- Email: [email protected]
Electronic transition moment directions in indoloindoles: the use of orientation amplifiers
Journal of Molecular Structure 475 (1999) 141–151
Electronic transition moment directions in indoloindoles: the use of orientation amplifiers Michal⁄ Gil a, Jan Marczyk a, Sergey Dobrin a,1, Piotr Kaszynski b, Jacek Waluk a,* a
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland b Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA Received 6 April 1998; accepted 14 May 1998
Abstract Indolo[3,2-b]indole and its four derivatives are studied by linear dichroism in stretched polymeric sheets, fluorescence anisotropy in glassy solutions, and by quantum chemical calculations. Poor alignment of the parent molecule in the stretched polymers does not allow the determination of the transition moment directions by standard procedures. However, this is made possible if long inert substituents are introduced into the molecule. Their role is to increase the alignment without modifying the electronic structure of the chromophore. For a well-oriented chromophore, principal orientation factors and polarizations of transitions may be obtained. The results show that for the four lowest observed excited singlet states of indolo[3,2-b]indole, all the pp * character, substitution by carboethoxy or alkyl groups does not alter the directions of the transition moments. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Linear dichroism; Fluorescence anisotropy; Indoloindoles
1. Introduction Determination of the directions of electronic transition moments is important for both spectroscopic and structural studies. First, it is often crucial for the assignment and characterization of electronic states. Moreover, it provides a possibility to obtain the degree of orientation of a particular chromophore in an aligned sample. The latter may be important in many different research areas, such as studies of intercalation into DNA, alignment in Langmuir–Blodgett films, surface adsorption, etc. [1]. * Corresponding author. Tel.: ⫹ 48 22 632 7269; Fax: ⫹ 48 39 120238; e-mail: [email protected] 1 On leave from N.N. Semenov Institute of Chemical Physics, Academy of Sciences of Russia, Kosygin Str.4, 117334 Moscow, Russia.
Unfortunately, the exact determination of the transition moments is straightforward only in cases in which their directions are dictated by sufficiently high molecular symmetry. For such symmetry species, C2v, D2h or D2, the transition moment can only lie in one of the three molecular directions which coincide with the symmetry axes. A well-established method used in such cases involves linear dichroism (LD) measurements on chromophores partially aligned in anisotropic media, such as stretched polymers or liquid crystals [1–3]. The results provide the average orientation of each of the molecular axes, usually expressed in terms of the socalled ‘orientation factors’ or related quantities. Ample experimental evidence showing that the alignment is related to the molecular shape was accumulated. Thus, the longest molecular axis orients the best, and the smallest, the worst. This observation,
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00500-6
142
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
combined with the experimental values of the orientation factors, provides an easy way to assign the absolute polarizations of transitions. For less symmetrical molecules, the situation is more complicated. Even if some symmetry elements remain, the number of possible transition moment directions may be infinite. Such is the case of structures which possess a plane of symmetry (Cs or C2h species). The transition moment can now be polarized perpendicular to the symmetry plane or assume any direction in the plane. Even for such cases, a fairly accurate estimation of the transition moment directions was shown to be feasible, provided the principal orientation factors — those that correspond to the best and worst aligned directions — can be evaluated. However, additional pieces of information are now required. A procedure has now been worked out, which combines the results of LD studies with measurements of fluorescence anisotropy and quantum chemical calculations. We have successfully used it for the determination of transition moment directions in benz[a]anthracene [4,5]
and its azaderivatives [6], chrysene [7], benz[a]pyrene [8], indoloquinoxalines [9], dibenzo[ j,lm]phenanthro[5,4,3-abcd ]perylene [10]. In this work, we use the basics of this technique, and expand it by studying molecules which contain what we call ‘orientation amplifiers’ —elongated substitutents which strongly enhance molecular alignment. The idea is to compare several molecules having essentially the same chromophore unit, but exhibiting various degrees of orientation. The large orientation factors obtained for well-orienting systems enable the determination of transition moment directions in the parent, unsubstituted compounds, for which the information obtained from LD is insufficient. The compounds we study (Scheme 1) are based on the indolo[3,2-b]indole chromophore (1). This molecule is a rigid analogue of trans-stilbene, whose photophysics is a textbook example of photoisomerization involving large amplitude molecular motion [11,12]. Recently, we studied the photophysical behavior of the systems presented in this work as well as
Scheme 1. 5,10-dihydroindolo[3,2-b]indole(1); 5,10-dihydro-N,N 0 -dimethylindolo-[3,2-b]indole (2); 5,10-dihydroindolo[3,2-b]indole-2,7dicarboxylate (3); diethyl 5,10-dihydro-N,N 0 -dimethylindolo[3,2-b]-indole-2,7-dicarboxylate (4); and diethyl 5,10-dihydro-N,N 0 -dihexylindolo-[3,2-b]indole-2,7-dicarboxylate (5).
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
143
Fig. 1. Absorption spectra of 1 and 2 in n-propanol at 293 K. Roman numbers mark the bands assigned to different electronic transitions. Estimated oscillator strengths are given in parentheses.
their analogues containing sulfur or selenium in place of one or both nitrogen atoms [13–15]. The present work will be followed by the series in which the influence of sulfur and selenium heterosubstitution upon transition moment directions will be studied.
2. Experimental and computational procedures The synthesis and purification of 1–5 were described elsewhere [16,17]. Absorption and LD spectra were recorded on a Shimadzu UV 3100 spectrophotometer, equipped, for LD measurements, with rotatable Glan prisms in both the sample and reference beams. Fluorescence and fluorescence anisotropy measurements were made in ethyl ether-isopentaneethanol 5:5:2 (EPA) or n-propanol glasses with an old [18] and new [19] type of Jasny spectrofluorimeter or with an Edinburgh Instruments FS900 steady state spectrometer. All solvents were of spectral grade. For LD measurements, the substances were introduced into polyethylene sheets from chloroform solutions. The sheets were then stretched about 400%. Some experiments were also done with stretched poly(vinyl alcohol) (PVA) as the orienting medium. In this case, the samples were produced from a stock
solution of PVA in water (10% v/v) to which a methanol solution of a given chromophore was added. The stretching was performed at elevated temperatures, using a hair dryer or water bath as a heating source. The INDO/S method [20] was used for the calculation of excited state energies, oscillator strengths and transition moment directions. Two hundred lowestenergy singly-excited configurations were taken into account in the CI procedure. In some cases, several doubly-excited configurations were also included. The input geometry was obtained by molecular mechanics, using the MMX force field [21].
