5802
J. Phys. Chem. 1995, 99, 5802-5808
Further Studies on the Nature of the Lowest Excited Triplet States of Haloanthraquinones: Triplet-Triplet Absorption and Phosphorescence Spectra of the a-Halo (1-Bromo, 1-Bromo-5-chloro,1,5-Dibromo, 1-Bromo-8-chloro, 1,8-Dibromo) and P-Halo (2-Bromo) Compounds Kumao Hamanoue," Toshihiro Nakayama, Iori Tsujimoto, Sadao Miki, and Kiminori Ushidat Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Received: July 25, 1994; In Final Form: December I , 1994@
For the lowest excited triplet (TI) states of the title a-haloanthraquinones, the following results are obtained: (1) The shapes of phosphorescence spectra are similar to those of the usual nn*-type phosphorescence spectra; (2) the phosphorescence quantum yields are very small; (3) the triplet lifetimes (0.32-260 ps) are much shorter than that (2.6-2.7 ms) of planar 2-bromoanthraquinone (the P-bromo compound) with a T I state of typical nn* character. Since calculation of the optimized geometries of a-haloanthraquinones indicates the nonplanarity in the central quinone structure, the distortion of the geometrical molecular structure caused by the steric hindrance between the oxygen and halogen atoms may give rise to the appearance of unusually broad phosphorescence spectra. On the basis of the fact that a log-log plot of the nonradiative triplet decay rate constants against the sum of the squares of atomic spin-orbit coupling constants gives an almost straight line, we conclude that not only a special nonradiative decay process (owing to the nonplanarity in the central quinone structure) but also the internal heavy-atom effect of the halogen atom affects the triplet lifetimes of a-haloanthraquinones.
Introduction
second excited triplet (Tz) states were 1 7 0 ps for the 1-chloro * . and 1,5-dichloro compounds and -700-750 ps for the 1,8dichloro c o m p o ~ n d . ~Since nanosecond laser photolysis of a-chloroanthraquinones at 77 K revealed the appearance of only the triplet-triplet absorptions due to the lowest excited triplet (TI) states, the T2 T1 internal conversion should be too rapid to observe any phosphorescences from the T2 states, i.e., the Tz-Tl energy gap should give a much greater T2 T1 internal conversion rate than that which can compete with the T2 ground-state radiative transition rate. Observation of any phosphorescences from the TZstates, therefore, may be impossible for a-chloroanthraquinones even at 77 K. In fact, our phosphorescence measurements for a-chloroanthraquinones (the 1-chloro, 1,5-dichloro, and l&dichloro compounds) and P-chloroanthraquinone (the 2-chloro compound) indicated that the anthraquinone-like phosphorescence spectrum observed for 1-chloroanthraquinone by Matsuzaki and Kuboyama should be ascribed to the phosphorescence from anthraquinone contained as an impurity! Our conclusion was that the emitting states of a- and P-chloroanthraquinones were always the lowest excited triplet states; the a-chloro compounds gave rise to the appearance of broad phosphorescence spectra with spectral profiles similar to those of the usual mixed nn*-nn*-type or nn*type phosphorescence spectra, while the phosphorescence spectrum of usual nn* character was observed for 2-chloroanthraquinone (the P-chloro compound). The triplet lifetimes of a-chloroanthraquinones, however, were unusually short compared with those of several excited triplet states with usual nn* character. As an extension of our studies on the photophysics of sterically strained haloanthraq~inones,3-~the present paper deals with the nature of the lowest excited triplet states of the 1-bromo (1B-AQ), 2-bromo (2B-AQ), 1-bromo-5-chloro (lB,SC-AQ), 1,5-dibromo (lB,SB-AQ), 1-bromo-8-chloro (lB,gC-AQ), and l&dibromo (lB,8B-AQ) compounds. Our conclusion is that the distortion of the geometrical molecular structure caused by the steric hindrance between the oxygen and halogen atoms in -,
When the excited nn* and nn* triplet states of aromatic carbonyl compounds are very close to each other, their relative energies are sometimes shifted by the chemical substitution in the molecule itself.' In connection with this, Matsuzaki and Kuboyama2 observed that the phosphorescence spectrum of 1-chloroanthraquinone had anthraquinone-like sharp emission peaks (with a lifetime of 3.5 ms) superposed on an unusually broad emission band (with a lifetime of 50.1 ms). For the phosphorescence spectrum of 1-bromoanthraquinone, however, no such a broad emission band was observed and the spectrum obtained was very similar to that obtained for anthraquinone in regard to the lifetime and the positions of emission peaks. These results were