2074
J. Phys. Chem. 1980, 84, 2074-2078
promoted onto the ligand system and those still remaining on the ( r ~ dcore. ) ~ The final observed electrostatic splitting of a given cluster of states is -100 cm-', whereas the contribution of spin-orbit coupling in the Hamiltonian of the (ndI6 system that separates the clusters is an order of magnitude larger than this. This view is just the reverse of the usual descriptions of excited states in which the gross splittings that produce the states from a given configuration are electrostatic and spin-orbit coupling plays a minor role in determining energies.13 We have used this ion-parent coupling model successfully to rationalize the existence, splittings, and behaviors of the low-lying CTTL excited states of Ru(I1) and Os(I1) complexes. Initially, because of the enormous increase in spin-orbit coupling that occurs when one switches from Ru(I1) to Os(II), we expected the coupling model to be far better suited for complexes of the heavier atom. Surprisingly, this did not happen. The expected dominance of spin-orbit coupling in Os(I1) complexes was mitigated by the greater extension of the 5d electrons onto the ligand system over that of the 4d set. This brings us to the central question: how does one choose between a singlet-triplet and an ion-parent coupling language to describe CTTL excited states? For a given molecule, experiment will dictate in the end, but some assertions can be made. A critical factor is the formal oxidation state of the central metal ion; a high formal oxidation state favors the coupling model. Important also is the magnitude of S; the spin-orbit coupling constant of the metal ion; the higher this constant is, the more appropriate becomes the ion-parent coupling description for the CTTL states. Thus, for complexes of this type involving the (5d)6 ions Pt(IV), Ir(III), Os(II), Re(I), and W(O), we expect the coupling model to work best for Pt(IV) but to fail as a first-order description for W(0) molecules. The CTTL excited states of these latter species are probably better described by the usual singlet-triplet formalism. For Re(1) we expect an intermediate situation to prevail.
An important manifestation of the degree of spin-orbit coupling in CTTL excited states of Ru(I1) and Os(I1) complexes of the type reported here, and also those of Ir(III),14is the rapid relaxation that occurs among the levels in the excited state manifold. All our decay-time measurements on these systems produced single exponentials even at temperatures of less than 2 K. Our conclusion is that equilibrium among the levels is maintained at all temperatures at all times. This behavior contrasts sharply with that displayed by organic systems, for which equilibrium is not maintained below -7 K.I3 In our view the substantial spin-orbit interactions in the complexes effectively tie the spins to the molecular framework and provide a mechanism for facile vibrational-electronic relaxation. Note Added in Proof. See Note Added in Proof of paper 1. References and Notes (1) (a) Research sponsored by the Air Force Offlce of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR 762932; (b) abstracted in part from the dissertation of D. E. Lacky submitted to the Graduate School of Washington State University In partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1975; (c) US. Senior Scientist (Humboldt Awardee), Unlversltat Hohenheim, West Germany, 1978-9. (2) B. J. Pankuch, D. E. Lacky, G. A. Crosby, J. phys. Chem.,preceding paper. (3) W. R. Ordorff and A. J. Hemmer, J . Am. Chem. Soc., 49, 1272 (1927). (4) J. N. Demas and G. A. Crosby, J. Am. Chem. Soc., 92, 7262 (1970). (5) J. N. DemasandG. A. Crosby, J. Am. Chem. Soc., 93, 2841 1971). (6) G. D. H aw and G. A. Crosby, J. Am. Chem. Soc., 97, 7031 (1975). (7) R. W. Harrigan, 0. D. Hager, G. A. Crosby, Chem. Phys. Lett., 21, 487 (1973); R. W. Harrigan and G. A. Crosby, J . Chem. Phys., 59, 3468 (1973). (8) K. W. Hipps and G. A. Crosby, J. Am. Chem. Soc.,97, 7042 (1975). (9) G. D. Hager, R. J. Watts, 0. A. Crosby, J . Am. Chem. Soc., 97, 7037 (1975). (10) M. J. D. Powell, Comput. J., 7, 303 (1965). (11) G. A. Crosbyand W. H. E M g , Jr., J. phys. Chem., 80, 2206 (1976). (12) M. K. DeArmond and J. E. Hlllis, J. Chem. Phys., 54, 2247 (1971). (13) M. A. El-Sayed, Acc. Chem. Res., 1, 8 (1968). (14) R. J. Watts and 0. A. Crosby, unpublished work, this laboratory.
