f=SOO mm, d -21 mm); L+, piano-convex quartz lens (f-200 mm, d=Zl mm); ST, dielectric beam splitter; VP, AR plate or “magic angle” polarization rotator; MS, opal glass; P,, .Ps Glan Polarizer; PD,, PD,, PDJ, photodiodes; RF, spatial frequency filter (d-O.1 mm); S1, &, slits (&>S&
reference beams on. This yields the beam splitter ratio SglS& (3) The probing light is often superimposed by stray fluorescence light from the sample. Therefore the normalized fluorescence spectra FR and Fp and the fluorescence intensity r, as a function of the excitation energy E must be measured. (4) In the last step, the signals Sp and &, with the sample excited, are measured. The absorbance is then calculated from [31]
In order to improve the signal-to-noise ratio all individual measurements are carried out 10-20 times and averaged. The literature values of the decadic extinction coefficients of the transients are as follows (the dependence on the solvent is assumed to be insignificant): first excited benzophenone singlet, ~(575) nm) a3600 M- 1 an-‘; first excited benzophenone triplet, ~(530 MI) =6500 M-’ cn-‘; ketyl radical, ~(545 nm)=4600 M-l cm-‘; ben-
zophenone
anion, ~(620-800) = 10 000 M-’ cm-’
PI* 3. Results 3.1. Ground stale absorption Ground state absorption measurements of B3D in the three solvents MCH, ACN and i-PrOH were carried out to determine whether some of the molecules are present as ground state complexes. In keeping with ref. 3, the spectra can be interpreted as the sum of the benzophenone spectrum and the spectrum of N,Wdimethyl-p-toiuidine (DMT). As with benzophenone alone in the different solvents, the n++ absorption band of B3D (340-380 nm) is shifted to shorter wavelengths with increasing polarity of the solvent, simultaneously with a red shift of the toluidine absorption edge. ln ACN and i-PrOH a weak absorption is recognized between 390 and 420 nm that cannot be explained by a benzophenone or DMT absorption. It may be assigned to a small fraction of ground state complexes as has also been found with free systems at high DMA concentrations
68
G. Hiittmunn et al. I Ion and ketyl radical fomration in benzo@e~one-dimethy!aniline
[9]. In MCH the absorption spectrum does not change in the temperature range 250-350 K. However, in ACN the absorption between 330 and 360 nm increases at low temperature. This effect may be caused by the stabilization of weakly bound complexes, but also may be due to an increase in the nti absorption as has been observed previously with benzophenone in ACN.
I................................I 470.
520.
570.
620.
670.
720.
770.
3.2. Transient absorption 3.2.1. Benzophenone in ACN, MCH and iP?OH
The transient absorption of benzophenone in the absence of a donor was measured for comparison with the polymethylene-linked system (B3D). The triplet spectrum appeared more rapidly than could be resolved by the apparatus, with a maximum at about 530 nm and somewhat shifted (AA=5 nm), but similar in shape, for all three solvents. Within the time range of measurement up to 8.4 ns no significant changes in the shape of the spectra could be observed (Fig. 2). The additional absorption at 470 and 580 nm within the time range where the pump and probe pulses overlap (5 ps) is caused by the first excited singlet state of benzophenone. From the absence of the ketyl band in all solvents used it can be concluded, in accordance with earlier measurements [32], that no appreciable hydrogen transfer from the solvent to benzophenone takes pIace within this time range. 3.2.2. 830 in MCH The absorption measurements carried out with B3D in MCH (see Fig. 3) showed the appearance of the benzophenone triplet, still within the excitation pulse in the first spectrum after 5 ps, superimposed with a small fraction of singlet abGptiofl. During the course of the measurement time .of 8.4 ns the maximum of the absorption shifts .to 545 nm accompanied by a change in the band shape. The spectral position and form allow the assignment of this band to the absorption of the intramolecularly generated ketyl radical [33] since, on the one hand, hydrogen abstractionfrom the solvent was not observed in pure benzophenone in this time range and, on the other, measurements with different concentrations of B3D (3X10m4 M to 6x low3 M) did not show differences in the spectra. The transient absorption spectra of B3D in MCH also exhibit a broad but weak absorption at about 700 nm. This cannot be attributed to a radical ion absorption, since the band does not change during the first nanosecond (see Fig. 3), i.e. the Bp- concentration remains constant in this time
15o.ps
559s
5.w 470.
520.
570.
620.
670.
720.
770.
Wavelength / [nm]
Fig. 2. Transient absorption spectra of benzophenone in MCH. Lower frame: time evolution of the absorption; the delay times between exciting and probe pulses are indicated in the figure. Upper frame: comparison of the spectra normalized at 530 am.
period. If ketyl radicals were generated from such radical ions, their slow formation with a constant radical ion concentration could not be explained. The weak broad band probably belongs to the triplet spectrum of B3D. In comparing these results with measurements reported on non-linked systems in non-polar solvents, it should be noted that there, in contrast with the measurements on B3D, electron transfer was observed [lo, 151. With DEA (0.75 M), having an oxidation potential similar to DMA [2], radical ions are formed in benzene or cyclohexane within 1 ns. Even with DPA, which has a distinctly higher oxidation potential than DMA, at a moderate concentration of 0.13 M, electron transfer at a nearly diffusion-limited rate is observed in isooctane [14].
89
G. Hiittmmm et al. / Ion and ketyl radical fomzntion in berrzophenone-dimethylaniline
70.
520.
570.
620.
670.
720.
770.
~................................I
I
470.
520.
570.
620.
720.
670.
770.
-X
64OO.ps -N
\
4400.ps
3504s
15o.ps
55.ps
5.ps 470.
529.
570.
620.
Wavelength
670.
720.
770.
470.
520.
720.
670.
770.
/ [nm]
Fig. 4. Transient absorption spectra of B3D in ACN. Lower frame; time evolution of the absorption; upper frame: comparison of the spectra normalized at 750 nm.
3.2.3 B3D in ACN In contrast with the measurements
in MCH, rapid electron transfer takes place in the highly polar solvent ACN (see Fig. 4). The triplet band disappears within a few hundred picoseconds and the absorption band of the benzophenone radical ion builds up at about 750 nm. Even at very early times the triplet spectrum is superimposed by a broad absorption, spanning the whole spectral range. By making use of the reference spectra for the B3D triplet absorption and the radical ion absorption, the time dependence of the relative transient concentrations can be determined (Fig. 5). The lifetime of the triplet and the rise time of the radical ion absorption are both 120 ps. However, a good fit is only obtained with the assumption that 80% of the radical ions are formed from the triplet, while 20% are generated during the wurse of excitation. An early triplet spectrum
620.
Wavelength
/ [nm]
Fig. 3. Transient absorption spectra of B3D in MCH. Lower frame: time evolution of the absorption; upper frame: comparison of the spectra normalized at 543 nm.
570.
