J. Phys. Chem. 1980, 84, 1361-1366
1381
Film Dichroism. 4.’ Linear Dichroism Study of Orientation Behavior of Planar Molecules in Stretched Poly(viny1 alcohol) Film Yuklo Matsuoka Deportment of Physical Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan (Received September 26, 1979)
A method for studying the orientation behavior of planar molecules in a stretched polymer film has been developed. In this method, the reduced dichroism (All- A,)/A could be related to the orientation angle +, the transition moment angle 8, and the orientation factor K,, as follows:
where the term (3K, - 1)/(3cos3 +z - 1)is called the orientation function of an assembly of like molecules and is equal to (3Ky- 1)/(3sin2 - 1)or 1- 3K,. The method was tested on 10-methylacridine(MeAcr) oriented in the unidirectionally stretched poly(viny1alcohol) film. The orientation factors K,,Ky,and K, and the Orientation angle +z of MeAcr were calculated from the reduced dichroism of ita short-axis and long-axis polarized transitions. The orientation behavior of acridine orange (AO), acridine yellow (AY), and crystal violet (CV) was also studied, MeAcr, AO, and AY were oriented with their in-plane molecular orientation axes predominantly aligning to the direction of stretch. The orientation angles decreased in the order of MeAcr (Ihl = 30°), A 0 (=18O),and AY(=Oo). On the other hand, CV was oriented with its molecular plane aligning to the stretch direction.
+,
Introduction Dichroism measurements have usually been expressed either in terms of the dichroic ratio:+ Rd = AII/A,, or the reduced d i c h r o i ~ m , ~ -AA/A ~ ~ ~ O= 3(All- A,)/(All + 2A,), where A and A, are the absorbances measured with light polarize$ parallel and perpendicular to the stretch direction of a film. It is well-known that Rd or AA/A of partially oriented rodlike molecules can be related to the transition moment angle e’, which is the angle between a transition moment vector of the molecule and its orientation axis, as follows:zJ1
or
--AA = 3 3 cos2 6’
-’
l)@
A
where @ is the orientation function of the guest molecules. Since the values of Rd and AAIA are experimentally available, the angle 6’ can be calculated from eq 1 or 2 if the value of 9 is obtained by some means. In 1959, an orientation factor T(S) was derived theoretically as a function of stretch ratio S of a film,5 and the transition moment angle 6’ was determined by the use of eq 1 (oneparameter method).6*6The factor T ( S )was related to @ by 9 = (3T(S) - 1)J2.2 The one-parameter method has treated quantitatively the dichroic data of guest molecules. However, the fact that the applicability of the method should be limited to rodlike molecules has not been appreciated properly. Sometimes the method has been applied uncorrectly to the dichroic analysis of the planar molecule which cannot be approximated by a rod. An alternative analytical approach is possible for dealing with dichroic data. If a transition moment angle of a particular molecule is given or assumed relative to the orientation axis from spectroscopic considerations, the value of @ can be calculated. On the basis that the long0022-3854/80/2084-1361$01 .OO/O
and short-axis polarized electronic transitions are distinguishable in the dichroic spectra of highly symmetric molecules, Thulstrup et al.12J3developed a new method (reduction procedure) for evaluating the orientation factors and the reduced spectra of planar molecules belonging to a Czvor Dah point symmetry group. This method was recently applied to the analysis of the linearly polarized absorption spectra of eight acridine dyes3 and three triphenylmethane dyes.l The results were quite satisfactory as regards the reduced spectra and the orientation factors of these dye molecules. As already pointed out: however, the reduction procedure cannot specify the degree of orientation of an assembly of like molecules. Furthermore, this method cannot determine unambiguously the direction of molecular orientation axis relative to the molecular framework for the Czv-or DZh-symmetricplanar molecule~,~~ The main objective of this paper is, therefore, to derive a new expression for the reduced dichroism under due consideration of the dependence of the orientation factors on the geometrical shape of the guest molecules. The new expression made it possible to specify the degree of orientation and molecular orientation axis of planar molecules. The orientation behavior of four dyes will be discussed fully on the basis of the orientation angle, the orientation factors, and the orientation parameters.
