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Control of ferromagnetic and antiferromagnetic couplings in a galvinoxyl–triplet chrysene encounter complex through solvent polarity
Chemical Physics Letters 372 (2003) 8–14 www.elsevier.com/locate/cplett
Control of ferromagnetic and antiferromagnetic couplings in a galvinoxyl–triplet chrysene encounter complex through solvent polarity Akio Kawai a
a,b
, Yasuyuki Watanabe a, Kazuhiko Shibuya
a,*
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan b PRESTO, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan Received 9 December 2002; in final form 24 February 2003
Abstract Energy difference, J, between the quartet and doublet spin states of a galvinoxyl radical–triplet chrysene pair as an encounter complex in solution was investigated by measuring chemically induced dynamic electron polarization (CIDEP) of galvinoxyl with a time-resolved ESR (TR-ESR) method. All the TR-ESR spectra of galvinoxyl show net type CIDEP signals due to doublet–triplet magnetic interactions. The CIDEP phase switches from absorption in non-polar solvent to emission in polar solvent. The signals were analyzed on the basis of the radical–triplet pair mechanism (RTPM) for CIDEP creation, the results of which indicate that galvinoxyl–triplet chrysene pairs show ferromagnetic and antiferromagnetic couplings in non-polar and polar solvents, respectively. This solvent polarity-controlled switching of magnetic coupling in the galvinoxyl–triplet chrysene pair was interpreted by introducing the effect of intermolecular charge transfer interaction on the J value. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction When a radical and a triplet molecule encounter, exchange interaction between them gives rise to energy separation of the quartet and doublet states. The energy separation is expressed by socalled J value: in the usual definition, the sign of J
*
Corresponding author. Fax: +81-3-5734-2655. E-mail addresses: [email protected] (A. Kawai), [email protected] (K. Shibuya).
is negative for the radical triplet molecule pairs, which couple antiferromagnetically with each other. Their characteristic state structure that the quartet state is located above the doublet has been analyzed through the chemically induced dynamic electron polarization (CIDEP) of radicals measured by time-resolved ESR (TR-ESR) methods [1–12]. Although the majority of the radical–triplet molecule pairs (RTPs) in fact show antiferromagnetic coupling ðJ < 0Þ, we have recently found that some RTPs exceptionally show ferromagnetic coupling ðJ > 0Þ [13–15]. Our TR-ESR study on
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00321-X
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
RTPs suggests that the exchange interaction induces normal antiferromagnetic coupling, while the intermolecular charge transfer (CT) interaction causes abnormal ferr- or antiferromagnetic coupling depending on the energy gap, DG, between CT and RTP states [14,15]. This feature is similar to the observation found in radical–ion pairs [16,17]. The most characteristic point in RTPs lies in that the J value is controlled by both exchange and CT interactions. It is well known that the CT state energy is largely shifted by changing the solvent polarity. Since the CT effect strongly depends on the DG value, we expect that solvent polarity will play an important role in controlling the sign of the J value. In this Letter, we demonstrate solvent polarity effect on the sign of the J value in a galvinoxyl radical–triplet chrysene system. This pair was used as a model system for solvent polarity effect because the DG value of this pair is close to zero and the sign of DG can be controlled by a choice of solvent. Analyses of CIDEP signals created in RTPs in a series of polar-to-non-polar solvents confirm our CT model for J value.
9
space. All the measurements were carried out at room temperature (293 K).
3. Results and discussion Fig. 1 shows TR-ESR spectra of galvinoxyl measured under the 297 nm laser excitation of chrysene in various solvents with different solvent polarity. In these measurements, the S1 state of chrysene undergoes intersystem crossing (ISC) with the rate of 2:0 107 s1 and the ISC quantum yield of 0.85 [18]. The lifetime of chrysene in the T1 state is reported to be 710 ls in Ar-bubbled n-hexane at room temperature [19] and the T1 chrysene may be quenched by galvinoxyl in solution. In Fig. 1, the polarity of solvent is con-
2. Experimental TR-ESR signals were measured by a conventional X-band ESR spectrometer (Bruker, ELEXIS 580E) combined with a boxcar integrator (Stanford, SR-250) for spectra or a digital oscilloscope (SONY/Techtroniks, TDS340) for time profiles. The excitation UV light at 297 nm was prepared by the frequency doubling (Inrad, R-6G crystal) of 594 nm light from a dye laser (Lambda Physik, Scanmate) pumped by the second harmonics of a YAG laser (Continuum, Powerlight 8000, 532 nm 100 mJ/pulse). The laser power was attenuated to be about 0.2 mJ/pulse. The signals were collected at a repetition rate of 10 Hz. The microwave power were 5 or 10 mW. For time profile measurements to determine triplet quenching rate constants, a wide-band preamplifier was utilized. All the chemicals (Tokyo Kasei) were used as received. Sample solutions were degassed by bubbling Ar gas and were flowed through a quartz flat cell with 0.5 mm interior
Fig. 1. TR-ESR spectra of galvinoxyl observed for a chrysene (2.5 mM)–galvinoxyl (0.56 mM) system in various solvents under the 297 nm laser excitation. The fractions of DCE in volume are described for DCE/o-Xyl mixture solutions. The gate was opened during 1:5–2:0 ls after laser excitation. Asterisks indicate the hyperfine line used for measurements of signal time evolutions.