0.6
0.5
s ‘G
I
I
I
-
0.4
-
0.3
-
I
I
I
(%p
.
triplet
.
radical ions (BP-)
I
I
)
2 5
e 8 L %
I
0.2 -
-200
0
200
400
Time
600
800
1000
1200
/ [PSI
Fig. 5. Relative concentration of triplet and radical ions of B3D in ACN. Deconvolution and fitting of exponentials yielded the functions (full line): Ctnplct= 0.56exp( -r/l2Ops),C,,,-0.29 -0.23 exp( -t/120 ps). Transient reference spectra were B3D in MCH at 150 ps and B3D in ACN at 8.4 ns.
90
G. Hiinmonn et al. / Ion and ketyl radical formation in benzophenone-dimethylaniline
of B3D iu MCH was used as a reference spectrum Identical decay and rise times, with a ratio of 30% to 70% for fast and slowly formed radical ions respectively, were determined if the reference spectrum of 3Bp* in ACN was used. The prompt formation of radical ions is in accord with measurements by Miyasaka et al. [9] with non-linked benzophenone and DMA, who also observed fast Bp- generation from the singlet state or a ground state complex. The rate of the electron transfer reaction of B3D in ACN is comparable with that of a free, non-linked system at a high donor concentration [9-Ill. The maximum of the Bp- band of B3D appears far in the red compared with the free system. A temporal shift of the band was not detected, although the blue shift observed by Mataga and coworkers [9, 141 and by Simon and Peters [ll] at 200 ps to 1 ns (see above) should also be seen in these measurements. The red shift seen by Devadoss and Fessenden [lo, 131, which was attributed to the transition from the CIP to the SSIP and took place within 50 ps, is three times faster than the electron transfer in our case and cannot be resolved+ Compared with measurements on free systems, the ketyl band is absent. According to the measurements of Mataga and coworkers [9, 141 on free systems of benzophenone and DMA, a proton is transferred within 200 ps in the radical ion pair of triplet spin multiplicity, essentially independent of the DMA concentration, and via the singlet radical ion pair the ketyl radical appears within 1.5 ns. 3.2.4. B3D in i-PrOH As in ACN electron transfer but no proton transfer is observed in the first 9 ns in the alcohol i-PrOH (Fig. 6). The kinetics are somewhat more intricate. In the transient absorption spectrum 15 ps before the full overlap of the pump and probe pulses (spectrum at - 15 ps), an extremely broad transient absorption appears that covers the whole wavelength range of the measurement. From this background the well-known triplet band emerges. During its decay within 1 ns the bands of the radical ion pair, Bp- and DMA+ build up. The strong blue shift of the Bp- band to 650 nm is due to hydrogen-bridge binding and is in accord with pulse radiolysis experiments on benzophenone in i-PrOH at low temperatures which yield benzophenone anions after irradiation with electrons [34]. This shift is a dynamic process that is terminated after 200 ps in non-linked benzophenone. In the B3D-i-PrOH system, the dynamics of the
470. 520. 570. 620. 670. 720. 770. I
3I_
’ I 64oo.ps 650.~~
3oo.ps
19o.ps
75.ps
5.ps
-1 !i.ps
470. 520. 570. 620.
670. 720. 770.
Wavelength / [nm] Fig. 6. Transient absorption spectra of B3D in i-PrOH. Lower frame: time evolution of the absorption; upper frame: comparison of the spectra normalized at 650 urn.
shift of the Bp- band cannot be resolved because the electron transfer reaction is too slow (approximately 700 ps). A kinetic analysis of the series of transient absorption spectra, using the end spectrum at 8.4 ns and the benzophenone triplet spectrum as reference spectra, shows an exponential decay of the triplet concentration with a time constant of about 670 ps (Fig. 7). The absorption within the radical ion band shows two components. One fraction, which is formed within the excitation pulse, decays relatively rapidly. At the same time, a second fraction appears with a rise time of about 500 ps. As in ACN, some of the radical ions must be formed either from the first excited singlet state or directly from charge transfer complexes of the ground state, whereas the slowly rising fraction is formed from the triplet state by electron transfer in a dynamic quenching process. Because of the slow electron transfer reaction in the triplet state,
G. Ii&mamz
et al. f Ion and keyI mdical formation in benzophenc+ae-dimethylaniline in ACN
0.30
or i-PrOH .‘i?iMDMA-
91
: _I
0.25
5 -E f E ; 2
0.20
0.15
0.10
in MCH
0.05
0.00 0
500
1000
Time /
1500
2000
[ps]
Fig. 7. Relative concentration of triplet and radical ions of B3D in I-PrOH. Deconvolution and fitting of exponentials yielded the functions (fill1 lie): Ctrfplct= 0.19 exp( -t/670 ps), CRIp=O.ls(l -exp(-r/500 ps)}+O.lO exp(-rfl30 ps). Transient reference spectra were ‘BP* in i-PrOH at 150 ps and B3D in i-PrOH at 8.4 ns.
the decay of the singlet radical ions, which can recombine in a spin-allowed process to the ground state, is directly observable in contrast with the measurements in ACN. Proton transfer is not observed within the first 9 ns. The fitting of the reference spectra used to calculate the relative concentrations cannot take into account a timedependent change in the absorption band shape. It is therefore not surprising that the decay time of the triplets and the rise time of the radical ion absorption do not correspond well. An uncertainty in the time constants of f 15% must be tolerated. 4. Discussion Our results can be summarized as shown in Scheme 6. As in the free system in ACN and iPrOH, the fraction of radical ion absorption which appears quasi-promptly with the excitation pulse can be distinguished from the other radical ion fraction which is formed from the locally excited triplet state. Within the limits of the time resolution (approximately 25 ps), further distinctions with respect to the origin of the prompt radical ions - excitation of a charge transfer complex or electron transfer from the locally excited singlet state - cannot be made. If the prompt radical ions come from the singlet excited state ‘B*3D, the electron transfer reaction from the singlet molecule will have to be more than one order of magnitude faster than that from the triplet molecule in order to compete with intersystem crossing (‘BP* to 3Bp*). From earlier studies of intra-
B~MDMA
Scheme 6.
