J. Phys. Chem. 1995,99, 17606-17614
17606
Diskotic Multiyne Langmuir-Blodgett Films. 2. Aggregate and Monomer UV-Vis Absorption and Fluorescence Radoslav Ionov* Institute of Applied Physics, Technical University, BG-1156 Sofia, Bulgaria
Angelina Angelova Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., B1. 21, BG-1113 Sofa, Bulgaria Received: August 23, 1994; In Final Form:April 4, 1995@
Absorption and fluorescence spectra of a new multiyne diskotic compound is studied in thin films and solutions. A detailed energy diagram of the monomer absorption transitions is derived. A correlation between the structural organization of the diskotic Langmuir-Blodgett films, characterized in part 1, and their optical properties is found. The significance of the n-n interactions in determining the type of the molecular aggregation is illustrated for the disk-shaped aromatic compounds considered in part 1. A set of rules are deduced on the basis of the electrostatic theory of the n-n interactions to predict the preferred type of diskotic liquid-crystalline phases depending on the molecular peculiarities and arrangement. The effect of the intraand interlayer molecular aggregation, environment and n-electron interactions on the optical properties of the diskotic films is discussed. The preferential orientations of the main transitional dipoles is determined using polarized absorption. The red shift of the J-aggregate bands evidence the nematic arrangement of the disk-like molecules in the monolayers. Anomalous fluorescence with quantum yield of 0.36 is found.
Introduction The optical properties resulting from the structural pe~uliaritiesl-~ of thin organic films are a sensitive tool for a film characterization. The absorption and fluorescence of bulk diskotic mesogenes have been investigated predominantly for columnar liquid-crystalline (LC) phases.I0-l2 In the limited number of studies of diskotic Langmuir-Blodgett (LB) films, attention has been mainly paid to the deposition conditions producing stable LB multilayers and to the structure of the films.I3 Optical properties of diskotic LB layers have been little ~tudied.’~-’~ The optical transitions of the aromatic molecules are determined by the specificity of their n-electron systems. Absorption and fluorescence of thin films containing aromatic molecules depend on the orientation of their transitional dipole moments and therefore on the molecular arrangement. This arrangement is sensitive to the electrostatic interactions between the n-electron systems.’8-22 In the present part 2 of this work, the absorption and fluorescence spectra of the novel disklike amphiphile pentakis((4-pentylpheny1)ethynyl)phenoxyundecanoic acid (compound 4b; Figure 1 of part 123and Figure 5b, inset) are studied in solutions and in LB films. The main characteristics of the monomer optical transitions involving n-electrons are determined. The established aggregation of the disklike molecules in the LB films as a consequence of the n-n interactions provides additional optical evidence for the induction of a new diskotic smectic-nematic phase, D S N ,in the multilayers.
Application of the Theory of the x-IC Interactions to the Diskotic Phases Effect of the z-x Interactions on the Molecular Arrangement. The arrangement and the optical properties of molecular
* Author for correspondence. @
Abstract published in Advance ACS Abstracts, May 15, 1995
aggregates possessing n-electron systems are considerably affected by the n-n interactions. Some aspects of the n-stacking have been explained by the ~ o l v o p h o b i celectron ,~~ donora ~ c e p t o r ,and ~ ~ atomic . ~ ~ chargez7models. The recently developed electrostatic modells has illuminated the physical nature of the n-n interactions. It explains much better the large variety of experimentally observed arrangements of molecules with n-systems. In this model, the n-system consists of a positively charged core s-framework (Figure 4a, inset), sandwiched between the two negatively charged n-electron clouds. The electrostatic and the dispersive van der Waals interactions play the most essential role in the pure n-n interactions.28The induction appears as a second-order term. The electrostatic interactions control the geometry of the n-n interactions, Le., the mutual position of the n-systems. The van der Waals contributiori, proportional to the area of the molecular overlap, dominates the net n-stacking energy. Generally, the “face-toface” stacked geometry is favored by the van der Waals interactions and disfavored by the repulsion of the n-electron clouds of neighboring nonpolar aromatic molecules. If the n-systems are polarized by a heteroatom, the electrostatic dipole-dipole interactions will influence the total energy of the x-n interactions. This energy will be affected by the heteroatoms even when the net molecular dipole moment is zero. The electrostatic model of the n-n interactionsIs has predicted a set of general rules (Al-B6) for the relationship between the arrangement of two aromatic molecules (Figure 4a, inset) and the favorable interactions: (A) For nonpolarized n-systems: (1) the “face-to-face” n-stacked geometry is disfavored by a dominating n-n repulsion; (2) the “edge-to-face’’ (T-shaped) geometry is favored by a dominating n-s attraction; (3) the large “offset” n-stacked geometry is favored by a dominating n-s attraction. (B) For polarized by a heteroatom n-systems: (4)chargecharge interactions dominate the interactions between highly charged n-systems. If the heteroatom is an acceptor, the
0022-3654/95/2099-17606$09.00/0 0 1995 American Chemical Society
Diskotic Multiyne Langmuir-Blodgett Films. 2 n-electron clouds of the two n-deficient systems attract each other in a “face-to-face” geometry, followed by an additional rotation around the disk axis. If the n-systems are polarized by a donor, the repulsion of the n-electron clouds favors an “offset” n-stacking. In a mixed couple of one neutral and one polarized n-system, the attraction is favourable if (5) the polarized n-system appears (i) a n-poor system in a “face-to-face” geometry or (ii) a n-poor “face-on” system or a n-rich “edge-on” system in a T-shaped geometry; (6) the s-framework of the polarized n-system appears (i) a positively charged system in a “face-to-face” geometry or (ii) a positively charged “edge-on” system or a negatively charged “face-on’’ system in a T-shaped geometry. The repulsion is favored if the polarization is reversed. The dipole-dipole interactions of disk-shaped molecules23 and the electron donor-acceptor interactions (yielding chargetransfer complexes)I8 could be considered as a special case of the n-n interactions. Since the intermolecular charge-transfer interactions are dependent on the molecular orbital symmetry and the geometry of the interactions, the electron donoracceptor conception should be applied with care18 in the interpretation of the optical results. The application of this concept should be based on structural investigations of the molecular arrangement which plays a crucial role in the donoracceptor interactions. JT-JT Interactions and Diskotic Liquid-Crystalline Phases. An important inference of the electrostatic model of the n-n interactionsI8 