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A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
trolled by changing the fraction of polar and nonpolar solvents in a mixure solvent. We chose o-xylene (o-Xyl) and 1,2-dichloroethane (DCE) as non-polar and polar solvents, respectively: the dielectric constants, e0r s, are 2.568 for o-Xyl and 10.37 for DCE [18]. These solvents have similar viscosities of 0.809 cp for o-Xyl and 0.800 cp for DCE [18], and the viscosities are essentially constant for all the mixture solvents studied here. This enables us to control the solvent polarity without any change in viscosity. The spectral assignment of galvinoxyl radical was made by judging the hyperfine structure of the spectrum which coincides with that in the CW-ESR spectrum of galvinoxyl. In non-polar o-Xyl solution, net absorption (Abs) CIDEP of galvinoxyl was observed. The similar net Abs CIDEP of galvinoxyl was observed in other non-polar solvents such as benzene and toluene. With increasing the DCE fraction in the DCE/o-Xyl mixture solution, the CIDEP phase changes drastically as shown in Fig. 1. In the solvent of 10% DCE, net Abs signal decreases in intensity. In the solvent of 30% DCE and 50% DCE, no clear CIDEP signals were observed in the spectra. Furthermore, the CIDEP signal turns out to be of net emission (Em) in the solvents of 73% DCE and 100% DCE. This result obviously suggests that the CIDEP phase would switch from Abs in non-polar solvents to Em in polar solvents. Fig. 2 shows the time profiles of the CIDEP signals against various galvinoxyl concentrations measured at the hyperfine line marked by asterisk in Fig. 1. In these measurements, optical density of chrysene is reduced to depress the triplet–triplet anihilation process. The observed time profiles are characterized by a fast rise and a slow decay, though the signal phase of Abs or Em depends on the solvent used. The time profiles are simulated as the My component of magnetization of galvinoxyl according to the Bloch and kinetic equations, which are based on radical–triplet pair mechanism (RTPM) as a CIDEP creation mechanism. The Bloch equations modified with chemical kinetics [4] are given by dMy My ¼ þ x1 Mz ; dt T2
ð1Þ
Fig. 2. CIDEP time profiles of galvinoxyl for a chrysene (0.27 mM)–galvinoxyl systems in (a) o-Xyl and (b) DCE. The concentrations of galvinoxyl were indicted in the figure. The broken lines are the simulation curves based on the Bloch and kinetic equations. The time profiles were measured at the peaks shown by asterisks in Fig. 1.
dMZ ðMz Peq ½galvinoxyl Þ ¼ x1 My T1 dt þ Pn kCIDEP ½galvinoxyl ½triplet ; d½triplet ¼ ðkT þ kq ½galvinoxyl Þ½triplet ; dt
ð2Þ ð3Þ
where Pn and kCIDEP are a spin polarization factor and a rate constant for CIDEP creation, respectively. The kT value corresponds to the triplet decay rate due to the processes except for galvinoxyl quenching. The kq values are the triplet quenching rate constants by galvinoxyl and were determined to be ð2:15 0:11Þ 109 M1 s1 in o-Xyl and ð2:57 0:12Þ 109 M1 s1 in DCE by transient
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
absorption method. All the other symbols follow their standard notation. The T2 value is estimated to be 200 ns from the linewidth of the CW-ESR spectrum and the x1 values are 1:0 106 and 0:5 106 rad s1 for the experiments in o-Xyl and in DCE, respectively. The initial concentration of the excited molecule is about 0.0014 mM estimated from the laser power of 0.2 mJ/pulse at 297 nm, the optical density of ca. 0.07, the volume of excitation region of 2:5 105 dm3 , and the ISC quantum yield of 0.85. Since no galvinoxyl signals due to thermal distribution were observed, we take an appropriate Pn kCIDEP value which is large enough to neglect the term of Peq in Eq. (2). A response function of our apparatus is also included for the simulation. The results are shown as broken lines in Fig. 2. The parameters estimated by these simulations are the T1 value of 300 ns and the kT values of 6 105 s1 in o-Xyl and 2 105 s1 in DCE. These kT values are larger than the literature value [19]. Since we did not try to remove dissolved O2 completely by Ar bubbling, the triplet lifetime is limited by the residual O2 . Hence, the kT values obtained for our system seems reasonable. The simulations well reproduce the CIDEP time profiles, which suggests that the CIDEP signals observed are created by RTPM. According to the sign rule for CIDEP in RTPM, net Em and Abs phases in triplet quenching processes are created in the RTPs with J < 0 and J > 0, respectively [1,2,5,15]. Therefore, we can determine the sign of J value for the gal-