molecular fluorescence quenching in compounds similar to B3D, namely pyrene (Py) or anthracene (An) linked with DMA through a (CH,), bridge, carried out in this laboratory [23-251, the following rate coefficients for electron transfer from the fh-st excited singlet state are known: Py3D (in ACN), 16 X 10’s_‘; Py3D (inn-PrOH) 2.4 X lo9 s-l; An3D (in ACN), 95X 10’ s-l; An3D (in ~-PI-OH), 6.7~ 10’ s-l. These values can be compared with the rate coefficients obtained from the present study for intramolecular triplet quenching of 3B*3D (see Figs. 5 and 7): 8.3~ IO9 s-l in ACN and 1.5X 109 s-l in i-PrOH. It is interesting to note that intermolecular quenching rate constants of approximately 2 X lOlo M-’ s-l in free systems of 3Bp* and DMA in ACN have been reported [9, lo]. From this we can conclude that a small fraction of singlet radical ion pairs may be formed from the locally excited singlet state. However, the short lifetime of the directly observed radical ions in iPrOH favours the concept of an immediately excited charge transfer ground state complex, as suggested by Miyasaka et al. [9] for free systems. It is worth noting that, in contrast with investigations of free systems in MCH, hydrogen transfer takes place in B3D without preceding radical ion formation, whereas in ACN and i-PrOH only electron transfer is observed. Electron transfer is obviously not a requirement but rather seems to impede ketyl formation of B3D. The function of the polymethylene chain between benzophenone and DMA is to keep the reactants in close proximity witbin a defined range. Moreover, it is expected that the aliphatic chain which substitutes one hydrogen atom has only a moderate influence on the physical and chemical properties of the donor and acceptor (singlet and triplet energy levels, redox potentials); however, it restricts the number
92
G. Hiittmann et 01 I Ion and ketyi radical formation in benzopherwne-dimethylaniline
of relative orientations of the molecules considerably. The different behaviour of free and polymethylene-linked systems may be explained by geometrical factors. It is generally assumed that, for proton transfer, the donor and acceptor must be at a very close distance, so that greater importance is attributed to the orientation of the molecules for proton transfer than for electron transfer. Electron transfer is generally not a necessary precondition for fast hydrogen transfer between benzophenone and DMA (i.e. within a few nanoseconds). On the contrary, it is conceivable that the large increase in the proton transfer rate caused by a preceding charge transfer, as observed in free systems, is due to electrostatic interaction which keeps the generated ion pairs, Bp- and DMA+, at an encounter distance for a prolonged time, thus increasing the chance for an orientation of donor and acceptor groups favourable for proton transfer. In the case of B3D in MCH, this function can be accomplished by the chain. During the course of the diffusional motion of the chain, a favourable conformation for hydrogen transfer is attained with a certain fmite probability. The low polarity of the solvent (MCH) makes electron transfer either very slow or energetically impossible (see ref. 35 for thermodynamic aspects), so that electron transfer can be neglected for ketyl formation. The highly polar solvents ACN and i-PrOH lower the energy of the radical ion pair state, stabilizing the charge transfer ground state complexes and allowing triplet ion pairs to be generated efficiently. The excited charge transfer complexes do not permit proton transfer because of their short lifetime. In addition, it is assumed that their structure is relatively rigid, possibly not allowing an alignment favourable for proton transfer [9]. Likewise, the triplet radical ions seem to be kept in a position, presumably due to the restricted degrees of freedom of the (CI-Q, chain, which does not favour proton transfer. Measurements on systems with different rigid and flexible molecular bridges may help to elucidate the mechanism of hydrogen abstraction in the benzophenone-DMA system in more detail. Such studies are in progress. Furthermore, compounds Bp (CH&-DMA (n =9, 16) have been synthesized and selected for magnetic field studies of the influence of intramolecular dynamics on the spin dynamics of radical ion pairs, not in the singlet [25] but in the triplet spin state.
Acknowledgments
We are grateful to B. Frederichs and H. Meyer for technical assistance. We acknowledge the valuable discussions with N. Ernsting, U. Werner and A. WieDner. This work was supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich SFB 93/D 1 “Photochemistry with Lasers”.
References
4 5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24
R. Hochstrasser. H. Lutz and G. W. Scott, Chem. Phys. Lett., 24 (1974) 162. B. M. P. Hendriks, R. I. Walter and I-X.Fischer, J. Am Chem. Sot., 101 (1979) 2378. H. Masuhara, Y. Maeda, H. Nakajo, N. Mataga, K. Tomita, H. Tatemitsu, Y. Sakata and S. Misumi, J. Am. Chem. Sot., 103 (1981) 634. M. W. Wolf, R. E. Brown and L. A. Singer, I. Am. Chem Sot., 99 (1977) 526. M. Hoshino, H. Seki and H. Shizuka, Chem Phys., I29 (1989) 395. M. Hoshino and K. Kogure, I. Phys. Chem., 93 (1989) 728. K R. Naqvi and U. P. Wild, Chem. Phys. Let& 41 (1976) 570. S. G. Cohen, A. Parola and G. H. Parson, Chem. Rev., 73 (1973) 141. H. Miyasaka, K. Morita, K. Kamada and N. Mataga, B&L Chem. sot. Jpn., 63 (1990) 3385. C. Devadoss and R. W. Fessenden, /. Phys. Chem., 95 (1991) 7253. J. D. Simon and K. S. Peters, /. Am. Chem Sot., 203 (1981) 6403. L. E. Manring and K. S. Peters, J. Am. Chem kc., 207 (1985) 6452. C. Devadoss and R. W. Fessenden, J, Phys Chem., 94 (1990) 4540. H. Miyasaka and N. Mataga, Bull. Chem. Sot. Jpr~, 63 (1990) 131. N. A. Borisevich, N. A. Lysak, S. A. Tiiomirov and G. B. Tolstorozhev, in T. Yajima, K. Yoshihara, C B. Harris and S. Shionoya (eds.), Ulinafast Phenomena vl, Springer Series in Physical Chemirby, Vol. 48, Springer, Berlin, Heidelberg, 1988, p. 565. J. D. Simon and X. S. Peters, 1. Am. Chem. Sot, 204 (1982) 6.542. J. D. Simon and K. S. Peters, Act. Chem. Res., 17 (1984) 277. M. Szwarc, Act. C&m. Res., 2 (1969) 87. E. Haselbach, P. Jaques, D. Pilloud, P. Suppan and E. Vauthey, J. phys. Chem., 95 (1991) 7115. S. Arimistsu and H. Tsubormura, Bull. Chem Sot. Jpn., 45 (1972) 1357. M. Schulz, H. Staerk and A. Weller, in J. B. Birks (ed.), Excited States of Biological Molecukv, Wiley, London, 1976. H. Staerk, R. Mitzkus, W. Kiihnle and A. Weller, in IL Eisenthal, R. Hochstrasser, W. Kaiser and A. Lauberau (eds.), Picosecond Phenomena HI, Springer Series in Chemical Physics, Vol. 23, Springer, Berlin, 1982, p. 205. J. Ihlemann, Dissertation, Univeristy of Glittingen, 1987. A. Hehnbold, D&&nuworlg University of GBttingen, 1988.
