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Fluorimetric and theoretical study of the rotamerism of trans-styrylanthracenes
Journal of Molecular Structure, 193 (1989) 173-183 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FLUORIMETRIC AND THEORETICAL STUDY ROTAMERISM OF trans-STYRYLANTHRACENES
173
OF THE
G. BARTOCCI, F. MASETTI and U. MAZZUCATO Dipartimento di Chimica, Universitti di Perugia, Z-06100 Perugia (Italy) I. BARALDI Dipartimento di Chimica, Universitti di Modena, Z-41100 Modena (Italy) E. FISCHER Department of Structural Chemistry, Weizmann Institute of Science, IL-76100 Rehovot (Israel) (Received 15 March 1988)
ABSTRACT The ground state conformational equilibrium of trans.n-styrylanthracenes (n= 1, 2, 9) has been studied by stationary and pulsed fluorimetric techniques in non-polar solvents. Parallel theoretical calculations of the potential energy curves for the internal rotation of the anthryl group have been carried out with a modified CS-INDO method. The behaviour of the mixture of almost isoenergetic rotamers of 2styrylanthracene has been particularly investigated to add some complementary and revised information to that recently reported in the literature.
INTRODUCTION
Ground state conformational equilibria in solutions of several stilbene-like molecules have been extensively investigated in the last decade in different laboratories by fluorimetric techniques [ 11.It has been shown that both photochemical and photophysical behaviour depend on the excitation energy and differ for the two (or more) rotamers derived by rotation of an aryl group around the single bond with the ethylenic bridge. A detailed photophysical study, including theoretical aspects, has been carried out in our laboratories for the overall series of trans-n-styrylnaphthalenes (n-StN, n=l, 2) [l-3] and n-styrylphenanthrenes (n-StPh, ~1, 2, 3, 4, 9) [ 41. More recently, the study has been extended to the trans-n-styrylanthracenes [ 5-71, to heteroaromatic compounds [ 81 and to the triplet behaviour of rotamers as investigated by flash photolysis [9,10]. In this paper, we compare the conformational equilibria of the three isomeric trans-n-styrylanthracenes (n-StA, n= 1, 2, 9) and in particular discuss the behaviour of 2-StA in the light of the new information obtained. This study,
QQ22-286Q/89/$03.5Q
0 1989 Elsevier Science Publishers B.V.
174
which includes photophysical measurements and theoretical calculations of the potential energy surfaces as a function of the torsional angle around the single bond with the styrene moiety, leads to a complete description of the conformational equilibrium of trans-StAs. EXPERIMENTAL
The tram-n-StAs were synthesized for previous work by methods described elsewhere [ 111. The experimental procedures (fluorescence spectra, quantum yields and decay profiles) have been reported in previous papers [2,12]. 2- (lNaphthyl)&phenyl-1,3,4-oxadiazole (cu-NPD) was used as standard for the QF measurements (@ r = 0.70 in cyclohexane) [ 41. For measurements of (IQ as a function of temperature, the (DF value at room temperature was used as reference, taking into account the changes in absorbance and refractive index with temperature. The uncertainty in the reported parameters is 4% for (DF and 7% for rr. RESULTS AND DISCUSSION
In the series of trans-n-StAs, the 1 and 9 isomers were found to display a normal fluorescence behaviour. In fact, their emission and excitation spectra do not depend on the excitation (A,,,) and emission (A,,) wavelength, respectively, and the emission decay profile is mono-exponential. In the case of lStA, the analogue of l-StN and 1-StPh, the conformational equilibrium is practically completely shifted towards the more stable rotamer, characterized by less severe steric interference between the aromatic and ethylenic hydrogens. In the case of the 9 isomer, two (nearly) equivalent rotamers are expected by rotation of the polycyclic group [ 71. Therefore, the 1 and 9 isomers behave as one-component systems. On the other hand, for 2-StA, the analogue of 2StN and 2- and 3-StPh, the fluorescence spectrum displays a dependence on A,,, and the emission decay profile is bi-exponential [ 5-71. Theoretical
analysis
The behaviour of the three isomeric trans-n-StAs is well described by the theoretical conformational analysis carried out using the CS-INDO method [ 131. For all molecules a rigid rotation around the A-St bond has been analyzed using the experimental geometry for the phenanthrene moiety [ 141 and assumed geometries for the styrene moiety. More precisely, the geometry of the styrene moiety in l- and 9-StA and in 2-StA is the same as that chosen in a previous study on the conformational properties of l-StN and 2-StN, respectively [ 31. The calculated points at different values of the torsional angle (@) are marked on the potential energy curves of Fig. 1.
175
(Cl
9-StA
18(
0
90
180
0
90
L
180
000 \
T 0
Fig. 1. Ground-state potential energy curves for internal rotation of trans-n-StAs “single” bond connecting the anthracenic group to the ethylenic bridge.