is the essential role of these interactions for the self-organization of the disk-shaped molecules in the diskotic LC. The more stable molecular arrangements are related to attraction between the aromatic species and they result from the tendency toward a minimization of the n-stacking energy. The T-shaped arrangement of the disklike molecules would lead to a formation of an isotropic phase or herringbone structures. The “face-to-face” intermolecular interaction is more interesting for the diskotic phases, and it will be considered more in debt here. The attractive “offset face-to-face” geometry of the n-n interactions would favor the formation of nematic phases. The attractive “face-to-face” geometry without, or with a relatively small, offset would lead to columnar or tilted columnar phases, respectively. The following rules of the electrostatic theory of the n-n interactions can be postulated for a “face-to-face” geometry of disk-shaped molecules: (Cl) The nematic diskotic arrangement is favored for diskshaped molecules without heteroatoms attached to the disk core (an outcome of the general rules A1 and A3). (C2) According to rule B4, the charge-charge attraction between n-poor aromatic cores or the repulsion between n-rich cores may lead to columnar or nematic phases, respectively. The attachment of acceptor atoms to the core favors an attractive “face-to-face” stacking. Therefore, columnar phases will form with a rotation of the molecules around the columnar axes (helicoidal columns). A nematic arrangement is predicted if the aromatic core is n-rich as a result of the attachment of a donor atom to it. (C3) The interaction of the electron cloud of one neutral n-system with that of another n-poor system (rule B5) or with a positively charged atom in the center of the aromatic core (rule B6) favors the columnar arrangement. (C4) The attractive dispersive van der Waals forces between the aromatic cores favour the formation of columnar phases. An additional consequence proceeds from the excluded volume effect:29 (C5) Steric interactions between ideally disk-shaped mol-
J. Phys. Chem., Vol. 99, No. 49, 1995 17607 ecules favor the formation of columnar phases only at a high density of the films. A nematic or an isotropic arrangement is preferred at a low density of the films. When the n-n interactions dominate the intermolecular interactions between the aromatic molecules, they may govem the structural arrangement of the disk-shaped molecules in their crystalline, LC, and isotropic state. However, these interactions are not the only factor affecting the structural arrangement. The geometrical and chemical peculiarities of the molecules, van der Waals interactions between their nonaromatic parts, ionic interactions, the extemal influence (magnetic or electric fields, external pressure in bulk state, or in Langmuir monolayers) may also affect the molecular arrangement. In principal, the arrangement of the molecules in thin films is a result of the balance of all these factors. Examples Based on the Bulk Diskotic Compounds 1-7 (Table 1 in Part l).23 The tendency of the compounds 1-3 toward a nematic arrangement is related to rule C1. The larger the disk-shaped n-system, the stronger the electrostatic repulsion of the n-electron clouds. Hence, a nematic phase is favored. The “starlike” distribution of the delocalized n-electrons or deviations from the disk shape (compound 2) may violate this tendency by involving a rotation around the axis perpendicular to the molecular plane. This leads to a “face-to-face” arrangement with a minimum overlap of the n-electron clouds. attached to The electron-donating alkoxy oxygen the aromatic core of the compounds 4 makes the core a n-rich system. According to rule C2, the nematic arrangement will be favored. This conclusion coincides with the predictions of the model of the dipole-dipole interaction^,^^ and it has the same electrostatic origin (rule B4). Compounds 4a-d exhibit a nematic bulk phase, and compound 4b exhibits a nematic arrangement in the monolayers. The absence of hydrocarbon wings in the compounds 4d-f reduces the van der Waals attraction. The T-shaped geometry, favored by the electrostatic repulsion of the n-electron clouds, contributes for the observation of an isotropic phase only. Owing to ionic interactions and the rule C2, a helicoidal columnar arrangement is favored for compounds 5. Recent application of the excitonic theory, which is based on other considerations, has yielded the same r e ~ u l t . ’ ~ . ’ ~ The decrease of the core radius reduces the electrostatic repulsion and increases the probability for a columnar arrangement. When the ratio of the core radius to the length of the wing groups becomes sufficiently small, even the tendency for an “offset face-to-face” stacking of the small cores is not able to disturb the columnar ordering. The columnar arrangement of the compounds 6 is supported additionally by rules C4 and c5. The n-rich oxygen atoms in the center of the core of compounds 7 favor the nematic arrangement (rule C2). Metal phthalocyaninesI2 or porphyrinsI7 form columnar phases when the metal ion is positioned in the center of the core (rule C3). The consideration of the n-n interactions contributes for a better understanding bf the physical origin of the different types of arrangementI4 of aromatic molecules: crystalline state, herringbone arrangement, H aggregates, brickstone work of J aggregates. The specific arrangement of the aromatic molecules governs the arrangement of their transitional dipole moments and, therefore, the optical peculiarities of their aggregates. The optical transitions involve n-electrons (n-n*and n-n* transitions). This is the physical basis to use the optical properties of the molecules for providing information about the molecular arrangement in aggregates and about the specificity of the n-n electron interaction.
Ionov and Angelova
17608 J. Phys. Chem., Vol. 99, No. 49, I995 a
Ib
parallel
C
$ .
.....
N>2
N=2
N>2
N=l
oblique
gQ9 A4 ra
N=2
Figure 1. Energy diagrams summarizing the effect of the aggregation on the optical transitions. (a) Parallel arrangement of the transitional dipoles. Left: brickstone work arrangement of J aggregate. Right: columnar arrangement of H aggregate. y (in degrees) is the characteristic angle of the aggregate. N is the number of molecules in the aggregate. The small solid arrows in the middle indicate the dimer dipole arrangement ( N = 2). The thick solid arrows are optical transitions (A, absorption; F, fluorescence; P, phosphorescence). The dashed arrows are nonradiative transitions. T denotes the triplet and S, the singlet levels. For the dimer ( N = 2) and the oligomer ( N > 2) singlet excited state, the solid and the dashed lines indicate the energetical levels of a higher and lower transition probability, respectively. (b) Monomer energy diagram ( N = 1). The characteristic shifts of the absorption bands of the J and H aggregates with respect to the monomer (M) one are illustrated, as well as the typical Davidov splitting (D)of nonparallel arranged transitional dipoles. , I is the wavelength. (c) Oblique arrangement of the transitional dipoles which may lead to Davidov splitting (D).