G. Hiittmann et ai. I Ion and ke&l m&al 25 H. Staerk, H.-G. Busmann, W. Kiihnle and R. Treichel, L Phys. Chem., 95 (1991) 19%. 26 H. Masuhara, T. Saito, Y. Maeda and N. Mataga, 1. Mol. sm&, 47 (1978) 243. 27 H. Masuhara, Y. Maeda, N. Mataga, K. Tomita, H. Tatemitsu, Y. Sakata and S. Misumi, Chem. Phys. Lett., 69 (1980) 182. 28 Z. Bor and B. Racz, Opt. Commun., 54 (1985) 165. 29 H. Staerk, J. Ihlemann and A, Helmbold, Las~?rOppoeleknoniIc, 20 (1988) 35 (in English).
formation in betuophenone-dirnethylaniline
93
30 Z. Bor, Opt. Left., I4 (1989) 119. 31 G. Hiittmann, Dissertution, University of GBttingen, 1992. 32 G. Porter and M. R. Topp, Proc. R Sot. London, Ser. A, 315 (1970) 163. 33 T. Shida and W. H. Hamill, J. Am. Chem. Sot., 88 (1966) 3683. 34 J. L. Marignier and B. Hickel, Chem Phys. Lett., 86 (1982) 95. 35 A. Weller, 2. Phys. Chem., 133 (1982)‘93.
83
Photoinduced ion and ketyl radical formation in covalently linked benzophenone-dimethylaniline studied by time-resolved transient absorption spectroscopy with picosecond resolution G. Hiittmann,
W. Kiihnle
and H. Staerk
Abteihmg Spelctroskopie,Mar-Planck-Institut flir Biophysikaltiche Chemie, Postfach 2841, W-3460 Gijttingen (Gomany) (Received May 29, 1992; accepted September 10, 1992)
Abstract Time-resohred transient absorption (TRABS) experiments were carried out on a picosecond to nanosecond time scale in order to determine the mechanism of photoinduced hydrogen transfer between benzophenone (Bp) and dhnethylaniline (DMA) covalently linked by a methylene chain (Bp(CXL&DMA), abbreviated B3D. The results obtained with the polymethylene-linked system B3D in methylcyclohexane (MCH), acetonitrile (ACN) and isopropanol (i-PrOH) have been compared with earlier investigations on free (unlinked) systems. With B3D in ACN and i-PrOH triplet formation and electron transfer were observed within a time window of O-9 ns, but no proton transfer was detected. In the non-polar solvent MCH intramolecular hydrogen transfer takes place leading, within 10 ns, to the ketyl radical, but with no radical ion formation as an intermediate reaction. These results are different from investigations reported on free systems, where radical ion formation is regarded as a requirement for subsequent efficient ketyl radical formation. Inferences can be drawn from this
study which point to the greater importance of relative geometrical orientation reactions compared with electron transfer reactions.
1. Introduction
Benzophenone is a molecule of great spectroscopic interest, since it allows the study of two fundamental photophysical processes simultaneously. After excitation, electron transfer and proton or hydrogen transfer can be induced. The very high intersystem-crossing rate limits the lifetime of the first excited singlet state of benzophenone to only 10 ps [l] and leads to a triplet yield of close to unity. Electron transfer is possible from both the S, and T, states; the two states are separated by only 0.25 eV and, in polar solvents, lie energetically above the radical ion pair state [2]. With the choice of appropriate donors benzophenone provides the interesting opportunity, in principle, to observe radical ion pairs and exciplexes in different spin states [3-6]. In suitable solvents the diphenyl ketyl radical (BpH’) is also observed after excitation of benzophenone [7]. It is formed either through hydrogen transfer to the neutral benzophenone or by protonation of the radical anion. If the reaction mechanism is not yet firmly established we refer in the following to a hydrogen transfer. lOlO-6030/93/$6.00
in hydrogen or proton transfer
1.1.The early model. Model 1 (Scheme 1) In the presence of aliphatic or aromatic amines (AH) with good electron donor properties the ketyl formation reaction proceeds unusually rapidly. Cohen et al. [8] therefore suggested that the hydrogen transfer takes place in two steps via a charge transfer species, according to Scheme 1 (model 1). The individual transients of this photoreaction can be identified easily using their characteristic absorption bands. The first excited singlet state of benzophenone (‘BP*) absorbs in a broad spectral range from 400 to 600 nm with a maximum at 575 run [9]. The triplet state (‘BP*) shows a very characteristic band with a maximum at 530 nm. The band of the ketyl radical (BpH) is found slightly shifted to the red. Et can be differentiated well by its maximum at 550 nm and its band shape. Triplet and ketyl absorption bands are largely independent of the solvent used, whereas the absorption maximum of the benzophenone anion (BP-) depends strongly on the polarity of the solvent. ‘8;
+ AH Ef
3[E3p-
AH+] A
S;ti
+
;
Scheme I.
@ 1993 - Elsevier Sequoia. All rights reserved
G. Hiittmann et al. I Ion and ketyl radical formation in bertzophenone-dimethylaniline
84
With the improved time resolution and sensitivity of transient absorption measurements in recent years it has been found that an increasing number of systems (benzophenone with different donors) exhibit radical ions as intermediate products preceding the protonation [9-151. Nevertheless, the exact mechanisms of electron and proton transfer of benzophenone are not sufficiently clear. Different models have been proposed which are based on contradictory experimental results. The starting point for our studies on benzophenone covalently bound to a donor molecule is the experience gained by others on free systems. The three most recent investigations and models derived from them are briefly summarized below. 1.2. Model 2 (Scheme 2) Simon and Peters [ll] have studied solutions of benzophenone (0.25 M) in acetonitrile (ACN) with the donors dimethylaniline (DMA) or diethylaniline (DEA) added at concentrations of 1 and 5 M. They observed that, immediately after excitation, the triplet absorption band at 525 nm, which decayed in 25 ps, was replaced by the absorption band of the ketyl radical. They noted a spectral shift of the Bp- absorption band from 720 to 690 nm which took place within 200 ps (with 1 M DEA or DMA as donor). The two different spectral positions of the Bp- band were assigned to the solvent-separated ion pair (SSIP) and contact ion pair (CIP) configurations. The interpretation of this set of measurements is summarized in Scheme 2. Electron transfer across a distance of about 7 8, leads to an SSIP which subsequently associates to a CIP. It was assumed that, in this tight complex, the proton is transferred from the donor to the acceptor (benzophenone). In ethanol, Simon and Peters [16] observed a Bpabsorption which shifted from 690 to 625 run. It was concluded that, in this protic solvent, weak ground state complexes (Bp-DMA), stabilized by hydrogen bonding, are present, and excitation leads directly to the CIP. Subsequent solvation and hydrogen-bridge binding of the keto oxygen to the OH group of the solvent shift the absorption maximum to 625 nm [17, 181. in
band in ACN, introduced in model 2, contradicts recent measurements by Devadoss and Fessenden [lo, 131 who observed a displacement of the Bpabsorption towards longer wavelengths, from 702 to 725 nm, within 50 ps; the concentration of the donors used (DEA and l+diazabicyclo[2.2.2]octane (DABCO)) was high (1 M). From this it can be concluded that, in contrast with model 2, a CIP-like state emerges after electron transfer which dissociates into an SSIP state in polar solvents. With all donors investigated these workers found indications for a CIP-like intermediate product prior to ketyl radical formation, in both polar and non-polar solvents. With the exception of triethylamine (TEA), which is not a good donor because of its high oxidation potential, the radical ion absorption could be followed spectroscopically, and Scheme 3 was established. After electron transfer the CIP is formed from which, in non-polar solvents, the BpK radical is generated with relatively high yield; however, in polar aprotic solvents the SSIP evolves from the CIP and predominantly recombines to the ground state. Only a small fraction is converted to the ketyl radical. However, in alcohols an ion pair (H-IP) bound by a hydrogen bridge is formed that can deactivate by recombination and also by ketyl radical formation. 1.4. Model
(Scheme 4)
Mataga and coworkers [9, 141 have communicated precise results of transient absorption measurements in the picosecond and femtosecond range with DMA and diphenylamine (DPA) as donors. By fitting the correct transient reference spectra, a detailed kinetic analysis of the absorption spectra was carried out which allowed the cal3B;
+ DEA m&$
.