around the
Figure 1 (a and c) shows the ground state CS-INDO potential energy curves describing the internal rotation of l- and 9-StA around the essential single bond connecting the anthryl and styryl groups. The results of the calculations confirm the expected existence of two equivalent minima at @= 52 ’ and @= 128” (noticeably shifted with respect to the planar configurations because of the steric interactions) in the case of 9-StA, while for l-StA two minima with an energy difference of ca. 3.1 kcal mol-’ were calculated. The latter value corresponds to a contribution of 0.6% of the distorted (115’ < @ < 120’ ) higher energy rotamer to the equilibrium mixture at room temperature. The equilibrium torsional angle calculated for the free molecule of 9-StA is a little smaller than that found by Becker et al. [ 151 in their crystallographic study (@=65.5’ ). However, it should be noted that in the crystallographic study of the related molecule di (9-anthryl) ethylene a value of ca. 55 ’ was found [ 161, nearer to that calculated in the present work. The notable difference between these experimental data indicates that intermolecular forces strongly affect the equilibrium value of @. Recently, the QCFF/PI method was applied to the study of the potential energy surfaces in the ground and lowest excited states of n-StAs. The optimized equilibrium geometry computed for the ground state configuration of trans-9-StA starting from the experimental geometries has the minimum at 59.8”. However, starting the optimization from the planar geometry, the minimum is located at 30.8” with an energy higher (by 0.7 kcal mol- ’ ) than the previous minimum [ 71. The anomalous dependence of the fluorescence spectrum of 2-StA on A,,, and the bi-exponential emission decay profile are in agreement with the CSINDO potential curve (Fig. 1 (b) ) which presents two almost isoenergetic min-
176
ima at $ = 0’ and @= 180’ separated by a barrier of ca. 5 kcal mol- I. The more stable rotamer is the one with $= 0’ (i.e. the more elongated one); the energy difference between the two rotamers is ca. 0.2 kcal mol-l. Therefore, 2-StA is expected to exist in solution as an equilibrium mixture with nearly equal contributions of the two coplanar rotamers at room temperature. Fluorimetric analysis of trans-2-StA The A,,, effect on the emission spectrum of 2-StA is shown in Fig. 2. The emission spectrum also changes with temperature as shown in Fig. 3. An accurate analysis of these spectra indicates the presence of three emission components with the following vibronic structure: component A, peaks at 422,449,480 nm; component B, peaks at 411,436,465 nm; component C, shoulder at 395 nm. The two first transitions are characterized by a vibronic progression of ca. 1400 cm-‘, which indicates an anthracenic character of the emission spectrum of 2-&A. The progression is not easily observable for the C component since it is overlapped by the other more intensely absorbing components. Transitions A and B correspond to those previously reported [ 5-71 and are clearly due to the existence of an equilibrium in solution between two almost isoenergetic conformers (A and B ) with different fluorescence spectra and lifetimes ( zA < rB). The almost pure A spectrum can be obtained in aerated solutions at room temperature using A,,, > 410 nm, at the threshold of its bathochromic absorption spectrum. The almost pure spectrum of B (the more stable, longer-lived conformer) can be obtained in de-aerated solutions at low temperature using A,,, = 355 nm.* The assignment of transition C is more difficult. That it is not due to an impurity is indicated by the ratio of the fluorescence intensity measured at 411 and 395 nm, corresponding to the B and C components, respectively, which remains constant as a function of A,,, but changes with the temperature (Table 1). C cannot be a component of the A spectrum because the emission at 395 nm disappears under the experimental conditions when one obtains the almost pure A spectrum (T> 200 K, A,,, > 410 nm). The lifetime and excitation spectrum of C (measured at &,, = 395 nm) are the same as measured for B in the experimental conditions when B predominates, thus indicating a common origin for the two transitions (see Table 2 and Fig. 4). On the other hand, the emission observed at 395 nm cannot be part of the “normal” B transition since the mixture is enriched in B on cooling whilst C disappears. Therefore, it appears that the B and C components of the emission originate from different excited states of rotamer B with the component C being an upper excited state *A and B refer to the L and S modifications described earlier [ 51.
1.0 P-StA
‘F
I-_-
304
“In
2 _____
335
nm
355
nm
4.
384
nm
5-
415
“Ill
3 _
_
-.
O.!
h (nm) Fig. 2. Corrected fluorescence spectra of trans-2-StA (curve 5 refers to the almost pure A spectrum).
in toluene at 293 K as a function
of I,,,
P-StA
1.
360K
2 .-._.-
288
3.____
238K
a._.
_.._
5._
400
450
500
K
197
K
147
K
550
A (nm) Fig. 3. Corrected fluorescence spectra of trans-2-StA in toluene at a,,,=355 temperature (curve 5 refers to the almost pure B spectrum).
nm as a function
of
178 TABLE 1 The ratio of the fluorescence intensity F(&,,, A,,) of trans-2-StA 411 and 395 nm as a function of A,,, at two temperatures
L, b-m)
305 315 326 335 342 355 360 365 373 380 385
JWx,, JW,,,,
in toluene measured at A,, of
411)/ 395)
293 K
343 K
3.91 4.00 3.92 3.89 3.85 3.76 3.82 3.85 3.81 3.92 3.95
2.92 2.93 2.92 2.86 2.83 2.76 2.86 2.81 2.79 2.96 2.96
TABLE 2 Fluorescence
decay parameters
of bans-2-StA
in toluene as a function of ,I,,, at 293 K”
L, (nm)
a,, (nm)
rA (ns)
TB(ns)
~bLxc,Ln)
/wL, &In)
x2
315 355 405 420 315 315 335 335 355 355
420 420 450 450 395 410 395 410 395 410
8.9 8.0 9.0 9.7
28.6 28.0 27.6
0.95 0.51 3.76
0.295 0.146 1.226
1.14 1.16 1.01 1.96 1.30 1.58 1.21 1.25 1.00 1.30
“For the meaning of (Yand /I parameters,
27.5 26.6 27.8 28.8 27.7 27.3 see refs. 2 and 17.