Basic Theory General Expression of Reduced Dichroism. The molar absorption coefficient q (i = x, y, and z denote the molecule-fixed Cartesian coordinate system) is proportional to (Ei.~)z, which is the square of the scalar product of the electric vector Ei of the incident light polarized parallel to the i axis and the transition moment p of the chromophore.14 For an assembly of like molecules oriented in a unidirectionally stretched polymer film, the component of absorption due to transitions polarized along the i axis can be represented byI5 eq 3 where k is a constant simAi = k(Ei.CO2 0 1980 American Chemical Society
(i = 1c, y , z )
(3)
1362
The Journal of Physical Chemistty, Vol. 84,
I
No. 11, 1980
Matsuoka
IMOA
b
f
%\I
/
Figure 1. The laboratory-fixed (0-X,, Y,,Z,) and the molecule-fixed (O-xl,yl,zI)coordinate systems. The partial and perfect alignment of the jth molecule are specified by angles (a) 0,and (b) #,, qY, I), respectively. MOA denotes the molecular orientation axis of the molecule. ZF Is taken as the stretch direction of a film.
ox,by,
plifying eq 3 (for the detail, see ref 14). The observed dichroic absorbances All and A, can be related to the absorbance Ai as follow^:^^^^^
C AiFi
Ail- A,
i =x ,y ,t
(4)
/T(l,O,O) KX
Figure 2. (a) Orientation triangle PQR in the coordinate system 0K,,Ky,Kz. The point K,represents a set of orientation factors (K,,K,,K,) at a given stretch ratio S. (b) Schematic illustration of the planar molecule with the coordinate system 0 - x , y , z . s* is the angle between the transition moment vector p and the x axis (out-of-plane axis). 0 Is the angle between the component of p projected onto the yz plane and the L axis.
P, are rewritten by $, qY,and #., The limiting value of the orientation factor is represented by eq 13. In Figure lim Ki = Mi = cos2 qi (i = x , y, z )
(5)
where Fi is the orientation parametePm and is connected to the orientation factor Ki as follows: (7)
The factors K,, Ky, and K, are given byz1 K, = (cos2 P,)
(9)
Ky = (cos2By)
(10)
K, = (cos2 P,)
(11)
where P,, By, and P, are the angles between the molecular xj, y, and zj axes and the stretch direction ZFof the film (seehgure la). With the aid of eq 4,5, and 7, the reduced dichroism AA/A = 3(All- AL)/(All+ 2AJZ2 can be expressed analytically in terms of Ai and Ki as follows: 3A, 3K,- 1 AA _ A A,+A,+A, 2 3Ay 3Ky- 1 3A, 3K2- 1 (12) A,+A,+A, 2 A, + A y + A, 2
+
-
+
It is interesting to note that each term on the right-hand side of eq 12 represents the product of two quantities, i.e., one for the optical property and the other for the orientational property. Equation 12 is the general expression for the reduced dichroism of an arbitrarily shaped molecule. Orientation Distribution of Planar Molecule. When each planar molecule is oriented by unidirectional stretch of a film, the angles Ox, P ,and P, change gradually keeping the relation K, + K + 2, = 1. In the vicinity of a limiting stretch (Le., S aj,the orientation distribution of each guest molecule will approach saturation (perfect orientation) as shown in Figure lb, where the angles p,, By, and
-
S--
(13)
lb, a particular axis, which is parallel to the stretch direction ZF, can be specified by the angles #, )I and 1c; relative to the molecular framework. This axis is cereatter called the molecular orientation axis (MOA). On the other hand, eq 8 represents a plane RTU as shown in Figure 2a, where a set of orientation factors (K,, Ky, K,) at a given S is symbolized by Ks. The point Ks should coincide with the point P (1/3, 1/3, 1/3) in the isotropic orientation distribution and with another point M (M,, My, M,) in the perfect orientation distribution. As the film is stretched, the point Ks will leave the point P for the limiting point M in an orientation triangle. There are six equivalent orientation triangles in the plane RTU; yet, if the labels of the molecular axes x, y, and z are taken as shown in Figure 2b, the location of the point Ks will be limited to the inside or the periphery in the orientation triangle PQR (Figure 2a). Some forms of orientation distribution of guest molecules in the stretched polyethylene film were reported:l6 (1) Three important limiting forms of orientation distribution correspond to the vertices, P, Q, and R of the triangle; (2) three more general limiting forms of orientation distribution correspond to the sides PQ,PR, and QR of the