Nonradiative Relaxation Process of the Higher Excited States of Meso-Substituted Anthracenes Kumao Hamanoue, Satoshi Hirayama,+ Toshlhiro Nakayama, and Hlroshl Teranlshi Deparlment of Chemistty, Faculty of Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan (Received: December 31, 1979)
Using pico- and nanosecond spectroscopic methods, we have measured the time-resolved absorption spectra for 9-nitroanthracene, 9-benzoyl-lO-nitroanthracene,and 9-cyano-10-nitroanthracene.Rather long buildup times of the triplet-triplet absorptions (72-86 ps) lead us to the conclusion that the observed buildup times do not reflect the lifetimes of the singlet states but might represent the rates of the internal conversion in the triplet manifold and that the indirect intersystem crossing S1(m*) Tn(nr*) Tl(m*) is the most important process to populate T1 in accordance with El-Sayed's rule.
-
Introduction Recently, the study of the nonradiative process of the fluorescing state in anthracene derivatives has received considerable attention from both experimental and t h o retical points of view.' For example, Bennett and 'Faculty of Textile Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan. 0022-3654/80/2084-2074$01.00/0
-
McCartin2 have concluded that, in those anthracene derivatives which show a significant variation of fluorescence yield with temperature, radiationless deactivation proceeds entirely by intersystem crossing (isc). The rate of isc is proportional to the vibrational overlap factor whose magnitude increases rapidly with decreasing energy gap between the two interacting states.3 As a result, the number and the order of the higher triplet state(s) lying 0 1980 American Chemical Society
The Journal of Physlcal Chemistty, Vol. 84, No. 16, 1980 2075
Excited States of Meso-SubstitutedAnthracenes
' =-L.-----I Figure 1. Schematic representation of a JEOL mode-locked ruby laser: (M1,M2)laser reflectors; (SA) dye cell; (T) telescope; (I) iris; (LR) laser C2, C,) coaxial cables; rod; (Pl, P2)polarizers, (PC) Pockels cell; IC1, (L) lens; (ND) neutral filter; (SG) spark gap; (HV) high voltage; (AMP) amplifier; (Rl, R2) prisms.
near or very slightly below the first excited singlet state may be expected to be important in the singlet-triplet isc. Such higher triplet states have been predicted by Parker4 for polyacenes and alternate hydrocarbons, and the existence of such a state has been observed for naphthalene (3B3u+),anthracene (3B1g+),2-methylanthracene (3B1'1, and tetracene (3B1,k)? For anthracene, therefore, it is cfear that isc is the most important nonradiative decay process from the fluorescing state. The importance of isc is amplified for the anthracene derivatives with nonbonding orbitals, since El-E3ayed's selection rulee predicts that isc between states arising from different types of electronic promotion (e.g., na* na*) should be at least an order of magnitude greater than isc between states of the same type of electronic configuration (e.g., nn* na* or %a* aa*). In the latter, direct spin-orbit coupling is very weak, and, therefore, the radiationless transition between the lowest excited singlet state and the nearby triplet state is expected to be dependent on the second-orderspin-orbit coupling which involves vibronic interaction between the lowest energy state and the higher-lying state of the different type of electronic configuration from that of the lowest energy state. This has been convincingly demonstrated for the nitrogen heterocyclics where it was shown that intermediate triplet states play an important role in the isc process if they are of different orbital configuration than S1.I We now wish to apply El-Sayed's rule to an important class of molecules, the meso-substituted nitroanthracene derivatives. Since they are usually nonfluorescent, it is expected that there exists a very fast nonradiative process from the lowest excited singlet state which may be followed by a picosecond laser photolysis. Since the lowest aa* electronic states in meso-substituted anthracenes are not significantly different in energy from that in anthracene, the important pathiway for isc in meso-substituted nitroanthracenes might be the one involving the 3(n7r*)state which is located near or very slightly below the lowest excited singlet state. Among several nitroanthracene derivatives, we selected 9-nitroanthrsicene(9-NOz-A),9-benzoyl-10-nitroanthracene (9-Bz-10-N02-A),aind 9-cyano-10-nitroanthracene (9-CN10-NOz-A) an which qualitative discussions as to the photochemical reactions have already been given elsewhere."1° Detailed information on the energy dissipation processes of these compounds not only is important in itself but also would be valuable in understanding their photochemical processes.1°
-
-
-
Experimental Section The picosecond mode-locked ruby laser system is shown in Figure 1. A ruby rod (6.5 mm i.d. X 110 mm) whose
Figure 2. Experimental setup for the picosecond spectroscopy: (OSCI.) laser oscillator; (AMP) amplifier; ( S a ) second-harmonlc generator; (DL) optical delay line; (BS1, ESP)beam splitters; (Ml, M,, M,, M,) mirrors; (Ll, Lp, L,, L,, LE)lenses; (SPM) self-phase modulatlon cell; (F)fitter; (SP) spectrograph; (C) sample cell; (Pl, P2) prisms.