interpreted as follows: (1) In 1-haloanthraquinone, one of the excited triplet nn* states, which was mainly localized at the carbonyl group adjacent to the halogen atom, might be considerably lower in energy than the other, which was mainly localized at the other carbonyl group. (2) Since the magnitude of the energy gap between these two excited triplet states was expected to increase as an increase in the van der Waals radii of the halogen atoms, it was proposed that the relatively larger energy gap for 1-bromoanthraquinonemade the internal conversion (from the upper to lower excited triplet states) of minute importance, while the internal conversion for l-chloroanthraquinone competed with the radiative process from the upper excited triplet state to the ground state, owing to the smaller energy gap. (3) As a result, two-component emissions originating from both the upper and lower excited triplet states were observed for 1-chloroanthraquinone, while only a one-component anthraquinone-like emission originating from the upper excited triplet state was observed for 1-bromoanthraquinone. Upon picosecond laser photolysis of a-chloroanthraquinones at room temperature, we observed that the lifetimes of the
'
Present address: The Institute of Physical and Chemical Research, Wako, Saitama, 351-01, Japan. @Abstractpublished in Advance ACS Abstracts, April 1, 1995.
0022-365419512099-5802$09.00/0
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0 1995 American Chemical Society
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Lowest Excited Triplet States of Haloanthraquinones a-haloanthraquinones (the bromo and/or chloro compounds) gives rise to the appearance of unusually short-lived and broad phosphorescence spectra with spectral profiles similar to those of the usual mixed m*-nn*-type or nn*-type phosphorescence spectra.
J. Phys. Chem., Vol. 99,No. 16, 1995 5803 0.6
(a12B-AQ
( b)
A
tn I
tA
Om4
z T =2.6 mr
Experimental Section The details of the method of purification of anthraquinone (CP-grade from Wako) and chloroanthraquinones [the 1-chloro, 2-chloro, and 1,5-dichloro compounds (EP-grade from Wako), and the 1,8-dichloro compound (EP-grade from Tokyo Kasei)] were given in the previous paper.6 1B-AQ, 2B-AQ, 1B,5BAQ, and lB,8B-AQ were synthesized from the corresponding chloroanthraquinones by substitution of the chlorine atom by the bromine atom7 and purified by column chromatography on 200-mesh alumina using toluene as the developer, followed by the successive recrystallization from toluene, acetic acid, and ethanol. By the Sandmeyer reaction, lB,SC-AQ and 1B,8CAQ were synthesized from 1-amino-5-chloroanthraquinone and l-amino-8-chloroanthraquinone,respectively, which were derived from the corresponding dichloroanthraquinones;’lB, 5CAQ and lB,8C-AQ thus obtained were purified by acetic acid and then by ethanol. The solvent used for the spectral measurements was a mixed solvent (EPA) of diethyl ether/isopentane/ethanol = 5:5:2 in volume ratio; diethyl ether (Merck) and spectral-grade ethanol (Nacarai) were used without further purification, and GR-grade isopentane (Wako) was purified by passing it through an alumina column. The sample solutions in a cell of 10-mm path length were degassed by several freeze-pump-thaw cycles, and all spectral measurements were performed at 77 K. The groundstate absorption spectra were recorded by a Hitachi 200-20 spectrophotometer. Nanosecond laser photolysis was carried out using a Q-switched ruby laser equipped with a multichannel analyzer which was composed of a polychromator (Unisoku M200), an image intensifier (Hamamatsu V3347U), and a linear position-sensitive detector (Unisoku USP501) controlled by a personal computer (NEC PC-9801RA).* The sensitivity of the multichannel analyzer was corrected using a halogen lamp as a standard light source, and sample excitation was performed by the second harmonic (347.2 nm) with a half-peak duration of 20 ns. The triplet-triplet (T’ TI) absorption spectra were recorded by operating the multichannel analyzer in the gated mode, where a pulsed xenon flash lamp (Unisoku USP543S) was used as a probing light source. For measurements of the phosphorescence spectra, however, the multichannel analyzer was operated in the continuous mode; using a Hitachi MPF-4 phosphorimeter equipped with a Hamamatsu R928 photomultiplier, the phosphorescence (and its excitation) spectra were also recorded. The decay curves of T’ T1 absorptions and phosphorescences were recorded by means of a combination of a photomultiplier (Hamamatsu R666 or RCA 8575) with a storage oscilloscope (Iwatsu TS-8 123) controlled by a personal computer.