triangle; and (3) more general types of orientation distribution correspond to the inside of the triangle. From the experimental result that each point Ks for an assembly of guest molecules in the stretched poly(viny1 alcohol) film lies nearly on a straight line passing through the isotropic point P (e.g., see Figure 4), the relations between the orientation factors K,, K y , and K, may be given by eq 14. Since a planar molecule as shown in 3K,-1 3Ky-1 3K,-1 ---=(14) 3My-1 3M2-1 3M,-1 Figure 2b is generally known to orient predominantly with its molecular plane parallel to the direction of stretch, the angle p, is assumed to approach 90' (i.e., #x = 90') as S m. This means that the molecular orientation axis is present in the yz plane. Under consideration that M, is zero in this caseF3 three important forms of orientation distribution are presented for the planar molecules in Table I, together with the possible regions the forms oc-
-
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980 1383
Planar Molecules in Stretched Poly(viny1 alcohol)
TABLE I: Three Important Forms of Orientation Distribution for the Planar Molecules with M , = 0 in the PQR Orientation Triangle rodlike orientation degree of alignment K,, K,, K, in PQR Kx =: K y= 0 Kz= 1
perfect
point R
disklike orientation K X ,K y , Kz in PQR
Kx= O K y = K,= 1 / 2
point
Q
line PQ
general planar orientation K,, K y , Kz in PQR any point M K x = 0,Ky = My, K . = Mz on line QR ( M y+ M z =1 ) ~ K Y 1 ~ K z 1 any line PM 1 - 3K, = -inside PQR 3My-1 3Mz-1 I__
P
cupy in the orientation triangle PQR. Reduced Dichroism of Planar Molecules. On the basis of the relations in ‘Table I, the expression for the reduced dichroism of planar molecules is derived. The relation 1 - 3K, = (3Ky- 1)/1(3My- 1) = (3Kz - 1)/(3M, - 1) reduces eq 12 to 3 A, Ay(3My- 1) A2(3M, - 1) -=--(1 - 3Kx) (15) A, A, A, A 2
+
+ + +
If a planar molecule has a transition moment 1.1polarized a t an angle { with respect to the x axis, the absorption components A,, A.y9and A, are given by eq 16-18 where A, = k p 2 cos2 [
(16)
A, = kp2 sin2 { sin2 6
(17)
A, = k p 2 sin2 { cos2 0
(18)
8 is the angle between the component of p projected onto the molecular plane and the z axis (Figure 2b). With the aid of eq 16, 17, and 18, eq 15 becomes PA - 5{3(My 3 sin2 8 + M, cos2 e) sin2 { - 1)(1 - 3K,) A (19)
--
In the case of th.e planar molecule whose out-of-plane polarized component A, can be ignored (i-e.,{ = goo), eq 19 becomes simply
Flgure 3. Three-dimensional illustrations of eq 20, 22, and 24 In the coordinate system 0-1,2,3. The quantities 101, CP, and A A / A are taken on the 1, 2, and 3 axes, respectively. Three examples are shown for the rodlike (M, = l), disklike (M, = 1/2), and planar (M, = 2/3) moleculeg with A, = 0.
= ?(3(2M2 - 1) cos2 8 - (3M, - 2)}@ (20) 2 where CP is the orientation function of an assembly of like molecules and is given by
cule which behaves like a disk in orientation (disklike molecule) can be given by substituting M, = 112 into eq 20 and 21 3 -AA- - -a A 4 CP = 1 - 3K, = 2(3K, - 1) = 2(3K, - 1) (25)
Since each point K5;lies on a straight line PM, the relations 0 5 K, 5 113, 5 Ky 5 1/3 (or 113 I K, IMy),and 113 IK, IM, hold in the orientation triangle PQR (see Figure 2a); therefore, 0 I CP 5 1. It is thus eq 20 that should be utilized for the analysis of the dichroic data of planar molecules with A, = 0. The expression for the reduced dichroism of the planar molecule which behaves like a rod in orientation (rodlike molecule) can be obtained by substituting M, = 1 into eq 20 and 21
Equation 24 indicates that the reduced dichroism of the disklike molecule with A, = 0 depends on the orientation function only. Equations 22 and 24 can be illustrated by the surfaces OA’C’E and OA”C’T3, respectively, in Figure 3, where another surface, OACB, is also shown for M, = 213. Figure 3 indicates that the negative reduced dichroism is not observed for a planar molecule whose Mz value is between 213 and 112, in other words, whose point KS lies anywhere in the triangle PQM’ (see Figure 4).24 For a disklike molecule whose out-of-plane absorption component A, cannot be ignored (Le., 5 # goo), the expression for the reduced dichroism can be derived newly by eq 26 and 27 where ( is the angle of the transition
A
My
3 _ *A - $3
A