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vinoxyl–triplet chrysene pair in each solvent as summarized in Table 1. It is noteworthy that the J values are found to be positive in relatively nonpolar solvents ðer < 3Þ while the J values become negative in rather polar solvents ðer > 5Þ. Because of weak CIDEP signals, we could not determine the sign of J value in some solvents whose polarities are intermediate ð3 < er < 5Þ. The switch from positive J values in non-polar solvents to the negative values in polar solvents seems to suggest the importance of CT interaction in a galvinoxyl–triplet chrysene pair. In order to examine the effect of a CT state on J value, the energy gap, DGðrÞ between zero-order RTP ðRTP0 Þ and CT ðCT0 Þ states is estimated by the equation DGðrÞ ¼ GðCT0 Þ GðRTP0 Þ ox red ðgalvinoxylÞ E1=2 ðtripletÞ ¼ fE1=2
þ DEcorr g þ fkðrÞ Ecoulomb ðrÞg DEðT1 Þ;
ð4Þ
ox red where E1=2 ðgalvinoxylÞ and E1=2 ðtripletÞ mean halfwave redox potentials of galvinoxyl (+0.07 V) in acetonitrile [20] and chrysene ()2.25 V) in N,Ndimethylformamide vs. SCE [18], respectively. Since these values are obtained in highly polar solvents ðer > 30Þ, we add a correction term, DEcorr , calculated using the Born equation for solvation energies of anion and cation by intro and the er values of ducing the ion radius of 3.5 A
Table 1 The values of DGðrÞ and the signs of J value of galvinoxyl–triplet chrysene encounter pair in various solvents Solvents DCE DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl o-Xyl Toluene Benzene a
mixture mixture mixture mixture mixture
(DCE (DCE (DCE (DCE (DCE
73%)a 50%)a 30%)a 15%)a 10%)a
er b
DEcorr (kcal mol1 )
kðrÞ (kcal mol1 )
Ecoulomb ðrÞ (kcal mol1 )
DGðrÞ (kcal mol1 )
Sign of J value
10.37 7.7 5.7 4.3 3.25 2.9 2.568 2.379 2.284
1.6 2.4 3.5 4.9 6.6 7.5 8.6 9.3 9.7
16.5 14.9 12.7 10.0 6.5 4.7 2.6 1.1 0.3
4.6 6.2 8.3 11.0 14.6 16.3 18.5 19.9 20.8
10.4 8.1 4.8 0.8 )4.6 )7.2 )10.4 )12.6 )13.8
) ) ) ? ? + + + +
The fractions of DCE in volume are represented in DCE/o-Xyl mixture solvents. Dielectric constants of DCE/o-Xyl mixture solutions are estimated using those in toluene/DCE mixture solvents obtained from [22]. All the others are obtained from [18]. b
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A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
35.94 for acetonitrile and 36.71 for N,N-dimethylformamide [18]. The symbol of kðrÞ is a solvent reorganization energy between the RTP0 and CT0 states, and Ecoulomb ðrÞ is the Coulomb energy of the CT0 state. The fkðrÞ Ecoulomb ðrÞg term depends on the radical–triplet distance, r. The kðrÞ values are calculated by using MarcusÕs formula [21] and , using the following values; rA ¼ rD ¼ 3:5 A , n ¼ 1:50 and er of solvents listed in rAD ¼ 7 A Table 1. The excitation energy of DEðT1 Þ is reported to be 2:0 104 cm1 [18]. The calculated DGðrÞ values are summarized in Table 1. Fig. 3 illustrates the relation between the calculated DGðrÞ and the observed sign of J values. The DGðrÞ values cover over a range from ca. )15 to 10 kcal mol1 and the signs of the J value are represented by filled (–) and open (+) bars. It is remarkable that the J values are positive for DGðrÞ < 6 kcal mol1 and negative for DGðrÞ > 5 kcal mol1 . This means that the sign switching of the J value occurs around DGðrÞ ¼ 0. As suggested by a large number of previous studies on radical–triplet interactions [1–12], J values in most encounter pairs are negative due to
Fig. 3. Plots of the signs of the J values vs. DG for galvinoxyl– triplet chrysene pairs in various solvents with different dielectric constants. Table 1 lists the values necessary for the plots.
