10322
J. Phys. Chem. 1991, 95, 10322-10325
Novel Rotational Isomerization of Anthracene Nucleus around Its Bond Connecting to C=N Bond in the Excited Singlet State of (E)-N-Methoxy-l-(2-anthryl)ethanimine Hideo Furuuchi, Tatsuo Arai,* Hirochika Sakuragi, Katsumi Tokumaru,* Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Yoshinobu Nishimura, and Iwao Yamazaki Department of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received: March 14, 1991)
The anthracene nucleus undergoes one-way rotational isomerization around its bond connecting to C=N bond in (E)-Nmethoxy-1-(2-anthry1)ethaniminefrom one rotational isomer to the other in the excited singlet state, as revealed by time-resolved fluorescence spectroscopy and decay kinetics in the picosecond to nanosecond time range.
Introduction Aromatic olefins can exist as different conformational isomers around the single bond connecting the aromatic ring and olefinic part as revealed by fluorescence studies.’-I* For example, the fluorescence spectrum of trans-2-styrylnaphthalene depends on excitation wavelength and decays with two components.6 In the ground state the rotational isomerization of the aromatic nucleus around the bond to the olefinic double bond is too fast in solution to be followed by spectroscopic methods, while in the excited state it has long been believed not to be able to take place within the lifetime of the excited state.l” However, BarbaraL3-I5and we16 independently reported an efficient rotational isomerization of anthracene nucleus in 2-vinylanthracene and its derivatives in the singlet excited state. We report that rotational isomerization of anthracene nucleus rakes place around the single bond connecting to a C=N double bond instead of a C=C bond within the lifetime of the excited singlet state of (E)-N-methoxy- 1-(2-anthryl)ethanimine ( ( E ) - I ) ; the lack of geometrical isomerization occurring from @)-I to (Z)-I, in contrast to the efficient isomerization in the opposite
( 1 ) Fisher, E. J . Phofochem. 1981, 17, 331. (2) Haas, E.; Fischer, G.; Fischer, E. J . Phys. Chem. 1978, 82, 1638. (3) Mazzucato, U. Pure Appl. Chem. 1982, 54, 1705. (4) Wismontski-Knittel, T.; Das, P. K.; Fischer, E. J . Phys. Chem. 1984, 88, 1163. (5) Ghiggino, K . P.; Skilton, P. F.; Fischer, E. J . A m . Chem. SOC.1986, 108, 1146. (6) Birks, J . B.; Bartocci, G.; Aloisi, G. G.; Dellonte, S.; Barigalleti, F. Chem. Phys. 1980, 51, 113.
(7) Sun, Y.-P.; Sears, D. F.; Saltiel, J.; Mallory, F. B.; Mallory, W. M.; Buster, C . A. J . Am. Chem. Soc. 1988, 110, 6974. (8) Bartocci, G.: Mazzucato, U.; Masetti, F.; Aloisi, G. G. Chem. Phys. 1986, 101, 461. ( 9 ) Bartocci, G.; Masetti, F.; Mazzucato, U.: Spaletti, A,; Baraldi, 1.; Momicchioli, F . J . Phys. Chem. 1987, 91, 4773. (IO) Lamotte, M.; Morgan, F. J.; Muszkat, K. A,; Wismontski-Knittel, T. J . Phys. Chem. 1990, 94, 1302. ( 1 I ) Cherkasov, A. S. Dokl. Acad. Sei. USSR ( E n g l . Transl.) 1962, 146. 852. (12) Mazurenko, Yu. T.; Udaltsov, V. S.: Cherkasov, A. 5. Opt. Spektrosk. 1979, 46, 389. (13) Brearley, A. M.; Stanjord. A. J. G.; Flom, S. R.; Barbara, P. F. Chem. Phys. Lett. 1985, 113, 43. (14) Flom, S. R.: Nagarajan, V.; Barbara, P. F. J . Phys. Chem. 1986, 90, iCY2. ( 1 5 ) Breariey, A. M.; Flom, S. R.; Nagarajan, V.: Barbara, P . F. J . Phys. Chem. 1986, 90, 2092. (16) Arai, T.; Karatsu, T.; Sakuragi, H.: Tokumaru, K.; Tamai, N.; Yamazaki, 1. Chem. Phys. Left. 1989, 158, 429. (17) Drew. J . ; Zerbetto, F.;Szabo, A. G.: Morand, P. J . Phys. Chem. 1990, 94, 4439. (18) Park, N . S.: Waldeck, D. H. Chem. Phys. Lett. 1990, 168, 379.