(S, ) thermally equilibrated with the S1 state of B above a certain temperature, say 200 K. A detailed kinetic analysis of this anomalous emission will be reported elsewhere [ 181. A rigorous fluorimetric analysis of the conformer mixture, as done for similar compounds using the Birks approach [2,8,17], could not be carried out in the present case, partly because of the complication of the C component, but mainly because the change in OF with a,,, is too small. This handicap persists also at ca. 170 K when the complication due to C disappears (see Table 1).
179
Fig. 4. Corrected fluorescence excitation (1,2) and emission (3,4) spectra of trans-2-StA in toluene at 293 K: curve 1 in the absence of oxygen, ;i,,,,= 395,409 nm (almost pure B ) ; curve 2 with 1 atm 02, /I,, = 453 nm; curve 3, I.,,, = 355 nm (rich in B ) and curve 4, IeX, = 420 nm (almost pure A), both in the absence of oxygen.
However, as mentioned above, it was possible, by an accurate choice of the experimental conditions, to obtain the almost pure fluorescence spectra of the two rotamers. On the basis of all our fluorescence data as a function of A,,, and temperature, it was thus possible to make some revisions to the analysis previously reported [ 61, as follows. The distinct fluorescence lifetimes of the two rotamers and the corresponding ratio of the pre-exponential factors were obtained from deconvolution of the decay curves. They are collected in Table 2, together with the x2 values, as a function of Lexc and A,,. The data obtained allow the following conclusions to be drawn: (a) the two rotamers are characterized by fairly different lifetimes (8.9 and 28.0 ns in toluene for species A and B, respectively), in very good agreement with the values previously reported [6]; (b) the decay curve obtained with &, 2 420 nm gives a practically mono-exponential decay with the lifetime of the shorter-lived rotamer A (;1,,=450 nm); (c) if the decay is followed at A,,= 395 nm and/or 410 nm, a monoexponential kinetics is again found at all A,,, values, this time with the lifetime of the longer-lived species B, in very good agreement with the spectral behaviour shown in Fig. 2. Since 0r and rr of the two rotamers are temperature independent (see Tables 3 and 4), it was possible, by a suitable choice of the experimental conditions (A,,, and/or temperature), to measure the @)Fvalues of each of the two
180 TABLE 3 Fluorescence quantum yield (@r) of trans-2-StA in toluene as a function of ,I,,, at two temperatures
L, (nm)
335 342 350 355 360 365 373 380 384
@F
L, (nm)
293 K
170 K
0.75
0.76 0.84 0.78 0.77 0.78 0.83 0.74 0.84 0.81
0.82
@F 293 K
386 392 395 400 405 410 415 420
0.80 0.88 1.00
170K 0.83 0.76 0.75 0.84 0.95 0.90 1.00
TABLE 4 Fluorescence decay parameters of trans-2-StA in toluene as a function of temperature (A,,,=315 nm; I,,=420 nm)a
T(K)
7A bs)
7B
353 299 293 273 253 223 193
8.3 8.2 8.2 7.6 8.1 8.3 10.1
27.8 27.7 28.0 25.9 26.5 25.8 26.3
(ns)
a
0’)
P
0.94 0.93 1.10 0.83 0.81 0.67 0.78
CT)
x2
1.10 1.10 1.10 1.16 0.98 1.23 1.05
0.281 0.275 0.322 0.243 0.247 0.215 0.299
“For the meaning of (Yand /3parameters, see refs. 2 and 17. TABLE 5 Kinetic parameters and fluorescence quantum yields (@r) of trans-n-StAs in MCH at 293 K Parameter
l-StA
2-StA(A)a
2-StA(B)a
9StA
@F
0.64 5.0 1.30
1.0 8.7 1.15
0.80 27.0 0.30
0.44 3.6 1.20
7F
(ns)
kF (loss-‘)
“Data in toluene.
181
1.0
i
I
0.5
r:: P-StA
1
x
5.4
L 450
500
1
A(nm) Fig. 5. Absorption (1) and emission (2) spectra of trans-2-StA in toluene at 150 K (A,,,=355 nm).