+ DMA -
Bp- + DMA+SSIPjl2O”rn)
By-... LIMA+ W(690nm)
B;H
t DbA
-----+
Bp
alcohol
Bp-
alcohols: Bp ,..
Scheme 2.
DMA%Bp
DMA -
Bf..Otd ‘IP&W~~i
-
H.
“Bp+ DM& I4 - IP(620”rn,
Scheme 3.
+
DEA’
-%p-
+
.
BP H+DEA
lIEA+) ====-
ACN: ‘8;
in
3 (Scheme 3) The observed blue. shift of the Bp-
1.3. Model
DEA’
+ DEA
G. Htittmann Ed aL I ion and ketyl radical formalion in beruoplzetwne-dimethyianiIine
culation of the rate constants and yields for the observed (or conjectured) reactions. The important result of this investigation is the distinction between three different types of radical ion pairs formed either by electron transfer in the ,excited singlet or tripiet state or by excitation of a charge transfer complex in the ground state. The evaluation of the measurements carried out with the donor DMA revealed that the ketyl radical is not formed directly but has a radical ion pair as an intermediate species. AI1 three types of radical ions exhibit different rates for the protonation and hence different ketyl radical yields (4) (see Scheme 4). By exciting the charge transfer complex in the ground state (which is formed to some extent between benzophenone and DMA at high concentrations) a very short-lived singlet radical ion pair is formed so that proton transfer cannot be accomplished during its lifetime (85 ps). Moreover, at high DMA concentration (1 M), radical ions are formed directly from the singlet state of benzophenone or, after intersystem crossing, from the triplet state. Since the multiplicity does not change with the electron transfer, the generated radical ion pairs must be in the corresponding overall spin state. Yet, protonation of the triplet radical ion pair is 7.5 times faster than protonation of the singlet radical ion pair. As in the study associated with model 2 (Simon and Peters [ll]), a shift of the Bp- band to shorter wavelengths is observed with 1 M DMA in the solvent ACN. It is explained as a superposition of the radical ion absorption (with a maximum at 740 nm), which originates from the charge transfer complex, with the bands of the radical ion species absorbing at shorter wavelengths (690-720 nm). The fast decay of the band of the excited charge transfer complex at 740 nm, which dominates the spectrum at early times, appears as a shift of the whole transient absorption. The conflicting shifts of the Bp- band, reported in the measurements associated with models 2 and 4 on the one hand and model 3 on the other,
‘(B~:..DMA+)
‘EL
+ DMA
\
“6;
Bp...DMA-
Scheme 4.
6,~ +DMA
+ DMA-fBpY
.OMA+)
019
85
may be explained by traces of water in the solvent ACN, since the groups associated with models 2 and 4 used the solvent without further purification, whereas the group associated with model 3 used dry ACN. It has been observed that water forms hydrogen bonds with benzophenone that lead, depending on the amount of water, to a blue shift of the Bp- absorption [13, 191. Measurements with the donor DPA revealed that electron transfer and ketyl formation must be competing processes originating from a nondetectable encounter complex, in contrast with ketyl formation via the radical ion pair in the case of DMA [14]. Charge transfer complexes in the ground state have also been observed in this example. In order to explain the different ketyl radical yields, the three observed types of radical ions and the different reaction mechanisms with DMA and DPA, it has been assumed that geometrical factors (relative orientation of donor and acceptor) strongly influence the proton and hydrogen transfer. 1.5. Experimental approach with a covaletily linked donor+zcceptor system Almost all measurements of benzophenone and aniline (donor) have been carried out on nonlinked systems in different polar and non-polar solvents where diffusion of the reacting pair precedes the actual reaction. If it is intended to resolve fast processes, high donor concentrations must be used to achieve a small mean distance between benzophenone and donor. In the measurements cited above the amine concentration was between 0.1 and 5 M. However, a concentration of 5 M DEA in ACN means that there are five quencher molecules to four solvent molecules [lo]. In this case the concept of an SSIP becomes questionable, and it is not guaranteed that only two molecules participate in the reaction. It is also conceivable that electron and proton transfer take place with two different amine molecules at a time. A further complication is the formation of ground state complexes which is favoured at high donor concentrations [3, 9, 16, 201. As in earlier studies of electron transfer reactions from the singlet state [21-251, it is possible to use bridged systems in investigations with benzophenone in order to keep donor and acceptor at a certain distance from each other. Only a few reports are available of studies on alkyl-linked benzophenone-DMA [3, 12, 17, 26, 271 and these are of qualitative character.
G. Hiimnann et al. 1 Ion and ketyr mdical fornation in &nzopIwnone-dimethylaniline
86
If bridged compounds show a different behaviour from free donor-acceptor systems, new inferences on the mechanism of the photoreaction of benzophenone may possibly be drawn. To clarify this question, time-resolved transient absorption measurements were carried out on systems containing DMA and benzophenone covalently linked by a 1,Zpropylene chain (see Scheme 5). The propylene chain was used since it allows a diffusional (though restricted) passage of the reactants and a very close proximity of benzophenone and DMA, as has been shown with pyrene (or anthracene) as acceptor, which facilitates efficient exciplex formation [21, 23, 241. 2. Experimental
details
2. I. Materials
Benzophenone (Bp) was obtained from J. T. Baker. Benzophenone-(CH,),-DMA ([4-[3-[4-(dimethylamino)phenyl]propyl]phenyl]phenyl-methanone, Reg. No. 73060-15-8, abbreviated here as l33D) was synthesized in this laboratory and purified by high performance liquid chromatography (HPLC). The mean distance between benzophenone and DMA in this compound corresponds to the mean distance of a non-linked system at 1 M quencher concentration. At a concentration of about 3 X lop3 M of B3D, intermolecular effects need not be considered within the time window (maximum, 8.4 ns) of this study. Methylcyclohexane (MCH) from Baker was purified by column chromatography and dried with a molecular sieve (MS, 4 A). Isopropanol (i-PrOH) (Uvasol from Merck) was dried (MS, 3 A) and distilled. ACN (Uvasol from Merck) was dried with a molecular sieve (3 A) on a vacuum line before the samples were degassed in the usual way (six freeze-pump-thaw cycles). 2.2. Instrumental To obtain transient absorption spectra, a doublebeam spectrometer was constructed (the apparatus is shown schematicalIy in Fig. 1) combined with a picosecond Nd:YAG laser. The Nd:YAG laser oscillator is both passively and actively mode
Scheme
5.
