J . Phys. Chem. 1989, 93, 7 7 5 7 - 7 7 5 9
7757
Direct Observation of the Triplet-Triplet Energy Transfer to the Second Excited Triplet State Tetsuo Okutsu, Akio Kawai, and Kinichi Obi* Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguroku, Tokyo 152, Japan (Received: August 9 , 1989)
The triplet-triplet energy-transfer mechanism was investigated in solution by time-resolved ESR. The acceptor molecule used here is acenaphthenequinone (ANQ) and the triplet sensitizers have all emissive spin polarization. Spin polarization of ANQ- produced in photosensitized reactions is emissive for the sensitizers with triplet energy lower than 21 300 cm-I, but enhanced absorption for those higher than 21 700 cm-l. This spin inversion is interpreted in term of T, -, T2 energy transfer, followed by the T2 -,T I internal conversion in ANQ. This is the direct experimental evidence of the T-T energy transfer to T2. The T, energy of ANQ is estimated to be 21 500 f 200 cm-I.
it is not easy to differentiate the energy transfer to T, ( n L 2) from that to T , in this method because of very short lifetime of T, and spectral overlap of T, with high vibrational levels of T,. The CIDEP technique makes it possible to observe directly the energy transfer to T, by using the spin polarization as a label. By choosing A N Q as a triplet acceptor and several compounds with different triplet energy as energy donors, we directly demonstrated T-T energy transfer to T2of A N Q and determined its energy.
Introduction The time-resolved ESR technique has widely been employed in order to identify short-lived paramagnetic chemical intermediates and to clarify the mechanisms of intramolecular relaxation process.' Buckley and McLauchlan studied the mechanism of intersystem crossing (ISC) of pyrazine and its methyl derivatives by the CIDEP technique., Reaction of triplet pyrazine with 2-propanol yielded absorptively polarized pyrazine neutral radicals, while reaction of triplet tetramethylpyrazine produced the emissively polarized neutral radicals. This changeover from absorptive polarization in pyrazine to an emissive one in tetramethylpyrazine was explained by the difference in ISC mechanism. In pyrazine and tetramethylpyrazine the signs of the zero-field coupling constants are different in 3nn* and 3nn* states, whose energy levels are inverted by methyl substitution because of proximity of these states. The lowest singlet states of these molecules are both Inn*. The Inn* state of tetramethylpyrazine transfers to the Y sublevel of the 3nn* lowest triplet state due to the selection rule of ISC. The Y sublevel lies above barycenter of sublevels and hence emissive CIDEP is generated. On the other hand, the Inn* state of pyrazine transfers to the Y sublevel of the high-lying 3 n ~ state * by ISC, followed by internal conversion to the 3nn* lowest triplet state. As the symmetry of sublevel is conserved during the internal conversion, the Y lowest sublevel of 3nn* is populated and the resultant CIDEP signals become absorptive. Shimoishi et aL3 also reported the inversion of spin polarization T I internal conversion of acenaphthenequinone during T, (ANQ). I n ANQ, the lowest singlet and triplet states are both nn* and the 3nn* state lies between 'n** and 3nn*. The Z highest sublevel of 3nn* populated by ISC relaxes to the Z lowest sublevel of 3nn* because of the opposite signs of D values of these states and conservation of symmetry. lmamura et aL4 reported that spin polarization was conserved in T-T energy transfer in acetophenone/naphthalene and benzophenone/biacetyl systems in an organic rigid matrix a t 7 7 K. Dexter exchange mechanism is experimentally demonstrated to be operative in T-T energy transfer. Conservation of spin polarization was confirmedS in the benzophenone/camphorquinone system in fluid solution at room temperature. In this work, the CIDEP technique is applied to reveal the detail of the T-T energy transfer. The T-T energy transfer has widely been studied by the transient optical spectroscopy.6 However,
Experimental Section All chemicals are G.R. grade. ANQ and triplet sensitizers were obtained from Tokyo Chemical Industry. ANQ, naphthalene, and phenanthrene were purified by recrystallization. Zone-refined phenanthrene was used without further purification. ATEMPO (4-amino-2,2,6,6-tetramethylpiperidinyl1-oxy), Wako Pure Chemicals, was used as received. Benzene, Kanto Chemicals, was used as a solvent without further purification. Oxygen dissolved in the sample solution was removed by bubbling nitrogen gas and all experiments were carried out a t room temperature with a flowing system. The solution flowed through a quartz flat cell ( 0 . 5 mm interior space) with a flow rate of ca. 2 mL min-I. The time-resolved ESR apparatus was described in a previous paper.' The samples in the cavity were excited by a nitrogen laser (Molectron UV 24) or XeCl excimer laser (Lambda Physik EMG 52 MSC).