species even without a rigorous quantitative analysis of the emission of the rotamer mixture. Thus, &.(A) was measured with ;1,,,> 415 nm, namely at the threshold of the absorption spectrum where the almost pure emission spectrum of A was observed (see Fig. 2 ) . On the other hand, @r (B ) was measured using A,,, = 355 nm at 130-150 K, where the almost pure spectrum of B was observed (see Fig. 3 ) . The CIQvalues determined under these conditions for the two rotamers are both high (1.0 for A and 0.8 for B). Moreover, the results obtained at different A,,, values (see Table 3 ) show that the quantum yield of the mixture, &, changes little with &.., at both room and low temperature, even at II,,, where fairly different emission spectra were observed (355, 304 and 410, 415, 420 nm); this behaviour again indicates that the two rotamers have similar (PIrvalues. The @r obtained in the present work at room temperature and A,,, = 355 nm (0.82) is in good agreement with those reported by Ghiggino et al. [ 61 (0.94 at II,,, = 295 nm) and by Das and co-workers [ 51 (0.82 at il...= 377 nm) and assigned to the longer-lived rotamer B. However, under conditions favouring observation of the shorter-lived A form, at A,,, > 410 nm, in dilute solutions ( < 4 x lop5 M), we found fluorescence yields of 0.8-1.0, i.e. considerably larger than those reported (0.64[6] and 0.52 [5] ). Indeed, the analysis of Ghiggino et al. [ 61 led to an intrinsic value of Qr (A) as low as 0.24. This discrepancy could be due to a concentration effect on @r which is only observable at long A,,, ( a400 nm) since, due to the small extinction coeffi-
182
cients ( < 2000) in this spectral region, one has to use more concentrated solutions, favouring association [ 191. In addition, it has to be noted that in the previous analysis [6] the presence of the third fluorescent component C was neglected. Table 5 reports the @r, rr and kF values for 2-StA in toluene at 293 K. As a result of the large differences between the present @F(A) values and those reported earlier [ 61, the conclusions regarding the radiative rate constants kF are quite different: whereas Ghiggino et al. [ 61 conclude rather similar kF values for A and B, the present results led us to conclude that kF for the A modification is about 1.1 x 10’ s-l, rather similar to l-StA and 9-StA, while kF for the B modification is about four times smaller, 3 x lo7 s-l, similar to Ghiggino’s kF for both A and B. This difference between k$ and kg is similar to our earlier findings with 2-styrylnaphthalene [2] and in agreement with the results of theoretical calculations which assign to the more stable (more elongated) rotamer B an S, state with a weaker oscillator strength [ 71. On cooling, the shape of the emission spectra changes towards that of the longer-lived rotamer B (see Fig. 3 ), thus showing that this is indeed the more stable species. Figure 5 shows the absorption and emission spectra at 150 K. The fact that: (a) the absorption spectrum in Fig. 5 still shows the shoulder at 407 nm, characteristic of the A species; (b) the fluorescence decay at il,,, = 315 nm remains bi-exponential also at low temperature (see Table 4); and (c) the ratios of preexponential factors do not show a clear trend with the temperature (as already found for styrylphenanthrenes) (see again Table 4), indicates that the enthalpy difference between the the two rotamers has to be small ( (0.4 kcal mol-‘) as confirmed by the value of ca. 0.2 kcal mol-l obtained by the CSINDO method (see above). This is also in good agreement with the value recently obtained by QCFF/PI and CNDO/S calculations [7] which gave AHE 0.3 kcal mol-‘, while the estimate of Ghiggino et al. was ca. 1 kcal mol-’
161. CONCLUSIONS
The 1 and 9 isomers of trans-n-StA behave in solution as one-component systems. The 2-StA displays a conformational equilibrium between two almost isoenergetic rotational isomers. They differ in the fluorescence lifetime mainly because of the difference in their radiative rate parameters. The present theoretical and fluorimetric analysis of the experimental data as a function of /2,,, and temperature has led to a revision of the previous conclusions [ 61. In particular, it shows that fluorescence is the main deactivation pathway for both rotamers, though with widely different rates, and that their ground state energy difference is even smaller than estimated before.
183 ACKNOWLEDGEMENT
Financial support by the Italian Consiglio Nazionale delle Ricerche and Minister0 per la Pubblica Istruzione (Roma) and stimulating discussions with Professors F. Momicchioli (Modena) and G. Orlandi (Bologna) are gratefully acknowledged.
REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
For review articles see: Yu.B. Scheck, N.P. Kovalenko and M.V. Alfimov, J. Lumin., 15 (1977) 157; E. Fischer, J. Photochem., 17 (1981) 331; J. Mol. Struct., 84 (1982) 219; U. Mazzucato, Pure Appl. Chem., 54 (1982) 1705 and references cited therein. G. Bartocci, F. Masetti, U. Mazzucato and G. Marconi, J. Chem., Sot., Faraday Trans. 2,80 (1984) 1093. I. Baraldi, F. Momicchioli and G. Ponterini, J. Mol. Struct. (Theochem), 110 (1984) 187. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti, I. Baraldi and F. Momicchioli, J. Phys. Chem., 91 (1987) 4733. T. Wismontski-Knittel, P.K. Das and E. Fischer, J. Phys. Chem., 88 (1984) 1163; see also G. Fischer and E. Fischer, J. Phys. Chem., 85 (1981) 1168. K.P. Ghiggino, P.F. Skilton and E. Fischer, J. Am. Chem. Sot., 108 (1986) 1146. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti, G. Orlandi and G. Poggi, J. Chem. Sot., Faraday Trans. 2,84 (1988) 385. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti and MC. Bruni, J. Chem. Sot., Faraday Trans. 2,82 (1986) 775. T. Wismontski-Knittel and P.K. Das, J. Phys. Chem., 88 (1984) 1168. V. Krongauz, N. Caste1and E. Fischer, J. Photochem., 39 (1987) 285. G. Galiazzo, A. Spalletti, F. Elisei and G. Gennari, submitted to Gazz. Chim. Ital. G. Bartocci, U. Mazzucato, F. Masetti and G. Galiazzo, J. Phys. Chem., 84 (1980) 847. F. Momicchioli, I. Baraldi and M.C. Bruni, Chem. Phys., 82 (1983) 229. D.W.J. Cruickshank, Acta Crystallogr., 9 (1956) 915. H.-D. Becker, V.A. Patrick and A.H. White, Aust. J. Chem., 37 (1984) 2215. H.-D. Becker, L.M. Englehardt, L. Ansen, V.A. Patrick and A.H. White, Aust. J. Chem., 37 (1984) 1329. J.B. Birks, G. Bartocci, G.G. Aloisi, S. Dellonte and F. Barigelletti, Chem. Phys., 51 (1980) 113. G. Bartocci et al., in preparation. F. Masetti, G. Bartocci, U. Mazzucato and E. Fischer, J. Chem. Sot., Perkin Trans. 2, (1983) 797.