3. Results and discussion Absorption spectra in isotropic solutions are presented in Figs. 1 and 2. At least five electronic transitions can be distinguished below 50 000 cm ⫺1 in the spectra of 1 and 2; in 3–5, the presence of six transitions is detected. The spectra are very similar. Substitution by carboethoxy ‘tails’ causes only minor changes: a red shift of the first absorption band and an increase of the intensity of the second transition. It is clear from the inspection of the spectra that the
144
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Fig. 2. Absorption spectra of 3, 4 and 5 in n-propanol at 293 K. See caption to Fig. 1.
identity of at least four lowest observed electronic transitions is preserved in all the molecules. Fig. 3 shows the LD curves recorded for 2 in a stretched polyethylene sheet. Two curves were obtained: EZ, measured with the electric vector of light parallel to the stretching direction and EY, obtained with the electric vector perpendicular to the stretching direction. The quantity which can be extracted from an LD spectrum for each particular electronic transition f, is the orientation factor, defined as Kf 具cos2 Z; M0f 典;
1
where M0f is the transition moment and Z the unique direction of the sample: in the case of polymer sheets, it coincides with the stretching direction. The value of the orientation factor, Kf, shows how well the transition f orients, on an average, along the unique direction of the sample: the larger the value, the better the orientation. The number of possible values of the orientation factors for a particular molecule is dictated by symmetry. For instance, in molecules of C2v or D2h symmetry, only three values are possible, as the
transition moments are restricted to lie along the three symmetry axes. Experimental evidence shows that the orientation is strongly correlated with molecular shape [1]. The molecular axis labeled z is defined as the one that orients the best. It usually corresponds with the longest axis in the molecule. The x-axis is defined as the one that orients the worst, usually it coincides with the shortest molecular axis. The y-axis is orthogonal to x and z. In molecules of lower symmetry, the set of principal x-, y- and z-axes is defined in the same way, but the location of the axes in the molecular frame is unknown. A common assumption is to approximate z, the ‘effective orientation axis’, as the direction perpendicular to the smallest cross-section of the molecule. The x-axis corresponds to that direction which among all directions orthogonal to z is aligned the worst. The important relation between the orientation factors Kx, Ky and Kz shows that only two of them are independent, as Kx ⫹ Ky ⫹ Kz 1:
2
Once the values of the principal orientation factors are established, the absolute value of the angle f f
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
145
Fig. 3. LD spectra measured for 2 in polyethylene at 293 K:Ez, solid lines; and Ey, dashed lines (top). Illustration of the stepwise reduction procedure (bottom). The values of K were varied from 0 to 1 in steps of 0.1.
between the transition moment M0f and the z-axis can be determined using the equation: tan2 M0f ; z tan2 ff Kz ⫺ Kf = Kf ⫺ Ky :
3
In order to obtain the numerical values of Kf from the spectra which contain broad, overlapping bands, a stepwise reduction (TEM) procedure is very useful [1,2]. At a given wavelength, the contribution from a transition f will disappear from the linear combination: 1 1 ⫺ Kf EZ l ⫺ Kf EY l: 2
4
In the TEM method, a series of curves corresponding to Eq. (4) is plotted by varying the values of Kf. The real value of Kf is obtained as the one that makes spectral features (peaks or shoulders) belonging to the transition f disappear. An example is presented in Fig. 3. The results of the LD and fluorescence anisotropy measurements are shown in Tables 1–5. Very different orientation factors are obtained. This is not surprising, as the molecular shape is also different. On the one hand, 2, and to a certain extent also 1, are disclike chromophores. On the other hand, 3 and 4 may be
considered rod-like structures in which large values of the orientation factors are expected for those transition moments that form small angles with the orientation axis. Indeed, the second and third observed electronic transitions reveal much larger values of the orientation factors in 4 than the other molecules (unfortunately, 3 could not be introduced into a polymer and hence was not measured). It should be stressed that such a comparison of orientation factors for different molecules is only valid if the polarizations of transitions remain the same for each chromophore. The correctness of such assumption is proved by the values of r, the anisotropy of fluorescence excitation, presented in Tables 1–5 and illustrated in Fig. 4. The angle (between the transition moment directions corresponding to the absorbing and emitting states is given by:
a cos⫺1 5r ⫹ 1=31=2 :
5
The shape of the anisotropy curves follows the same pattern in all the molecules. It follows from Tables 1–5 that the angles a are the same, up to within a few degrees, at least in the region of the first four electronic transitions. Thus, in order to
146
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Table 1 Transition energies, orientation factors (Kf), fluorescence anisotropy values (rf), angles between the moments of the absorbing and emitting transitions [a (⬚)] and the angles between the transition moments and the effective orientation axis [f f (⬚)] observed for 1 兩f f兩
兩a 兩 d
f fe
Electronic transition a
Energy (10 3 cm ⫺1)
I
28.2 29.4
0.50 ^ 0.05
0.40
26
0
⫹ 26 ^ 10
II
30.8 31.5 32.3
0.54 ^ 0.02
0.26
5
29
⫺ 5 ^ 10
III
38.7
0.52 ^ 0.02
0.22
18
33
⫺ 18 ^ 10
IV
42.9
0.50 ^ 0.05
0.29
26
25
⫹ 26 ^ 10
V
48.8
t 53
55
⫹ 53 ^ 20
Kf b
rf c
t 0.40
t0
a
Measured in n-propanol. Measured in PVA. c Accuracy: ^ 0.02. d Accuracy: ^ 5⬚. e Estimated direction of the effective orientation axis: ⫹ 5⬚ from the a-axis; positive sign for counterclockwise rotation (see Scheme 1). b
determine the absolute polarizations, it is sufficient to do it just for one of the molecules under study. Obviously, this should be a molecule for which the LD data leads to the most accurate estimation of the principal orientation factors. While such estimation would hardly be possible from the data for 1 and 2, the molecule 4, for which the orientation was
enhanced by the carboethoxy substituents, provides the essential information. It reveals the highest K values and the largest spread of the observed orientation factors. By definition, the true value of Kz must be either larger or equal to the largest observed Kf. This yields Kz ⱖ 0:70; which implies Ky ⱖ 0:15 and Kx ⱕ 0:15. The x-axis most probably corresponds to
Table 2 The results of measurements for 2. See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf
rf
兩ff 兩
兩a 兩
I
27.2 28.3
0.42 ^ 0.02 b 0.43 ^ 0.03 c
0.36
44
15
⫹ 44 ^ 10
II
30.5 31.2 32.0
0.52 ^ 0.02 b 0.55 ^ 0.03 c
0.22
12
33
⫹ 12 ^ 10
III
38.0
0.51 ^ 0.01 b 0.55 ^ 0.05 c