Molecular Aggregates. Usually, only the monomer absorption and fluorescence bands are observed in the spectra of aromatic compounds dissolved in good solvents at concentrations of about 1 x M. The position and the intensity of these bands for nonpolar solvents, without specific solutesolvent interactions, are determined mainly by the specificity of the transitions between the n,n*,and n molecular orbitals. At higher solution concentrations and in solid films, aggregation may take place both in the ground and in the excited
the transitional dipole moments (“offset face-to-face” geometry; Figure la). Another characteristic feature is that the absorption bandwidth of the J aggregate is narrower than the monomer one. The H aggregates show a blue (hypsochromic) shift of the low-energy absorption band and an absence or a low quantum yield of fluorescence (Figure la,b). The relaxation of the excited H aggregate is nonradiative to the triplet state (T) followed by phosphorescence transitions. The optical properties of the H aggregates are a consequence of the columnar (“face-to-face” without or with a small “offset”) arrangement of the transitional dipole moments. (ii) Nonparallel Dipoles. This type of arrangement includes an oblique (Figure IC) or overhead crossed arrangement of the transitional dipoles. The first case corresponds usually to a h e r ~ i n g b o n e ~molecular ’,~~ arrangement, while the second one corresponds to a helicoidal c o l ~ m n a r ’arrangement. ~ The absorption bands exhibit the typical David0v4~splitting (Figure lb) corresponding to the two allowed transitions (Figure IC). The above correlations between the optical properties of the J and H aggregates and their structural properties have been confirmed for a variety of rodlike (e.g., polymetine dyes’$37) and disk-shaped porphyrin^^-^^) molecules. Solvatochrortlic Effect. The solvent polarity may also cause a considerable (blue or red) shift of the monomer bands (solvatochromic effect). The degree, and even‘the direction, of the shift of the aggregate band is relative to the monomer band position. The latter should always be determined in nonpolar solvents. The environmental polarity in aggregates of polar molecules differs from that in nonpolar solvents. Therefore, the solvatochromic effect should be taken into account in the consideration of the aggregates band shift. Different theoretical based on the concept of the point dipole, have been developed for an explanation of the solvatochromic effect. Using the Kawski model!’ the following system of equations, determining the relative frequency position of the absorption V I A and fluorescence Y I F bands with respect to the corresponding Y2A and Y ~ bands F in nonpolar solvents, was obtained:
-4.31 -39
A molecule in an aggregate can interact effectively with ( N
- 1) of its neighbors forming dimers ( N = 2) or oligomers ( N > 2 ) . The position and the intensity of the absorption and the
fluorescence bands of the aggregate depend critically on the structural arrangement of the molecules. Generally, there are two models of the transitional dipole moments arrangement (Figure 1); ( i ) Parallel Dipoles. The main characteristic parameter of the aggregate is the angle y between the line connecting the dipole centers and the direction of the parallel transitional dipoles (Figure la, top middle). The ratio between the angle y and the critical angle40 yo, yo = arc~os(l/(3”~)) = 54.7”, determines the assignment of the aggregates to J (0 y < yo) or to H (n/2 > y > yo) type. This is illustrated on Figure l a by the variation of the excited-state singlet energy of the dimers ( N = 2) with the angle y . The J aggregates show a red (bathochromic) shift of the lowenergy absorption and fluorescence bands (Figure la,b). It is determined by the typical brickstone work31,37arrangement of
+
+ +
Here, the reaction field factors areJ = (2n2 1 ) ( ( ~- 1)/(~ 2 ) - (n2 - l)/(n2 2))/(n2 2 ) and gi = 1.5(n4 - l)/(n2 2)2;n and E are the refractive index and permittivity of the solute environment (polar ( i = 1) or nonpolar ( i = 2 ) solvent); pe and p, are the excited- and ground-state dipole moments of the solute molecule, respectively; p = 2n~ohc= 1.10511 x 10-35C?; r is the van der Waals radius of the solute. The ground and the excited state dipole moments, pg.e,of the molecules can be determined on the basis of the solvatochromicmethod of temary solutions5’ by plotting P versus Q:
+
+
p A s F = Q - pg,:(dp - d,)/a (2) where PA,F= ln(l/AvA,F- l), Q = In(l/x - 1) +,u:ddaP ,un2d,,/ap, and A v ~=%( v~, , ~ ’-~ V ~ , ~ ) / ( Y-, ,vpA’F), ~ ~ ~ dn,p = - l)/(kT(~,,~2)). p,, andpp are the mean dipole moments of the nonpolar (n) and the polar (p) solvents, ai = 4n€or?, ri is the Onsager cavity radius, and x is the molar fraction of the polar solvent in the nonpolar/polar mixtures.
+
J. Phys. Chem., Vol. 99, No. 49, 1995 17609
Diskotic Multiyne Langmuir-Blodgett Films. 2
Experimental Section
E
The experimental details for the preparation of the diskotic LB films of the compound 4b, pentakis((4-pentylpheny1)ethyny1)phenoxyundecanoic acid, are presented in part 1.23 The optical investigation refers to the same diskotic films which were characterized structurally and morphologically by means of X-ray diffraction and optical microscopy (see Table 3 of ref 23). The solvents used for the preparation of the 4b solutions were of spectroscopic grade (products of Merck). Absorption spectra were measured by means of Perkin-Elmer Lambda 19 double-beam spectrometer. Identical bare solid substrates, or quartz cuvettes with the corresponding solvents, were placed in the reference beam. The polarized absorption was recorded at normal incidence and at a grazing incidence angle of 78" (Le.,3!, = 12" in the inset to Figure 5c). The polarization angle 8 (with 8 = 0 for p-polarization) was varied with a step of 1". Fluorescence spectra were recorded using Perkin-Elmer LS 50 spectrometer. An angle of excitation of 60" at the wavelength corresponding to the absorption maximum of each sample was applied.
