J. Phys. Chem. 1981, 85, 2611-2613
rent (6d) basis yields the best total energy and hence is the basis of choice. A fundamental problem with double zeta contractions of the augmented basis sets is that they fix the ratio of coefficients between all original primitives and allow only the relative coefficients of the (slightly used) diffuse augmenting primitives to vary. While the contraction of this augmented basis by means of orbitals from the 4s13d"-l state leads to perhaps acceptable atomic splittings [errors of 0.49 eV (5d) and 0.65 eV (Sd)], it is not flexible enough to handle the general contraction or expansion of the 3d orbitals that result from a charge transfer to and from the ligands in molecules. For this reason we find the reoptimized (5d) basis contracted (4,l) to be the most suitable for routine molecular calculations. Further, we suggest that the (4d) basis for calculations on large transition metal complexes is a useful basis (errors
2611
in excitation energies of less than 1.5 eV and no difficulty contracting to double zeta). Acknowledgment. This research was supported in part by the U.S.Department of Energy (Contract No. EX-76G-03-1305). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE. Partial support was also provided by the National Science Foundation (Grant No. CH&80-17774). The research reported in this paper made use of the Dreyfus-NSF Theoretical Chemistry Computer which was funded through grants from the Camille and Henry Dreyfus Foundation, the National Science Foundation (Grant No. CHE78-20235), and the Sloan Fund of the California Institute of Technology.
Conformational Equilibria in trans-I ,2-Diarylethylenes Manifested in Their Emission Spectra. 4.' 3-Anthryl and 3-Pyrenyl Derivatives Gabrlella Flscher and Ernst Flscher" Depadment of Structural Chemistry, Weizmann Institute of Science, Rehovot, Israel (Received: December 18, 1980; In Final Form: May 12, 1981)
The emission spectra of the title compounds vary with the wavelength of excitation. They can be described as superpositions of two spectra shifted 8-15 nm with respect to each other and contributing to the overall emission to an extent varying with the excitation wavelength. The phenomenon is ascribed to the existence, in solution, of an equilibrium mixture of two almost isoenergeticrotational conformers with slightly different absorption and emission spectra.
In earlier papers by us1 and by Sheck et aL2emissionspectroscopic evidence was described for the existence of conformational equilibria in solutions of 2-naphthyl and 3-phenanthryl analogues of stilbene. In order to check
T A B L E I: Emission Peaks (in n m ) of the Two Modifications J. and t of Various trans-(3-Anthry1)ethylenes under t h e Conditions Described in Figure la
compd
la
lb
the generality of the underlying concepts, we have now investigated a number of 3-anthryl derivatives. As expected by analogy, we observed in all of them a variation of the emission spectra with the wavelength of the exciting light, which was most pronounced at long excitation wavelengths, in the tail of the absorption bands. In all (1) (a) Fischer, E. J. Phys. Chem. 1980, 84, 403-410. (b) Bull. SOC. Chirn. Belg. 1979, 88, 889-895. (c) Haas, E.; Fischer, G; Fischer, E. J. Phys. Chem. 1978,82,1638-43. (d) Fischer, G Fischer, E. Unpublished observations at 25 O C , not at reduced temperatures. (Improved experimental conditions have enabled ua to make these observations recently.) (2) Scheck, Yu. B.; Kovalenko, N. P.; Alfimov, M. V . J. Lumin. 1977, 15. 157. 0022-3654/81/2085-26 11$0 1.2510
1 t 1 t
IC
.1
Id
t J. t
2a
4
2b
t J. t
2c
1
t 2d
&
t
413 426 407 416 414 4 24 423 430 422 431 4 15 428 426 433 423 433
436 452 (432) 444 438 451 448 458 450 460 442 457 450 459 450 462
462 482 (461) 414 (468) 482 (479) (492) 480 490 470 488 (483) (493) 419 497
Values in parentheses are n o t well defined.
cases the emission spectra can be described by a superposition of two sets of peaks, shifted by 8-15 nm with respect to each other, and with the relative contributions of each set to the overall spectrum varying with the wavelength of excitation. Figure 1describes the results obtained at reduced temperatures, where the spectra are sharper, but qualitatively similar results were obtained also at room temperature. 0 1981 American Chemical Society
2612
The Journal of Physical Chemistry, Vol. 85,No. 18, 1981
Fischer and Fischer
W
>
5 i
J W
rr
,
I . ,
,
Figure 1. Uncorrected emission spectra and partial absorption spectra of trans-3anthrylethylene derivatives in the solvents and at the temperatures indicated. Excitation wavelengths as given in the description of the curves. Dotted lines denote interpolated parts of the emission curves, in those wavelength regions Inaccessible because of scattered exciting light. At still shorter excitation wavelengths the emission spectra resemble those = 380-390 nm. Observations of the short wavelength part of emission spectra exclted at long wavelengths, such as the f spectra shown for A, in Figures 1 and 2 and t t in Figure 2, are difficult because of scattered exciting light. However, it is of particular Importance nevertheless that the observed short wavelength edges of the 7 and t t spectra clearly show that the short wavelength peak of 1 is either absent or contrlbutes little to the t and t t spectra.