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Results and Discussion It is necessary to know the spin polarization of triplet sensitizers in fluid solution before we proceed to study the T-T energy transfer. The spin polarization of triplet state was previously demonstrated to transfer to doublet radicals.' Spin polarization of triplet sensitizers used in this work was determined in sensitizer/ATEMPO systems. A T E M P O is one of stable nitroxide radicals and known as a good quencher of triplet aromatic hydrocarbons.8 Six compounds with triplet energy from 19 200 to 24 200 cm-' were used as triplet sensitizers. The time-resolved ESR spectra of 1-nitronaphthalene (I-NO,-NP) (1 X M ) / A T E M P O (2 X M), and phenanthrene (1 X lo-, M)/ATEMPO (2 X M) in benzene are shown as examples in Figure 1, a and b, respectively. The C W ESR spectrum of ATEMPO in benzene is shown in Figure I C in the first derivative form as a reference. Emissive signals of ATEMPO were observed for both sensitizers. The other four sensitizers also showed emissive signals similar to these two compounds. As the signal polarity (emission) of ATEMPO reflects the polarity of the donor molecule,
( I ) Muss, L. T.; Atkins, P. W.; McLauchlan, K. A,; Pedersen, J. B. Chemically Induced Magnetic Polarization; Reidel: Dordrecht, 1977. Lepley, A. R.; Closs, G. L. Chemically Induced Magnetic Polarization; Wiley-lnterscience: New York, 1973. (2) Buckley, C. D.; McLauchlan, K. A. Chem. Phys. 1984, 86, 323. (3) Shimoishi, H.; Akiyama, K.; Tero-Kubota, S.; Ikeaami, Y. Chem. Lett. 1988, 25 I . (4) Imamura, T.; Onitsuka, 0.; Murai, H.; Obi, K. J . Phys. Chem. 1984. 88, 4028. ( 5 ) Depew, M. C.; Wan, J . K. S. J . Phys. Chem. 1986, 90, 6597.
(6) Birks, J . B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. Turro, N. J. Modern Molecular Photochemistry; The Benjamin/Cummings: Menlo Park, CA, 1978. (7) Imamura, T.; Onitsuka, 0.;Obi, K. J . Phys. Chem. 1986, 90, 6741. (8) Gijzeman, 0. L. J.; Kaufman, F.; Porter, G. J . Chem. Soc., Faraday Trans. 2 1973,69,727. Kuzumin, V. A,; Tatikolov, A. S. Chem. Phys. Lett. 1978, 53, 606.
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7758 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
a) b)
v
Em
Letters A N Q l TEA
a) '-N02-NP
v
Em.
0
0.4 mM 1-
10 G
Figure 1. Time-resolved ESR spectra obtained in the laser photolysis of (a) I-nitronaphthalene (1 X IO-) M)/ATEMPO ( 2 X IO-' M) and (b) phenanthrene ( I X M)/ATEMPO (2 X IO-' M). The CW ESR spectrum of ATEMPO is shown in (c).
40mM
nit ronaph t halene concentration
0 CJ
1 OmM
2 mM
40 mM
phenanthrene concentration Figure 3. Time-resolved ESR spectra of ANQ- produced in the photosensitized reactions of (a) 1-nitronaphthalene and (b) phenanthrene.
-nitro nap ht ha L e n e
Figure 4. Model proposed to explain the spin polarization in T-T energy transfer to ANQ.
10 G
Figure 2. Time-resolved ESR spectrum obtained in the laser photolysis of ANQ ( 2 x 10-3 M)/TEA ( 1 x 10-1 M).