186.0
a
2206 1572 2222 1191 2221 1677 2222 CeC A r C*C C-0-C C r C Ar C=C
a 0
300
400
500
600
wavelength (nm)
b
Results Figure 2a presents the experimental absorption spectrum of the investigated diskotic compound 4b in a chloroform (1 x M) solution. The background was subtracted. The best fit for the background was obtained using a polinom of a sixth power in the program5*FIT. The spectrum was decomposed into 10 Gaussian bands so that the difference between the experimental and the simulated spectrum reached a minimum. The spectral characteristics of the obtained Gaussian bands are given in Table 1. Figure 2a shows also the energy differences of the vibrational transitions (Table 1) and their assignment to vibrations of specific chemical groups in agreement with the infrared data.53 The optical transitions derived from the absorption (Figure 2a) and fluorescence spectra (Figure 3) of the 4b solution are presented on Figure 2b. Figure 3a shows the absorption and fluorescence spectra of the compound 4b at 1 x M concentration in hexanel chloroform mixtures. The plot of P versus Q (eq 2) is presented as an inset (Figure 3b). The following values for the nonpolar and the polar solvents54were used: (i) in hexane, p,,= 0 D, E,, = 1.89, n, = 1.3754, vnA = 2.993 x lo6 m-l, vnF = 2.262 x lo6 m-l; (i) in chloroform,pu,= 1.1 D, cp = 4.806, np = 1.446, vpA= 2.962 x lo6 m-l, vpF = 2.230 x lo6 m-I, rp = 0.18 nm, T = 22 "C. Figure 4 compares the nonpolarized absorption spectra of the three groups investigated diskotic films (see Table 3 in part l).23 The films of the first group show well-expressed vibrational structure of the absorption spectra similarly to the monomer absorption (Figure 4a). However, the vibrational structure of the spectra is strongly smeared for the other two groups of films. Broadening and reduction of the intensity of the main absorption band was found for the samples of the second group (Figure 4b). All films show red-shifted absorption bands with respect to the monomer absorption band in hexane (Figure 4d). The red shift increases linearly with the decrease of the average transfer ratio of the films. A largest red shift was observed with the alternating multilayer (AM) sample. The polarized absorption spectra of the alternating multilayer (sample Q1 in Table 3 of part 1) are shown on Figure 5 . The main band is strongest and well expressed at normal incidence and at a polarization angle 8 = 0" (i.e., in dipping direction). This allows the spectrum to be decomposed into Gaussian bands
SO Figure 2. (a) Decomposed monomer absorption spectrum of the compound 4b in 1 x M chloroform solution. The background is subtracted. The energy differences (in cm-I) of the transitions 1-10 are assigned to vibrations of specific chemical groups (Ar, aromatic; C E C ; C-0-C). The inset shows the typical energy diagram, E vs r, exhibiting a relative shift of the excited state energy minimum toward larger radii. The main absorption transition is N = 4. (b) Detailed energy diagram of the 4b monomer absorption transitions 1-10 (solid arrows). S, denote the singlet states (i = 0-3) and F the main fluorescence transition. The small numbers denote the vibrational levels, vi,, of the corresponding singlet states, S,. In the presence of spectroscopically sensitive impurities, S? and S3 singlet states should be replaced by SIand S2, respectively (see the text). TABLE 1: Spectral Characteristics of the Gaussian Bands, N = 1-10, of the Decomposed Monomer Absorption Spectrum of the Diskotic Compound 4b (Figure 2aP N
1
2
3
4
5
6
7
8
9
10
A
249.2 263.7 322.3 339.5 367.2 384.0 419.8 453.3 490.6 551.0 D 1488 2138 4649 14133 3347 3161 558 279 1023 186 2W 11.7 18.7 25.4 27.0 19.6 16.0 13.8 12.4 14.6 61.3
a d (nm) is the wavelength position, D (L/Mcm) the opitcal density, and 2W (cm-' the full width at the half-maximum.
of well-defined width and intensity. The same fitting procedure was applied as for the spectrum on Figure 2a. The main absorption band of the decomposed spectrum (Figure 5a) is redshifted with about 2100 cm-l with respect to the band in hexane. A new band at 373 nm was observed at polarization making 8 = 30" with the dipping direction (Figure 5b). Figure 5c shows the polarized absorption spectra of the AM sample in the optical geometry shown on the inset. The band of the less-permitted transition N = 9 is not red-shifted in the aggregated state. This transition is oriented in a completely different direction than the main transition N = 4 (Figure 5b, inset). The band 9 is well expressed in the Ah4 sample only at grazing incidence @
17610 J. Phys. Chem., Vol. 99, No. 49, I995
Ionov and Angelova n
h
-0 N
1.0%
0.3
0.85
0.2
E
x
c
0.6
’i
0.1
u c C
0.4 -
0.0
0 0
0.2
0.6
C
g
?
VI
0.0
400
300
500
600
0
?
W
LL
0 .c
2 -
wavelength (nm)
B
Figure 3. (a) Monomer absorption and fluorescence spectra of the compound 4b in 1 x M solution in chloroform (solid line) and in hexane/chloroform (4/1 molar ratio) mixture (dashed line). (b) Plot of P versus Q,of the eq 2 for the main absorption (A) and fluorescence (F) band positions.
n
E
0.4
2..
....
0.2
4:
0.1
0.2
0.1
0.0
300
400
500
wavelength ( n m )
Figure 5. Polarized absorption spectra of the alternating multilayer (Ql): (a) Decomposed absorption spectrum (with subtracted background). The beam hitting the sample at normal incidence is polarized parallel to the dipping direction (8 = 0). (b) Experimental absorption spectra at normal incidence with different polarization angles 8. The inset shows the probable orientation of the transitional dipole moments 4 and 9 within the 4b molecular framework. (c) Polarized absorption spectra of the AM sample in the geometry shown on the inset @ = 12”;d denotes the dipping direction, and p the p-polarized light). The arrow indicates the transition N = 9. The spectra are normalized to equal areas of illumination.
b 2
0.2 C
.-.0u
a
0.1
ln
n 4
0.2 600 n
?
0.1
0
W
.~
400
Y
fl C
300
400
500
wavelength ( n m )
Figure 4. Absorption spectra of the diskotic films of the first (a), second (b), and third (c) group 4b samples. For the sample notations and characteristic parameters, see Table 3 in part l.23 Additional notations: (a) C denotes the cast film absorption and M the 4b monomer M hexane/chloroform mixture (4/1 molar ratio). absorption in 1 x The inset shows “face-to-face” geometry of two z-systems (1); “faceto-edge’’ (T-shaped) geometry (2) and “offset face-to-face” geometry (3). S denotes the s framework and z the n-electron clouds. (d) Dependence of the absorption wavelength maximum, I , on the average transfer ratio, a,of the LB films. The absorption maxima of the cast film (full circle), alternating multilayer, AM (full square), and 4b monomer in 1 x M hexane solution (open circle) are shown as well.
= 12”). Its maximal intensity is found at polarization making
8 = 9” with the dipping direction.
0
= 200 c
0
400
500
60C
wavelength (nm)
Figure 6. Fluorescence spectra of the monomer (M) (1 x M solution in chloroform) and the LB films of the compound 4b. The sample notations are the same as in Table 3 in part 1.23
The 4b monomer fluorescence, presented on Figures 3 and 6 , shows a considerable Stokes shift of about 7200 cm-I. The vibrational structure of the fluorescence spectra is not resolved. The fluorescence bands of all thin films (Figure 6 and 7) are red-shifted with respect to the monomer one in hexane. The
Diskotic Multiyne Langmuir-Blodgett Films. 2
600 ?