0.21
17
34
⫹ 17 ^ 10
IV
41.7
0.40 ^ 0.05 b 0.43 ^ 0.05 c
0.25
49
30
⫹ 49 ^ 10
V
48.7
0.40 ⫺ 0.45 b
0.11
38–51
44
⫹ (38 ⫺ 51) ^ 20
Estimated direction of the effective orientation axis: ⫺ 13⬚ from the a-axis. In polyethylene. c In PVA. a
b
ff a
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151 Table 3 The results of measurements for 3. See caption to Table 1 for details Electronic transition
Energy [103 cm ⫺1]
rf
兩a 兩
I
23.9 24.8
0.38
11
II
28.2 29.3
0.26
29
III
35.7
0.26
29
IV
40.7
V
44.1 ⬎ 50.0
VI
the out-of-plane direction, while the y-axis lies in the molecular plane. As 4 is well approximated by a rodlike shape, it can be assumed that x- and y-axes are oriented to a similar degree. The starting values for Kz, Ky and Kx are thus 0.70, 0.15 and 0.15, respectively. Using these in Eq. (3), one obtains the following values for the angles between the effective orientation axis and the transition moment directions: 43⬚ (transition I), 0⬚ (transition II), 11⬚ (transition III) and 47⬚ (transition IV). These may now be compared with the results of anisotropy measurements which yield the same values for a , about 30⬚ for the transitions II and III. From the initially assumed set of Kz, Ky and Kx, we get a 43⬚ for transition II and a 32⬚ for
147
transition III, respectively. The agreement between the LD and anisotropy results becomes much better if one allows for a slightly larger Kz and, a correspondingly smaller Ky. For Kz 0:72, Ky 0:14 one obtains a 33⬚ for transition II and a 29⬚ for transition III, which is in excellent agreement with the LD results. Further increase of Kz decreases the agreement, as the a values calculated from the orientation factors become too small. In conclusion, the values of Kz 0:72 ^ 0:02, Ky 0:14 ^ 0:02 are most compatible with the results of fluorescence anisotropy measurements. It should be noted that the aforementioned procedure should be applied only to those transitions which do not significantly overlap with other ones, which could possibly have different polarizations. Therefore, we used it only for the two strongest transitions, II and III and not, e.g. for the transition IV, for which the anisotropy is most probably lowered because of spectral overlap with the transition III. It should be stressed that both anisotropy and LD measurements yield only the absolute values of the angles, but not their signs. However, the a values obtained independently from LD and anisotropy measurements are compatible with each other only if the transitions I, II and III lie on the same side of the effective orientation axis. The other alternative would lead to a much too high value for a , extracted from the LD parameters. This is an important result, as it allows the assignment of the absolute orientation
Table 4 The results of measurements for 4. See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf a
rf
兩ff 兩
I
23.5 24.3
0.44 ^ 0.05
0.39
44
7
⫹ 44 ^ 10
II
28.1 29.4
0.70 ^ 0.02
0.27
11
28
⫹ 11 ^ 10
III
35.2
0.68 ^ 0.02
0.25
15
30
⫹ 15 ^ 10
IV
39.9
0.41 ^ 0.03
0.30
47
24
⫹ 47 ^ 10
V
44.1
0.4 ^ 0.1
0.22
48
23
⫹ 48 ^ 20
VI a b
48.9
–
In polyethylene. Estimated direction of the effective orientation axis: ⫺ 10⬚ from the a-axis.
兩a 兩
ff b
148
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Table 5 The results of measurements for 5 See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf a
rf
兩ff 兩
兩a 兩
ff b
I
23.2 24.3
0.44 ^ 0.02
0.40
44
0
⫹ 44 ^ 10
II
28.0 29.2
0.54 ^ 0.02
0.24
12
31
⫹ 12 ^ 10
III
35.1
0.52 ^ 0.02
0.24
18
31
⫹ 18 ^ 10
IV
39.6
0.5 ^ 0.1
0.30
28
24
⫹ 28 ^ 20
V
44.1
0.50 ^ 0.05
0.22
28
33
⫹ 28 ^ 20
VI a b
t 50.0 In polyethylene. Estimated direction of the effective orientation axis: as in 2.
of the transition moments. In order to do that, at least one transition moment direction has to be ‘anchored’ in the molecular frame. For this, we use the data obtained from calculations (Table 6). The theoretical predictions for each of the first three calculated allowed transitions lead to the same result (except for the second calculated transition in 1). The final assignment of the location of the transition moment directions corresponding to the four lowest observed singlet states in the indolo[3,2b]indole chromophore is given in Fig. 5, and the comparison with theory is presented in Fig. 5 and Table 6. All the low-lying calculated transition with non-negligible intensity are of pp * character. The predicted energies are well within the accuracy of the INDO/S method, 2000–3000 cm ⫺1. The transition moment directions are reproduced by calculations with errors of about 20⬚ for transitions I and IV, 30⬚–40⬚ for transitions II and III. Some part of this discrepancy may be because of our choice of the effective orientation axis, which is probably accurate to about 10⬚. Indeed, the calculated values of the relative angles a are in better agreement with experiment (for instance, for transitions corresponding to the observed transition III, the calculated values are 45⬚, 42⬚ and 44⬚ for 1, 2 and 3, respectively, which is to be compared with the experimental values of about 30⬚). The only large difference between the experiment and calculations is obtained for the calculated moment for the second transition in 1. An interesting observation
is that the calculations predict the wrong ratio of the intensities of the first and second electronic transitions in all the compounds. Without knowing the transition moment directions, one could suspect that the calculated positions of these two states can be reversed. However, this is not the case as is shown by comparison of the experimental and calculated transition moment directions, and also by theoretical prediction that on carboethoxy substitution, the lowest excited singlet state should be stabilized much more than the second one. This is indeed observed (cf. Figs. 1 and 2). In order to find the source of the discrepancy in the calculated and observed intensity ratio between S1 ← S0 and S2 ← S0 transitions, calculations were repeated for different molecular geometries, various CI sizes, and also by using different methods, PPP and PM3. None of these modifications altered the original results. The only change towards the correct values was obtained when several low-lying doubly excited configurations were included in the CI procedure. For instance, standard INDO/S calculation for 1 which only included singly excited configurations gave the oscillator strengths of 0.79 and 0.02 for the first and second transition, respectively (see Table 6). Inclusion of some doubly excited configurations changed these values to 0.21 and 0.16, respectively. Since the results presented in this work are based on the assumption of the same polarization of several electronic transitions in different compounds, it is important to check the validity of this conjecture in
⫺ 67 ⫹ 32 42.90 (0.12) 44.41 (0.89) 12 14 b
a
0.50 (2–2) ⫺ 0.49 (2–5) ⫹ 41 46.22 (0.80) 12
Oscillator strengths in parentheses. With respect to the a-axis, see Scheme 1; positive values for counterclockwise rotation. c HOMO 1, sHOMO 2, LUMO ⫺ 1, sLUMO ⫺ 2, etc.