the exchange interaction between radical and triplet molecules. On the other hand, some galvinoxyl–triplet systems with their intermolecular CT states closely locating near their RTP states have been reported to indicate positive J value [13–15]. According to the mechanism proposed for the observed positive J values, their doublet CT0 ð2 CT0 Þ states cause blue- or red-shifts of their doublet RTP0 ð2 RTP0 Þ states depending on their DGðrÞ values between RTP0 and CT0 states. Examples of these effects are schematically described in Fig. 4 where their intermolecular states are shown along solvent coordinates. RTP0 states 0 have the energy minima at rRT s along solvent coordinates and are energetically split into 4 RTP0 and 2 RTP0 states by exchange interaction. Figs. 4a, c represent the potentials of RTP0 and CT0 of the galvinoxyl–triplet chrysene pairs in non-polar ðDGðrÞ < 0Þ and in polar ðDGðrÞ > 0Þ solvents, respectively. It is noteworthy that the solvent reorganization energy is larger while the minimum energy of the CT0 state is lower in polar solvents. The balance between the reorganization energy and the minimum energy of CT0 state dominantly controls the sign of DGðrÞ value at rRT . Potential curves in Figs. 4a, c are converted into Figs. 4b, d, respectively, when electron correlation between CT0 and RTP0 states is considered. In Fig. 4b, their 2 RTP state is higher in energy than the 4 RTP state, which corresponds to the RTP with J > 0. On the other hand, the 2 RTP state is still lower in energy than the 4 RTP state in Fig. 4d for DGðrÞ > 0, indicating that J < 0. If the interaction energy between the 2 RTP0 and 2 CT0 states for DGðrÞ < 0 dominates over the exchange interaction energy, the sign of the J value becomes positive [13–15]. This is exactly the case for a galvinoxyl–triplet chrysene pair in non-polar solvents where the estimated DGðrÞ value is negative, the CT effect causes positive J value, and apparent ferromagnetic interaction is observed to occur in RTP. On the other hand, the J value of the pair with positive DGðrÞ value in polar solvents is primarily controlled by the exchange interaction and possibly by an additional effect of the CT interaction. The resultant J value is negative and apparent antiferromagnetic coupling is observed to take place in RTP (Fig. 4d). It is interesting to note
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
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Fig. 4. Schematic description of potential surfaces of RTP and CT states along the solvent coordinate. The zero-order states with negative and positive DGðrÞ values are shown in (a) and (c), respectively. When electron correlation between 2 RTP0 and 2 CT0 states is considered, the potential surfaces in (a) and (c) are converted into those shown in (b) and (d), respectively. The 2 RTP state shown in (b) is higher in energy than the 4 RTP0 state at rRT for the potential minimum of RTP. This situation corresponds to the positive sign of J value in RTP.
that we observed very weak CIDEP signals for the pairs with the DGðrÞ value of ca. )5 to 4 kcal mol1 and could not determine the sign of J values. In this DGðrÞ region, 2 CT0 states locate so closely in energy to 2;4 RTP0 states that the interactions among the 2 CT0 and 2;4 RTP0 states are expected to be large. We cannot discuss the complicated dynamics of such a radical–triplet pair at this moment. The present result clearly indicates a switch in the sign of J value of a galvinoxyl–triplet chrysene encounter pair depending on the DGðrÞ value. This observation strongly supports our model for positive J value for RTP, namely, the 2 CT0 state causes the spin selective energy shift of the 2 RTP0 state. A galvinoxyl–triplet chrysene pair is not the only example which shows a switch in the sign of J value by changing the solvent from non-polar to polar ones. We found similar switching phenomena in pairs of galvinoxyl–triplet naphthalene and
galvinoxy–triplet 4-bromobiphenyl. Extended studies on the switching in the sign of J value depending on the solvent polarity are now in progress to understand the effect of CT interaction in RTP, in particular the complicated interaction of the pairs where the energies of RTP0 states nearly equal to those of CT0 states.
Acknowledgements We would like to express our thanks to Prof. Kinichi Obi for his encouragement in this study. The present work is partly defrayed by the Grantin-Aid for Scientific Research (Nos.13740327 and 13127202) from the Ministry of Education, Sports, Science and Culture of Japan. This study was performed using one of on-campus cooperative research facilities in Tokyo Institute of Technology, Ôa pulsed ESR systemÕ.
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References [1] C. Bl€ attler, F. Jent, H. Paul, Chem. Phys. Lett. 166 (1990) 375. [2] A. Kawai, T. Okutsu, K. Obi, J. Phys. Chem. 95 (1991) 9130. [3] A. Kawai, K. Obi, J. Phys. Chem. 96 (1992) 5701. [4] G.-H. Goudsmit, H. Paul, A.I. Shushin, J. Phys. Chem. 97 (1993) 13243. [5] A. Kawai, K. Obi, Res. Chem. Intermed. 19 (1993) 866. [6] N.J. Turro, I.V. Khudyakov, D.W. Dwyer, J. Phys. Chem. 79 (1993) 10530. [7] C. Corvaja, L. Franco, L. Pasimeni, A. Toffoletti, L. Montanari, Chem. Phys. Lett. 210 (1993) 355. [8] J. Fujisawa, K. Ishii, Y. Ohba, Y. Iwaizumi, S. Yamauchi, J. Phys. Chem. 99 (1995) 17082. [9] K. Ohara, N. Hirota, Bull. Chem. Soc. Jpn. 69 (1996) 1517. [10] A. Kawai, T. Yamamoto, T. Okutsu, K. Obi, Bull. Chem. Soc. Jpn. 72 (1999) 2625. [11] A. Blank, H. Levanon, J. Phys. Chem. A 104 (2000) 794.