0 v
>r
.-
400
Y
In
c a
= 200 .w
440
460 wavelength (nm)
Figure 7. Histogram of the fluorescence intensity versus the maximum wavelength positions for the samples of the first and third groups. M denotes the monomer fluorescence in 1 x M hexane solution of 4b, C1 the cast film, and Q1 the AM fluorescence.
shift for the sample G20 is 820 cm-I, and for the sample Q1, it is 250 cm-I. The intensities and the fluorescence band positions of the first and third groups samples are well defined (Figure 7). The band positions of the second group samples are distributed in the region 450-480 nm. Their intensities show intermediate values between those of the groups I and 111.
Discussion Monomer Absorption. The absorption spectrum of the diskotic compound 4b on Figure 2 resembles the spectra of the porphyrin^.^^ For example, bands 4 and 9 look like the Soret (B-band) and the Q-band, respectively. However, the frequency position of the absorption band 9 is lower than the fluorescence frequency. The bands 2, 4, and 9 were attributed to the three main electronic transitions (Figure 2b) because of the following features: (i) a large spectral distance between these bands (8500-9000 cm-I); (ii) a higher band intensity in comparison to the neighboring vibrational bands; (iii) a red shift of the bands 3-7 in the spectra of the thin films and a lack of shift of the other bands; (iv) dependence of the bands 4 and 9 of the alternating multilayer on the polarization. The observed absorption spectrum (Figure 2a) shows a welldefined vibrational structure in the low-energy side of the main absorption transition 4. This is typical for band diagrams with a relative shift of the excited state energy minimum toward longer radii (Figure 2a, inset) and appears a consequence of the Franck-Condon p r i n ~ i p l e . ~The ~ . ~ assignment ~ of the vibrations accompanying the electronic transitions (Figure 2a) suggests that the n-electrons in the aromatic core of 4b are predominantly involved in electronic transitions. Therefore, the n-n* transitions will be accompanied primary by the vibrations of the chemical units within the core (aromatic, Ar,and CEC) or directly attached to it (C-0-C). Taking into account the relatively high energy of these vibrations and the Boltzmann statistics, the absorption spectrum reflects the vibrational structure of the excited states. Figure 2b presents a more detailed diagram of the probable electronic transitions responsible for the observed absorption spectrum. The assignment of the bands is confirmed by the validity of the Lambert-Beer’s law in the concentrational region of 1 x lOTS-1 x M 4b in chloroform. According to this diagram, the main transition 4 occurs from the ground singlet state (SO,vm to the fourth vibrational level in the second singlet state (SZ,u23). The vibrational level ~ 2 could 3 be presented as
J. Phys. Chem., Vol. 99, No. 49, 1995 17611 a combination of two vibrations of the CM! group (2200 cm-’) and one of the C-0-C group (1200 cm-I). Therefore, the most probable electronic transition 4 is accompanied by simultaneous vibrations of the Cand C-0-C groups. The absorption spectrum shows that all electronic transitions are accompanied by vibrations of the C- groups which indirectly demonstrates the n-nature of the transitions. The effective maximum of the experimental spectrum (Figure 2a) is at 338 nm and it has molar optical density (extinction), D,of 15 620 L/Mcm. Therefore, its effective cross sections0 of absorption is u = 0.006 nm2.Taking into account that the n-electrons, involved in the optical transitions, occupy the aromatic core of cross sectional area of about 2.1 1 nm2, it can be estimateds0 that the molecule of the compound 4b absorbs about 0.3% of the impinging photons at 338 nm. Monomer Absorption Transition 9. Usually, the fluorescence occurs from the lowest energy vibrational level of the first singlet state due to the fast relaxation processes in the excited state. The observation of lower frequencies of the absorption transitions 8-10, as compared to the main fluorescence transition at 448 nm in chloroform solution (Figures 2, 3, and 6), is n o n t r i ~ i a l . ~The ~ transitions 8-10 could be assigned to triplet or symmetry-forbiddensinglet transitions. The singlet-triplet transitions are spin-forbidden with a spinforbiddness factoP of lo8. The spin-orbit coupling due to “heavy atom” or “oxygen perturbation” effectsss could enhance these transitions by 4-5 orders of magnitude. The symmetry forbiddness factor of the singlet-singlet transitions is about 10lo3 due to the vibrational modification of the symmetry of the pure electronic states. The electronic transition N = 9 is the strongest among the transitions 8-10 (Figure 2a). The transitions 8 and 10 determine its vibrational structure. The forbiddeness factor of the transition 9, estimated from its optical density (Table l), is about 14. From this value, one obtains a unreasonably high enhancement of about lo7 for the singlet-triplet transition. Therefore, the transition 9 should be assigned to a symmetry-forbidden singlet-singlet transition (Figure 2b). The main fluorescence transition S2 SOis probably anomalous similarly to the azulene fluorescen~e.~~ An altemative explanation, which would allow the fluorescence to be considered as normal one, SI SO, is based on an assignment of the bands 8-10 as owing to the presence of small amount of impurity or formation of charge transfer complexes (e.g., with oxygen from the air or protonation of the n-systems by HCl traces in the solvent). However, the presence of band 9 both in strongly diluted solutions of different solvents and in the solid samples, as well as its polarization oriented perpendicular to the solid supports (see below), indicate that chargetransfer complexes probably do not form. The chemical purification6’ of the compound 4b has been thoroughly performed. Nevertheless, the spectroscopic purity could differ from the chemical one. Eventual trace amount of impurity might not influence the structural properties of the diskotic films, but it could be spectroscopically active. In this case, bands 8-10 could be related to impurities. Then, S2 and S3 should be replaced by SIand S2,respectively in Figure 2b. This point needs further clarification. Solvatochromic Contribution to the Optical Transitions in Aggregated State. The molecules in the thin diskotic films are aggregated. The net absorption and fluorescence band shifts are result of both the solvatochromic (environmental) and the aggregate band shifts. The contribution of the solvatochromic shift can be determined if one considers the molecules in the thin 4b films as dissolved in a solvent of the same molecules.