all sensitizers are known to have enhanced population in higher triplet sublevels. The triplet state of A N Q was reported3 to react with triethylamine (TEA) and yield ANQ- and TEA' by electron-transfer reaction. Figure 2 shows the CIDEP spectrum of the ANQ/TEA system obtained by direct photoexcitation. The spectrum was observed a t a microwave power of 15 m W by opening the gate of the boxcar integrator between 1 . 1 and 2.1 ps after the laser shot. This spectrum is the same as that of the ANQ- radical reported by Shimoishi et aL3 The polarization of ANQ- was almost absorptive which was caused by triplet mechanism; the lower triplet sublevel was dominantly populated by ISC and the /3/3 spin state was enriched. Figure 3 shows the CIDEP spectra of ANQ- in the sensitizer/ANQ/TEA systems obtained by varying the sensitizer concentration. In the presence of l-NOz-NP, the phase of ANQchanges from absorption to emission by the increase in the sensitizer concentration as shown in Figure 3a. The inversion of the phase is interpreted by the spin polarization transfer from triplet I-NOZ-NPwith emissive character to the acceptor, ANQ. The mechanism is illustrated in Figure 4. While the results were completely different when we used a sensitizer with the same phase of polarization but higher triplet energy, phenanthrene. As seen in Figure 3b, the absorptive signals of ANQ- were enhanced by the increase in the phenanthrene concentration. Shimoishi et aL3 reported about the ISC mechanism of ANQ: ISC from S l ( l n r * ) preferentially populated the T, sublevel in T2, followed by the internal conversion with spin conservation leading to a large population in the T, sublevel in Ti(3nr*). They also reported that the order of spin sublevels in the 3 n r * state was different from that in the 37rn-* state. According their mechanism, the spin
TABLE I: CIDEP Signals of ANQ- for Several Triplet Sensitizers
sensitizers benzophenone phenanthrene naphthalene 2-chloronaphthalene 1 -bromonaphthalene 1 -nitronaphthalene
triplet energy, cm-' 24 200 21 700 21 300 21 100 20 650 19200
CIDEP enhanced A enhanced A E E
E E
polarization is expected to be inverted during I C from T 2 ( r a * ) to Tl(nr*). Our results shown in Figure 3b directly demonstrate the energy transfer to Tz and the spin polarization inversion during the T2 TI internal conversion. Triplet phenanthrene transfers its excited energy to the highest triplet sublevel in Tz of A N Q according to the spin polarization conservation in T-T energy transfer. The T, state populated in the highest spin sublevel relaxes to the Ti state with enhanced population in the lowest sublevel (TJ. Considering spin conservation in internal conversion, the highest sublevel in T2 should be T,. The sign of the D value of T2 is experimentally demonstrated to be the opposite of that of T I : positive in T i ( n r * ) and negative in T,(aa*). In order to estimate the energy of T2, we examined CIDEP by using six sensitizers with different triplet energy from 19 200 to 24 200 cm-'. Table I summarizes the phase of spin polarization of ANQ- generated in the triplet photosensitized reaction. All the triplet states of the energy donors used here have emissive character. When the triplet energy was lower than 21 500 cm-I, the phase of donors was conserved in the product anion radical, ANQ-. This indicates that the donors transfer their energy to T I of ANQ. On the other hand, the donors with higher energy than 21 500 cm-' yielded enhanced absorption which was opposite in phase to the donors themselves. These higher energy sensitizers produce T, of A N Q by the energy transfer, which is followed by T, TI IC. The T2 energy of A N Q is therefore estimate to be 21 500 i 200 cm-'. Shimoishi et al.3 carried out experiments in the A N Q / TEA/benzophenone system using a XeCl excimer laser as an excitation light source. The CIDEP signal of ANQ- gradually
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7759
J . Phys. Chem. 1989, 93, 7759-7760 changed from absorption to emission when benzophenone was added. They interpreted the inversion of the phase by electron transfer from polarized ketyl radical to ANQ. This mechanism is, however, ruled out under our experimental condition, because C I D E P signal of the ketyl radical could not be detected in the absence of ANQ. The opposite tendency observed here would result from the different experimental conditions; the ratio of optical density of benzophenone to A N Q was 0-0.4 in our experiments, but 0.3-2.4 in theirs. Actually we could observe the emissive signals when the ratio was raised up to about 0.8. Therefore, the enhancement of absorptive signal by the addition of benzophenone reflects the energy transfer to T, of ANQ.
In conclusion, the triplet-triplet energy transfer to the second excited triplet state of A N Q was directly demonstrated by the time-resolved ESR measurements. The spin polarization of ANQproduced by the sensitized reaction with emissive triplet was emission for lower energy sensitizers but enhanced absorption for higher ones. This spin inversion is interpreted in terms of TI T, energy transfer, followed by the Tz TI internal conversion. The second excited triplet energy of A N Q is estimated to be 21 500 f 200 cm-I.
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Acknowledgment. This work was supported in part by a Grant-in-Aid from the Ministry of Education (No. 63430002).