ends were cut at Brewster's angle and coated by antireflecting films was pumped by a helical flash lamp in a water-cooled laser head which was placed between two reflectors. Both the front (50% reflection) and the rear reflectors (100% reflection) of the laser cavity were wedged at the angle of 6 = tan-' 1/30. The mode locking was achieved by the methanol solution of DDI (1,l'-diethyl2,2'-dicarbocyanine iodide). A beam-expanding telescope was put in the cavity to avoid the lens effect of the ruby rod and the damage of all of the optics due to the selftrapping of intense laser light. The following procedures were carried out to get a stabilized laser oscillation: (1)The pumping flash lamp was fired only above the threshold voltage, that is, above 00.03 kV with the accuracy of hO.01 kV. The interval between shots was kept constant (5 min). (2) The saturable dye solution (DDI in methanol) was circulated in the dye cell with a 1.0-mm path length constructed by a 100% reflector and a glass (BK-7). (3) The room temperature was kept constant (18-20 "C), and the temperature of the cooling water of the laser system was kept constant within f 3 "C of the room temperature. It was found that the disturbance of the mode-locked pulse train on the oscilloscope (Tektronix 7844) was caused by the mechanical vibrations and loosening of the reflectors of the laser cavity. We, therefore, constructed an optics composed of three etalon plates, by which we could easily readjust the optical alignment without changing the arrangement of the optical system behind the laser cavity, and more than 80% of the laser shots were found to give good mode-locked pulse trains. A train of about 25-30 pulses, separated by the round-trip time, i.e., -7 ns, was produced at the output of the oscillator and had an energy of ca. 184 mJ. A single pulse was taken out from the pulse train by using a Pockels cell and was amplified by passing twice through an amplifier ruby rod (15 mm i.d. X 150 mm). The resulting single pulse had an energy of ca. 162 mJ. The experimental setup for a picosecond photolysis is shown in Figure 2, which is essentially the same as that of Magde and Windsor.ll The 694.3-nm pulse, on emerging from the amplifier, was frequency doubled by passing it into a phase-matched KDP crystal. The 694.3- and 347.2-nm pulses were then separated by a dielectric-coated beam splitter (BS1) which reflected 347.2-nm light but at the same time allowed 694.3-nm light to be transmitted. This second-harmonic pulse with energy of ca. 1.6 mJ excited the sample, the 347.2-nm path length being variable by means of combination of a mirror (MJ and a dielectric-coated beam splitter (BSJ controlled by a micrometer. The monitoring-light pulse was continuous self-phasemodulated (SPM) light obtained by passing the focused
2070
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
Hamanoue et al.
10.06 02c
0.6
1
.
10.04
1
tl 450
400
Wavelength(nrn)
Flgure 3. Time-resolved absorption spectrum of 9-N02-A at 170-ps delay. (Dottedline is the result obtained by a conventional Rash photolysis at 77 K In EPA.)