-
-
17612 J. Pkys. Ckem., Vol. 99, No. 49, 1995
From the cutoff of the ordinate axis, P, of Figure 3b, values of lpgl = 27.9 D and Ipel = 28.6 D were obtained on the basis of eq 2 using r = 0.8 x 1.5 = 1.2 nm for an ellipsoid of rev0lution.4~Although a high pgvalue could be expected, taking into account the dimensions of the molecule studied (Figure 1 in part 1)23 and its strongly polarizable aromatic core, the individual values of pg and pe seem to be too high. Due to the uncertainty in the value of the radius r and the limitations of the point dipole model^,^^,^' the solvatochromic method used could produce systematic errors in the determination of the individual values of the dipole moments (keeping their difference Ip, - pgl correct). The small difference between the excited- and ground-state dipole moments determines a little effect of the environment on the band position of the compound 4b. The expected band positions according to eqs 1 and LA = 334.4 nm and LF = 442.7 nm. They were estimated using the mean values56of E = 2.42 and n = 1.56 determined57for a similar diskotic compound 3 (Figure 2, part 1). The contribution of the solvatochromic effect to the red shift of the absorption and fluorescence bands of the thin 4b films with respect to the monomer band position in hexane is only about 28 and 31 cm-I, respectively. Therefore, the considerable red band shift observed with the films of the compound 4b appears of predominantly other origin. Absorption of the Diskotic Langmuir-Blodgett Films. The red shift of the absorption maxima observed for all thin discotic films (Figure 4d) is typical for the brickstone work arrangement of the molecules in J aggregates. This result considerably supports the two dimensional nematic-like in-plane arrangement of the diskliie molecules in the LB films discussed in the structural investigation (see Figure 13 of part l).23 The body-centered crystalline unit cell in the samples of the first group appears an example of the “offset face-to-face” arrangement of the disklike molecules which differs from the “headto-head’’ bilayer arrangement typical for the samples of the third group. Alternating Multilayer. The disk-shaped 4b molecules in the adjacent diskotic monolayers of the altemating multilayer (AM) are of equivalent “edge-on” orientation with respect to the substrate. Their carboxylic groups contact those of the neighboring barium arachidate monolayers deposited on withdrawal. The lamellas are well expressed. It is simpler to study the optical properties of the altemating LB system, than of the single diskotic bilayers, because of the diminished mutual influence of the adjacent diskotic monolayers. The largest red shift observed with the AM (Figure 4d) indicates a smallest angle y of the J aggregate and, therefore, a maximum “offset” arrangement of the molecules within the diskotic monolayers. The transitional dipole moment of the main band 4 has a preferential orientation in a dipping direction, 8 = 0” (Figure 5b). In this direction, the intensity of the main band is highest and the reduced bandwidth of the J aggregate is well expressed (Figure 5a). Therefore, the polarized spectra indicate that the transitional dipole moment 4 is oriented parallel to the substrate and in dipping direction. The number, N , of the molecules associated in a J aggregate can be evaluated5*from the approximate relationship between the full bandwidths at half-maximum of the J aggregate band, 2wJ, and the monomer one in a nonpolar solution, 2Ws:
2WJ2WS= I?-’’2 This relationship yields about three to four molecules ( N = 3.7) with associated transitional dipole moments. The new transition at 373 nm, resolved only in the AM sample, is oriented parallel to the substrate and it makes an angle
Ionov and Angelova
8 of 30” with the dipping direction (Figure 5b). This band could be related to the existence of domains, discussed in the structural in~estigation,~~ with another orientation of the transitional dipole moments. The polarization of the transition 9 (Figure 5c) shows that its transitional dipole is oriented approximately normal to the substrate making an angle of about 9” with the surface normal. Assuming that the transitional dipoles lie in the disk planes, as usual for the disk-shaped molecule^?^ the orientation of the main transitional dipoles in the molecular framework are indicated on the inset to Figure 5b. The established tilt of the 4b molecules is in an excellent agreement with the value of 9” estimated from the X-ray diffraction data.23 The orientation of the main transition 4, parallel to the substrate and in the dipping direction, demonstrates the preferential orientation of the disk planes parallel to the dipping direction during the monolayer transfer on a solid support. The absorption spectrum of the AM sample exhibits vibrational structure only for the main transition 4. The vibrational structures of the electronic transitions 2 and 9 are smeared due to the increased packing density of the diskotic monolayers and intermolecular interaction. Films of the First Group. The structural arrangement of the 4b molecules in the cast film (C) corresponds to an equilibrium state of minimum energy of the intermolecularinteractions. The diminished interactions in the loose-packed body-centered orthorhombic cell23 allow the observation of well-expressed vibrational structure of the absorption spectrum (Figure 4a). The smaller red shift of the absorption band of the cast film in comparison to that of the AM (Figure 4d) is related to the different structure arrangement of the 4b molecules. This shift probably results from a higher angle, y , between the transitional dipoles in the crystalline cast film. The absorption spectra of the dendritic films, deposited at low transfer ratios, demonstrate that the body-centered cell, evidenced by means of X-ray d i f f r a ~ t i o nis, ~distorted ~ to some extent. The position of the main band is intermediate between those of the cast film and the AM (Figure 4d). A vibrational structure only of the main band is found (Figure 4a). However, the relatively low compression during the monolayer transfer and the resulting low transfer coefficients do not cause a complete distortion of the crystalline cell. Films of the Second Group. The absorption spectra of the films prepared at intermediate transfer ratios (Figure 4b) demonstrate a considerably smeared vibrational structure. This behavior can be well explained considering the increased packing density of the layers and the resulting enhanced intermolecular interactions. The distortion of the crystalline structure and the instability of the new structural arrangement is related to the fluctuations of the intermolecular distances and the angles between the transitional dipole moments. This leads to fluctuations of the frequency positions of the main absorption band and therefore to its broadening and reduced intensity. Films of the Third Group. The induced new diskotic smectic nematic “DSNsolid” structure arrangement of the 4b molecules is better expressed at higher transfer ratios of the films. This results in the stabilization of the frequency position and reduction of the band widths (Figure 4c). Similar changes of the absorption spectra, due to the different compression of the monolayers, have been reported for porphyrins.44 The more ordered, bilayer, structures of the diskotic films of the third group exhibit a smallest red shift of the absorption maxima (Figure 4d). This red shift increases almost linearly with the decrease of the average transfer ratio of the films. The largest red shift, measured with the alternating multilayer
