1638
E. Haas, G. Fischer, and ,E. Fischer
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
Conformational Equilibria in 1,P-Diarylethylenes Manifested in Their Emission Spectra and Lifetimes Ellsha Haas, Gabrlela Fischer, and Ernst Fischer" Departments of Chemical Physics and Structural Chemistry, The Weizrnann Institute of Science, Rehovof, Israel (Received February 1, 1978)
Downloaded by NEW YORK UNIV on September 6, 2015 | http://pubs.acs.org Publication Date: July 1, 1978 | doi: 10.1021/j100503a014
The emission spectra of trans-l-(2-naphthyl)-2-arylethylenes (where aryl = phenyl, 1-naphthyl, or 2-naphthyl) vary with the wavelength of excitation, indicating the presence of two species of each compound, with slightly shifted absorption and emission peaks. The emission decay curves of two of these compounds can be expressed only as the sum of at least two exponentials,the contribution of each varying with the wavelength of excitation. Both phenomena are restricted to those trans diarylethylenes capable of existing as a mixture of two or three almost isoenergetic conformers, and it is therefore suggested that such conformers should be assigned to the two species postulated above. These differ in their emission spectra, decay rates, and quantum yields. In stilbene and in l-phenyl-2-(l-naphthyl)and 1,2-di-(l-naphthyl)ethylene the same emission spectra were observed even when exciting all through the range of the first emission band.
Scheme I
In two earlier publications1"l2 dealing with diarylethylenes we mentioned briefly evidence from solution emission experiments for the existence of at least two conformers of the trans isomers of l-phenyl-2-(2naphthy1)ethylene (Ph-2N), of 1,2-di-(2-naphthyl)ethylene
&@ (Ph-Phl
(0)
I
Ph-Ph
1N-1N
2N-2N
Ph-1N
if (IN-IN)
(2N-2N)
1N-2N
Ph-2N
(2N-2N), and of 1N-2N, but not of those of Ph-lN, lN-lN, and Ph-Ph, i.e., stilbene. This parallels our observations with the corresponding cis isomers, where the evidence was mainly by way of the photocyclization products.lb We assumed this to be due to the fact that only in the first three compounds the trans isomers may in principle exist as a mixture of two or three coplanar and almost strainless and nearly isoenergetic conformer^.^^ In Ph-Ph only one coplanar form is possible,& while in Ph-1N and 1N-lN one conformer is strained in the coplanar form much less than the other(s), and is therefore expected to predominate in solution (cf. Scheme I). In a recent paper4 emission spectroscopic evidence was provided for the existence of several conformers not only in Ph-2N and in 2N-2N as expected, but also in the last three compounds. In the present paper we shall provide detailed experimental evidence for our earlier conclusions, which concur with those of the Russian authors4only in regard to Ph-2N and 2N-2N.
I. Results (a) Emission Spectra and Quantum Yields. The absorption and emission spectra of Ph-2N and 2N-2N in solution possess pronounced vibrational structure, in 0022-365417812082- 1638$01.0010
A
&&p B
B
B (Ph-IN1
(IN-2N)
( Ph-PNI
particular at low temperatures2 (Figures 1-4). A variation of the shape of the fluorescence spectra with the wavelength of the exciting light was observed even at room temperature (Figures 2-4). The spectra obviously result from the superposition of two sets of emission peaks, denoted in the figures by set 1 and set t, with the relative contribution of each set varying with the wavelength of the exciting light. The results are most striking in 2N-2N, where the shift of one set toward the other is larger (about 11 nm, vs. 8 in Ph-2N). Only the emission of 2N-2N excited at 366 nm and above seems to be almost pure f, as indicated by the absence of peaks at 360 and 380 nm. The emission spectra described in Figures 2a, 3a, 3b, and 4 were measured with argon-flushed solutions. In order to check for possible preferential oxygen quenching of one 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
Conformational Equilibria in 1,a-Diaryiethylenes
1639
1.0 0.8 0)
0.60
; ln
2
0.40.2 -
0
I
I '
Downloaded by NEW YORK UNIV on September 6, 2015 | http://pubs.acs.org Publication Date: July 1, 1978 | doi: 10.1021/j100503a014
250
I
I
,
350
350 250 nm
Figure 1. Absorption spectra at -180 OC. Solutions in 2:l mixtures of MCH and 2- or 3-methylpentane. Approximate concentrations were M; 2N-2N, 1.1 X M. Ph-PN, 1.6 X
5
z W
k-
f W
>
t v) z W I-
L z
5
0 v) v,
z
W
i
i
i
i
9
I
5a W
c
t
0 360 380 400 420 440 n m
W
Flgure 3. (a) Emission spectra of 2N-2N in MCHIMP, 4-9 X lo-' M, at room temperature with excitation as follows: (1) 294 and 355 nm; (2) 313 and 329 nm. (b) Emission spectra at room temperature with excitation as follows: (1) 366 and 368 nm; (2) 364; (3) 362; (4) 360 nm. (c) Emission spectra at -185 O C with excitation as follows: (1) 337; (2) 368 and 370 nm.