FLUORIMETRIC AND THEORETICAL STUDY ROTAMERISM OF trans-STYRYLANTHRACENES
173
OF THE
G. BARTOCCI, F. MASETTI and U. MAZZUCATO Dipartimento di Chimica, Universitti di Perugia, Z-06100 Perugia (Italy) I. BARALDI Dipartimento di Chimica, Universitti di Modena, Z-41100 Modena (Italy) E. FISCHER Department of Structural Chemistry, Weizmann Institute of Science, IL-76100 Rehovot (Israel) (Received 15 March 1988)
ABSTRACT The ground state conformational equilibrium of trans.n-styrylanthracenes (n= 1, 2, 9) has been studied by stationary and pulsed fluorimetric techniques in non-polar solvents. Parallel theoretical calculations of the potential energy curves for the internal rotation of the anthryl group have been carried out with a modified CS-INDO method. The behaviour of the mixture of almost isoenergetic rotamers of 2styrylanthracene has been particularly investigated to add some complementary and revised information to that recently reported in the literature.
INTRODUCTION
Ground state conformational equilibria in solutions of several stilbene-like molecules have been extensively investigated in the last decade in different laboratories by fluorimetric techniques [ 11.It has been shown that both photochemical and photophysical behaviour depend on the excitation energy and differ for the two (or more) rotamers derived by rotation of an aryl group around the single bond with the ethylenic bridge. A detailed photophysical study, including theoretical aspects, has been carried out in our laboratories for the overall series of trans-n-styrylnaphthalenes (n-StN, n=l, 2) [l-3] and n-styrylphenanthrenes (n-StPh, ~1, 2, 3, 4, 9) [ 41. More recently, the study has been extended to the trans-n-styrylanthracenes [ 5-71, to heteroaromatic compounds [ 81 and to the triplet behaviour of rotamers as investigated by flash photolysis [9,10]. In this paper, we compare the conformational equilibria of the three isomeric trans-n-styrylanthracenes (n-StA, n= 1, 2, 9) and in particular discuss the behaviour of 2-StA in the light of the new information obtained. This study,
QQ22-286Q/89/$03.5Q
0 1989 Elsevier Science Publishers B.V.
174
which includes photophysical measurements and theoretical calculations of the potential energy surfaces as a function of the torsional angle around the single bond with the styrene moiety, leads to a complete description of the conformational equilibrium of trans-StAs. EXPERIMENTAL
The tram-n-StAs were synthesized for previous work by methods described elsewhere [ 111. The experimental procedures (fluorescence spectra, quantum yields and decay profiles) have been reported in previous papers [2,12]. 2- (lNaphthyl)&phenyl-1,3,4-oxadiazole (cu-NPD) was used as standard for the QF measurements (@ r = 0.70 in cyclohexane) [ 41. For measurements of (IQ as a function of temperature, the (DF value at room temperature was used as reference, taking into account the changes in absorbance and refractive index with temperature. The uncertainty in the reported parameters is 4% for (DF and 7% for rr. RESULTS AND DISCUSSION
In the series of trans-n-StAs, the 1 and 9 isomers were found to display a normal fluorescence behaviour. In fact, their emission and excitation spectra do not depend on the excitation (A,,,) and emission (A,,) wavelength, respectively, and the emission decay profile is mono-exponential. In the case of lStA, the analogue of l-StN and 1-StPh, the conformational equilibrium is practically completely shifted towards the more stable rotamer, characterized by less severe steric interference between the aromatic and ethylenic hydrogens. In the case of the 9 isomer, two (nearly) equivalent rotamers are expected by rotation of the polycyclic group [ 71. Therefore, the 1 and 9 isomers behave as one-component systems. On the other hand, for 2-StA, the analogue of 2StN and 2- and 3-StPh, the fluorescence spectrum displays a dependence on A,,, and the emission decay profile is bi-exponential [ 5-71. Theoretical
analysis
The behaviour of the three isomeric trans-n-StAs is well described by the theoretical conformational analysis carried out using the CS-INDO method [ 131. For all molecules a rigid rotation around the A-St bond has been analyzed using the experimental geometry for the phenanthrene moiety [ 141 and assumed geometries for the styrene moiety. More precisely, the geometry of the styrene moiety in l- and 9-StA and in 2-StA is the same as that chosen in a previous study on the conformational properties of l-StN and 2-StN, respectively [ 31. The calculated points at different values of the torsional angle (@) are marked on the potential energy curves of Fig. 1.
175
(Cl
9-StA
18(
0
90
180
0
90
L
180
000 \
T 0
Fig. 1. Ground-state potential energy curves for internal rotation of trans-n-StAs “single” bond connecting the anthracenic group to the ethylenic bridge.