a 0.2
400
500
600
700
Wavelength / nm Figure 1. Transient absorption spectrum at 40-ns delay (a) and decay curve (-) of transient absorption at 390 nm (b) obtained by nanosecond laser photolysis of 2B-AQ. In b, the dashed curve is a singleexponential function with a lifetime of t~ = 2.6 ms.
I
1B-AQ C
0.3 -
A
-
-
Results and Discussion
-
Comparison of the T TIAbsorption and Phosphorescence Spectra of 2B-AQ with Those of a-aaloanthraquinones. Figure 1 shows the transient absorption spectrum (a) and the decay profile of transient absorption (b) obtained by nanosecond laser photolysis of 2B-AQ; the spectrum in a was recorded at the end of pulse excitation (40-ns delay), and the decay of transient absorption in b was monitored at 390 nm. Clearly, the decay curve (solid line) of transient absorption could be reproduced by a single-exponential function (dashed line)
0.1
y
y 400
500 600 700 Wavelength I nm
800
Figure 2. Transient absorption spectra obtained by nanosecond laser photolysis of a-haloanthraquinones at a delay time of 40 ns.
with a lifetime of t~ = 2.6 ms and this lifetime was found to be independent of the monitoring wavelengths. We thus assigned the spectrum shown in Figure l a to the T‘ T1 absorption originating from the lowest excited triplet (TI) state of 2B-AQ, because the spectrum obtained was very similar to the T‘ T1 absorption spectrum of anthraquinone (AQ) in regard to the spectral profile and the lifetime.4 As shown in Figure 2, the transient absorption spectra obtained for a-haloanthraquinones (1B-AQ, lB,SC-AQ, lB,SB-AQ, lB,8C-AQ, and lB$B-AQ) also had absorption bands of A, B, and C; for 1B,8B-AQ, moreover, an additional
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Hamanoue et al.
5804 J. Phys. Chem., Vol. 99, No. 16, 1995 0 - 3 ( ~ ~ - ~ ~
1
2B-AQ 1580 Cm", 1600 cm"
(a)
f
I
II I I
\
.-
)I
I
I
I
U
I
I
v)
I
C
I I
I I
-e!
I
1
E
1B,SC-A0
400 zT=0.98 pS
'T
= 0.64 pS
500 Wavelc 19th / nm
600
a '
0
2
4
6
8
0
Time I p s Figure 3. Decay curves (-) of transient absorptions obtained for a-haloanthraquinones. The monitoring wavelength is 390 nm, and the dashed curves are single-exponential functions with lifetimes (TT)
indicated. absorption band (B') could be seen. However, in comparison with the spectrum shown in Figure la, the spectra shown in Figure 2 revealed that the intensities of bands B and C relative to that of band A were enhanced and the positions of bands B and C were shifted to longer wavelengths; the shift of band B increased in the order of 1B-AQ < lB,SC-AQ < lB,SB-AQ < lB,8C-AQ < lB,SB-AQ. Since the transient absorptions monitored at 390 nm decayed following single-exponential functions (cf. Figure 3) and the lifetimes (ZT) obtained were found to be independent of the monitoring wavelengths, we T1 assigned the spectra shown in Figure 2 to the T' absorptions in accordance with the results obtained for the 1-chloro (1C-AQ), 1,5-dichloro (lC,SC-AQ), and 1,8-dichloro (lC,8C-AQ) compound^.^ Figure 4a shows the emission spectrum obtained by nanosecond laser photolysis of 2B-AQ. One can clearly observe the principal vibrational structure corresponding to the carbonyl stretching frequency of 1580-1600 cm-'. As shown in Figure 4b, moreover, the emission decay curve (solid line, monitored at 490 nm) could be reproduced by a single-exponentialfunction (dashed line) with a lifetime (zp) of 2.7 ms which was almost equal to that ( t=~ 2.6 ms) of the T' TI absorption (cf. Figure lb). By using a commercial (Hitachi MPF-4) phosphorimeter, a similar emission spectrum was also obtained and the corresponding excitation spectrum (solid line) was identical with the ground-state absorption spectrum (dotted line) as shown in Figure 4c. Hence, the spectrum shown in Figure 4a was assigned to the phosphorescence of 2B-AQ originating from its lowest excited triplet state of usual nn* character. In fact,