> l-
a
0
360
I I I I 380 400 420 440 nm
Figure 2. (a) Emission spectra of Ph-2N in MCHIMP, 5-9 X M, 290, 306, and 316 nm; (2) 260, 270, at room temperature: (1),,A, 280, 330 nm; (3) 350, 355,360 nm. (b) Emission spectra at -185 'C: (1) excited at 360 nm; (2) 309,320, 336, 340 nm. At other ,A, the spectra are somewhere between the two curves.
of the two species assumed to be responsible for the two sets of emission peaks, measurements were also performed in oxygen-flushed solutions. In both compounds oxygen caused a preferential quenching of 1, indicated by changes in the shape of the emission spectra similar to curve 2 curve 3 in Figure 2a and curve 2 curve 1 in Figure 3a. Again the effect is more pronounced in 2N-2N. Table I summarizes the results obtained at room temperature with
-
-
2N-2N dissolved in methylcyclohexane, MCH. The relative ratio A between the two species 1 and t is estimated from the ratio between the emission peaks at 360 (5. form) and 370 nm (t form). We conclude that species 1 is longer lived than t. The last column in Table I shows that the effect of oxygen is roughly constant over a wide range of A values. The separation between the two species is much more pronounced a t low temperatures. trans-2N-2N tends to precipitate on cooling because of its very low solubility. We therefore applied a technique described earlier:2s5 a solution of cis-2N-2N was cooled to -185 "C and converted into trans by 366-nm irradiation. Under these conditions no aggregation or crystallization takes place because of the combined effect of high viscosity and low t e m p e r a t ~ r e . ~
1640
E. Haas, G. Fischer, and E. Fischer
The Journal of Physical Chemistry, Vo/. 82, No. 14, 1978 I
r r !
I
c
I
I
1.0
a
.-
I
I
I
I
I
I
I
I
I
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4-
E -
+ C
Y
-
u c 0 .n
.-E 5 VI .-
E
W
-
L
n
0) >
.c
-0
0.6
0.4
a
-
0.2
cr"I
I
I
I
I
I
i
440 480 nm Flgure 4. Emission spectra of 1N-2N in MCH at room temperature, 5 X lo-' M. Excitation was at the. wavelengths indicated.
360
400
0.0
I
300
4 00
I
5 O(
nm Absorption and emission spectra of IN-1N in MCH/MP at -125 "C with a concentration (absorption spectra) of 4 X lo-' M.
Downloaded by NEW YORK UNIV on September 6, 2015 | http://pubs.acs.org Publication Date: July 1, 1978 | doi: 10.1021/j100503a014
Figure 5.
TABLE I: Relative Ratios between the i and t Species of 2N-2N in MCH at Room Temperature Estimated from the Fluorescence Spectra in Argon- or Oxygen-Flushed Solutions Excited at Various Wavelengths
13. I/[ f 1 = A
hc, nm
Gas
280 294
Ar Ar
0.71
0, Ar
0.96
300 313
Ar 0 2
320 329
Ar Ar 0 2
340 355
Ar Ar
350a
0.64
t
0.72
I
0.73
0.75 1.00
1*01 0.73 0.84
0*57t
0.62
0 2
O.l6 0.10
0.64
Ar
0.92
0 2
366
::;:t
Ao,/AA,
Ar
0.3 5
I
a Excited by light from a 150-W xenon arc passed through a cobaltlnickel filter transmitting below 350 nm.
The same technique was also applied to Ph-2N although in this compound no precipitation takes place, and measurements of a trans solution cooled directly to -140 or -185 "C gave rather similar results. Figures 2b and 3c indicate that excitation at the long wavelength edge of the absorption spectrum results in almost pure f emission, while in the emission spectra excited at shorter wavelengths the 4 form predominates, with relatively small variation with the exciting wavelength. Incidentally, the technique involving the cis isomers also serves to check for possible impurities. These should show up at room temperature, where cis-Ph-2N and cis-2N-2N are practically nonfluorescent.lbv6 As expected, the excitation spectra of 2N-2N monitored at the emission peaks of 360 and 383 nm are rather similar, but differ greatly from that monitored at 390 nm. The relative fluorescence quantum yields of 2N-2N were determined at several excitation wavelengths, by comparing the total areas under emission curves roughly similar to curve 2 in Figure 3a and curve 1in Figure 3b: he, 340 nm, 1.00,349nm, 0.99; 368 nm, 0.50. Since the emission spectra excited at the two shorter wavelengths are actually superpositions of two or three emissions, the observed ratio of two between the quantum yields constitutes a lower limit. Solutions of 2N-2N in toluene and in acetonitrile exhibit a variation of the emission spectra with the excitation