It is interesting to note that eq 22 is equal to eq 2 (i.e., 0 = 8’) since the molecular orientation axis of the rodlike molecule agrees with its z axis. On the other hand, the expression for the reduced dichroism of the planar mole-
cos2 [ - 1)CP’
moment vector with respect to the x axis, while CP’ is a new orientation function of the molecule (-0.5 5 a’ 5 O).’ It is worth noting that a very close relation has been derived for the orientation function CP’ of the disklike molecule in
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Matsuoka
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980
TABLE 11: Stretch Ratio Dependence of the Observed AA/A and the Calculated @ z , 0 ,and Ki (i = x, y , z ) for MeAcr AAIA 412 nm 263 nm S ( y polarized) (z polarized) $ z , deg Q, Kx KY Kz 1.0 0 0 0 113 113 113 1.7 -0.054 0.271 30.1 0.145 0.285 0.321(0.32)" 0.394(0.40) 2.3 -0.105 0.486 29.5 0.2 54 0.249 0.310(0.31) 0.441(0.45) 3.0 -0.123 0.588 29.6 0.310 0.230 0.306(0.31) 0.464(0.47) 3.7 -0.136 0.638 29.5 0.333 0.222 0.303( 0.30) 0.475(0.48) 4.3 -0.152 0.704 29.5 0.368 0.211 0.299(0.30) 0.490(0.49) " The values in the parentheses were determined by the reduction procedure.
the study of electric birefringence.26
Experimental Section Materials. The dyes used were 10-methylacridine (MeAcr), acridine orange (AO), acridine yellow (AY), and crystal violet (CV), which are all in the monocationic form, the anion being chloride. All these dyes were purified as described bef0re.l~~The powdered poly(viny1 alcohol), PVA, sample with a nominal degree of polymerization of 1680 was obtained from Kuraray Co. (Kuraray Poval 117-H). Both sample and reference films were prepared as described b e f ~ r e . ~ ~ ~ Measurements. The linear dichroic absorption spectra ( A spectrum and A , spectrum) of MeAcr, AO, AY, and Cs were measured on a Hitachi EPS-3T double-beam recording spectrophotometer equipped with a mechanical stretcher designed and constructed in our laboratory? The sample and reference films were stretched simultaneously a t about 80 "C. The stretch ratio S of the PVA film was defined as described e l ~ e w h e r e . ~ ? ~
'Results and Discussion Determination of Orientation Angle and Orientation Factors. In the dichroic absorption spectra of MeAcr measured at S = 4.3, the value of AAIA was nearly constant in the 460-390-nm (about -0.152) and 270-250-nm (about 0.704) regions. This means that the 412- and 263-nm bands belong to the pure y- (0 = 90" in eq 20) and z-axis (0 = 0") polarizations, respectively.26 Therefore, by the use of eq 20 two simultaneous equations, -0.152 = 1.5(2 - 3 cos2$z)@ and 0,704 = 1.5(3cos2 $, - 1)@, hold for those two absorption bands. When these equations were solved, the orientation angle $, and the orientation function @ were determined (l$,l = 29.5" and 3 = 0.368). The same quantities determined at five different S values are given in Table 11, where $, is nearly constant regardless of S but @ varies with S. With these values the orientation factors can now be calculated from eq 21. The calculated values of Ky and K , in Table I1 were in good agreement with the values of K y and K, determined by the reduction procedure. In the observed dichroic spectra of A 0 at S = 4.3, AAIA varied from about 4 . 5 2 0 (near 330 nm) to 1.541 (near 502 nm). This result indicates that these two transitions are y- and z-axis polarized, respectively,z6but they cannot be considered as the pure transitions because the observed AAIA was not constant in those wavelength regions. In this case the trial-and-error reduction search by ~omputer'~ was employed to determine the orientation factors. The reduced spectra of A 0 showed that the 330- and 502-nm bands are not pure transitions but probably contain some contributions from either the z- or y-axis polarized component? Therefore, in order to apply eq 20 to the 330- and 502-nm bands of AO, the apparent angles of 0 = 77" (for the 330-nm band) and 0 = 7" (for the 502-nm band) were used instead of 0 = 90" and 0". The apparent angles can
TABLE 111: Stretch Ratio Dependence of the Observed AA/A and the Calculated @ ' and Ki (i = x, y , z ) for CV AAIA
S
550 nm (in-plane polarized)
K,
9'
K y = K,
0 0 113 113 0.135 -0.135 0.273(0.26)" 0.364(0.37) 0.231 -0.154 0.231(0.24) 0.385(0.38) 0.300 -0.200 0.20010.20) 0.40010.40) 0.361 -0.241 0.173(0.16j 0.414(0.42j 0.421 -0.281 0.146(0.14) 0.427(0.43) a The values in the parentheses were determined by the reduction procedure.