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5 10 I f Time / ms
Wavelength I nm
Figure 4. (a) Phosphorescence spectrum obtained by nanosecond laser photolysis of 2B-AQ. (b) Phosphorescence decay curve (-) of 2BAQ monitored at 490 nm and a single-exponential function (- - -) with a lifetime (tp) of 2.7 ms. (c) Phosphorescence excitation (-) and ground-state absorption (..-) spectra of 2B-AQ obtained with a Hitachi MPF-4 phosphorimeter (with the sector) and a Hitachi 200-20 spec-
trophotometer, respectively. the emission spectrum obtained was very similar to the usual m*-type phosphorescence spectrum of AQ with a short lifetime of 3.2 m ~ . ~ As shown by solid lines in Figure 5 , the emission spectra obtained by nanosecond laser photolysis of a-haloanthraquinones (1B-AQ, IB,SC-AQ, lB,SB-AQ, lB,8C-AQ, and 1B,8BAQ) were very broad and the general spectral features were greatly different from those of the usual nn*-type phosphorescence spectra obtained for typical aromatic carbonyl compounds such as anthraquinone: benzophenone? and 1-indanone.lo In Figure 5 , the dotted lines are the emission spectra of a-haloanthraquinones recorded by a Hitachi MPF-4 phosphorimeter without the sector: no emission spectra of these compounds could be recorded with the sector, whereas the phosphorescence spectra of AQ and its P-halo derivatives (2B-AQ and 2C-AQ) could be recorded irrespective of the presence or absence of the sector. Owing to the insufficient sensitivity correction of the photomultiplier (Hamamatsu R928) used in the Hitachi MPF-4 phosphorimeter, the spectral profiles of emission spectra (dotted lines) recorded by this phosphorimeter were slightly different from those (solid lines) obtained by nanosecond laser photolysis using a multichannel analyzer. Since the emission excitation spectra obtained by the Hitachi MPF-4 phosphorimeter were confirmed to be identical with the ground-state absorption spectra and since the emission intensities decayed following single-exponential functions with lifetimes (zp) nearly equal to those ( t ~of) the T' T1 absorptions (cf. Figure 6 and Table l), the spectra shown in Figure 5 were ascribed to the genuine phosphorescences originating from the lowest excited triplet states of a-haloanthraquinones (1B-AQ, lB,SC-AQ, lB, 5B-AQ, lB,8C-AQ, and lB,8B-AQ) in accordance with the results obtained for 1C-AQ, lC,SC-AQ, and 1C,8C-AQ.4
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Lowest Excited Triplet States of Haloanthraquinones
J. Phys. Chem., Vol. 99, No. 16, 1995 5805
(a) 1B-AQ
I
I B-AQ
I
1B,8C-AQ tp= 0.98 ps
c E
0
2
4
6
8
1B,5B-AQ tp= 0.32 p
0
Wavelength / nm Figure 5. Phosphorescence spectra of a-haloanthraquinones obtained by nanosecond laser photolysis (-) and using a Hitachi MPF-4 phosphorimeter without the sector The excitation wavelength for the dotted spectra is 350 nm. (.e*).