0.58 (2–5) ⫹ 28 45.83 (0.60)
⫹ 39 41.54 (0.38) ⫹ 27 42.57 (0.31) 9
9
⫺ 24 37.90 (0.75) ⫺ 31 38.88 (0.65) 5
5
0.96 (1–1) 0.70 (1–2) ⫺ 0.60 (2–1) 0.64 (2–1) ⫹ 0.62 (1–2) 0.66 (2–2) ⫹ 14 ⫹ 40 30.43 (0.79) 31.20 (0.02) 1 2
12
9
42.12 (0.18)
⫺4
⫺ 0.55 (2–1) ⫹ 0.68 (1–2) 0.47 (1–5) ⫹ 0.43 (2–3) 0.69 (1–5) 0.67 (2–3) ⫺ 37 38.49 (0.68) 7
0.94 (1–1) 0.70 (2–1) ⫹7 ⫺ 47 27.83 (1.02) 30.56 (0.05)
0.96 (1–1) 0.65 (1–2) ⫺ 0.66 (2–1) 0.66 (2–1) ⫹ 0.63 (1–2) 0.69 (2–2) ⫹ 18 ⫺ 29 1 2
30.40 (0.72) 30.81 (0.02)
Leading config. 2 f (⬚) E (10 3 cm ⫺1) Leading config. c 1 f b (⬚) E (10 3 cm ⫺1) a
Table 6 INDO/S calculated S0 –Sn transitions for 1, 2, and 3. Only transitions with oscillator strength larger than 0.02 are given
1 4
E (10 3 cm ⫺1)
3 f (⬚)
Leading config.
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
149
several independent ways. One of them is provided by the same values of anisotropies in all molecules, as already discussed. The other is the transferability of the results obtained from the experimental data for 4 to other molecules. For 2, the direction of the effective orientation axis is predicted to differ from that in 4 by only 3⬚. Thus, not only the same values of a , but also of f as in 4 are to be expected, even though the orientation factors are completely different. One may use the values of Kf observed for 2 and the values of f observed for 4, to determine the principal orientation factors in 2 from Eq. (3). The values of Kz 0:53, Ky 0:30 are obtained for 2, which lead to a I; II 32⬚ ^ 10⬚, a I; III 27⬚ ^ 10⬚, which is in excellent agreement with the values obtained from anisotropy measurements. Even more convincing are the LD results obtained for 2 in a different anisotropic matrix, stretched PVA. The orientation of 2 in PVA is better than in polyethylene, which is revealed by larger values of the K factors corresponding to ‘long-axis’ polarized transitions II and III (Table 2). Assuming, in this case, Kz 0:57, Ky 0:30 leads to the values of 46⬚, 16⬚, 16⬚ and 46⬚, for the angles (of the first four observed transitions in 2. These values are not only practically the same as that observed for the same compound in polyethylene, but also the same as that obtained for a different compound (4) in a different matrix. This result proves that the transition moment directions remain the same in the two molecules. It also implies that the direction of the effective orientation axis in 2 is the same in two different
Fig. 4. Absorption (293 K) and fluorescence (77 K) of 2 in EPA (top). Anisotropy of fluorescence excitation. The emission was monitored at 24 400 cm 1 (bottom).
150
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Fig. 5. Assignment of the location of the transition moment directions in the indolo[3,2-b]indole chromophore (top) and comparison with the results of INDO/S calculations (bottom).
polymers. Therefore, we can safely compare the LD results obtained for 1 in a PVA matrix, with the data for other molecules, obtained from measurements in polyethylene. It should also be noted that the Kz, Ky values obtained in polyethylene for 2 and 4 are very similar to those observed in the same medium for pyrene and diphenyldiacetylene, respectively [1]. In the latter two molecules, the ratios of the long to the short in-plane dimensions closely resemble those found in 2 and 4, providing yet another argument for the close relationship between the molecular shape and alignment. The Kf values obtained for 5 require some comment. They are considerably smaller than their counterparts in 4 and only slightly larger than those of 2. Evidently, addition of two n-hexyl substituents to a rod-like, well-orienting molecule 4 decreases the alignment. Moreover, one cannot exclude the possibility of the existence of various conformers, each with different orientational properties. Various conformers are also possible with respect to the position of the
carboethoxy groups in 3–5. However, in this case differences in the estimated directions of the effective orientation axes are of the order of at most 10⬚.
4. Summary and conclusions The approach presented in this work allowed us to determine the directions of transition moments in molecules, for which the standard LD measurements could not provide sufficient information. However, this task was made possible after the introduction of long substituents which significantly enhanced the alignment without changing the transition moment directions. The procedure is quite general and can be extended to many other not-well-orienting molecules. The ideal ‘orientation amplifier’ should interact as little as possible with the investigated system. From this point of view, long alkyl chains seem to be a good choice (we have recently used n-propyl substituents to induce the alignment of porphycene in polyethylene
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
[22]). However, their flexibility can lead to the presence of various conformers which orient to a different degree. Therefore, more rigid substituents, such as the carboethoxy chains used in this work seem better suited for the purpose, as long as they do not modify the electronic structure of the investigated chromophore. Acknowledgements We are very grateful to Graz˙yna Orzanowska for the technical help. Part of the measurements was made possible because of the donation from the Foundation for Polish Science (‘Fastkin’ program). References [1] J. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light, VCH Publishers, New York, 1995. [2] E.W. Thulstrup, J. Michl, Elementary Polarization Spectroscopy, VCH Publishers, New York, 1989. [3] B. Norde´n, M. Kubista, T. Kurucsev, Quart. Rev. Biophys. 25 (1992) 51. [4] J. Waluk, E.W. Thulstrup, Chem. Phys. Lett. 135 (1987) 515.
151
[5] J. Waluk, A. Mordzin´ski, J. Spanget-Larsen, E.W. Thulstrup, Chem. Phys. 116 (1987) 411. [6] J. Waluk, A. Mordzin´ski, J. Spanget-Larsen, E.W. Thulstrup, Chem. Phys. 124 (1988) 103. [7] J. Spanget-Larsen, J. Waluk, E.W. Thulstrup, J. Phys. Chem. 94 (1990) 1800. [8] J. Spanget-Larsen, J. Waluk, S. Eriksson, E.W. Thulstrup, J. Am. Chem. Soc. 114 (1992) 1942. [9] J. Waluk, E.W. Thulstrup, Spectrochim. Acta 44A (1988) 1335. [10] J. Marczyk, J. Waluk, J.C. Fetzer, Acta Phys. Polonica A88 (1995) 295. [11] D.H. Waldeck, Chem. Rev. 91 (1991) 415. [12] H. Go¨rner, H.J. Kuhn, Adv. Photochem. 19 (1995). [13] S. Dobrin, P. Kaszynski, S. Ikeda, J. Waluk, Chem. Phys. 216 (1997) 179. [14] S. Dobrin, P. Kaszynski, J. Waluk, J. Photochem. Photobiol. A105 (1997) 149. [15] S. Dobrin, A. Starukhin, P. Kaszynski, J. Waluk, Opt. Spectr. 83 (1997) 669. [16] P. Kaszynski, D.A. Dougherty, J. Org. Chem. 58 (1993) 5209. [17] S. Hu¨nig, H.-C. Steinmetzer, Liebigs, Ann. Chem. (1976) 1090. [18] J. Jasny, J. Luminescence 17 (1978) 149. [19] J. Jasny, J. Waluk, Rev. Sci. Instr. (in press). [20] J.E. Ridley, M.Z. Zerner, Theor. Chim. Acta 32 (1973) 111. [21] PCMODEL, Serena Software, Bloomington, IN. [22] K.B. Anderson, E. Vogel, J. Waluk, Chem. Phys. Lett. 271 (1997) 341.