The two sets of peaks are denoted by 1 and t, respectively. In Figure 1the emission spectra are recorded as obtained in the solvents, and by excitation at the wavelengths, indicated. The excitation wavelengths given in Figure 1were chosen to represent mixtures consisting of (a) mainly 4, -, (b) mainly t, - - -, and (c) a mixture of both 1 and f in about equal proportions, - - - Most compounds have kindly been supplied by Professor Siegrist, who has described them in his paper.3 Concentrations were l X 10-6-2 x 10-5 M. In view of the similarity with our previous results with related compounds, we restrict ourselves to the factual description provided in Figure 1 and in Table I, in which we summarize the wavelengths of the first three emission peaks in each of the two sets. The compounds are defined
-.
(3) Siegrist, A. E.; Liechti, P.; Meyer, H. R.; Weber, K. Helu. Chirn. Acta 1969,52, 2524. (4)Fischer, G.; Seger, G.; Muszkat, K. A,; Fischer, E. J. Chern. Soc., Perkin Trans 2 1975, 1569.
30
400
nm
500
540
Flgure 2. Uncorrected emission spectra and partial absorption spectrum (heavy line) of frans-l-(2-naphthyl)2-(3-pyrenyl)ethylene. Excitatlon wavelengths as indicated.
in Figure 1. In Table I they are assigned the numbers used in Figure 1.
Conformational Equilibria in frans-l,2-Diarylethylenes
The fluorescence decay curves were not determined for these compounds, with the exception of IC. However, a comparison of the emission spectra in argon-flushed vs. oxygen-flushed solutions showed preferential quenching of the J. modification, Le., when argon is replaced by oxygen, the emission spectrum changes in favor of the ? spectrum. This indicates that the t modification has a shorter fluorescence decay time, as already observed with some naphthyl derivatives.'* In compound IC the emission decay was measuredlCat 20 "C and found to be biexponential, with ( ~ 3=) 17 ~ and (T& = 5.6 ns. The contribution of the two modifications at 20 OC was estimated as 80% for 1 and 20% for t. The 3-pyrenyl derivative (3) described in Figure 2
U
3c
presents some special features. According to the working hypothesis presented earlier' we would expect in this case, just as in the 1-naphthyl analogue l-(l-naphthyl)-2-(2naphthyl)ethylene,lConly two more or less isoenergetic conformers 3A and 3B, since 3C may be expected to deviate more strongly from coplanarity and should therefore be less favored energetically. Actually we observed, in addition to the two spectra 1 and t described in Figure 2, a third spectrum at much longer wavelengths. The three sets of peaks were as follows:
The Journal of Physical Chemistty, Vol. 85,
No. 18, 1981 2613
419,447, 475 nm t 428,458, 490 nm t t 465,498,533 nm The third set of peaks is thus shifted by more than 45 nm relatively to the J set. In the cases described hitherto we observed evidence for the existence of three rotamers, i.e., three sets of emission peaks, only with 1,2-di-(2naphthy1)ethylene,ld (4), and its heterocyclic analogue, 1,2-di-(7-q~inolyl)ethylene,'~ where three almost isoenergetic rotamers can be postulated.lC In these compounds the three sets of peaks were shifted relative to each other to about the same extent. The values for 41d at room temperature were as follows: set 1 360, 380, 404 nm set 2 370, 392, 416 nm set 3 (380))403, 425 nm We propose that in the pyrenyl derivative the Tt set be assigned to the less favored rotamer 3C,since it may be expected to differ considerably from 3A and 3B, and also to exhibit a larger Stokes shift, in analogy to sterically hindered trans-stilbene derivative^.^ Indeed, the assumption of the existence of conformers in all these compounds is still only a reasonable but unproven explanation of the mounting volume of similar observations. It is to be hoped that 13C NMR measurements, or possibly other physical methods, will eventually provide more direct evidence. The same holds for eventual calculations of the expected differences among the absorption and emission spectra of the postulated conformers. Regarding other aspects of these phenomena see the Discussion in ref l a and IC. Acknowledgment. The authors gratefully acknowledge the gift of most compounds by Professor A. E. Siegriet, Basel, the gift of compound l b by Professor W. H. Laarhoven, Nijmegen, the synthesis of 2b by Mr. M. Kaganowitch, the measurement of the decay curves by Dr. E. Haas, and the technical assistance of Mrs. Nelly Castel.