locked, having a repetition rate of 0.3 s-l. One pulse is selected with an opto-electronic pulse slicer, amplified and split with a beam splitter. One part of the 1064 nm pulse is polarization rotated 90” with a A/2 plate, delayed 2 ns and reflected back in the original direction. The two pulses polarized perpendicular to each other are further amplified and then split with a polarizing beam splitter. The vertically polarized pulse is frequency tripled with KDP I/II crystals yielding 0.5 al at 354 nm and is used as the excitation pulse (pump pulse). The horizontally polarized pulse (probe pulse), 10-20 mJ at 1064 nm, is provided for continuum generation in the doublebeam spectrometer (see Fig. 1). The continuum is obtained by focusing the laser pulse into a 5 cm cell containing a 1:l mixture of D20-H,O. The continuum is split into two parts by a half mirror. The probe beam is focused onto the sample cell. The reference beam penetrates the sample at a non-excited area. The use of field lenses (Fl,, 2) is important. However, chromatic and spatial group velocity dispersion [28-301 leads to a travelling time difference in the wavelength range 700 nm (arrives first) to 470 nm (arrives last) of approximately 6 ps between the centre of the continuum generator and the sample cuvette [31]. A doublediode array (DDA) at the output of the polychromator is used as the detector in the spectrometer whose layout is shown schematically in Fig. 1 [31]. Signal averaging is carried out by integration over 7 pixels of the DDA corresponding to 5 nm. 2.3. Data evahation Data acquisition is accomplished with a personal computer equipped with a multi-I/O card and an IEEE bus. The transient absorption spectrum at a delay time I is calculated, in principle, from the signals of the probe beam and the reference beam (subscripts P and R) with and without excitation (superscript 0) of the sample. The absorbance A is A@#)=-log
$$ R
x $#) f
P
(1)
In practice, however, corrections have to be made. Generally, four different pairs of signals must be measured separately in order to obtain a correct absorption spectrum at a tied delay time. (1) The dark signals of the DDA are read without excitation of the sample and with probing beams off, yielding the background “spectra” U,(h) and UR(h) for probe and reference channel. (2) The signals of both detector arrays are read with the excitation beam off but probing and
G. Hiittmann et al. I Ion and J&y1 miical I !
Sample
Pump
fomhbn iv benwphenvne-dhethylan.%e
Pulse
iI
Probe
87
Pulse
Cuvette
CP200
Fig. 1. Double-beam spectrometer for time-resolved (picosecond to nanosecond) broad-band transient absorption measurements: BI, aperture (0.5 mm); 6, aperture (4 mm); B,, aperture (1x2 nun’); B,, aperture; Bs, aperture (5 mm); B, aperture; CP 200, supercorrected holographic grating spectrograph (Jobin Yvon); DDA, double-diode array (Spectroscopy Instruments); F,, filter (KG3, 2 mm or dielectric mirror); Fz, edge filter and various attenuation filters; Fs, filter (UG2, 1 mm); FL,,, FL, plano-convex field lenses (f- 60 mm, d-21 mm); LI, plaoo-uxwex lens (f=lOOO nun); L,, L,, acbromat cf==60 nun, d=18 mm); L,, achromat (f-80 mm, d - 18 mm); L,, achromat (f= 100 mm, d = 18 mm); L6, achromat (f= 120 mm, d =34 mm); L,, achromat (f= 80 mm, d = 34 mm); L, piano-convex quartz lens (f=SOO mm, d -21 mm); L+, piano-convex quartz lens (f-200 mm, d=Zl mm); ST, dielectric beam splitter; VP, AR plate or “magic angle” polarization rotator; MS, opal glass; P,, .Ps Glan Polarizer; PD,, PD,, PDJ, photodiodes; RF, spatial frequency filter (d-O.1 mm); S1, &, slits (&>S&
reference beams on. This yields the beam splitter ratio SglS& (3) The probing light is often superimposed by stray fluorescence light from the sample. Therefore the normalized fluorescence spectra FR and Fp and the fluorescence intensity r, as a function of the excitation energy E must be measured. (4) In the last step, the signals Sp and &, with the sample excited, are measured. The absorbance is then calculated from [31]
In order to improve the signal-to-noise ratio all individual measurements are carried out 10-20 times and averaged. The literature values of the decadic extinction coefficients of the transients are as follows (the dependence on the solvent is assumed to be insignificant): first excited benzophenone singlet, ~(575) nm) a3600 M- 1 an-‘; first excited benzophenone triplet, ~(530 MI) =6500 M-’ cn-‘; ketyl radical, ~(545 nm)=4600 M-l cm-‘; ben-
zophenone
anion, ~(620-800) = 10 000 M-’ cm-’
PI* 3. Results 3.1. Ground stale absorption Ground state absorption measurements of B3D in the three solvents MCH, ACN and i-PrOH were carried out to determine whether some of the molecules are present as ground state complexes. In keeping with ref. 3, the spectra can be interpreted as the sum of the benzophenone spectrum and the spectrum of N,Wdimethyl-p-toiuidine (DMT). As with benzophenone alone in the different solvents, the n++ absorption band of B3D (340-380 nm) is shifted to shorter wavelengths with increasing polarity of the solvent, simultaneously with a red shift of the toluidine absorption edge. ln ACN and i-PrOH a weak absorption is recognized between 390 and 420 nm that cannot be explained by a benzophenone or DMT absorption. It may be assigned to a small fraction of ground state complexes as has also been found with free systems at high DMA concentrations
68
G. Hiittmunn et al. I Ion and ketyl radical fomration in benzo@e~one-dimethy!aniline
[9]. In MCH the absorption spectrum does not change in the temperature range 250-350 K. However, in ACN the absorption between 330 and 360 nm increases at low temperature. This effect may be caused by the stabilization of weakly bound complexes, but also may be due to an increase in the nti absorption as has been observed previously with benzophenone in ACN.
I................................I 470.
520.
570.
620.
670.
720.
770.