Electronic transition moment directions in indoloindoles: the use of orientation amplifiers Michal⁄ Gil a, Jan Marczyk a, Sergey Dobrin a,1, Piotr Kaszynski b, Jacek Waluk a,* a
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland b Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA Received 6 April 1998; accepted 14 May 1998
Abstract Indolo[3,2-b]indole and its four derivatives are studied by linear dichroism in stretched polymeric sheets, fluorescence anisotropy in glassy solutions, and by quantum chemical calculations. Poor alignment of the parent molecule in the stretched polymers does not allow the determination of the transition moment directions by standard procedures. However, this is made possible if long inert substituents are introduced into the molecule. Their role is to increase the alignment without modifying the electronic structure of the chromophore. For a well-oriented chromophore, principal orientation factors and polarizations of transitions may be obtained. The results show that for the four lowest observed excited singlet states of indolo[3,2-b]indole, all the pp * character, substitution by carboethoxy or alkyl groups does not alter the directions of the transition moments. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Linear dichroism; Fluorescence anisotropy; Indoloindoles
1. Introduction Determination of the directions of electronic transition moments is important for both spectroscopic and structural studies. First, it is often crucial for the assignment and characterization of electronic states. Moreover, it provides a possibility to obtain the degree of orientation of a particular chromophore in an aligned sample. The latter may be important in many different research areas, such as studies of intercalation into DNA, alignment in Langmuir–Blodgett films, surface adsorption, etc. [1]. * Corresponding author. Tel.: ⫹ 48 22 632 7269; Fax: ⫹ 48 39 120238; e-mail: [email protected] 1 On leave from N.N. Semenov Institute of Chemical Physics, Academy of Sciences of Russia, Kosygin Str.4, 117334 Moscow, Russia.
Unfortunately, the exact determination of the transition moments is straightforward only in cases in which their directions are dictated by sufficiently high molecular symmetry. For such symmetry species, C2v, D2h or D2, the transition moment can only lie in one of the three molecular directions which coincide with the symmetry axes. A well-established method used in such cases involves linear dichroism (LD) measurements on chromophores partially aligned in anisotropic media, such as stretched polymers or liquid crystals [1–3]. The results provide the average orientation of each of the molecular axes, usually expressed in terms of the socalled ‘orientation factors’ or related quantities. Ample experimental evidence showing that the alignment is related to the molecular shape was accumulated. Thus, the longest molecular axis orients the best, and the smallest, the worst. This observation,
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00500-6
142
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
combined with the experimental values of the orientation factors, provides an easy way to assign the absolute polarizations of transitions. For less symmetrical molecules, the situation is more complicated. Even if some symmetry elements remain, the number of possible transition moment directions may be infinite. Such is the case of structures which possess a plane of symmetry (Cs or C2h species). The transition moment can now be polarized perpendicular to the symmetry plane or assume any direction in the plane. Even for such cases, a fairly accurate estimation of the transition moment directions was shown to be feasible, provided the principal orientation factors — those that correspond to the best and worst aligned directions — can be evaluated. However, additional pieces of information are now required. A procedure has now been worked out, which combines the results of LD studies with measurements of fluorescence anisotropy and quantum chemical calculations. We have successfully used it for the determination of transition moment directions in benz[a]anthracene [4,5]
and its azaderivatives [6], chrysene [7], benz[a]pyrene [8], indoloquinoxalines [9], dibenzo[ j,lm]phenanthro[5,4,3-abcd ]perylene [10]. In this work, we use the basics of this technique, and expand it by studying molecules which contain what we call ‘orientation amplifiers’ —elongated substitutents which strongly enhance molecular alignment. The idea is to compare several molecules having essentially the same chromophore unit, but exhibiting various degrees of orientation. The large orientation factors obtained for well-orienting systems enable the determination of transition moment directions in the parent, unsubstituted compounds, for which the information obtained from LD is insufficient. The compounds we study (Scheme 1) are based on the indolo[3,2-b]indole chromophore (1). This molecule is a rigid analogue of trans-stilbene, whose photophysics is a textbook example of photoisomerization involving large amplitude molecular motion [11,12]. Recently, we studied the photophysical behavior of the systems presented in this work as well as
Scheme 1. 5,10-dihydroindolo[3,2-b]indole(1); 5,10-dihydro-N,N 0 -dimethylindolo-[3,2-b]indole (2); 5,10-dihydroindolo[3,2-b]indole-2,7dicarboxylate (3); diethyl 5,10-dihydro-N,N 0 -dimethylindolo[3,2-b]-indole-2,7-dicarboxylate (4); and diethyl 5,10-dihydro-N,N 0 -dihexylindolo-[3,2-b]indole-2,7-dicarboxylate (5).
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
143
Fig. 1. Absorption spectra of 1 and 2 in n-propanol at 293 K. Roman numbers mark the bands assigned to different electronic transitions. Estimated oscillator strengths are given in parentheses.
their analogues containing sulfur or selenium in place of one or both nitrogen atoms [13–15]. The present work will be followed by the series in which the influence of sulfur and selenium heterosubstitution upon transition moment directions will be studied.
2. Experimental and computational procedures The synthesis and purification of 1–5 were described elsewhere [16,17]. Absorption and LD spectra were recorded on a Shimadzu UV 3100 spectrophotometer, equipped, for LD measurements, with rotatable Glan prisms in both the sample and reference beams. Fluorescence and fluorescence anisotropy measurements were made in ethyl ether-isopentaneethanol 5:5:2 (EPA) or n-propanol glasses with an old [18] and new [19] type of Jasny spectrofluorimeter or with an Edinburgh Instruments FS900 steady state spectrometer. All solvents were of spectral grade. For LD measurements, the substances were introduced into polyethylene sheets from chloroform solutions. The sheets were then stretched about 400%. Some experiments were also done with stretched poly(vinyl alcohol) (PVA) as the orienting medium. In this case, the samples were produced from a stock
solution of PVA in water (10% v/v) to which a methanol solution of a given chromophore was added. The stretching was performed at elevated temperatures, using a hair dryer or water bath as a heating source. The INDO/S method [20] was used for the calculation of excited state energies, oscillator strengths and transition moment directions. Two hundred lowestenergy singly-excited configurations were taken into account in the CI procedure. In some cases, several doubly-excited configurations were also included. The input geometry was obtained by molecular mechanics, using the MMX force field [21].