1.0 1.7 2.3 3.0 3.7 4.3
be determined easily from the reduced spectra, since the relation tan2 ~9= Ay/A, holds from eq 17 and 18 at any wavelength. In the experimental dichroic spectra of AY at S = 4.3, the highest value of AAIA was about 2.095 at 467 nm, and the lowest one was about 0.214 at ca. 315 nm. However, these two bands could not be considered as the pure transitions by the same reason as AO; therefore, the factors K,, Ky,and K, were determined by the trial-anderror reduction search. Since CV probably belongs to a C3"point symmetry group, it can be assumed to indicate a disklike orientation. If only the in-plane transitions are present in the observed wavelength region, the wavelength dependence of MIA at a given S should become flat through the region concerned as indicated by eq 24. On the contrary, if both the in-plane and the out-of-plane transitions are overlapped with each other, the former is located in the large AAIA region and the latter is located in the small AAIA region (see eq 26). The observed AAIA of CV at S = 4.3 varied from about -0.073 (near 280 nm) to 0.421 (near 550 nm) but was nearly constant (ca. 0.421) in the 555-520-nm region.' This indicates that the 550-nm band is a pure in-plane transition (f = 90° in eq 26), and, therefore, the relation 0.421 = -1.53' holds for CV from eq 26. The values of 3' and K,. K.,, and K , calculated from eq 27 are given in Table 111:' The orientation factors, K, and K,, of MeAcr, AO, AY, and CV are shown in the oriintationiriangle PQR (Figure 4). Since the points Ks for AY are nearly on the line PR (K,= 1- 2Ky),this dye can be approximated as a rodlike molecule regarding its orientation distribution. The points Ks for CV are on the line PQ (K, = Ky),Le., it follows the disklike orientation. The points Ks for MeAcr and A 0 are inside the triangle and indicate a general planar orientation. The straight line passing through the series of points Ks of MeAcr or A 0 can be easily extrapolated to the line QR. The coordinates of the crossing point M give the values of My and M,; they are 0.25 and 0.75 for MeAcr and 0.10 and 0.90 for AO. With the aid of eq 14 the line PM is expressed by K, = 2 - 5Ky for MeAcr or 7K, = 8 - 17Ky for AO. The angles $, of MeAcr, AO, and AY were calculated from their M, values according to eq 13. The .Jl
The Journal of fhysical Chemistry, Vol. 84, No. 11, 1980 1366
Planar Molecules In Stretched Poly(viny1 alcohol) -r
I
I
I
I
10
I
1
I
'
a MeAcr
L
0
A
1
I
I
0.2
I
0.4
I
1
KY
Flgure 4. The values of orientation factors K, and K, of MeAcr, AO, AY, and CV at six different Svalues. Each set of (KY,Kz)$is represented by a point Ks in the Orientation triangle PQR. Each point Ks lies on the line PR (AY), PM(MeAcr and AO), or PQ (CV). The size of point Ks nearly corresponds to experimental uncertainty. The dotted lines In the yz plane of MeAcir, AO, and AY denote the molecular orientation axis.