fundamental through a 15-cm cell containing carbon tetrachloride. After sampling simultaneously an excited volume of 0.03-cm2cross section in the center and unexcited reference volumes both above and below the excited region, we fed this probe pulse into a Mizojiri-SD-12V spectrograph with a recording film of Tri-X-Pan. A 10" angle between the probe and the exciting pulses kept the latter out of the spectrograph. When the probe beam was imaged onto the film, decreased transmission through the sample was revealed by a bright central streak, corresponding to a transient absorption spectrum sandwiched between two spectra which show no absorption where the ground-state absorption is absent. A densitometer trace vertically across the photographic spectrum at a given wavelength was recorded with a modified Sakura PDM-5 microdensitometer. Thus, we could record a reference spectrum for the probe beam on each shot along with the transient absorption spectrum. The concentrations of the samples were 1.59 X 10-3-4.8 X M, making the absorbances of the solution, at 347.2 nm, 1.58-2.18 in a cell of 2-mm path length. The sample solutions were not deaerated and were excited at room temperature. The pulse width (mean width) was measured by a twophoton fluorescence method using a methanol solution of rhodamine 6G,12and the pulse width was determined to be ca. 30 ps. We have also measured the overlap of exciting and probe pulses by a technique of Hochstrasser et d.13 by measuring the buildup of S, S1absorption at 560 nm in a benzene solution of 9-cyanoanthracene, and the overlap was well represented by a 26-ps pulse width with an estimated error of 4 ps. For a nanosecond photolysis, a Q-switched ruby laser was constructed in this laboratory. The ruby laser was equipped with a 10 mm i.d. X 100 mm parallel-cut ruby rod, and it generated a pulse containing about 1.5 J of red light of wavelength 694.3 nm whose half-peak duration was 22 ns. The KDP frequency doubler converted a few percent of the energy of the 694.3-nm beam to that of the 347.2-nm beam. The transient spectra and decay times at various wavelength were observed by using a RCA 8575 photomultiplier and a Tektronix 475A oscilloscope. The monitoring-light pulse was obtained from a Xe-flash lamp (Nikon SD-X). The experiments were carried out at room temperature with deaerated sample solutions. Conventional flash photolysis experiments were carried out at 77 K in deaerated EPA (ether/isopentane/ethanol = 5:5:2 in volume ratio) solution.
-
Results and Discussion Transient Absorptions. Figures 3 and 4 are the timeresolved absorption spectra in picosecond photolysis for 9-N02-Aand 9-Bz-10-N02-Ain benzene, respectively. We
'b
.
450
Wavelength(nm)
500'
Figure 4. Time-resolved absorption spectrum of 9-Bz- 10-N02-A: (0) at 170-ps delay in ethanol by picosecond photolysis; (0) at 0.4-ps delay in benzene by nanosecond photolysis: (0)in EPA at 77 K by a conventional flash photolysis.
I
I
400
450
Wavelength(nrn)
Flgure 5. Time-resolved absorption spectrum of 9-CN-10-N02-A. (Dotted line is the result at 3-ps delay in benzene obtained by nanosecond photolysls.)
also show the results obtained by a nanosecond ruby laser photolysis at room temperature and a conventional flash photolysis at 77 K in the deaerated system. All of the transient spectra show the characteristic absorption bands around 450 nm.14 The lifetimes of the transient species at 77 K are 17.7 ms for 9-NO2-A and 16.0 ms for 9-Bz10-N02-A. They are about a half of the lifetime of the lowest triplet state of anthracene. On the basis of these results, the observed spectra of the two compounds can be assigned to the T, TI absorptions which undoubtedly originate from the lowest triplet state T1 ( S T * ) . The time-resolved absorption spectra of 9-CN-10-N02-A in pico- and nanosecond photolyses are a little different from those of 9-N02-Aand 9-Bz-10-N02-A as shown in Figure 5. One reason may be due to the fact that below 430 nm 9-CN-10-N02-A shows the strong ground-state absorption, and hence the monitoring light intensity is too weak to observe the transient absorption. Moreover, the T, T1 absorption itself is very weak, and no triplet absorption could be detected at 77 K by a conventional flash photolysis. In a nanosecond pulse radiolysis using a Febetron 70716 we can get an absorption spectrum due to the anion radical of 9-CN-10-N02-Awhen a small amount of triethylamine is added to the solution. In the absence of triethylamine, is obtained the transient spectrum which is very similar to that obtained by nanosecond laser photolysis both with regard to the position of the absorption band and its intensity distribution. Since triethylamine works as a good cation radical scavenger, the transient spectrum obtained in pulse radiolysis may well be due to the triplet state of 9-CN-10-N02-Aoriginated from the recombination between an anion radical of 9-CN-10-N02-Aand a solvent cation radical. Thus, the observed spectrum in picosecond photolysis may also be ascribed to the T, TI absorption. We can safely assume, therefore, that the rate of increase in the absorption intensity (AOD) at ca. 450 nm measures the population growth rate of the lowest triplet state.