0022-3654/91/2095-10322$02.50/0
direction from (Z)-I to (E)-I,19has enabled us to observe clearly the rotational isomerization.
m
C=N
CHI’
(/?).I
‘ hv
-//c -
C=N
\OCH3 (€)-I
(s-trans)
m Ctl,/
C=N CH,’
\OCH, (€)-I
a
(S-CIS)
,OCH,
C=N
CH,/
(Z)-I
Experimental Section 1. Materials. A mixture of ( E ) - and (Z)-I was prepared from 2-acetylanthracene with 0-methylhydroxylamine and sodium acetate trihydrate in ethanol under reflux. Each isomer was separated by column chromatography and purified by recrystallization from a mixture of hexane and benzene (1:l). In the absorption and fluorescence studies toluene (Dotite Spectrosol) was used as a solvent. The sample solution was prepared to have an absorbance of ca. 0.1 at the excitation wavelength (316 nm) to avoid spectral distortion by reabsorption of fluorescence, and deaerated by bubbling with nitrogen or argon for 20 min or degassed by freeze-pump-thaw cycles. 2. Fluorescence Studies. Fluorescence spectra were measured in toluene on a Hitachi F-4000 fluorescence spectrophotometer. Time-resolved fluorescence spectra and fluorescence decay curves were measured in toluene on a picosecond single photon counting apparatus exciting at 316 nm.*’ Scattered laser light was measured with a pulse width of 30 ps (fwhm). Results and Discussion Absorption and Fluorescence Spectra. Figure 1 depicts fluorescence spectra observed on 316- and 400-nm excitation of (E)-I. Excitation at the shorter wavelength (316 nm) exhibits a broad-band fluorescence spectrum with maxima at 394, 413, and 429 nm, while excitation at the longer wavelength (400 nm) gave a fluorescence spectrum at longer wavelength exhibiting a well-defined vibrational structure with peaks at 407,43 1, and 457. The observed effect of excitation wavelength on the fluorescence (19) Furuuchi, H.; Arai, T.; Kuriyama, Y . ;Sakuragi, H.: Tokumaru, K. Chem. Lett. 1990, 847. (20) Yamazaki, 1.; Tamai, N.; Kume, H.; Tsuchiya. H.; Oba, K . Reo. Sci. Instrum. 1985, 56, 1187.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10323
Rotational Isomerization of Anthracene Nucleus
d d
TcmpnturePC
-
E
V
2
2w
\
5
, Jj $
350
1
400
550
500
450
Vnm
Figure 1. Corrected fluorescence spectra of (E)-N-methoxy-l-(2anthry1)ethanimine in deaerated toluene at room temperature. The spectra are normalized at the peak wavelength. Notations S and L are explained in the text. 0
300
320
340
360
380
400
420
Vnm
Figure 3. Temperature dependence of absorption spectrum of (E)+methoxy-l-(2-anthryl)ethanimine in toluene.
!I
280
300
320
340
360
380
400
420
Nnm
Figure 2. Fluorescence excitation spectra of (E)-N-methoxy-l-(2anthry1)ethaniminein deaerated toluene at room temperature.
is attributed to the existence of the two conformationally isomeric forms having different absorption and fluorescence spectra. The observation of a well-resolved vibrational structure in the latter fluorescence spectrum indicates that the spectrum is attributed to only one rotational isomer (denoted as L). Subtraction of the latter spectrum, after multiplication by an appropriate factor, from the former spectrum gives a fluorescence spectrum attributed to the other rotamer (defined as S) which now exhibits a vibrational progression with peaks at 394,415, and 436 nm shifted to shorter wavelength than L. The excitation spectrum, particularly around 400 nm, is also dependent on the monitoring wavelength, indicating existence of different ground-state rotational isomers (Figure 2). The excitation spectrum measured at 430 nm gave essentially the same spectral pattern as the absorption spectrum (Figure 3), while that measured at 390 nm exhibits a lower intensity at 390-410 nm. Figure 3 also shows the temperature effect on the absorption spectrum of (/?)-I in toluene, which indicates that two forms S and L must exist in the ground state. The absorbance at 400 nm, which is mostly due to the absorption by L, increased with increasing temperature, suggesting that L is less stable than S in energy. The isosbestic point appears only at 395 nm and the absorbance at the wavelength shorter than 395 nm decreased monotonically with increasing temperature. The integral of the absorption spectrum of @)-I in the region of 310-420 nm decreases with increasing temperature, even after the temperature effect of the density of toluene is corrected. These results suggest that L is less stable and has a smaller integrated molar extinction coefficient in the region of 310-420 nm than S. Although the quilibrium constant between L and S could not be estimated because of the difference in the integrated molar extinction coefficients between S and L, the temperature effect indicates at least that S is more stable in energy than L in the ground state. Picosecond Measurements. Figure 4 shows time-resolved fluorescence spectra of (@-I at varying temperatures. At 58 O C ,
1.
375
,
425
400
.