calculated values clearly show that the direction of the molecular orientation axis deviates from the z axis in the order of AY (I$,I = OO), A 0 = 18O), and MeAcr (I$,I = 30°) (see Figure 4). I t should also be noted that the value of lqZl=30° for MeAcr compares favorably with the values of $, given in Table 11. This close agreement is a strong support for the use of the extrapolated values My and M, in conjunction with eq 13. The molecular orientation axis of CV could not be assigned uniquely owing to its possible alignment in any direction in the molecular plane. Orientation Behavior of MeAcr, AO, AY, and CV. An average orientation of a dye molecule in the stretched PVA film can be specified by giving a set of the orientation parameters F,, Fy, and FZ:I each of which represents the molecular orientation along the x , y , or z axis. By use of eq 7, Fi (i = x , y , z ) can be calculated from Ki (i = x , y , z ) which were determined in the preceding section. The value of 9 given by eq 21 may be considered as a measure of the orientation degree of the guest molecules since it represents the molecular orientation along the orientation axis. The values of Fi (i = x , y , z ) and CP calculated for MeAcr, AO, AY, and CV are shown at various S in Figure 5a-d. In the case of MeAcr and A 0 (Figure 5, a and b), they cannot be approximated by the rodlike molecule since the value of F differs from that of F, at five different S. The values of %i' (i = x , y, z ) at the limiting stretch (i.e., S m) were calculated by eq 7 from the values of M,, My, and M,;they are F, = -0.50, Fy = -0.13, and F, = 0.63 for MeAcr, and F, = -0.50, Fy = -0.35, and F, = 0.85 for AO. The values of F, of these two dyes indicate that MeAcr is less orientable than A 0 with respect to the z axis. For the orientation parameters of MeAcr and AO, the relation = F, + (F, - F,) held a t each S within the experimental uncertainty. In this relation the quantity (F - F,) denotes the orientation around the z axis. From dgure 5, a and b, the orientation distribution of MeAcr and A 0 can be considered as follows. The molecular yz plane tends to align along the stretch direction; however, they and z axes are not equivalent hut the z axis is favored more than the y axis in the orientation distribution. The geometrical
-
Flgure 5. The values of orientation parameters F, (0),F, (a),and Fz (0)for (a) MeAcr, (b) AO, (c) AY, and (d) CV at SIXdifferent S values. Solid curve in (c) represents a theoretical Orientation function.
(A),and the Orientation function
shape of MeAcr and A 0 obviously affects the molecular orientation axis which is the director in the orientation distribution of these dyes (see Figure 4). The value of F, of AY is nearly equal to the value of @ at each S, and also Fy .= F, (Figure 5c). This means that the molecular orientation axis of AY is equal to the z axis (the orientation around the z axis is apparently zero). Accordingly, the z axis of AY predominantly aligns to the direction of stretch in the orientation distribution. The solid curve in Figure 5c denotes the theoretical orientation function @(S)= (3T(S)- 1)/2 determined for the hypothetical assembly of unit vector^.^^^ The discrepancy between the theoretical and the calculated values of @ for AY is clear and is taken as an example of the inadequacy of the theoretical function 9(S)(for the detail, see ref 3). The orientation parameters Fi (i = x , y , z ) of CV are shown in Figure 5d. Their limiting values a t S are F, = -0.50 and Fy = F, = 0.25. Figure 5d indicates that the value of (F, - F,)is about three times larger than that of F, at each stretch ratio. This means that CV is oriented with its molecular plane rather than the z axis to the direction of stretch (the y and z axes are equivalent in their orientation distribution). Thus the idea of the molecular orientation axis and the oriental function facilitated the study of the orientation behavior of planar molecules. The direction of the molecular orientation axis and the value of the orientation function can be calculated by eq 20 for a dye molecule with purely polarized transitions. In this connection, it is interesting to extend eq 20 to a dye with overlapped transitions and moreover to a dye of low symmetry. The details will be reported in future publications.
-
Conclusions The new expressions for the reduced dichroism of planar molecules in the stretched film were derived (eq 19, 20, and 26). The dichoric data of the planar molecules (MeAcr, AO, and AY) with long- and short-axis polarized transitions could be analyzed by eq 20. The dichroic data of the disklike molecule (CV) with out-of-plane and inplane polarized transitions were analyzed by eq 26. The orientation behavior of MeAcr, AO, AY, and CV in the stretched PVA film could be specified by the orientation
J. PhyS. Chem. 1980, 84, 1300-1371
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factors or the orientation parameters under due considerations of their molecular orientation axes. Acknowledgment. The author is sincerely grateful to Assistant Professor Kiwamu Yamaoka of Hiroshima University for his helpful discussions and critical reading of the manuscript. References and Notes (1) Part 3: Y. Matsuoka and K. Yamaoka, Bull. Chem. SOC.Jpn., 52, 2244 (1979). (2) K. Yamaoka and Y. Matsuoka, J. Sci. Hbhlm Unlv., Ser. A: phys. Chem., 40, 105 (1976). (3) Y. Matsuoka and K. Yamaoka, Bull. Chem. SOC.Jpn., 52, 3163 (1979). (4) R. D. B. Fraser, J . Chem. Phys., 28, 1113 (1958). (5) Y. Tanlzakl, Bull. Chem. Soc.Jpn., 32, 75 (1959); 38, 1798 (1965). (6) Y. Tanlzakl and S. Kubodera, J. Mol. Spectrosc., 24, 1 (1967). (7) 8. Nordbn, Chem. Scr., 1, 145 (1971). (8) A. Yogev, L. Margulles, B. Strasberger,and Y. M u ,J. Phys. Chom., 78, 1400 (1974). (9) C. C. Bott and T. Kurucsev, J. Chem. Sac., Faraday Trans. 2, 71, 749 (1975). (10) C. Horcssler, B. Hardy,andE. Frederlcq, 6@o&mers, 13, 1141 (1974). (11) A. R. Foweraker and B. R. Jennlngs, Appl. Opt., 12, 1983 (1973).