around the
Figure 1 (a and c) shows the ground state CS-INDO potential energy curves describing the internal rotation of l- and 9-StA around the essential single bond connecting the anthryl and styryl groups. The results of the calculations confirm the expected existence of two equivalent minima at @= 52 ’ and @= 128” (noticeably shifted with respect to the planar configurations because of the steric interactions) in the case of 9-StA, while for l-StA two minima with an energy difference of ca. 3.1 kcal mol-’ were calculated. The latter value corresponds to a contribution of 0.6% of the distorted (115’ < @ < 120’ ) higher energy rotamer to the equilibrium mixture at room temperature. The equilibrium torsional angle calculated for the free molecule of 9-StA is a little smaller than that found by Becker et al. [ 151 in their crystallographic study (@=65.5’ ). However, it should be noted that in the crystallographic study of the related molecule di (9-anthryl) ethylene a value of ca. 55 ’ was found [ 161, nearer to that calculated in the present work. The notable difference between these experimental data indicates that intermolecular forces strongly affect the equilibrium value of @. Recently, the QCFF/PI method was applied to the study of the potential energy surfaces in the ground and lowest excited states of n-StAs. The optimized equilibrium geometry computed for the ground state configuration of trans-9-StA starting from the experimental geometries has the minimum at 59.8”. However, starting the optimization from the planar geometry, the minimum is located at 30.8” with an energy higher (by 0.7 kcal mol- ’ ) than the previous minimum [ 71. The anomalous dependence of the fluorescence spectrum of 2-StA on A,,, and the bi-exponential emission decay profile are in agreement with the CSINDO potential curve (Fig. 1 (b) ) which presents two almost isoenergetic min-
176
ima at $ = 0’ and @= 180’ separated by a barrier of ca. 5 kcal mol- I. The more stable rotamer is the one with $= 0’ (i.e. the more elongated one); the energy difference between the two rotamers is ca. 0.2 kcal mol-l. Therefore, 2-StA is expected to exist in solution as an equilibrium mixture with nearly equal contributions of the two coplanar rotamers at room temperature. Fluorimetric analysis of trans-2-StA The A,,, effect on the emission spectrum of 2-StA is shown in Fig. 2. The emission spectrum also changes with temperature as shown in Fig. 3. An accurate analysis of these spectra indicates the presence of three emission components with the following vibronic structure: component A, peaks at 422,449,480 nm; component B, peaks at 411,436,465 nm; component C, shoulder at 395 nm. The two first transitions are characterized by a vibronic progression of ca. 1400 cm-‘, which indicates an anthracenic character of the emission spectrum of 2-&A. The progression is not easily observable for the C component since it is overlapped by the other more intensely absorbing components. Transitions A and B correspond to those previously reported [ 5-71 and are clearly due to the existence of an equilibrium in solution between two almost isoenergetic conformers (A and B ) with different fluorescence spectra and lifetimes ( zA < rB). The almost pure A spectrum can be obtained in aerated solutions at room temperature using A,,, > 410 nm, at the threshold of its bathochromic absorption spectrum. The almost pure spectrum of B (the more stable, longer-lived conformer) can be obtained in de-aerated solutions at low temperature using A,,, = 355 nm.* The assignment of transition C is more difficult. That it is not due to an impurity is indicated by the ratio of the fluorescence intensity measured at 411 and 395 nm, corresponding to the B and C components, respectively, which remains constant as a function of A,,, but changes with the temperature (Table 1). C cannot be a component of the A spectrum because the emission at 395 nm disappears under the experimental conditions when one obtains the almost pure A spectrum (T> 200 K, A,,, > 410 nm). The lifetime and excitation spectrum of C (measured at &,, = 395 nm) are the same as measured for B in the experimental conditions when B predominates, thus indicating a common origin for the two transitions (see Table 2 and Fig. 4). On the other hand, the emission observed at 395 nm cannot be part of the “normal” B transition since the mixture is enriched in B on cooling whilst C disappears. Therefore, it appears that the B and C components of the emission originate from different excited states of rotamer B with the component C being an upper excited state *A and B refer to the L and S modifications described earlier [ 51.
1.0 P-StA
‘F
I-_-
304
“In
2 _____
335
nm
355
nm
4.
384
nm
5-
415
“Ill
3 _
_
-.
O.!
h (nm) Fig. 2. Corrected fluorescence spectra of trans-2-StA (curve 5 refers to the almost pure A spectrum).
in toluene at 293 K as a function
of I,,,
P-StA
1.
360K
2 .-._.-
288
3.____
238K
a._.
_.._
5._
400
450
500
K
197
K
147
K
550
A (nm) Fig. 3. Corrected fluorescence spectra of trans-2-StA in toluene at a,,,=355 temperature (curve 5 refers to the almost pure B spectrum).
nm as a function
of
178 TABLE 1 The ratio of the fluorescence intensity F(&,,, A,,) of trans-2-StA 411 and 395 nm as a function of A,,, at two temperatures
L, b-m)
305 315 326 335 342 355 360 365 373 380 385
JWx,, JW,,,,
in toluene measured at A,, of
411)/ 395)
293 K
343 K
3.91 4.00 3.92 3.89 3.85 3.76 3.82 3.85 3.81 3.92 3.95
2.92 2.93 2.92 2.86 2.83 2.76 2.86 2.81 2.79 2.96 2.96
TABLE 2 Fluorescence
decay parameters
of bans-2-StA
in toluene as a function of ,I,,, at 293 K”
L, (nm)
a,, (nm)
rA (ns)
TB(ns)
~bLxc,Ln)
/wL, &In)
x2
315 355 405 420 315 315 335 335 355 355
420 420 450 450 395 410 395 410 395 410
8.9 8.0 9.0 9.7
28.6 28.0 27.6
0.95 0.51 3.76
0.295 0.146 1.226
1.14 1.16 1.01 1.96 1.30 1.58 1.21 1.25 1.00 1.30
“For the meaning of (Yand /I parameters,
27.5 26.6 27.8 28.8 27.7 27.3 see refs. 2 and 17.