[12] T. Okutsu, M. Ooyama, K. Tani, H. Hiratsuka, A. Kawai, K. Obi, J. Phys. Chem. A 105 (2001) 3741. [13] A. Kawai, K. Shibuya, K. Obi, Appl. Magn. Reson. 18 (2000) 343. [14] A. Kawai, Y. Watanabe, K. Shibuya, Mol. Phys. 100 (2002) 1225. [15] A. Kawai, K. Shibuya, J. Phys. Chem. A 106 (2002) 12305. [16] S. Sekiguchi, Y. Kobori, K. Akiyama, S. Tero-Kubota, J. Am. Chem. Soc. 120 (1998) 1325. [17] Y. Kobori, S. Sekiguchi, K. Akiyama, S. Tero-Kubota, J. Phys. Chem. A 103 (1999) 5416. [18] S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, Marcel Dekker, New York, 1993. [19] W. Heinzelmann, H. Labhart, Chem. Phys. Lett. 4 (1969) 20. [20] A. Samanta, K. Bhattacharyya, P.K. Das, P.V. Kamat, D. Weir, G.L. Hug, J. Phys. Chem. 93 (1989) 3651. [21] R.A. Marcus, J. Chem. Phys. 24 (1956) 966. [22] M.T. Ratzsch, E. Rickelt, H. Rosner, Z. Phys. Chem., Leipzig 256 (1975) 349.
Control of ferromagnetic and antiferromagnetic couplings in a galvinoxyl–triplet chrysene encounter complex through solvent polarity Akio Kawai a
a,b
, Yasuyuki Watanabe a, Kazuhiko Shibuya
a,*
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan b PRESTO, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan Received 9 December 2002; in final form 24 February 2003
Abstract Energy difference, J, between the quartet and doublet spin states of a galvinoxyl radical–triplet chrysene pair as an encounter complex in solution was investigated by measuring chemically induced dynamic electron polarization (CIDEP) of galvinoxyl with a time-resolved ESR (TR-ESR) method. All the TR-ESR spectra of galvinoxyl show net type CIDEP signals due to doublet–triplet magnetic interactions. The CIDEP phase switches from absorption in non-polar solvent to emission in polar solvent. The signals were analyzed on the basis of the radical–triplet pair mechanism (RTPM) for CIDEP creation, the results of which indicate that galvinoxyl–triplet chrysene pairs show ferromagnetic and antiferromagnetic couplings in non-polar and polar solvents, respectively. This solvent polarity-controlled switching of magnetic coupling in the galvinoxyl–triplet chrysene pair was interpreted by introducing the effect of intermolecular charge transfer interaction on the J value. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction When a radical and a triplet molecule encounter, exchange interaction between them gives rise to energy separation of the quartet and doublet states. The energy separation is expressed by socalled J value: in the usual definition, the sign of J
*
Corresponding author. Fax: +81-3-5734-2655. E-mail addresses: [email protected] (A. Kawai), [email protected] (K. Shibuya).
is negative for the radical triplet molecule pairs, which couple antiferromagnetically with each other. Their characteristic state structure that the quartet state is located above the doublet has been analyzed through the chemically induced dynamic electron polarization (CIDEP) of radicals measured by time-resolved ESR (TR-ESR) methods [1–12]. Although the majority of the radical–triplet molecule pairs (RTPs) in fact show antiferromagnetic coupling ðJ < 0Þ, we have recently found that some RTPs exceptionally show ferromagnetic coupling ðJ > 0Þ [13–15]. Our TR-ESR study on
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00321-X
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
RTPs suggests that the exchange interaction induces normal antiferromagnetic coupling, while the intermolecular charge transfer (CT) interaction causes abnormal ferr- or antiferromagnetic coupling depending on the energy gap, DG, between CT and RTP states [14,15]. This feature is similar to the observation found in radical–ion pairs [16,17]. The most characteristic point in RTPs lies in that the J value is controlled by both exchange and CT interactions. It is well known that the CT state energy is largely shifted by changing the solvent polarity. Since the CT effect strongly depends on the DG value, we expect that solvent polarity will play an important role in controlling the sign of the J value. In this Letter, we demonstrate solvent polarity effect on the sign of the J value in a galvinoxyl radical–triplet chrysene system. This pair was used as a model system for solvent polarity effect because the DG value of this pair is close to zero and the sign of DG can be controlled by a choice of solvent. Analyses of CIDEP signals created in RTPs in a series of polar-to-non-polar solvents confirm our CT model for J value.
9
space. All the measurements were carried out at room temperature (293 K).