J. Phys. Chem., Vol. 99, No. 49, 1995 17613
Diskotic Multiyne Langmuir-Blodgett Films. 2 structure, results from the diminished interaction between the adjacent diskotic monolayers due to their separation by optically inactive arachidate monolayers. The smallest red shift of the third group samples is not due to reduced intermolecular distances in the 4b monolayers, as with the porphyrins,44since their in-plane packing density is approximately the same as in the AM sample. The n-electrons of the strongly polarizable aromatic cores of the adjacent diskotic monolayers can interact effectively in the multilayers due to the reduced distances between them in the bilayer “head-to-head” region (Figure 13 in part 1). Therefore, both the in-plane and the interlayer interaction of the disk-shaped n-electron systems of the bilayers should be taken into account in the spectroscopic explanation of the shift of the absorption maxima of the third group samples. The interplay of the two factors is related to the (i) formation of in-plane “extended2 dipoles” of about three molecules with J-type aggregate arrangement in the monolayers and (ii) H-type aggregate arrangement of the “extended dipoles” (of y > 54.7”) in the bilayers normally to the substrate. The first factor plays the major role for the optical features because of the smaller “face-to-face” distance between the aromatic cores and stronger interaction of their n-electron systems. According to the twodimensional (2D) extended dipole model of the J aggregates,’S2 this factor determines the resultant red shift of the absorption maxima of all diskotic samples. The second factor plays an important, but minor role, because of the larger “side-by-side” distance between the aromatic cores of the adjacent monolayers. The resulting additional interaction between the 2D “extended dipoles” of the adjacent monolayers in the bilayer structures causes a reduction of the red shift of the absorption maxima. The decrease of the red shift on increase of the average transfer ratios (Figure 4d) indicates different degrees of interaction of the “extended dipoles”, arranged in H-type aggregates normally to the substrate with an angle y bigger than the critical angle yo = 54.7”. The increased n-electron interactions in the bilayer structures of the second and third groups of diskotic films contribute for a strong smearing of the vibrational structure of their absorption spectra (Figure 4b,c). Monomer Fluorescence. Fluorescence quantum yield of 0.36 was estimated for the 4b monomer in 1 x M chloroform solution. It was determined according to the M quinine sulfate procedure described in ref 55 using 1 x in H2S04 as a quantum counter. The “anomalous” monomer fluorescence from the second singlet state S2 SO,discussed above (Figure 2b), indicates the existence of a certain forbidnessM for the relaxation to the first singlet state. However, the value of the fluorescence quantum yield is more likely for the SO, and supports the impurity normal fluorescence, S I hypothesis. The independence of the fluorescence spectra on the excitation wavelength and the considerable Stokes shift of the fluorescence band (Figure 3) demonstrate the existence of fast relaxation processes to the lowest vibration level, 190, of the S2 (Figure 2b). The long low-energy tail of the fluorescence results from the transitions to the vibrational levels of the ground state SO. The fluorescence spectra (Figures 3 and 6) do not follow the mirror rule.50 This usually indicates different geometrical positions of the fluorescent nuclei in the ground and the excited state. A typical example is the d i ~ h e n y l e n e .The ~ ~ shift of the nuclei could occur because of the relatively high lifetime of the excited-state S2 in comparison to the time of the electronic transitions. As a result, the orientation of the dipole moments in the excited state differs from that in the ground state. Aggregate Fluorescence. The structural changes occurring
-
-
in the excited state could cause changes in the chromophores aggregation. This will result in a different behavior of the fluorescence bands as compared to the absorption ones. Indeed, all diskotic films investigated show a red shift of the fluorescence maxima (Figures 6 and 7), thus demonstrating a brickstone work arrangement of the excited state dipoles. However, the magnitude of the shift is smaller than that of the absorption bands. The smaller red shift could be due to both a reduction of the aggregation number (dimer formation) and changes of the characteristic angle, y , of the aggregates in the excited state (Figure 1). The red shift for the films of the third group is larger than that for the alternating multilayer Q1 (Figure 7). The structural changes in the excited state of the AM sample are probably related to an increase of the characteristic angle y of the in-plane formed dimers which leads to a smaller red shift. The stable structure arrangement of the diskotic samples of the groups I and 111 results in a stable frequency position of the fluorescence bands (Figure 7). However, the unstable teared structure of the samples of the group I1 causes fluctuations of the band positions. The fluorescence intensity decreases essentially on increase of the packing density of the diskotic layers (Figures 6 and 7). This is probably due to self-quenching. The intensity of the bands of the first group samples is considerably lower than that of the solution (M). The higher in-plane packing density of the 4b molecules in the sample Q1 results in stronger quenching as compared to the other samples of the Fist group. The highest fluorescence quenching, obtained for the third group films, is due to their highest packing density and strongest intermolecular n-electron interactions.
Conclusion The rules postulated on the basis of the electrostatic theory of the n-n interactions for the diskotic phases illuminate the physical origin of the structural organization of the aromatic compounds considered. These results can predict the type of the molecular aggregation and, therefore, the optical properties of the disk-shaped molecules. For the investigated multiyne amphiphile 4b, the rule C2 predicts a nematic in-plane arrangement. This is confirmed experimentally by the red shift of the absorption and fluorescence bands typical for J aggregates. It is estimated that three to four 4b molecules are associated in the aggregates. The contribution of the solvatochromic effect to the shift of the bands is negligibly small. The magnitude of the shift is reduced by the n-electron interlayer interactions in the films of bilayer structure. The monomer absorption spectrum and the energy diagram demonstrate that the electronic transitions are accompanied by strong vibrations of the chemical groups within, or attached to, the aromatic core of 4b. The main absorption transition occurs to the second singlet state, while the transitions to the first singlet state are symmetry-forbidden. It is concluded that the fluorescence of 4b occurring from the second singlet state is anomalous. The fluorescence quantum yield is determined to be 0.36. Both the absorption and the fluorescence spectra depend on the conditions for deposition of the diskotic thin films and follow their structural changes. This illustrates the correlation between the structural organization of the diskotic films and their optical properties.
Acknowledgment. The authors are indebted to Prof. L. Brehmer for initiating the work on the diskotic compound and to Dr. W. Regenstein for the kind permission to use the optical equipment in his outstanding laboratory. We would like to thank the reviewer for the constructive comments.