-
-
-
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2077
Excited States of Meso-SubstitutedAnthracenes
OJ
oi
I
I
I
I
I
400
200
I
600
Delay time(psec)
I
I
800
Figure 6. Absorbances AOD(t)/AOD(m) at 452 nm due to the excitdstate absorption of 9-N02-A in benzene resulting from singlet-state excitation at 347.2 nm. The CP are normalized absorbances, and the smooth curve is the calculated value.
T
1
f l
I
200
0' 0
1
I
f I
400
r
1
600
I
I
800
Delay tirne(psec)
Flgure 7. Absorbances AOD(t)/AOD(m) at 452 nm due to the excited-state absorption of 9-BZ-10-No2-A in benzene resulting from singlet-state excitation at 347.2 nm. The CP are normalized absorbances, and the smooth curve is the calculated value.
TABLE I: Buildup Times of T, 9-NO,-A (at 452 nm) 9-Bz-10-N02-A (at 452 nm) 9-CN-10-N02-A (at 470 nm)
86 f 76 f 72 f 82 f 76 f
6 ps in 6 ps in 6 ps in 6 ps in 6 ps in
+
T, Absorptions
ethanol benzene or bromobenzene ethanol benzene or bromobenzene benzene or bromobenzene
However, we cannot explain why its triplet-state lifetime at 77 K is so short compared with those of 9-NO2-Aand 9-Bz-10-N02-A. The typical time evolutions of the T, T1absorptions are shown in Figures 6-8. The experimental points,shown in these figures, give the average AOD(t)/AOD(m) of all shots at each delay time, and the error bars give the standard deviation from the average. AOD(m) was determined by averaging data for t = 250-1500 ps at each shot. It is evident that the band growth has reached plateau after ca. 200 ps and that its intensity remains constant up to 1500 ps. The smooth curves correspond to the theoretical absorbance calculated with a well-known convolution method,13J6by assuming a single exponential population of the absorbing triplet state and 26-ps Gaussian probe and excitation pulse shapes. The buildup times of the T, T1 absorption for the nitroanthracene derivatives are given in Table I, all of which are approximately the same within a factor of 1.2. There is some slight solvent dependence of these buildup times, but the exact source of the solvent dependence is not clear. One can see no external heavy atom effect on the buildup time of T1 population. Intersystem Crossing Process. The S1 states of the nitroanthracenes studied here are undoubtedly of a m* character by judging from the absorption spectra and the
-
-
-
A,
,
I
,
I
I)
Figure 8. Absorbances AOD(t)lAOD(m) at 470 nm due to the excited-state absorption of 9-CN-10-N02-A in benzene resulting from singlet-state excitation at 347.2 nm. The @ are normalized absorbances, and the smooth curve is the calculated value.
substitution effect on them. The absorption spectra at 77 K are very similar to that of anthracene in structure, and no hump or shoulder assignable to nr* state was observed. Compared with 9-N02-A,the 0-0 band of 9-CN-10-NOz-A shifts to the red greatly because of a cyano group, making a (hidden) h a * state much higher than a lmr* state in a comparative sense. (At 77 K in EPA, the 0-0 bands are about 25 800,25 600, and 24 300 cm-' for 9-NO2-A,9-Bz10-NO2-A,and 9-CN-10-N02-A,respectively.) Since the lowering of Sl(m*) does not essentially affect the decay mechanism of the three compounds stated here, the same electronic state which is of a m*character could be at+signed to S1. When the lowest excited singlet state is of a mr* character, direct isc to T1will be of minute importance because of the extremely unfavorable Franck-Condon factors for such a process, as has been previously discussed on the anthracene derivatives. Moreover, there may be the external heavy atom effect in this case. Provided that isc is the slowest one among the processes in populating the lowest triplet state T,, it is easily seen that the observed buildup time should reflect the lifetime of the S1state. Therefore, if an ultrafast internal conversion from S1to the ground state exists and if such a process is responsible for the lack of fluorescence, the buildup time of the triplet-triplet absorption will be much shorter than ca. 70 ps. This is unlikely because the rate constant of S1 So internal conversion is expected to be of the order of lo6 s-l for anthracene and its analogues." If we assume the fluorescence rate constant to be of the order of lo8 s-l and that the observed buildup time corresponds to the rate constant of isc, the calculated fluorescence quantum yield is expected to be greater than 0.007, which means that the S1 So fluorescence should be detectable with the present sensor device.18 Since no such emission has ever been detected even at 77 K, isc from S1(m*) to the triplet manifold must be fast with a rate constant 110" s-'. The ultrafast isc would be related to the presence of an n?r* triplet state near or very slightly below the lowest excited singlet state, the transition to which from Sl(mr*) is very rapid because of a favorable spin-orbit coupling and large Franck-Condon factor.20 It is concluded, therefore, that the measured buildup times (7)should correspond to the rate constants ki,( = T - ~ = 101os-l)of the internal conversion in the triplet manifold (e.g., 3na* T1(m*)). Since the large S1-T1 splitting makes it likely that there is a second or third triplet state between S1and T1, the internal conversion from T,(na*) to Tl may well pass through these states. Since vibronic interaction between the two electronic states depends on the overlap integraP and it is small between nr* and m* states, the vibronic interaction between these states is expected to be weak.