3. Results and discussion Fig. 1 shows TR-ESR spectra of galvinoxyl measured under the 297 nm laser excitation of chrysene in various solvents with different solvent polarity. In these measurements, the S1 state of chrysene undergoes intersystem crossing (ISC) with the rate of 2:0 107 s1 and the ISC quantum yield of 0.85 [18]. The lifetime of chrysene in the T1 state is reported to be 710 ls in Ar-bubbled n-hexane at room temperature [19] and the T1 chrysene may be quenched by galvinoxyl in solution. In Fig. 1, the polarity of solvent is con-
2. Experimental TR-ESR signals were measured by a conventional X-band ESR spectrometer (Bruker, ELEXIS 580E) combined with a boxcar integrator (Stanford, SR-250) for spectra or a digital oscilloscope (SONY/Techtroniks, TDS340) for time profiles. The excitation UV light at 297 nm was prepared by the frequency doubling (Inrad, R-6G crystal) of 594 nm light from a dye laser (Lambda Physik, Scanmate) pumped by the second harmonics of a YAG laser (Continuum, Powerlight 8000, 532 nm 100 mJ/pulse). The laser power was attenuated to be about 0.2 mJ/pulse. The signals were collected at a repetition rate of 10 Hz. The microwave power were 5 or 10 mW. For time profile measurements to determine triplet quenching rate constants, a wide-band preamplifier was utilized. All the chemicals (Tokyo Kasei) were used as received. Sample solutions were degassed by bubbling Ar gas and were flowed through a quartz flat cell with 0.5 mm interior
Fig. 1. TR-ESR spectra of galvinoxyl observed for a chrysene (2.5 mM)–galvinoxyl (0.56 mM) system in various solvents under the 297 nm laser excitation. The fractions of DCE in volume are described for DCE/o-Xyl mixture solutions. The gate was opened during 1:5–2:0 ls after laser excitation. Asterisks indicate the hyperfine line used for measurements of signal time evolutions.
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A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
trolled by changing the fraction of polar and nonpolar solvents in a mixure solvent. We chose o-xylene (o-Xyl) and 1,2-dichloroethane (DCE) as non-polar and polar solvents, respectively: the dielectric constants, e0r s, are 2.568 for o-Xyl and 10.37 for DCE [18]. These solvents have similar viscosities of 0.809 cp for o-Xyl and 0.800 cp for DCE [18], and the viscosities are essentially constant for all the mixture solvents studied here. This enables us to control the solvent polarity without any change in viscosity. The spectral assignment of galvinoxyl radical was made by judging the hyperfine structure of the spectrum which coincides with that in the CW-ESR spectrum of galvinoxyl. In non-polar o-Xyl solution, net absorption (Abs) CIDEP of galvinoxyl was observed. The similar net Abs CIDEP of galvinoxyl was observed in other non-polar solvents such as benzene and toluene. With increasing the DCE fraction in the DCE/o-Xyl mixture solution, the CIDEP phase changes drastically as shown in Fig. 1. In the solvent of 10% DCE, net Abs signal decreases in intensity. In the solvent of 30% DCE and 50% DCE, no clear CIDEP signals were observed in the spectra. Furthermore, the CIDEP signal turns out to be of net emission (Em) in the solvents of 73% DCE and 100% DCE. This result obviously suggests that the CIDEP phase would switch from Abs in non-polar solvents to Em in polar solvents. Fig. 2 shows the time profiles of the CIDEP signals against various galvinoxyl concentrations measured at the hyperfine line marked by asterisk in Fig. 1. In these measurements, optical density of chrysene is reduced to depress the triplet–triplet anihilation process. The observed time profiles are characterized by a fast rise and a slow decay, though the signal phase of Abs or Em depends on the solvent used. The time profiles are simulated as the My component of magnetization of galvinoxyl according to the Bloch and kinetic equations, which are based on radical–triplet pair mechanism (RTPM) as a CIDEP creation mechanism. The Bloch equations modified with chemical kinetics [4] are given by dMy My ¼ þ x1 Mz ; dt T2
ð1Þ
Fig. 2. CIDEP time profiles of galvinoxyl for a chrysene (0.27 mM)–galvinoxyl systems in (a) o-Xyl and (b) DCE. The concentrations of galvinoxyl were indicted in the figure. The broken lines are the simulation curves based on the Bloch and kinetic equations. The time profiles were measured at the peaks shown by asterisks in Fig. 1.