(S, ) thermally equilibrated with the S1 state of B above a certain temperature, say 200 K. A detailed kinetic analysis of this anomalous emission will be reported elsewhere [ 181. A rigorous fluorimetric analysis of the conformer mixture, as done for similar compounds using the Birks approach [2,8,17], could not be carried out in the present case, partly because of the complication of the C component, but mainly because the change in OF with a,,, is too small. This handicap persists also at ca. 170 K when the complication due to C disappears (see Table 1).
179
Fig. 4. Corrected fluorescence excitation (1,2) and emission (3,4) spectra of trans-2-StA in toluene at 293 K: curve 1 in the absence of oxygen, ;i,,,,= 395,409 nm (almost pure B ) ; curve 2 with 1 atm 02, /I,, = 453 nm; curve 3, I.,,, = 355 nm (rich in B ) and curve 4, IeX, = 420 nm (almost pure A), both in the absence of oxygen.
However, as mentioned above, it was possible, by an accurate choice of the experimental conditions, to obtain the almost pure fluorescence spectra of the two rotamers. On the basis of all our fluorescence data as a function of A,,, and temperature, it was thus possible to make some revisions to the analysis previously reported [ 61, as follows. The distinct fluorescence lifetimes of the two rotamers and the corresponding ratio of the pre-exponential factors were obtained from deconvolution of the decay curves. They are collected in Table 2, together with the x2 values, as a function of Lexc and A,,. The data obtained allow the following conclusions to be drawn: (a) the two rotamers are characterized by fairly different lifetimes (8.9 and 28.0 ns in toluene for species A and B, respectively), in very good agreement with the values previously reported [6]; (b) the decay curve obtained with &, 2 420 nm gives a practically mono-exponential decay with the lifetime of the shorter-lived rotamer A (;1,,=450 nm); (c) if the decay is followed at A,,= 395 nm and/or 410 nm, a monoexponential kinetics is again found at all A,,, values, this time with the lifetime of the longer-lived species B, in very good agreement with the spectral behaviour shown in Fig. 2. Since 0r and rr of the two rotamers are temperature independent (see Tables 3 and 4), it was possible, by a suitable choice of the experimental conditions (A,,, and/or temperature), to measure the @)Fvalues of each of the two
180 TABLE 3 Fluorescence quantum yield (@r) of trans-2-StA in toluene as a function of ,I,,, at two temperatures
L, (nm)
335 342 350 355 360 365 373 380 384
@F
L, (nm)
293 K
170 K
0.75
0.76 0.84 0.78 0.77 0.78 0.83 0.74 0.84 0.81
0.82
@F 293 K
386 392 395 400 405 410 415 420
0.80 0.88 1.00
170K 0.83 0.76 0.75 0.84 0.95 0.90 1.00
TABLE 4 Fluorescence decay parameters of trans-2-StA in toluene as a function of temperature (A,,,=315 nm; I,,=420 nm)a
T(K)
7A bs)
7B
353 299 293 273 253 223 193
8.3 8.2 8.2 7.6 8.1 8.3 10.1
27.8 27.7 28.0 25.9 26.5 25.8 26.3
(ns)
a
0’)
P
0.94 0.93 1.10 0.83 0.81 0.67 0.78
CT)
x2
1.10 1.10 1.10 1.16 0.98 1.23 1.05
0.281 0.275 0.322 0.243 0.247 0.215 0.299
“For the meaning of (Yand /3parameters, see refs. 2 and 17. TABLE 5 Kinetic parameters and fluorescence quantum yields (@r) of trans-n-StAs in MCH at 293 K Parameter
l-StA
2-StA(A)a
2-StA(B)a
9StA
@F
0.64 5.0 1.30
1.0 8.7 1.15
0.80 27.0 0.30
0.44 3.6 1.20
7F
(ns)
kF (loss-‘)
“Data in toluene.
181
1.0
i
I
0.5
r:: P-StA
1
x
5.4
L 450
500
1
A(nm) Fig. 5. Absorption (1) and emission (2) spectra of trans-2-StA in toluene at 150 K (A,,,=355 nm).