3.2. Transient absorption 3.2.1. Benzophenone in ACN, MCH and iP?OH
The transient absorption of benzophenone in the absence of a donor was measured for comparison with the polymethylene-linked system (B3D). The triplet spectrum appeared more rapidly than could be resolved by the apparatus, with a maximum at about 530 nm and somewhat shifted (AA=5 nm), but similar in shape, for all three solvents. Within the time range of measurement up to 8.4 ns no significant changes in the shape of the spectra could be observed (Fig. 2). The additional absorption at 470 and 580 nm within the time range where the pump and probe pulses overlap (5 ps) is caused by the first excited singlet state of benzophenone. From the absence of the ketyl band in all solvents used it can be concluded, in accordance with earlier measurements [32], that no appreciable hydrogen transfer from the solvent to benzophenone takes pIace within this time range. 3.2.2. 830 in MCH The absorption measurements carried out with B3D in MCH (see Fig. 3) showed the appearance of the benzophenone triplet, still within the excitation pulse in the first spectrum after 5 ps, superimposed with a small fraction of singlet abGptiofl. During the course of the measurement time .of 8.4 ns the maximum of the absorption shifts .to 545 nm accompanied by a change in the band shape. The spectral position and form allow the assignment of this band to the absorption of the intramolecularly generated ketyl radical [33] since, on the one hand, hydrogen abstractionfrom the solvent was not observed in pure benzophenone in this time range and, on the other, measurements with different concentrations of B3D (3X10m4 M to 6x low3 M) did not show differences in the spectra. The transient absorption spectra of B3D in MCH also exhibit a broad but weak absorption at about 700 nm. This cannot be attributed to a radical ion absorption, since the band does not change during the first nanosecond (see Fig. 3), i.e. the Bp- concentration remains constant in this time
15o.ps
559s
5.w 470.
520.
570.
620.
670.
720.
770.
Wavelength / [nm]
Fig. 2. Transient absorption spectra of benzophenone in MCH. Lower frame: time evolution of the absorption; the delay times between exciting and probe pulses are indicated in the figure. Upper frame: comparison of the spectra normalized at 530 am.
period. If ketyl radicals were generated from such radical ions, their slow formation with a constant radical ion concentration could not be explained. The weak broad band probably belongs to the triplet spectrum of B3D. In comparing these results with measurements reported on non-linked systems in non-polar solvents, it should be noted that there, in contrast with the measurements on B3D, electron transfer was observed [lo, 151. With DEA (0.75 M), having an oxidation potential similar to DMA [2], radical ions are formed in benzene or cyclohexane within 1 ns. Even with DPA, which has a distinctly higher oxidation potential than DMA, at a moderate concentration of 0.13 M, electron transfer at a nearly diffusion-limited rate is observed in isooctane [14].
89
G. Hiittmmm et al. / Ion and ketyl radical fomzntion in berrzophenone-dimethylaniline
70.
520.
570.
620.
670.
720.
770.
~................................I
I
470.
520.
570.
620.
720.
670.
770.
-X
64OO.ps -N
\
4400.ps
3504s
15o.ps
55.ps
5.ps 470.
529.
570.
620.
Wavelength
670.
720.
770.
470.
520.
720.
670.
770.
/ [nm]
Fig. 4. Transient absorption spectra of B3D in ACN. Lower frame; time evolution of the absorption; upper frame: comparison of the spectra normalized at 750 nm.
3.2.3 B3D in ACN In contrast with the measurements
in MCH, rapid electron transfer takes place in the highly polar solvent ACN (see Fig. 4). The triplet band disappears within a few hundred picoseconds and the absorption band of the benzophenone radical ion builds up at about 750 nm. Even at very early times the triplet spectrum is superimposed by a broad absorption, spanning the whole spectral range. By making use of the reference spectra for the B3D triplet absorption and the radical ion absorption, the time dependence of the relative transient concentrations can be determined (Fig. 5). The lifetime of the triplet and the rise time of the radical ion absorption are both 120 ps. However, a good fit is only obtained with the assumption that 80% of the radical ions are formed from the triplet, while 20% are generated during the wurse of excitation. An early triplet spectrum
620.
Wavelength
/ [nm]
Fig. 3. Transient absorption spectra of B3D in MCH. Lower frame: time evolution of the absorption; upper frame: comparison of the spectra normalized at 543 nm.
570.
0.6
0.5
s ‘G
I
I
I
-
0.4
-
0.3
-
I
I
I
(%p
.
triplet
.
radical ions (BP-)
I
I
)
2 5
e 8 L %
I
0.2 -
-200
0
200
400
Time
600
800
1000
1200
/ [PSI
Fig. 5. Relative concentration of triplet and radical ions of B3D in ACN. Deconvolution and fitting of exponentials yielded the functions (full line): Ctnplct= 0.56exp( -r/l2Ops),C,,,-0.29 -0.23 exp( -t/120 ps). Transient reference spectra were B3D in MCH at 150 ps and B3D in ACN at 8.4 ns.
90
G. Hiinmonn et al. / Ion and ketyl radical formation in benzophenone-dimethylaniline
of B3D iu MCH was used as a reference spectrum Identical decay and rise times, with a ratio of 30% to 70% for fast and slowly formed radical ions respectively, were determined if the reference spectrum of 3Bp* in ACN was used. The prompt formation of radical ions is in accord with measurements by Miyasaka et al. [9] with non-linked benzophenone and DMA, who also observed fast Bp- generation from the singlet state or a ground state complex. The rate of the electron transfer reaction of B3D in ACN is comparable with that of a free, non-linked system at a high donor concentration [9-Ill. The maximum of the Bp- band of B3D appears far in the red compared with the free system. A temporal shift of the band was not detected, although the blue shift observed by Mataga and coworkers [9, 141 and by Simon and Peters [ll] at 200 ps to 1 ns (see above) should also be seen in these measurements. The red shift seen by Devadoss and Fessenden [lo, 131, which was attributed to the transition from the CIP to the SSIP and took place within 50 ps, is three times faster than the electron transfer in our case and cannot be resolved+ Compared with measurements on free systems, the ketyl band is absent. According to the measurements of Mataga and coworkers [9, 141 on free systems of benzophenone and DMA, a proton is transferred within 200 ps in the radical ion pair of triplet spin multiplicity, essentially independent of the DMA concentration, and via the singlet radical ion pair the ketyl radical appears within 1.5 ns. 3.2.4. B3D in i-PrOH As in ACN electron transfer but no proton transfer is observed in the first 9 ns in the alcohol i-PrOH (Fig. 6). The kinetics are somewhat more intricate. In the transient absorption spectrum 15 ps before the full overlap of the pump and probe pulses (spectrum at - 15 ps), an extremely broad transient absorption appears that covers the whole wavelength range of the measurement. From this background the well-known triplet band emerges. During its decay within 1 ns the bands of the radical ion pair, Bp- and DMA+ build up. The strong blue shift of the Bp- band to 650 nm is due to hydrogen-bridge binding and is in accord with pulse radiolysis experiments on benzophenone in i-PrOH at low temperatures which yield benzophenone anions after irradiation with electrons [34]. This shift is a dynamic process that is terminated after 200 ps in non-linked benzophenone. In the B3D-i-PrOH system, the dynamics of the
470. 520. 570. 620. 670. 720. 770. I
3I_
’ I 64oo.ps 650.~~
3oo.ps
19o.ps
75.ps
5.ps
-1 !i.ps
470. 520. 570. 620.
670. 720. 770.