dMZ ðMz Peq ½galvinoxyl Þ ¼ x1 My T1 dt þ Pn kCIDEP ½galvinoxyl ½triplet ; d½triplet ¼ ðkT þ kq ½galvinoxyl Þ½triplet ; dt
ð2Þ ð3Þ
where Pn and kCIDEP are a spin polarization factor and a rate constant for CIDEP creation, respectively. The kT value corresponds to the triplet decay rate due to the processes except for galvinoxyl quenching. The kq values are the triplet quenching rate constants by galvinoxyl and were determined to be ð2:15 0:11Þ 109 M1 s1 in o-Xyl and ð2:57 0:12Þ 109 M1 s1 in DCE by transient
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
absorption method. All the other symbols follow their standard notation. The T2 value is estimated to be 200 ns from the linewidth of the CW-ESR spectrum and the x1 values are 1:0 106 and 0:5 106 rad s1 for the experiments in o-Xyl and in DCE, respectively. The initial concentration of the excited molecule is about 0.0014 mM estimated from the laser power of 0.2 mJ/pulse at 297 nm, the optical density of ca. 0.07, the volume of excitation region of 2:5 105 dm3 , and the ISC quantum yield of 0.85. Since no galvinoxyl signals due to thermal distribution were observed, we take an appropriate Pn kCIDEP value which is large enough to neglect the term of Peq in Eq. (2). A response function of our apparatus is also included for the simulation. The results are shown as broken lines in Fig. 2. The parameters estimated by these simulations are the T1 value of 300 ns and the kT values of 6 105 s1 in o-Xyl and 2 105 s1 in DCE. These kT values are larger than the literature value [19]. Since we did not try to remove dissolved O2 completely by Ar bubbling, the triplet lifetime is limited by the residual O2 . Hence, the kT values obtained for our system seems reasonable. The simulations well reproduce the CIDEP time profiles, which suggests that the CIDEP signals observed are created by RTPM. According to the sign rule for CIDEP in RTPM, net Em and Abs phases in triplet quenching processes are created in the RTPs with J < 0 and J > 0, respectively [1,2,5,15]. Therefore, we can determine the sign of J value for the gal-
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vinoxyl–triplet chrysene pair in each solvent as summarized in Table 1. It is noteworthy that the J values are found to be positive in relatively nonpolar solvents ðer < 3Þ while the J values become negative in rather polar solvents ðer > 5Þ. Because of weak CIDEP signals, we could not determine the sign of J value in some solvents whose polarities are intermediate ð3 < er < 5Þ. The switch from positive J values in non-polar solvents to the negative values in polar solvents seems to suggest the importance of CT interaction in a galvinoxyl–triplet chrysene pair. In order to examine the effect of a CT state on J value, the energy gap, DGðrÞ between zero-order RTP ðRTP0 Þ and CT ðCT0 Þ states is estimated by the equation DGðrÞ ¼ GðCT0 Þ GðRTP0 Þ ox red ðgalvinoxylÞ E1=2 ðtripletÞ ¼ fE1=2
þ DEcorr g þ fkðrÞ Ecoulomb ðrÞg DEðT1 Þ;
ð4Þ
ox red where E1=2 ðgalvinoxylÞ and E1=2 ðtripletÞ mean halfwave redox potentials of galvinoxyl (+0.07 V) in acetonitrile [20] and chrysene ()2.25 V) in N,Ndimethylformamide vs. SCE [18], respectively. Since these values are obtained in highly polar solvents ðer > 30Þ, we add a correction term, DEcorr , calculated using the Born equation for solvation energies of anion and cation by intro and the er values of ducing the ion radius of 3.5 A
Table 1 The values of DGðrÞ and the signs of J value of galvinoxyl–triplet chrysene encounter pair in various solvents Solvents DCE DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl DCE/o-Xyl o-Xyl Toluene Benzene a
mixture mixture mixture mixture mixture
(DCE (DCE (DCE (DCE (DCE
73%)a 50%)a 30%)a 15%)a 10%)a
er b
DEcorr (kcal mol1 )
kðrÞ (kcal mol1 )
Ecoulomb ðrÞ (kcal mol1 )
DGðrÞ (kcal mol1 )
Sign of J value
10.37 7.7 5.7 4.3 3.25 2.9 2.568 2.379 2.284
1.6 2.4 3.5 4.9 6.6 7.5 8.6 9.3 9.7
16.5 14.9 12.7 10.0 6.5 4.7 2.6 1.1 0.3
4.6 6.2 8.3 11.0 14.6 16.3 18.5 19.9 20.8
10.4 8.1 4.8 0.8 )4.6 )7.2 )10.4 )12.6 )13.8
) ) ) ? ? + + + +
The fractions of DCE in volume are represented in DCE/o-Xyl mixture solvents. Dielectric constants of DCE/o-Xyl mixture solutions are estimated using those in toluene/DCE mixture solvents obtained from [22]. All the others are obtained from [18]. b
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35.94 for acetonitrile and 36.71 for N,N-dimethylformamide [18]. The symbol of kðrÞ is a solvent reorganization energy between the RTP0 and CT0 states, and Ecoulomb ðrÞ is the Coulomb energy of the CT0 state. The fkðrÞ Ecoulomb ðrÞg term depends on the radical–triplet distance, r. The kðrÞ values are calculated by using MarcusÕs formula [21] and , using the following values; rA ¼ rD ¼ 3:5 A , n ¼ 1:50 and er of solvents listed in rAD ¼ 7 A Table 1. The excitation energy of DEðT1 Þ is reported to be 2:0 104 cm1 [18]. The calculated DGðrÞ values are summarized in Table 1. Fig. 3 illustrates the relation between the calculated DGðrÞ and the observed sign of J values. The DGðrÞ values cover over a range from ca. )15 to 10 kcal mol1 and the signs of the J value are represented by filled (–) and open (+) bars. It is remarkable that the J values are positive for DGðrÞ < 6 kcal mol1 and negative for DGðrÞ > 5 kcal mol1 . This means that the sign switching of the J value occurs around DGðrÞ ¼ 0. As suggested by a large number of previous studies on radical–triplet interactions [1–12], J values in most encounter pairs are negative due to
Fig. 3. Plots of the signs of the J values vs. DG for galvinoxyl– triplet chrysene pairs in various solvents with different dielectric constants. Table 1 lists the values necessary for the plots.