-
-
-
2078
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
-
Kokubun et al.24have determined the quantum yields of T, S1 intersystem crossing of anthracene, 9methylanthracene, 9-phenylanthracene, 9,lO-dichloroanthracene, and 9,lO-dibromoanthracene by a double excitation method, and the rate of T2 T1 internal conversion has been estimated to be ;=lo1' s-l for all compounds studied. However, from the studies on the photochemical sensitization, Liu and co-workers26obtained the lifetimes of the T2state to be 7(T2)= 200-300 ps for several mesosubstituted anthracenes (9-phenylanthracene, 9,lO-dichloroanthracene, 9,10-dibromoanthracene, 9,lO-diphenylanthracene). Gillispie and Lim26also suggested 7(T2)= 200 ps from an empirical energy gap correlation of internal conversion rate for 9-bromoanthracene and 9,lO-dibromoanthracene. Much stronger confirmation of our data has been reported by Hirayama and K ~ b a y a s h i . ~ These ~ authors T1 absorption in 9studied the buildup times of T, acetylanthracene, 9-benzoylanthracene, 9-benzoyl-10chloroanthracene, and 9-benzoyl-10-cyanoanthraceneand found T(T,)between 20 and 300 ps. These rates are more than an order of magnitude slower than the rates of S1 T isc in these molecules. The slower rate process is thus assigned as an internal conversion to T1 from a higher triplet state T(n?r*) initially populated via fast isc from
-
-
-
SI(7r7r*). We have presented a picture of the nitroanthracene isc where the involvement of T, cannot be neglected. We have also made the suggestion that the indirect path S1(m*) T,(n?r*) T1 (m*)arises as a consequence of the strong spin-orbit coupling between S1and T, and that vibronic mixing between T, and T1is not large. These conclusions are entirely consistent with other observations 3n~* that have confirmed the importance of the l m * or ln.lr* %7r* decay channel in planar heteroaromatics,7 nitronaphthalene,1e*22and aromatic ketone^.^^^^^
-
-
-
-+
Acknowledgment. This research was partly financed by Grant in Aid for Scientific Research from the Ministry of Education of Japan (No. 320911). References and Notes (1) J. B. Birks, "Photophysics of Aromatic Molecules", Wlley-Interscience, New York, 1970, p 142. (2) R. G. Bennett and P. J. McCartin, J. Chem. Phys., 44, 1969 (1966). (3) G. W. Robinson and R. P. Frosch, J. Chem. Phys., 37, 1962 (1962); 38, 1187 (1963). (4) R. Pariser, J. Chem. Phys., 24, 250 (1956). (5) D. M. Hanson and G. W. Robinson, J. Chem. Phys., 43,4174 (1965); R. E. Kellog, ibhl., 44, 411 (1960); R. Astier, A. Bokobza, and Y. H. Meyer, ibid., 51, 5174 (1969).
Hamanoue et al.