No Existence of the Phosphorescence Originating from the Upper Excited Triplet State of 1B-AQ. The phosphorescence spectrum of 1B-AQ (shown in Figure 5a) was greatly different from that obtained in methylcyclohexaneby Matsuzaki and Kuboyama.2 They found that the phosphorescence spectrum of 1B-AQ recorded with or without the sector of the phosphorimeter was very similar to that of AQ in regard to the lifetime and the positions of emission peaks, i.e., no broad-band phosphorescence spectrum as observed by us could be observed. Since it was reported that the spectral profiles of phosphorescence spectra obtained for aromatic carbonyl compounds (with very closely lying excited nz* and nz* triplet states) were strikingly dependent on the nature of en~ironments,ll-~~ we also recorded the phosphorescence spectrum of 1B-AQ in methylcyclohexane without the sector of the phosphorimeter and found that the spectrum obtained in methylcyclohexane was identical with that obtained in EPA. Although Matsuzaki and Kuboyama assigned the anthraquinone-like sharp spectrum to the phosphorescence originating from the upper excited triplet state of 1B-AQ, our conclusion is that the emitting states of a- and P-haloanthraquinones are always their lowest excited triplet states and that the anthraquinone-likephosphorescence spectrum obtained for 1B-AQ by Matsuzaki and Kuboyama is due to the phosphorescence from anthraquinone which is contained as an impurity. Since the phosphorescence quantum yield obtained for 1B-AQ is about 1.6 x times smaller than that obtained for anthraquinone as shown later (cf. Table l), the absence of the broad-band emission in the spectrum measured by Matsuzaki and Kuboyama can be explained by the lack of sensitivity of a 1P28 photomultiplier whose spectral response suddenly drops above 650 nm. This conclusion is really supported by the results
0.5
1
1.5
lime Ips Figure 6. Phosphorescence decay curves (-) of a-haloanthraquinones. The monitoring wavelengths are 550 nm (lB,SC-AQ), 560 nm (1BAQ and lB,SB-AQ), 580 nm (lB,K-AQ), and 600 nm (lB,8B-AQ), and the dashed curves are single-exponential functions with lifetimes ( t p ) indicated.
TABLE 1: Lifetimes of Triplet-Triplet Absorptions (ZT) and Phosphorescences (Q, Phosphorescence Quantum Yields (@PI, and Radiative (k,)and Nonradiative (k,) Triplet Decay Rate Constants Obtained for Anthraquinone and Baloanthraquinones in EPA at 77 K' tT/Ps tP/,s @P Us-' k&-' AQ 3400 3200 2C-AQ 3400 3300 2B-AQ 2700 2600 1C-AQ 170 160 1B-AQ 0.68 0.66 lC,SC-AQ 260 250 lB, 5C-AQ 0.98 0.98 lB, 5B-AQ 0.69 0.70 lC,8C-AQ 40 50 lB, 8C-AQ 0.64 0.63 lB, 8B-AQ 0.33 0.32
2.9 x lo-' 3.1 x lo-' 2.8 x lo-' 4.1 x 4.6 x 4.3 x 3.9 x 3.8 x 2.6 x 1.7 x 6.2 x
8.8 x 9.2 x 1.1 x 2.5 x 6.9 x 1.7 x 3.9 x 5.4 x 5.9 2.7 x 1.9 x
2.1 x 2.1 x 2.7 x 5.8 x 1.5 x 3.9 x 1.0 x 1.4 x 2.3 x lo2 1.6 x lo2 3.1 x 10 10 lo2 10 10' 10 lo2 lo2
lo2 10' 102 lo3 lo6 lo3 lo6 lo6 104 lo6 lo6
The values of ZT and t p for AQ, 1C-AQ, 2C-AQ, lC,SC-AQ, and 1C,8C-AQ were taken from ref 4. shown in Figure 7, where 1B-AQ was purified following the method by Matsuzaki and Kuboyama, and the spectral recording was performed without the sector of a Hitachi MPF-4 phosphorimeter equipped with a Hamamatsu R928 photomultiplier whose spectral response extended up to 850 nm: (1) The phosphorescence spectrum (solid line) recorded by 330-nm excitation was somewhat different from that recorded by 350nm excitation (cf. dotted line in Figure 5a), and the positions of emission peaks P1-P4 were identical with those observed for the phosphorescence spectrum of anthraquinone; Matsuzaki and Kuboyama obtained only the sharp spectrum consisting of emission peaks P1-P4. (2) The excitation spectrum (dashed
5806 J. Phys. Chem., Vol. 99, No. 16, 1995
Excitation
Hamanoue et al.