the exchange interaction between radical and triplet molecules. On the other hand, some galvinoxyl–triplet systems with their intermolecular CT states closely locating near their RTP states have been reported to indicate positive J value [13–15]. According to the mechanism proposed for the observed positive J values, their doublet CT0 ð2 CT0 Þ states cause blue- or red-shifts of their doublet RTP0 ð2 RTP0 Þ states depending on their DGðrÞ values between RTP0 and CT0 states. Examples of these effects are schematically described in Fig. 4 where their intermolecular states are shown along solvent coordinates. RTP0 states 0 have the energy minima at rRT s along solvent coordinates and are energetically split into 4 RTP0 and 2 RTP0 states by exchange interaction. Figs. 4a, c represent the potentials of RTP0 and CT0 of the galvinoxyl–triplet chrysene pairs in non-polar ðDGðrÞ < 0Þ and in polar ðDGðrÞ > 0Þ solvents, respectively. It is noteworthy that the solvent reorganization energy is larger while the minimum energy of the CT0 state is lower in polar solvents. The balance between the reorganization energy and the minimum energy of CT0 state dominantly controls the sign of DGðrÞ value at rRT . Potential curves in Figs. 4a, c are converted into Figs. 4b, d, respectively, when electron correlation between CT0 and RTP0 states is considered. In Fig. 4b, their 2 RTP state is higher in energy than the 4 RTP state, which corresponds to the RTP with J > 0. On the other hand, the 2 RTP state is still lower in energy than the 4 RTP state in Fig. 4d for DGðrÞ > 0, indicating that J < 0. If the interaction energy between the 2 RTP0 and 2 CT0 states for DGðrÞ < 0 dominates over the exchange interaction energy, the sign of the J value becomes positive [13–15]. This is exactly the case for a galvinoxyl–triplet chrysene pair in non-polar solvents where the estimated DGðrÞ value is negative, the CT effect causes positive J value, and apparent ferromagnetic interaction is observed to occur in RTP. On the other hand, the J value of the pair with positive DGðrÞ value in polar solvents is primarily controlled by the exchange interaction and possibly by an additional effect of the CT interaction. The resultant J value is negative and apparent antiferromagnetic coupling is observed to take place in RTP (Fig. 4d). It is interesting to note
A. Kawai et al. / Chemical Physics Letters 372 (2003) 8–14
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Fig. 4. Schematic description of potential surfaces of RTP and CT states along the solvent coordinate. The zero-order states with negative and positive DGðrÞ values are shown in (a) and (c), respectively. When electron correlation between 2 RTP0 and 2 CT0 states is considered, the potential surfaces in (a) and (c) are converted into those shown in (b) and (d), respectively. The 2 RTP state shown in (b) is higher in energy than the 4 RTP0 state at rRT for the potential minimum of RTP. This situation corresponds to the positive sign of J value in RTP.
that we observed very weak CIDEP signals for the pairs with the DGðrÞ value of ca. )5 to 4 kcal mol1 and could not determine the sign of J values. In this DGðrÞ region, 2 CT0 states locate so closely in energy to 2;4 RTP0 states that the interactions among the 2 CT0 and 2;4 RTP0 states are expected to be large. We cannot discuss the complicated dynamics of such a radical–triplet pair at this moment. The present result clearly indicates a switch in the sign of J value of a galvinoxyl–triplet chrysene encounter pair depending on the DGðrÞ value. This observation strongly supports our model for positive J value for RTP, namely, the 2 CT0 state causes the spin selective energy shift of the 2 RTP0 state. A galvinoxyl–triplet chrysene pair is not the only example which shows a switch in the sign of J value by changing the solvent from non-polar to polar ones. We found similar switching phenomena in pairs of galvinoxyl–triplet naphthalene and
galvinoxy–triplet 4-bromobiphenyl. Extended studies on the switching in the sign of J value depending on the solvent polarity are now in progress to understand the effect of CT interaction in RTP, in particular the complicated interaction of the pairs where the energies of RTP0 states nearly equal to those of CT0 states.
Acknowledgements We would like to express our thanks to Prof. Kinichi Obi for his encouragement in this study. The present work is partly defrayed by the Grantin-Aid for Scientific Research (Nos.13740327 and 13127202) from the Ministry of Education, Sports, Science and Culture of Japan. This study was performed using one of on-campus cooperative research facilities in Tokyo Institute of Technology, Ôa pulsed ESR systemÕ.
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