Wavelength / [nm] Fig. 6. Transient absorption spectra of B3D in i-PrOH. Lower frame: time evolution of the absorption; upper frame: comparison of the spectra normalized at 650 urn.
shift of the Bp- band cannot be resolved because the electron transfer reaction is too slow (approximately 700 ps). A kinetic analysis of the series of transient absorption spectra, using the end spectrum at 8.4 ns and the benzophenone triplet spectrum as reference spectra, shows an exponential decay of the triplet concentration with a time constant of about 670 ps (Fig. 7). The absorption within the radical ion band shows two components. One fraction, which is formed within the excitation pulse, decays relatively rapidly. At the same time, a second fraction appears with a rise time of about 500 ps. As in ACN, some of the radical ions must be formed either from the first excited singlet state or directly from charge transfer complexes of the ground state, whereas the slowly rising fraction is formed from the triplet state by electron transfer in a dynamic quenching process. Because of the slow electron transfer reaction in the triplet state,
G. Ii&mamz
et al. f Ion and keyI mdical formation in benzophenc+ae-dimethylaniline in ACN
0.30
or i-PrOH .‘i?iMDMA-
91
: _I
0.25
5 -E f E ; 2
0.20
0.15
0.10
in MCH
0.05
0.00 0
500
1000
Time /
1500
2000
[ps]
Fig. 7. Relative concentration of triplet and radical ions of B3D in I-PrOH. Deconvolution and fitting of exponentials yielded the functions (fill1 lie): Ctrfplct= 0.19 exp( -t/670 ps), CRIp=O.ls(l -exp(-r/500 ps)}+O.lO exp(-rfl30 ps). Transient reference spectra were ‘BP* in i-PrOH at 150 ps and B3D in i-PrOH at 8.4 ns.
the decay of the singlet radical ions, which can recombine in a spin-allowed process to the ground state, is directly observable in contrast with the measurements in ACN. Proton transfer is not observed within the first 9 ns. The fitting of the reference spectra used to calculate the relative concentrations cannot take into account a timedependent change in the absorption band shape. It is therefore not surprising that the decay time of the triplets and the rise time of the radical ion absorption do not correspond well. An uncertainty in the time constants of f 15% must be tolerated. 4. Discussion Our results can be summarized as shown in Scheme 6. As in the free system in ACN and iPrOH, the fraction of radical ion absorption which appears quasi-promptly with the excitation pulse can be distinguished from the other radical ion fraction which is formed from the locally excited triplet state. Within the limits of the time resolution (approximately 25 ps), further distinctions with respect to the origin of the prompt radical ions - excitation of a charge transfer complex or electron transfer from the locally excited singlet state - cannot be made. If the prompt radical ions come from the singlet excited state ‘B*3D, the electron transfer reaction from the singlet molecule will have to be more than one order of magnitude faster than that from the triplet molecule in order to compete with intersystem crossing (‘BP* to 3Bp*). From earlier studies of intra-
B~MDMA
Scheme 6.
molecular fluorescence quenching in compounds similar to B3D, namely pyrene (Py) or anthracene (An) linked with DMA through a (CH,), bridge, carried out in this laboratory [23-251, the following rate coefficients for electron transfer from the fh-st excited singlet state are known: Py3D (in ACN), 16 X 10’s_‘; Py3D (inn-PrOH) 2.4 X lo9 s-l; An3D (in ACN), 95X 10’ s-l; An3D (in ~-PI-OH), 6.7~ 10’ s-l. These values can be compared with the rate coefficients obtained from the present study for intramolecular triplet quenching of 3B*3D (see Figs. 5 and 7): 8.3~ IO9 s-l in ACN and 1.5X 109 s-l in i-PrOH. It is interesting to note that intermolecular quenching rate constants of approximately 2 X lOlo M-’ s-l in free systems of 3Bp* and DMA in ACN have been reported [9, lo]. From this we can conclude that a small fraction of singlet radical ion pairs may be formed from the locally excited singlet state. However, the short lifetime of the directly observed radical ions in iPrOH favours the concept of an immediately excited charge transfer ground state complex, as suggested by Miyasaka et al. [9] for free systems. It is worth noting that, in contrast with investigations of free systems in MCH, hydrogen transfer takes place in B3D without preceding radical ion formation, whereas in ACN and i-PrOH only electron transfer is observed. Electron transfer is obviously not a requirement but rather seems to impede ketyl formation of B3D. The function of the polymethylene chain between benzophenone and DMA is to keep the reactants in close proximity witbin a defined range. Moreover, it is expected that the aliphatic chain which substitutes one hydrogen atom has only a moderate influence on the physical and chemical properties of the donor and acceptor (singlet and triplet energy levels, redox potentials); however, it restricts the number
92
G. Hiittmann et 01 I Ion and ketyi radical formation in benzopherwne-dimethylaniline
of relative orientations of the molecules considerably. The different behaviour of free and polymethylene-linked systems may be explained by geometrical factors. It is generally assumed that, for proton transfer, the donor and acceptor must be at a very close distance, so that greater importance is attributed to the orientation of the molecules for proton transfer than for electron transfer. Electron transfer is generally not a necessary precondition for fast hydrogen transfer between benzophenone and DMA (i.e. within a few nanoseconds). On the contrary, it is conceivable that the large increase in the proton transfer rate caused by a preceding charge transfer, as observed in free systems, is due to electrostatic interaction which keeps the generated ion pairs, Bp- and DMA+, at an encounter distance for a prolonged time, thus increasing the chance for an orientation of donor and acceptor groups favourable for proton transfer. In the case of B3D in MCH, this function can be accomplished by the chain. During the course of the diffusional motion of the chain, a favourable conformation for hydrogen transfer is attained with a certain fmite probability. The low polarity of the solvent (MCH) makes electron transfer either very slow or energetically impossible (see ref. 35 for thermodynamic aspects), so that electron transfer can be neglected for ketyl formation. The highly polar solvents ACN and i-PrOH lower the energy of the radical ion pair state, stabilizing the charge transfer ground state complexes and allowing triplet ion pairs to be generated efficiently. The excited charge transfer complexes do not permit proton transfer because of their short lifetime. In addition, it is assumed that their structure is relatively rigid, possibly not allowing an alignment favourable for proton transfer [9]. Likewise, the triplet radical ions seem to be kept in a position, presumably due to the restricted degrees of freedom of the (CI-Q, chain, which does not favour proton transfer. Measurements on systems with different rigid and flexible molecular bridges may help to elucidate the mechanism of hydrogen abstraction in the benzophenone-DMA system in more detail. Such studies are in progress. Furthermore, compounds Bp (CH&-DMA (n =9, 16) have been synthesized and selected for magnetic field studies of the influence of intramolecular dynamics on the spin dynamics of radical ion pairs, not in the singlet [25] but in the triplet spin state.
Acknowledgments
We are grateful to B. Frederichs and H. Meyer for technical assistance. We acknowledge the valuable discussions with N. Ernsting, U. Werner and A. WieDner. This work was supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich SFB 93/D 1 “Photochemistry with Lasers”.
References
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16 17 18 19 20 21 22
23 24
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