3. Results and discussion Absorption spectra in isotropic solutions are presented in Figs. 1 and 2. At least five electronic transitions can be distinguished below 50 000 cm ⫺1 in the spectra of 1 and 2; in 3–5, the presence of six transitions is detected. The spectra are very similar. Substitution by carboethoxy ‘tails’ causes only minor changes: a red shift of the first absorption band and an increase of the intensity of the second transition. It is clear from the inspection of the spectra that the
144
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Fig. 2. Absorption spectra of 3, 4 and 5 in n-propanol at 293 K. See caption to Fig. 1.
identity of at least four lowest observed electronic transitions is preserved in all the molecules. Fig. 3 shows the LD curves recorded for 2 in a stretched polyethylene sheet. Two curves were obtained: EZ, measured with the electric vector of light parallel to the stretching direction and EY, obtained with the electric vector perpendicular to the stretching direction. The quantity which can be extracted from an LD spectrum for each particular electronic transition f, is the orientation factor, defined as Kf 具cos2 Z; M0f 典;
1
where M0f is the transition moment and Z the unique direction of the sample: in the case of polymer sheets, it coincides with the stretching direction. The value of the orientation factor, Kf, shows how well the transition f orients, on an average, along the unique direction of the sample: the larger the value, the better the orientation. The number of possible values of the orientation factors for a particular molecule is dictated by symmetry. For instance, in molecules of C2v or D2h symmetry, only three values are possible, as the
transition moments are restricted to lie along the three symmetry axes. Experimental evidence shows that the orientation is strongly correlated with molecular shape [1]. The molecular axis labeled z is defined as the one that orients the best. It usually corresponds with the longest axis in the molecule. The x-axis is defined as the one that orients the worst, usually it coincides with the shortest molecular axis. The y-axis is orthogonal to x and z. In molecules of lower symmetry, the set of principal x-, y- and z-axes is defined in the same way, but the location of the axes in the molecular frame is unknown. A common assumption is to approximate z, the ‘effective orientation axis’, as the direction perpendicular to the smallest cross-section of the molecule. The x-axis corresponds to that direction which among all directions orthogonal to z is aligned the worst. The important relation between the orientation factors Kx, Ky and Kz shows that only two of them are independent, as Kx ⫹ Ky ⫹ Kz 1:
2
Once the values of the principal orientation factors are established, the absolute value of the angle f f
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
145
Fig. 3. LD spectra measured for 2 in polyethylene at 293 K:Ez, solid lines; and Ey, dashed lines (top). Illustration of the stepwise reduction procedure (bottom). The values of K were varied from 0 to 1 in steps of 0.1.
between the transition moment M0f and the z-axis can be determined using the equation: tan2 M0f ; z tan2 ff Kz ⫺ Kf = Kf ⫺ Ky :
3
In order to obtain the numerical values of Kf from the spectra which contain broad, overlapping bands, a stepwise reduction (TEM) procedure is very useful [1,2]. At a given wavelength, the contribution from a transition f will disappear from the linear combination: 1 1 ⫺ Kf EZ l ⫺ Kf EY l: 2
4
In the TEM method, a series of curves corresponding to Eq. (4) is plotted by varying the values of Kf. The real value of Kf is obtained as the one that makes spectral features (peaks or shoulders) belonging to the transition f disappear. An example is presented in Fig. 3. The results of the LD and fluorescence anisotropy measurements are shown in Tables 1–5. Very different orientation factors are obtained. This is not surprising, as the molecular shape is also different. On the one hand, 2, and to a certain extent also 1, are disclike chromophores. On the other hand, 3 and 4 may be
considered rod-like structures in which large values of the orientation factors are expected for those transition moments that form small angles with the orientation axis. Indeed, the second and third observed electronic transitions reveal much larger values of the orientation factors in 4 than the other molecules (unfortunately, 3 could not be introduced into a polymer and hence was not measured). It should be stressed that such a comparison of orientation factors for different molecules is only valid if the polarizations of transitions remain the same for each chromophore. The correctness of such assumption is proved by the values of r, the anisotropy of fluorescence excitation, presented in Tables 1–5 and illustrated in Fig. 4. The angle (between the transition moment directions corresponding to the absorbing and emitting states is given by:
a cos⫺1 5r ⫹ 1=31=2 :
5
The shape of the anisotropy curves follows the same pattern in all the molecules. It follows from Tables 1–5 that the angles a are the same, up to within a few degrees, at least in the region of the first four electronic transitions. Thus, in order to
146
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Table 1 Transition energies, orientation factors (Kf), fluorescence anisotropy values (rf), angles between the moments of the absorbing and emitting transitions [a (⬚)] and the angles between the transition moments and the effective orientation axis [f f (⬚)] observed for 1 兩f f兩
兩a 兩 d
f fe
Electronic transition a
Energy (10 3 cm ⫺1)
I
28.2 29.4
0.50 ^ 0.05
0.40
26
0
⫹ 26 ^ 10
II
30.8 31.5 32.3
0.54 ^ 0.02
0.26
5
29
⫺ 5 ^ 10
III
38.7
0.52 ^ 0.02
0.22
18
33
⫺ 18 ^ 10
IV
42.9
0.50 ^ 0.05
0.29
26
25
⫹ 26 ^ 10
V
48.8
t 53
55
⫹ 53 ^ 20
Kf b
rf c
t 0.40
t0
a
Measured in n-propanol. Measured in PVA. c Accuracy: ^ 0.02. d Accuracy: ^ 5⬚. e Estimated direction of the effective orientation axis: ⫹ 5⬚ from the a-axis; positive sign for counterclockwise rotation (see Scheme 1). b
determine the absolute polarizations, it is sufficient to do it just for one of the molecules under study. Obviously, this should be a molecule for which the LD data leads to the most accurate estimation of the principal orientation factors. While such estimation would hardly be possible from the data for 1 and 2, the molecule 4, for which the orientation was
enhanced by the carboethoxy substituents, provides the essential information. It reveals the highest K values and the largest spread of the observed orientation factors. By definition, the true value of Kz must be either larger or equal to the largest observed Kf. This yields Kz ⱖ 0:70; which implies Ky ⱖ 0:15 and Kx ⱕ 0:15. The x-axis most probably corresponds to
Table 2 The results of measurements for 2. See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf
rf
兩ff 兩
兩a 兩
I
27.2 28.3
0.42 ^ 0.02 b 0.43 ^ 0.03 c
0.36
44
15
⫹ 44 ^ 10
II
30.5 31.2 32.0
0.52 ^ 0.02 b 0.55 ^ 0.03 c
0.22
12
33
⫹ 12 ^ 10
III
38.0
0.51 ^ 0.01 b 0.55 ^ 0.05 c
0.21
17
34
⫹ 17 ^ 10
IV
41.7
0.40 ^ 0.05 b 0.43 ^ 0.05 c
0.25
49
30
⫹ 49 ^ 10
V
48.7
0.40 ⫺ 0.45 b
0.11
38–51
44
⫹ (38 ⫺ 51) ^ 20
Estimated direction of the effective orientation axis: ⫺ 13⬚ from the a-axis. In polyethylene. c In PVA. a
b
ff a
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151 Table 3 The results of measurements for 3. See caption to Table 1 for details Electronic transition