(6) M. A. El-Sayed, J . Chem. Phys., 38, 2834 (1963); S. K. Lower and M. A, El-Sayed, Chem. Rev., 88, 199 (1966). (7) P.E. Zinsli and M. A. ECSayed, Chem. Phys. Lett., 34, 304 (1975); 36, 290 (1975). (8) N. C. Yung, Pure Appl. Chem., 9,591 (1964); N. C. Yung, M. Nwsim, M. J. Jorgenson, and S.Murov, Tetrahedron Lett., 3657 (1964). (9) 0. L. Chapman, D. C. Heck&, J. W. Reasoner, and S. P. Thackabeny, J. Am. Chem. Soc., 88, 5550 (1960). 10) S. Hrayama, K. Hamanoue, H. ohya, and H. Terankhi, to be submitled for publlcation; paper presented at the joint meetlng of the American Chemlcai Soclety and the Chemical Society of Japan, Honolulu, HI, 1979, Physical Chemistry, No. 38. 11) D. Madge and M. W. Windsor, Chem. Phys. Lett., 27, 31 (1974). 12) J. A. Giordmaine, P. M. Rentzepis, S. L. Shaplro, and K. W. Wecht, ADD/. Phvs. Lett.. 11. 216 (1967). (13) R: W. Anberson Jr., R: M. Hochstiasser, H. Lutz, and G. W. Scott, J. Chem. Phys., 61, 2500 (1974). (14) I n Figure 4 the transient spectrum In a picosecond photolysis is a little different from that in a'conventional flash photolysis, suggesting that there is a possibility of the formation of oxy1 radical. However, the spectrum was nearly the same as those of 9-benzoylanthracene and 9-acetylanthracene. 15) K. Hamanoue, H. Ohya, H. Teranishi, M. Washio, S. Tagawa, and Y. Tabata, to be submitted for publicatlon; paper presented at the 22nd Symposium on Radiation Chemistry, Nagoya, Japan, 1979. 16) R. W. Anderson Jr., R. M. Hochstrasser, H. Lutz, and G. W. Scott, Chem. Phys. Lett., 28, 153 (1974). 17) See also ref 1, p 185. 18) The fluorescence quantum yield of 9-bromoanthracene is reported to be O.Ol.'' The emisslon from this compound is easily measurable with a standard fluorimeter such as a Shimadzu RF 502 spectrofluorophotometer, and hence there Is no dlfficulty In determinlng the fluorescence quantum yield as low as The nltroanthracenes studied here do not show any emission measurable with a Shlmadzu RF 502 spectrofiuorophotometer at temperatures down to 77 K, indicating their fluorescence quantum ylekls are far lower than lo4. (19) S.Schoof, H. Gusten, and C. von Sonntag, Ber. Bunsenges. Phys. Chem., 81, 305 (1977). (20) The role of a carbonyl or a nitro group is so dramatic in enhanclng a radiatlonless process that neither carbonyl- nor nitro-substltuted anthracene has ever been known to be fluorescent at room temperature. Even the bromine substitution does not Increase Isc to such a great extent. Actually, both 9-bromo- and 9,lOdibromoanthracene fluoresce strongly enough to be observed, and the rlse tlmes of thelr T, TI absorptions are measured to be 0.4 and 0.6 ns, respectlvely.2' Thus, It Is most reasonable to ascrlbe the lack of fluorescence In the nltroanthracenes to the efficlent k c to T(nn*). The existence of such a T(nn*) is also suggested by Mikula et ai. for 1- and 2-nitroanthra~ene.~~ (21) K. Harnanoue, S. Hirayarna, S. Tal, and H. Teranishi, to be submitted for publicatlon; paper presented at the 3rd International Congress of Quantum Chemistry, 24-10, Kyoto, Japan, 1979. (22) J. J. Mikula, R. W. Anderson Jr., and L. E. Harris, A&. Mol. Rebxatbn Processes, 5, 193 (1973). (23) E. C. Lim and J. M. H. Yu, J . Chem. Phys., 47, 3270 (1967). (24) S. Kobayashi, K. Kikuchl, and H. Kokubun, Chem. Phys., 27, 399 (1978). (25) R. S. H. Liu and P. E. Keliog, J. Am. Chem. Soc., 91, 250 (1969); R. S. H. Liu and J. R. Edman, iba.,91, 1492 (1969); R. 0. Campbell and R. S. H. Liu, bid., 95, 6560 (1973); C. C. Ladwig and R. S. H. Liu, bid., g8, 6210 (1974). (26) G. D. Glllispie and E. C. Lim, Chem. Phys. Lett., 63, 193 (1979). (27) S. Hlrayama and T. Kobayashi, Chem. Phys. Lett., 52, 55 (1977). (28) D. E. Damschen, C. D. Merritt, D. L. Perry, G. W. Scott, and L. D. Tally, J. Phys. Chem., 82, 2268 (1978). +-