17614 J. Phys. Chem., Vol. 99, No. 49, 1995
References and Notes (1) Kuhn, H.; Mobius, D.; Bucher, H. In Physical Methods of
Chemistry; Weissenberger, A., Rossiter, B., Eds.; Interscience: New York, 1972; Vol. I, Part IIIB, p 577. (2) Nakahara, H.; Fukuda, K.; Mobius, D.; Kuhn, H. J . Phys. Chem. 1986, 90, 6144. (3) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Lungmuir 1988, 4 , 583. (4) Kuhn, H.; Mobius, D. Angew. Chem. 1971, 83, 672. (5) Befort, 0.;Mobius, D. Thin Solid Films 1994, 243, 553. (6) Nakahara, H.; Hayashi, K.; Shibasaki, Y.; Fukuda, K.; Ikeda, T.; Sisido, M. Thin Solid Films 1994, 244, 1055. (7) Nakahara, H.; Endo, H.; Fukuda, K.; Ikeda, T.; Sisido, M. Mol. Cryst. Liq. Cryst. 1992, 218, 177. (8) Biesmans, G.; Verbeek, G.; Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Thin Solid Films 1989, 169, 127. (9) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385. (IO) Markovitsi, D.; Lecuyer, I. Chem. Phys. Lett. 1988, 149, 330. (1 1) Markovitsi. D.: Rieaut. F.: Mouallem. M.: Malthete. J. Chem. Phvs. Lett. 1987, 135, 236. " (12) Markovitsi, D.; Lecuyer, I.; Simon, J. J. Phys. Chem. 1991, 95, 362n
(13) See the references of part lZ3where the structural investigations of the diskotic LB films are summarized. (14) Ecoffet, C.; Markovitsi, D.; Vandevyver, M.; Barraud, A,; Veber, M.; Jallabert, C.; Strzelecka, H.; Albouy, P. A. Thin Solid Films 1992,210/ 211, 586. (15) Ecoffet. C.: Markovitsi. D.: Millie. Ph.: Lemaistre. J.-D. Chem. Phys. 1993, 177, 629. (16) Ecoffet, C.; Markovitsi. D.: Jallabert, C.; Strzelecka. H.; Veber, M. Thin Solid Films 1994, 242, 83. (17) Bonosi, F.; Ricciardi, G.; Lelj, F.; Martini, G. J . Phys. Chem. 1993, 97, 9181. (18) Hunter, Ch. A,; Sanders, K. M. J. Am. Chem. SOC.1990,112,5525. (19) Hunter, Ch.; Meah, M.; Sanders, J. J . Am. Chem. SOC. 1990, 112, 5773. (201 Anderson. H.: Hunter. Ch.: Meah. M.: Sanders. J. J . Am. Chem. SOC.1990, 112, 5780. (21) Burlev, S. K.; Petsko, G. A. Science 1985, 229, 23. (22) Rigby, M.; Smith, E.; Wakeham, W.; Maitland, G. The Forces between Molecules; Clarendon: Oxford, 1986. (23) Ionov, R.; Angelova, A. J . Phys. Chem., previous paper in this issue. (24) Schneider. H.-J.: PhiliDDi. .I K.: Pohlmann. J. Anaew. Chem.. In?. Ed. Engl. i984, 23, 908. (25) Zimmerman. S. C.: VanZvl, C. M.; Hamilton, G. S. J . Am. Chem. SOC.1988, 111, 1373. (26) Strong, R. L. Intermolecular Forces; Pullman, B., Ed.; D. Reidel: Dordrecht, 1981. (27) Muehldorf, A.; Engen, D.; Warner, J.; Hamilton, A. J . Am. Chem. SOC. 1988, 110, 6561. (28) Interactions between molecules with n-electron systems without a consideration of additional environmental effects.
Ionov and Angelova (29) Veerman, J. A.; Frenkel, D. Phys. Rev. A 1992, 45, 5632. (30) Kurosawa, S.; Kamo, N. Lungmuir 1992, 8, 254. (31) Czikkley, V.; Forsterling, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6 , 207. (32) Kuhn, H. J . Chem. Phys. 1970, 53, 101. (33) Kasha. M.: El-Bavoumi. M.: Rhodes. W. J . Chim. Phvs. , Phvs. , Biol. 1961, 58, 916. (34) McRae. E. G.: Kasha. M. J . Chem. Phvs. 1958. 28. 721. (35) Vuorimaa, E.; Ikonen, h.;Lemmetyinen, H. Thin Solid Films 1992, 214, 243. (36) Song, Q.; Evans, C. E.; Bohn, P. W. J . Phys. Chem. 1993, 97, 13736. (37) Tyutyulkov, N.; Fabian, J.; Mehlhorn, A.; Dietz, F.; Tadjer, A. Polymerine dyes; St. Kliment Ohridski University Press: Sofia, 1991. (38) Mahrt, J.; Willig, F.; Storch, W.; Weiss, D.; Kietzmann, R.; Schwarzburg, K.; Tufts, B.; Trosken, B. J . Phys. Chem. 1994, 98, 1888. (39) Van der Auweraer, M.; Willig, F. lsr. J . Chem. 1985, 25, 274. (40) yo = 54.7' in the framework of the point dipole approximation and the assumption of only nearest-neighbor interactions. (41) Kirstein, S.; Mohwald, H. Chem. Phys. Lett. 1992, 189, 408. (42) Bliznyuk, V. N.; Kirstein, S.; Mohwald, H. J . Phys. Chem. 1993, 97, 569. (43) Davydov, A. S. Theory ofMolecular Excitons; Plenum: New York, 1971. (44) Adachi, M.; Yoneyama, M.; Nakamura, S. Langmuir 1992,8,2240. (45) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L.; Crossley, M. J . Phys. Chem. 1993, 97, 12862. (46) Schick, G.; Schreiman, I.; Wagner, R.; Lindsey, J.; Bocian, D. J . Am. Chem. SOC.1989, 111, 1344. (47) Kawski, A. Progress in Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1992. (48) Mc Rae, E. J . Phys. Chem. 1957, 61, 562. (49) Magata, N. Bull. Chem. SOC. Jpn. 1963, 36, 1607. (50) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (51) Gorodyskii, V.; Bakhshiev, N. Opt. Spectrosc. 1971, 31, 117. (52) Petkov, V.; Bakaltchev, N. J . Appl. Crystallogr. 1990, 23, 138. (53) Ionov, R.; Angelova, A., to be published. (54) Reichardt, Ch. Solvents and Solvent Effects in Organic Chemistry; VCH: Weinheim, 1988. (55) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (56) It was assumed that E and n of the compounds 3 and 4b do not differ essentially. (57) Heppke, G.; Kitzerow, H.; Oestreicher, F.; Quentel, S.; Ranft, A. Mol. Cryst. Liq. Cryst. Lett. 1988, 6 , 71. (58) Knapp, E. W. Chem. Phys. 1984, 85, 73. (59) Thulstrup, E. W.; Michl, J. Elementary Polarization Spectroscopy; VCH: New York, 1989. (60) Investigations of the time-resolved fluorescence are of interest for a clarification of its origin. (61) Angelova, A.; Reiche, J.; Ionov, R.; Janietz, D.; Brehmer, L. Thin Solid Films 1994, 242, 289. JP942294E