wavelength qualitatively similar to that observed in MCH solutions. Solutions of 1N-2N in MCH behave in a very similar way, as shown in Figure 4. Again, two sets of peaks were observed, with the relative contribution of each set varying with A,,. Here too the spectral definition of the two sets was much better at low temperatures. In view of the results reported4 for Ph-lN, lN-lN, and Ph-Ph, we repeated our relevant emission measurements with these compounds under strict precautions to avoid artifacts due to scattered exciting light, built-in filters, reabsorption effects, etc. No variation of the emission spectra with exciting wavelength could be observed, though e.g., with 1N-1N excitation was at up to 404 nm, Le., right through the range of the shortest emission peak at 401 nm (Figure 5). Specifically we could not confirm the disappearance of this emission peak, as well as of the emission peak of Ph-1N at 371 nm, and of the emission shoulder of Ph-Ph at about 330 nm, when exciting in the long wavelength edge of the absorption spectra of these three compounds. As a further check we measured the emission spectrum of 1N-1N at -120 "C, where it has maximal vibrational structure,2 as a function of the exciting wavelength up to 380 nm. No variation was found. We conclude that there is no spectral evidence for the existence of conformers in these three compounds. We did not investigate solutions in plastic media: Some variation of the relative fluorescence quantum yields with exciting wavelength was observed. For trans-1N-1N QFrel values were as follows: 300 nm, 0.78; 310 nm, 0.81; 320 nm, 0.90; 330 nm, 1.00; 340 nm, 1.02; 350 nm, 0.97; 360 nm, 0.97; 370 nm, 1.13; 380 nm, 1.18. In trans-Ph-Ph the following were observed 300 nm, 0.85; 315 nm, 1.01; 320 nm, 1.00; 325 nm, 1.06; 330 nm, 1.10, These results are in qualitative agreement with the relevant ones reported: but a quantitative comparison is not feasible (cf. Experimental Section). (b) Emission Decay Curves, These were measured for all six compounds, at room temperature in argon-flushed solutions, in cells subsequently fused-off from a high vacuum line. Monoexponential decay curves were observed for four compounds,with the following decay times: Ph-lN, 1.7 ns; 1N-lN, 2 ns; 1N-2N, 0.95 ns; Ph-Ph, 0.12 n ~ With . ~ Ph-2N and 2N-2N the decay could be expressed only by a sum of at least two exponentials I ( t ) = Io[alexp(-t/TJ + a2 exp(-t/~Jl (1)
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1641
Conformational Equilibria in 1,2-Diarylethylenes
TABLE 11: Fluorescence Decay Results for Solutions of trans-2N-2N in MCH at Room Temperaturea 3.62-
hex,,
nm
a,/a,A-I
Remarks
a1
aa
280 300 300
0.46 0.41 0.235
0.09 0.115 0.17
5.1 3.6 1.5
320 320
0.385 0.35
0.125 0.125
3.1 3.5 2.8
320
0.44
0.07
6.3
320
0.40
0.09
4.4
0.46 0.485 0.44 0.57 0.32
0.10 0.07 0.05 0.04 0.12
340 354 358 C and in (1N-2N), B > A. Yu. B. Scheck, N. P.Kovalenko, and M. V. Alfimov, J. Lumin., 15, 157 (1977). E. Fischer, J. Phys. Chem., 77, 859 (1973). J. Klueger, G. Fischer, E. Fischer, Ch. Goedicke, and H. Stegemeyer, Chem. Phys. Lett., 8, 279 (1971). (a) This result was considered to be only a very rough estimate, but recent results obtained with picosecond pulse are in surprisingly good agreement with our value obtained” with pulses of 5-7 ns (I). (b) M. Sumitani, N. Nakashima, K. Yoshihara, and S. Nagakura, Chem. Phys. Lett., 51, 183 (1977). In 1,2di(2-naphthyl)cyclopentene(a frozen-in analogue of cis-2N-2N), which is already fluorescent at room temperature, no emissionspectroscopic evidence was found for the exlstence of conformers, despite clear-cut photochemical evidence.’ However, a distinct variation of OF with the excltation wavelength was o b s e r ~ e d . ~ T. Wismonski-Knittel, G. Seger, M. Kaganowitch, and E. Fischer, forthcoming paper. M. Kaganowitch, G. Fischer, E. Fischer, Ch. Goedicke, and H. Stegemeyer, Z. Phys. Chem. (Frankfurtam Main), 78, 79 (1971). N. P. Kovalenko, Yu. B. Sheck, L. Ya. Malkes, and M. V. Alfimov, Bull. Acad. Sci. USSR. Ser. Chem., 298 (1975). D. J. S. Birch and J. B. Birks, Chem. Phys. Lett., 38, 432 (1976). Unpublished results by E. Lippert, Berlin, and U. Wild, Zurich. G. Hazan, E. Haas, and I. 2. Steinberg, Biochim. Biophys. Acta, 434, 144 (1976). L. Hundley, T. Coburn, E. Garwin, and L. Stryer, Rev. Sci. Instrum., 38, 488 (1967). G. Hazan, A. Grinwald, M. Maytal, and I. 2 . Steinberg, Rev. Sci. Instrum., 45, 1602 (1974). A. Grinwald and I. 2. Steinberg, Anal. Biochem., 59, 583 (1974). D. W. Marquardt, KIN (1966), submitted by Share Programme Libraty, SDA 3094.01.