1B-AQ
Emission
Wavelength I nm Figure 7. Phosphorescence (-) and its excitation (- - -, .-*) spectra of 1B-AQ obtained using a Hitachi MPF-4 phosphorimeter (without the sector); 1B-AQ was purified following the method by Matsuzaki and Kuboyama.* The excitation wavelength for the phosphorescence spectrum is 330 nm, and the monitoring wavelengths for the phosphorescence excitation spectra are 600 nm (- - -) and 491 nm (. .).
line) monitored at 600 nm (peak P5) was identical with the ground-state absorption spectrum of 1B-AQ, while the excitation spectrum (dotted line) monitored at 491 nm (peak Ps) was very similar to the excitation and ground-state absorption spectra of anthraquinone. The Origin for the Appearance of Unusually Broad Phosphorescence Spectra of a-Haloanthraquinones. For several aromatic carbonyl compounds, it is well-known that the lowest excited triplet states of usual nn* character give rise to the appearance of broad phosphorescence spectra with relatively long lifetimes compared with the usual nn*-type phosphorescence spectra with lifetimes of a few millise~onds.'~J~ For anthraquinone (AQ) and its /?-halo derivatives (2C-AQ and 2BAQ) in a strong hydrogen-bonding solvent such as 2,2,2trifluoroethanol (TFE) containing water (H20), we have really observed the usual nn*-type broad phosphorescence spectra with relatively long lifetimes, Le., 117.0 ms for AQ in TFE/ H20 (15 vol %), 113.5 ms for 2C-AQ in TFEM20 (1.2 vol %), and 11.1 ms for 2B-AQ in TFEM20 (0.9 vol %). l7 As listed in Table 1, however, the phosphorescence lifetimes of a-haloanthraquinones were very short compared with those of AQ and its /?-halo derivatives (2B-AQ and 2C-AQ). We thus believe that the nature of the lowest excited triplet states of a-haloanthraquinones (giving rise to the appearance of unusually short-lived and broad phosphorescence spectra) is remarkably different from that of the well-known mixed m*-xn*-type or nn*-type excited triplet states, because it has been confirmed that the very short triplet lifetimes and the small phosphorescence quantum yields obtained for a-haloanthraquinones are not due to photochemical reactions.l8 For /?-haloanthraquinones, the steric hindrance between the oxygen and halogen atoms is expected to be very small based on the fact that the photophysical and photochemical behavior of /?-haloanthraquinones is different from that of a-haloanthraquinones but similar to that of anthraquinone which has a lowest excited triplet state of usual nn* character. Hence, it seems appropriate to propose that the distortion of the geometrical molecular structure caused by the steric hindrance between the oxygen and halogen atoms in a-haloanthraquinones is the origin for the appearance of unusually short-lived and broad phosphorescence spectra with spectral profiles similar to those of the usual mixed nn*-nn*type or mc*-type phosphorescence spectra. In order to confirm the structural deformation, we thus calculated the optimized geometries of haloanthraquinones by
a MNDO methodlg using an APMAC program. For /?-haloanthraquinones (2B-AQ and 2C-AQ), the structural deformation was found to be negligibly small; a typical example for the optimized structural model of 2B-AQ is shown in Figure 8a. For a-haloanthraquinones,however, the distortion from planarity in the central quinone structure was observed and the origin for this structural deformation may be the repulsive interaction between the lone-pair electrons on the oxygen and halogen atoms, giving rise to the rearrangement of the three sp2 hybrid a-orbitals and the n-orbital at the meso-position. A typical example for the optimized structural model of 1B,8B-AQ is shown in Figure 8b indicating a bird-like molecular structure, Le., the central quinone structure was nonplanar, losing the aromaticity, and the two oxygen atoms bent upward, while the outer two planar benzene rings bent downward from the original molecular plane; the deviation of the halogen atom from the plane of the corresponding outer benzene ring was very small. Although the deviation of the halogen atom from the plane of the outer benzene ring might minimize the repulsive interaction between the lone-pair electrons on the oxygen and halogen atoms, the n-orbital conjugation in the central quinone structure seems to be much smaller than that between the outer benzene ring and the halogen atom, resulting in a small deviation of the halogen atom from the plane of the outer benzene ring. The bond angles and the bond lengths obtained by the present calculation for a-haloanthraquinones (1B-AQ, 1C-AQ, 1B,5BAQ, lC,SC-AQ, and lC$C-AQ) were found to be consistent with those obtained by X-ray structural analysis for crystalline 1B-AQ,20 lC-AQ,*l 1B,5B-AQ,22 1C,5C-AQ,23 and 1C,8CAQ.24 We thus conclude that the nonplanarity in the central quinone structure of a-haloanthraquinones gives rise to the appearance of unusually short-lived and broad phosphorescence spectra. In connection with our previous results for a-chloroanthraquinones," Yamauchi and HirotaZ5performed a time-resolved EPR study and concluded that (1) the lowest excited triplet (TI) states of 1C-AQ and lC,SC-AQ had intermediate character between those of AQ and 1C,8C-AQ and (2) the TI state of lC,8C-AQ was a modified nx* triplet state, where the n orbital was a linear combination of those on the oxygen and chlorine atoms. If this conclusion is correct, highly strained tertbutylanthraquinones [the 1,2,3-tri-tert-butyland 1,2-di-tert-butyl3-(trimethylsilyl) compounds] should not give rise to the appearance of unusually short-lived and broad phosphorescence
J. Phys. Chem., Vol. 99, No. 16, 1995 5807
Lowest Excited Triplet States of Haloanthraquinones
(a) 2B-AQ
(b) lB,8B=AQ Figure 8. Optimized structural models of 2B-AQ (a) and lB,8B-AQ (b).