species even without a rigorous quantitative analysis of the emission of the rotamer mixture. Thus, &.(A) was measured with ;1,,,> 415 nm, namely at the threshold of the absorption spectrum where the almost pure emission spectrum of A was observed (see Fig. 2 ) . On the other hand, @r (B ) was measured using A,,, = 355 nm at 130-150 K, where the almost pure spectrum of B was observed (see Fig. 3 ) . The CIQvalues determined under these conditions for the two rotamers are both high (1.0 for A and 0.8 for B). Moreover, the results obtained at different A,,, values (see Table 3 ) show that the quantum yield of the mixture, &, changes little with &.., at both room and low temperature, even at II,,, where fairly different emission spectra were observed (355, 304 and 410, 415, 420 nm); this behaviour again indicates that the two rotamers have similar (PIrvalues. The @r obtained in the present work at room temperature and A,,, = 355 nm (0.82) is in good agreement with those reported by Ghiggino et al. [ 61 (0.94 at II,,, = 295 nm) and by Das and co-workers [ 51 (0.82 at il...= 377 nm) and assigned to the longer-lived rotamer B. However, under conditions favouring observation of the shorter-lived A form, at A,,, > 410 nm, in dilute solutions ( < 4 x lop5 M), we found fluorescence yields of 0.8-1.0, i.e. considerably larger than those reported (0.64[6] and 0.52 [5] ). Indeed, the analysis of Ghiggino et al. [ 61 led to an intrinsic value of Qr (A) as low as 0.24. This discrepancy could be due to a concentration effect on @r which is only observable at long A,,, ( a400 nm) since, due to the small extinction coeffi-
182
cients ( < 2000) in this spectral region, one has to use more concentrated solutions, favouring association [ 191. In addition, it has to be noted that in the previous analysis [6] the presence of the third fluorescent component C was neglected. Table 5 reports the @r, rr and kF values for 2-StA in toluene at 293 K. As a result of the large differences between the present @F(A) values and those reported earlier [ 61, the conclusions regarding the radiative rate constants kF are quite different: whereas Ghiggino et al. [ 61 conclude rather similar kF values for A and B, the present results led us to conclude that kF for the A modification is about 1.1 x 10’ s-l, rather similar to l-StA and 9-StA, while kF for the B modification is about four times smaller, 3 x lo7 s-l, similar to Ghiggino’s kF for both A and B. This difference between k$ and kg is similar to our earlier findings with 2-styrylnaphthalene [2] and in agreement with the results of theoretical calculations which assign to the more stable (more elongated) rotamer B an S, state with a weaker oscillator strength [ 71. On cooling, the shape of the emission spectra changes towards that of the longer-lived rotamer B (see Fig. 3 ), thus showing that this is indeed the more stable species. Figure 5 shows the absorption and emission spectra at 150 K. The fact that: (a) the absorption spectrum in Fig. 5 still shows the shoulder at 407 nm, characteristic of the A species; (b) the fluorescence decay at il,,, = 315 nm remains bi-exponential also at low temperature (see Table 4); and (c) the ratios of preexponential factors do not show a clear trend with the temperature (as already found for styrylphenanthrenes) (see again Table 4), indicates that the enthalpy difference between the the two rotamers has to be small ( (0.4 kcal mol-‘) as confirmed by the value of ca. 0.2 kcal mol-l obtained by the CSINDO method (see above). This is also in good agreement with the value recently obtained by QCFF/PI and CNDO/S calculations [7] which gave AHE 0.3 kcal mol-‘, while the estimate of Ghiggino et al. was ca. 1 kcal mol-’
161. CONCLUSIONS
The 1 and 9 isomers of trans-n-StA behave in solution as one-component systems. The 2-StA displays a conformational equilibrium between two almost isoenergetic rotational isomers. They differ in the fluorescence lifetime mainly because of the difference in their radiative rate parameters. The present theoretical and fluorimetric analysis of the experimental data as a function of /2,,, and temperature has led to a revision of the previous conclusions [ 61. In particular, it shows that fluorescence is the main deactivation pathway for both rotamers, though with widely different rates, and that their ground state energy difference is even smaller than estimated before.
183 ACKNOWLEDGEMENT
Financial support by the Italian Consiglio Nazionale delle Ricerche and Minister0 per la Pubblica Istruzione (Roma) and stimulating discussions with Professors F. Momicchioli (Modena) and G. Orlandi (Bologna) are gratefully acknowledged.
REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
For review articles see: Yu.B. Scheck, N.P. Kovalenko and M.V. Alfimov, J. Lumin., 15 (1977) 157; E. Fischer, J. Photochem., 17 (1981) 331; J. Mol. Struct., 84 (1982) 219; U. Mazzucato, Pure Appl. Chem., 54 (1982) 1705 and references cited therein. G. Bartocci, F. Masetti, U. Mazzucato and G. Marconi, J. Chem., Sot., Faraday Trans. 2,80 (1984) 1093. I. Baraldi, F. Momicchioli and G. Ponterini, J. Mol. Struct. (Theochem), 110 (1984) 187. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti, I. Baraldi and F. Momicchioli, J. Phys. Chem., 91 (1987) 4733. T. Wismontski-Knittel, P.K. Das and E. Fischer, J. Phys. Chem., 88 (1984) 1163; see also G. Fischer and E. Fischer, J. Phys. Chem., 85 (1981) 1168. K.P. Ghiggino, P.F. Skilton and E. Fischer, J. Am. Chem. Sot., 108 (1986) 1146. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti, G. Orlandi and G. Poggi, J. Chem. Sot., Faraday Trans. 2,84 (1988) 385. G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti and MC. Bruni, J. Chem. Sot., Faraday Trans. 2,82 (1986) 775. T. Wismontski-Knittel and P.K. Das, J. Phys. Chem., 88 (1984) 1168. V. Krongauz, N. Caste1and E. Fischer, J. Photochem., 39 (1987) 285. G. Galiazzo, A. Spalletti, F. Elisei and G. Gennari, submitted to Gazz. Chim. Ital. G. Bartocci, U. Mazzucato, F. Masetti and G. Galiazzo, J. Phys. Chem., 84 (1980) 847. F. Momicchioli, I. Baraldi and M.C. Bruni, Chem. Phys., 82 (1983) 229. D.W.J. Cruickshank, Acta Crystallogr., 9 (1956) 915. H.-D. Becker, V.A. Patrick and A.H. White, Aust. J. Chem., 37 (1984) 2215. H.-D. Becker, L.M. Englehardt, L. Ansen, V.A. Patrick and A.H. White, Aust. J. Chem., 37 (1984) 1329. J.B. Birks, G. Bartocci, G.G. Aloisi, S. Dellonte and F. Barigelletti, Chem. Phys., 51 (1980) 113. G. Bartocci et al., in preparation. F. Masetti, G. Bartocci, U. Mazzucato and E. Fischer, J. Chem. Sot., Perkin Trans. 2, (1983) 797.