Energy [103 cm ⫺1]
rf
兩a 兩
I
23.9 24.8
0.38
11
II
28.2 29.3
0.26
29
III
35.7
0.26
29
IV
40.7
V
44.1 ⬎ 50.0
VI
the out-of-plane direction, while the y-axis lies in the molecular plane. As 4 is well approximated by a rodlike shape, it can be assumed that x- and y-axes are oriented to a similar degree. The starting values for Kz, Ky and Kx are thus 0.70, 0.15 and 0.15, respectively. Using these in Eq. (3), one obtains the following values for the angles between the effective orientation axis and the transition moment directions: 43⬚ (transition I), 0⬚ (transition II), 11⬚ (transition III) and 47⬚ (transition IV). These may now be compared with the results of anisotropy measurements which yield the same values for a , about 30⬚ for the transitions II and III. From the initially assumed set of Kz, Ky and Kx, we get a 43⬚ for transition II and a 32⬚ for
147
transition III, respectively. The agreement between the LD and anisotropy results becomes much better if one allows for a slightly larger Kz and, a correspondingly smaller Ky. For Kz 0:72, Ky 0:14 one obtains a 33⬚ for transition II and a 29⬚ for transition III, which is in excellent agreement with the LD results. Further increase of Kz decreases the agreement, as the a values calculated from the orientation factors become too small. In conclusion, the values of Kz 0:72 ^ 0:02, Ky 0:14 ^ 0:02 are most compatible with the results of fluorescence anisotropy measurements. It should be noted that the aforementioned procedure should be applied only to those transitions which do not significantly overlap with other ones, which could possibly have different polarizations. Therefore, we used it only for the two strongest transitions, II and III and not, e.g. for the transition IV, for which the anisotropy is most probably lowered because of spectral overlap with the transition III. It should be stressed that both anisotropy and LD measurements yield only the absolute values of the angles, but not their signs. However, the a values obtained independently from LD and anisotropy measurements are compatible with each other only if the transitions I, II and III lie on the same side of the effective orientation axis. The other alternative would lead to a much too high value for a , extracted from the LD parameters. This is an important result, as it allows the assignment of the absolute orientation
Table 4 The results of measurements for 4. See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf a
rf
兩ff 兩
I
23.5 24.3
0.44 ^ 0.05
0.39
44
7
⫹ 44 ^ 10
II
28.1 29.4
0.70 ^ 0.02
0.27
11
28
⫹ 11 ^ 10
III
35.2
0.68 ^ 0.02
0.25
15
30
⫹ 15 ^ 10
IV
39.9
0.41 ^ 0.03
0.30
47
24
⫹ 47 ^ 10
V
44.1
0.4 ^ 0.1
0.22
48
23
⫹ 48 ^ 20
VI a b
48.9
–
In polyethylene. Estimated direction of the effective orientation axis: ⫺ 10⬚ from the a-axis.
兩a 兩
ff b
148
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Table 5 The results of measurements for 5 See caption to Table 1 for details Electronic transition
Energy (10 3 cm ⫺1)
Kf a
rf
兩ff 兩
兩a 兩
ff b
I
23.2 24.3
0.44 ^ 0.02
0.40
44
0
⫹ 44 ^ 10
II
28.0 29.2
0.54 ^ 0.02
0.24
12
31
⫹ 12 ^ 10
III
35.1
0.52 ^ 0.02
0.24
18
31
⫹ 18 ^ 10
IV
39.6
0.5 ^ 0.1
0.30
28
24
⫹ 28 ^ 20
V
44.1
0.50 ^ 0.05
0.22
28
33
⫹ 28 ^ 20
VI a b
t 50.0 In polyethylene. Estimated direction of the effective orientation axis: as in 2.
of the transition moments. In order to do that, at least one transition moment direction has to be ‘anchored’ in the molecular frame. For this, we use the data obtained from calculations (Table 6). The theoretical predictions for each of the first three calculated allowed transitions lead to the same result (except for the second calculated transition in 1). The final assignment of the location of the transition moment directions corresponding to the four lowest observed singlet states in the indolo[3,2b]indole chromophore is given in Fig. 5, and the comparison with theory is presented in Fig. 5 and Table 6. All the low-lying calculated transition with non-negligible intensity are of pp * character. The predicted energies are well within the accuracy of the INDO/S method, 2000–3000 cm ⫺1. The transition moment directions are reproduced by calculations with errors of about 20⬚ for transitions I and IV, 30⬚–40⬚ for transitions II and III. Some part of this discrepancy may be because of our choice of the effective orientation axis, which is probably accurate to about 10⬚. Indeed, the calculated values of the relative angles a are in better agreement with experiment (for instance, for transitions corresponding to the observed transition III, the calculated values are 45⬚, 42⬚ and 44⬚ for 1, 2 and 3, respectively, which is to be compared with the experimental values of about 30⬚). The only large difference between the experiment and calculations is obtained for the calculated moment for the second transition in 1. An interesting observation
is that the calculations predict the wrong ratio of the intensities of the first and second electronic transitions in all the compounds. Without knowing the transition moment directions, one could suspect that the calculated positions of these two states can be reversed. However, this is not the case as is shown by comparison of the experimental and calculated transition moment directions, and also by theoretical prediction that on carboethoxy substitution, the lowest excited singlet state should be stabilized much more than the second one. This is indeed observed (cf. Figs. 1 and 2). In order to find the source of the discrepancy in the calculated and observed intensity ratio between S1 ← S0 and S2 ← S0 transitions, calculations were repeated for different molecular geometries, various CI sizes, and also by using different methods, PPP and PM3. None of these modifications altered the original results. The only change towards the correct values was obtained when several low-lying doubly excited configurations were included in the CI procedure. For instance, standard INDO/S calculation for 1 which only included singly excited configurations gave the oscillator strengths of 0.79 and 0.02 for the first and second transition, respectively (see Table 6). Inclusion of some doubly excited configurations changed these values to 0.21 and 0.16, respectively. Since the results presented in this work are based on the assumption of the same polarization of several electronic transitions in different compounds, it is important to check the validity of this conjecture in
⫺ 67 ⫹ 32 42.90 (0.12) 44.41 (0.89) 12 14 b
a
0.50 (2–2) ⫺ 0.49 (2–5) ⫹ 41 46.22 (0.80) 12
Oscillator strengths in parentheses. With respect to the a-axis, see Scheme 1; positive values for counterclockwise rotation. c HOMO 1, sHOMO 2, LUMO ⫺ 1, sLUMO ⫺ 2, etc.
0.58 (2–5) ⫹ 28 45.83 (0.60)
⫹ 39 41.54 (0.38) ⫹ 27 42.57 (0.31) 9
9
⫺ 24 37.90 (0.75) ⫺ 31 38.88 (0.65) 5
5
0.96 (1–1) 0.70 (1–2) ⫺ 0.60 (2–1) 0.64 (2–1) ⫹ 0.62 (1–2) 0.66 (2–2) ⫹ 14 ⫹ 40 30.43 (0.79) 31.20 (0.02) 1 2
12
9
42.12 (0.18)
⫺4
⫺ 0.55 (2–1) ⫹ 0.68 (1–2) 0.47 (1–5) ⫹ 0.43 (2–3) 0.69 (1–5) 0.67 (2–3) ⫺ 37 38.49 (0.68) 7
0.94 (1–1) 0.70 (2–1) ⫹7 ⫺ 47 27.83 (1.02) 30.56 (0.05)
0.96 (1–1) 0.65 (1–2) ⫺ 0.66 (2–1) 0.66 (2–1) ⫹ 0.63 (1–2) 0.69 (2–2) ⫹ 18 ⫺ 29 1 2
30.40 (0.72) 30.81 (0.02)
Leading config. 2 f (⬚) E (10 3 cm ⫺1) Leading config. c 1 f b (⬚) E (10 3 cm ⫺1) a
Table 6 INDO/S calculated S0 –Sn transitions for 1, 2, and 3. Only transitions with oscillator strength larger than 0.02 are given
1 4
E (10 3 cm ⫺1)
3 f (⬚)
Leading config.