spectra similar to those observed for a-haloanthraquinones. In contrast, the phosphorescence spectra of these highly strained tert-butylanthraquinones were really short-lived and broad, while planar tert-butylanthraquinones (the 1-tert-butyl and 1,2-di-tertbutyl compounds) gave rise to the appearance of sharp phosphorescence spectra similar to those of AQ and its P-halo derivatives (2B-AQ and 2C-AQ).26 Since our simple molecular mechanics calculation using a commercial MM2 program revealed that the steric hindrance among three substituents in the 1,2,3-tri-tert-butyl and 1,2-di-tert-buty1-3-(trimethylsilyl) compounds caused the distortion of the geometrical molecular structure,27we refute the conclusion by Yamauchi and Hirota that the existence of modified nn* triplet states described above is the origin for the appearance of unusually short-lived and broad phosphorescence spectra. The Internal Heavy-Atom Ef€ectof the Halogen Atom on the Triplet Lifetimes of a-Haloanthraquinones. It is generally believed that the internal heavy-atom effect on the lifetime of a mr*-type excited triplet state is larger than that on the lifetime of an nn*-type excited triplet state.** In fact, as listed in Table 1, the triplet lifetimes (ZT zp) of AQ and its P-halo derivatives (2B-AQ and 2C-AQ) were nearly on the same order. In contrast, the triplet lifetimes of the a-bromo compounds (1BAQ, lB,SB-AQ, and lB,8B-AQ) were shorter than those of the corresponding a-chloro compounds; also, the triplet lifetime of lB,8B-AQ (or lC,8C-AQ) was shorter than that of 1B-AQ (or 1C-AQ). Hence, there might exist an internal heavy-atom effect of the halogen atom on the triplet lifetimes of a-haloanthraquinones. However, the triplet lifetime of 1C,SC-AQ was longer than those of 1C-AQ and lC,8C-AQ; the triplet lifetime of lB,SB-AQ was also longer than that of 1B78B-AQbut almost
= I
v)
O$)
5 3
' 5
4/
7
I
I
I
I
6
7
log (1c2/ cme2) Figure 9. Log-log plots of k,, (0)and kr (0)against Cc2;(1) AQ, (2) 2C-AQ, (3) 2B-AQ, (4) 1C-AQ, (5) 1B-AQ, (6) IC,SC-AQ, (7) lB,SC-AQ, (8) lB,SB-AQ, (9) lC,8C-AQ, (IO) lB,8C-AQ, and (11) lB,8B-AQ.
equal to that of 1B-AQ. In order to find out the much more quantitative internal heavy-atom effect, the T I ground-state
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Hamanoue et al.
5808 J. Phys. Chem., Vol. 99, No. 16, 1995 radiative (k,) and nonradiative (km) decay rate constants listed in Table 1 were calculated as follows: (1) The phosphorescence spectra (recorded against the wavenumber) were obtained by nanosecond laser photolysis. (2) Integrating the phosphorescence spectra thus obtained over wavenumbers and assuming the fluorescence quantum yield of 9,lO-diphenylanthraceneto be unity,29 the phosphorescence quantum yields (Qp) of AQ and haloanthraquinones were determined. (3) Taking the average of ZT and ZP listed in Table 1, Le., kT-' = ( t-t ~ zp)/2, and assuming the efficiency for the population of the lowest excited triplet state via the lowest excited singlet state to be unity, k, and kN were calculated by
k, = k, - k, Figure 9 shows the log-log plots of knr(filled circles) and k, (open circles) against the sum of the squares of atomic spinorbit coupling constants (