Heat of Formation for the Hydroxymethylene Radical Cation. The Importance of Reverse Activation Energy Ngai Ling Ma, Brian J . Smith, Michael A. Collins, John A. Pople,' and Leo Radom* Research School of Chemistry, Australian National University. Canberra, A.C.T, 2601, Australia (Received: September 14, 1989)
The importance of taking into account reverse activation energy and isotope effects in calculating heats of formation from appearance energy measurements is demonstrated in the particular case of HCOH" produced from CH30H. New heats of formation for HCOH" of 971 kJ mol-' (AH;,,) and 968 kJ mol-' (AH;,,,) are obtained on this basis. The discrepancy between theoretical and experimental estimates of the energy difference between HCOH" and H2CO'+ is res.)lved.
Introduction The heat of formation (AHf'298) of the hydroxymethylene radical cation (HCOH") has recently been determined2 from appearance energy measurements as 996 kJ mol-'. This places HCOH'+ 55 kJ mol-' higher in energy than its isomer, the formaldehyde radical cation (H,CO*+), for which the well-established AHf0298value (calculated by using (H,CO) = -109 kJ mol-', IE,(H,CO) = 10.88 eV = 1050 kJ n10l-l)~ is 941 kJ mol-'. O n the other hand, several recent high-level a b initio molecular orbital calculations4" have given an energy difference between HCOH" and H2CO'+ of just 10-15 kJ mol-'. An error of 40 kJ mol-' would be unlikely at the levels of theory employed and suggests that a reexamination of the experimental data might be worthwhile. The results of such a reexamination, leading to a resolution of the apparent discrepancy between theory and experiment, are presented here. Derivation of the experimental AH? for HCOH'+ from appearance energy (AE) measurements on methanol uses the following formula' (see Figure 1) AH?(HCOH'+) = AHfO(CH3OH) + AE(HC0H") - E: - E' (1) where E: is the reverse activation energy, E , the so-called kinetic shift, is the energy in excess of AE(HCOH'+) necessary to drive ( I ) Permanent address:
Department of Chemistry, Carnegie-Mellon
University, Pittsburgh, PA 15213. (2) Burgers, P. C.; Mommers, A. A,; Holmes, J . L. J . A m . Chem. SOC. 1983, 105, 5976. (3) (a) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, 1.; Bailey, S.M.; Churney, K. L.; Nuttall, R. L. J . Phys. Chem. Ref. Data 1982, / I , Suppl. 2. (b) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Dura 1977, 6, Suppl. 1. (4) Frisch, M. J.; Raghavachari, K.; Pople, J. A,; Bouma, W. J.; Radom, L. Chem. Phys. 1983, 75, 323. (5) DeFrees, D. J.; McLean, A . D.; Herbst, E. Asfrophys. J . 1984, 279,
322.
(6) Bouma, W. J.; Burgers, P. C.; Holmes, J. L.; Radom, L. J . Am. Chem. SOC.1986, 108, 1767. (7) For an excellent discussion, see: Holmes, J. L. Org. Mass Specrrom. 1985, 20, 169.
0022-3654/89/2093-7759$01.50/0
TABLE I: Calculated Total Energies (E,,, hartrees)' and Temwrature Corrections (H,- - HmkJ mol-')
species HCOH'+ H2CO'+ H2 TS
EO
H298
- HO
-1 13.923 53
10.2
-113.93341 -1.165 01 -1 15.077 24
10.1
8.7 11.1
"GI level of theory, see text.
TABLE 11: Calculated and Experimental Thermochemical Data (kJ mol-') 0K 298 K
AH(HCOH'+-H2CO'+)"
26 30
E:" AHID(CH30H)b
-190
AE(DCOH'+)C
1 I97
AE(HCOH-+)~
1191 97 1 944 27
AHfO(HCOH'+)e,f AH; (H2CO'+)bf AHI"(HCOH'+) - AHfO(H2CO")
26 22 -201 1 I97 1191 968 94 1
27
"Calculated from theoretical energy data in Table I . bFrom ref 3. From ref 2. Our calculated temperature corrections indicate that AE(DCOH'+) or AE(HCOH'+) are almost temperature independent in this temperature range. dCorrected for isotope effects, see text. e Values derived using eq 1 . /Stationary electron convention used, see ref 3. the reaction a t an observable rate, and quantum mechanical tunnelling is assumed to be negligible. The threshold AE (corresponding to ES = 0) can in principle be obtained through appropriate experiments.8 However, experimental estimation of E: is more difficult. In the absence of information to the contrary, E: is generally assumed to be zero in situations of this type. The validity of this assumption depends on the particular system under examination. We show here that it is a poor approximation in the case of production of HCOH" from CH30H. Neglect of E: (8) Lifschitz, C. Mass Spectrom. Rev. 1982, I , 309.
0 1989 American Chemical Society