450
475
i
l
0-0.25 n i
500
525
Nnm
I
P
I
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A
1
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375
400
425
450
475
Nnm
500
525
375
400
425
450
475
500
525
Nnm
Figure 4. Time-resolved fluorescence spectra of (E)-N-methoxy-142anthry1)ethaniminein deaerated toluene at -90, 7.8. 23.8, and 58 OC.
the spectrum observed at 0-250 ps is composed mostly of S and converted to L at the nanosecond time scale. The time-resolved fluorescence spectra show that the conversion of the rotational isomer of (E)-I in the excited singlet state takes place in a one-way manner from IS* to IL*. The conversion rate of the spectrum decreased with decreasing temperature, and finally no spectral change was observed at -90 'C in toluene. The observed temperature effect indicates involvement of an activation process in the decay of singlet excited state of S. Typical decay curves observed at 390 and 460 nm at 58 and -90 O C are shown in Figure 5. At 58 O C the time profile at 460 nm fits two components: a rise (3.6 ns) and a decay component
10324 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
Furuuchi et al.
112
- -
+
102
Sor3S'
ki
k4
1L*
ks
L+/IVL
LorlL*
Time/ns
TABLE I: Fluorescence Decay Parameters of (E)-N-Methoxv-l-(2-anthrvl)etba~mine 10
0
20
40
30
Time/ns
58 44.5 36.5 29.8 23.8
15 30 40 Time/ns Figure 5. Fluorescence decay and rise curves of (E)-N-methoxy-l-(2anthry1)ethanimineat 390 and 460 nm in deaerated toluene at 58 and 10
0
20
-90 'C.
-30 -60 -90
19.2
18.8
3.56 3.63 (-) 4.52 4.33 (-) 5.07 5.00 (-) 5.25 5.65 (-) 5.6 5.9 (-) 5.88 6.41 (-) 6.46 6.45 (-)
7.12 7.34 8.41 7.61 8.63 7.82 8.15 8.0 8.3 8.3 8.34 8.79 8.64 8.97
10.0 10.6 10.4 10.4 10.2 10.4
OThe sign (-) means the rise component.
2 18.6 9
7.8
390 460 390 460 3 90 460 390 460 390 460 390 460 390 460 390 460 390 460 390 460
18.4
radiative and nonradiative decay rate constants, and the rate constant for rotational isomerization from IS*to I L * , respectively. If we assume that temperature affects only the conversion rate from IS* to IL* (k,) in eq 1 and that the time constant at -90
18 2 18 17.8
I? 6
3
3.1
3.2
3.3
3.4
3.5
3.6
1rri1O3K
Figure 6. Plot of In k3 vs 1/T.
(7.3 ns); however, that observed at 390 nm fits two decay components of 3.6 and 7.1 ns. Accordingly, the observed shorter and longer time constants are reasonably assigned to the lifetimes of IS* and 'L*, respectively. The rise time constant observed at 460 nm and the shorter decay constant observed at 390 nm agree well with each other and increase with decreasing temperature: 3.6 ns at 58 O C to 10.4 ns at -90 O C . On the other hand, the longer time constant changes only slightly between 7.5 (58 "C) and 8.8 ns (7.8 O C ) . At -30 to -90 O C the decay curves observed at both 390 and 460 nm almost fit one component with a decay time constant of 10.4 ns. The observed results are summarized in Table I. The observation of the rise component at 460 nm and the temperature effect on this time constant in addition to the effect of excitation wavelength on the fluorescence spectrum strongly indicate that rotational isomerization from one isomer ( S ) to the other (L) takes place as an activation process at the excited singlet state within its lifetime. The rotational isomerization occurs at higher temperature; however, it does not practically occur below -30 "C, since the decay curves are composed of only a single component at any wavelength. These observations are explained by the processes shown in Scheme 1, where k l , k2, and k3 are
1 / r S = kl
+ k2 + k3 = kl + k2 + A exp(-E,/RT)
(1)
OC is kl + k2 (=9.6 X lo7s-I; k3 = 0), we can estimate k3 changing from 0.52 X lo8 s-l at 7.8 O C to 1.82 X lo8 s-I a t 58 OC. The Arrhenius plot of the k3 values (Figure 6) gave an activation barrier E, for the conversion from IS* to IL* as 4.6 kcal mol-l and a preexponential factor ( A ) as 1.8 X lo1' s-l. Noticeably, these activation parameters are essentially the same magnitude as observed in 2-vinylanthracene, E, = 3.9 kcal mol-l and A = 1.7 X 10" Potential Energy Surfaces of Rotational Isomerization in the Ground and Excited Singlet State. The singlet excitation energies of S and L forms were estimated from the 0-0 band of the fluorescence spectra to be 72.6 (394 nm) and 70.3 kcal mol-l (407 nm), respectively. The timeresolved fluorescence spectra observed on 3 16-nm laser excitation and the steady-state fluorescence spectrum observed on 400-nm excitation indicate that the conversion from L to S in the excited singlet state (k-3) must be much slower than the S to L process, probably k-,