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
149
several independent ways. One of them is provided by the same values of anisotropies in all molecules, as already discussed. The other is the transferability of the results obtained from the experimental data for 4 to other molecules. For 2, the direction of the effective orientation axis is predicted to differ from that in 4 by only 3⬚. Thus, not only the same values of a , but also of f as in 4 are to be expected, even though the orientation factors are completely different. One may use the values of Kf observed for 2 and the values of f observed for 4, to determine the principal orientation factors in 2 from Eq. (3). The values of Kz 0:53, Ky 0:30 are obtained for 2, which lead to a I; II 32⬚ ^ 10⬚, a I; III 27⬚ ^ 10⬚, which is in excellent agreement with the values obtained from anisotropy measurements. Even more convincing are the LD results obtained for 2 in a different anisotropic matrix, stretched PVA. The orientation of 2 in PVA is better than in polyethylene, which is revealed by larger values of the K factors corresponding to ‘long-axis’ polarized transitions II and III (Table 2). Assuming, in this case, Kz 0:57, Ky 0:30 leads to the values of 46⬚, 16⬚, 16⬚ and 46⬚, for the angles (of the first four observed transitions in 2. These values are not only practically the same as that observed for the same compound in polyethylene, but also the same as that obtained for a different compound (4) in a different matrix. This result proves that the transition moment directions remain the same in the two molecules. It also implies that the direction of the effective orientation axis in 2 is the same in two different
Fig. 4. Absorption (293 K) and fluorescence (77 K) of 2 in EPA (top). Anisotropy of fluorescence excitation. The emission was monitored at 24 400 cm 1 (bottom).
150
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
Fig. 5. Assignment of the location of the transition moment directions in the indolo[3,2-b]indole chromophore (top) and comparison with the results of INDO/S calculations (bottom).
polymers. Therefore, we can safely compare the LD results obtained for 1 in a PVA matrix, with the data for other molecules, obtained from measurements in polyethylene. It should also be noted that the Kz, Ky values obtained in polyethylene for 2 and 4 are very similar to those observed in the same medium for pyrene and diphenyldiacetylene, respectively [1]. In the latter two molecules, the ratios of the long to the short in-plane dimensions closely resemble those found in 2 and 4, providing yet another argument for the close relationship between the molecular shape and alignment. The Kf values obtained for 5 require some comment. They are considerably smaller than their counterparts in 4 and only slightly larger than those of 2. Evidently, addition of two n-hexyl substituents to a rod-like, well-orienting molecule 4 decreases the alignment. Moreover, one cannot exclude the possibility of the existence of various conformers, each with different orientational properties. Various conformers are also possible with respect to the position of the
carboethoxy groups in 3–5. However, in this case differences in the estimated directions of the effective orientation axes are of the order of at most 10⬚.
4. Summary and conclusions The approach presented in this work allowed us to determine the directions of transition moments in molecules, for which the standard LD measurements could not provide sufficient information. However, this task was made possible after the introduction of long substituents which significantly enhanced the alignment without changing the transition moment directions. The procedure is quite general and can be extended to many other not-well-orienting molecules. The ideal ‘orientation amplifier’ should interact as little as possible with the investigated system. From this point of view, long alkyl chains seem to be a good choice (we have recently used n-propyl substituents to induce the alignment of porphycene in polyethylene
M. Gil et al. / Journal of Molecular Structure 475 (1999) 141–151
[22]). However, their flexibility can lead to the presence of various conformers which orient to a different degree. Therefore, more rigid substituents, such as the carboethoxy chains used in this work seem better suited for the purpose, as long as they do not modify the electronic structure of the investigated chromophore. Acknowledgements We are very grateful to Graz˙yna Orzanowska for the technical help. Part of the measurements was made possible because of the donation from the Foundation for Polish Science (‘Fastkin’ program). References [1] J. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light, VCH Publishers, New York, 1995. [2] E.W. Thulstrup, J. Michl, Elementary Polarization Spectroscopy, VCH Publishers, New York, 1989. [3] B. Norde´n, M. Kubista, T. Kurucsev, Quart. Rev. Biophys. 25 (1992) 51. [4] J. Waluk, E.W. Thulstrup, Chem. Phys. Lett. 135 (1987) 515.
151
[5] J. Waluk, A. Mordzin´ski, J. Spanget-Larsen, E.W. Thulstrup, Chem. Phys. 116 (1987) 411. [6] J. Waluk, A. Mordzin´ski, J. Spanget-Larsen, E.W. Thulstrup, Chem. Phys. 124 (1988) 103. [7] J. Spanget-Larsen, J. Waluk, E.W. Thulstrup, J. Phys. Chem. 94 (1990) 1800. [8] J. Spanget-Larsen, J. Waluk, S. Eriksson, E.W. Thulstrup, J. Am. Chem. Soc. 114 (1992) 1942. [9] J. Waluk, E.W. Thulstrup, Spectrochim. Acta 44A (1988) 1335. [10] J. Marczyk, J. Waluk, J.C. Fetzer, Acta Phys. Polonica A88 (1995) 295. [11] D.H. Waldeck, Chem. Rev. 91 (1991) 415. [12] H. Go¨rner, H.J. Kuhn, Adv. Photochem. 19 (1995). [13] S. Dobrin, P. Kaszynski, S. Ikeda, J. Waluk, Chem. Phys. 216 (1997) 179. [14] S. Dobrin, P. Kaszynski, J. Waluk, J. Photochem. Photobiol. A105 (1997) 149. [15] S. Dobrin, A. Starukhin, P. Kaszynski, J. Waluk, Opt. Spectr. 83 (1997) 669. [16] P. Kaszynski, D.A. Dougherty, J. Org. Chem. 58 (1993) 5209. [17] S. Hu¨nig, H.-C. Steinmetzer, Liebigs, Ann. Chem. (1976) 1090. [18] J. Jasny, J. Luminescence 17 (1978) 149. [19] J. Jasny, J. Waluk, Rev. Sci. Instr. (in press). [20] J.E. Ridley, M.Z. Zerner, Theor. Chim. Acta 32 (1973) 111. [21] PCMODEL, Serena Software, Bloomington, IN. [22] K.B. Anderson, E. Vogel, J. Waluk, Chem. Phys. Lett. 271 (1997) 341.