(12) E. W. Thulstrup and J. H. Eggers, Chem. Phys. Lett., 1,690 (1968). (13) E. W. Thulstrup, J. Mill, and J. H. E m s , J. Phys. Chem., 74, 3868 (1970); J. Mlchl, E. W. Thulstrup, and J. H. Eggers, IbM., 74, 3878 (1970). (14) J. A. Schellman, Chem. Rev., 75, 323 (1975). (15) B. Nordbn, G. Llndblom, and I. Jon& J. Phys. Chem., 81, 2086 (1977). (16) J. Mlchl and E. W. Thulstrup, Spectrosc. Lett., 10, 401 (1977). (17) 8. Nordbn, “Proceedingsof Nobel Workshop on Linear Dichroism Spectroscopy”,Lund University Press, 1977, pp 1-34. (18) A. Davldsson and B. Nordbn, Chem. Phys. Lett., 28, 221 (1974). (19) E. W. Thulstrup and J. Mlchl, J. Am. Chem. Scc., BEr 4533 (1976). The q uam F, Is also called ^‘orderparameter” h ref 15 or “*ntatlon tensor” In ref 20. (20) A. Saup, Mol. Cryst., 1, 527 (1966). (21) Factors K and K,are equal to K2and K,, respectively,in ref. 13. (22) This quanhy can be converted Into the dlchrolc ratlo Rd by R, = A I / A L = (3 -I-2AAIA)/(3 - A A I A ) (see ref 2). (23) If b e molecular orientatlon axis is absent In the molecular plane, the value M, does not become zero. (24) For example, acenaphthylene may be considered as a molecule of thls type (see ref 19). (25) M. J. Shah, J. Phys. Chem., 87, 2215 (1963). (26) The Intensity of the out-of-plane component was assumed to be negligible, If any. (27) R. S. Ste!n and 8. E. Read, J. Appl. &&m. Scl., App!. &&m. Symp., 8, 255 (1969).
Acid-Base Properties of l-Naphthol.’ Proton-Induced Fluorescence Quenching Chris M. Harrls and Ben K. Sellnger” Chemlstty Department, Australlen Natlonal University, Canberra, Australla (Received September 19, 1979)
An explanation is presented for the almost 30-year-oldparadox of Forster, which says that l-naphthol (RQH), the species excited below pK, lives long enough to produce excited l-naphtholate (RO-*) but does not itself fluoresce in water. We report the dependence on pH of the very weak fluorescence from ROH* and the much more intense fluorescence from RO* in aqueous solution. The s u m of the relative quantum yields of fluorescence is less than unity at low pH. This result is attributed to diabatic quenching of ROH* (1.7 f 0.5 X lo9 8-l M-I) and RO-* (2.8 f 0.5 X 1O1O s-l M-l) by Hag+. The neglect of these rate constants for this system has led to errors in the rate constant for reprotonation. The combination of steady-stateand time-dependentmeasurements of fluorescence allows the direct determination of the pK in the excited state, pK*, as 0.5 f 0.2.
Introduction The dissociation of 1-naphthol in the excited state was investigated fmt by Forster2 and later by Weller.31~ Forster explained that the presence of fluorescence from RO-* (the deprotonated naphthol in the excited state) in the pH range pK (9.4) to pK* (2.0) was due to the limited excited-state lifetime which allowed insufficient time for complete dissociation. Forster2 and Weller3noted that ROH* (the naphthol in the excited state) did not fluoresce in water because it was quenched by water molecules but that the dissociation to RO-* in neutral and weakly acidic solutions was faster than the quenching process; otherwise, the fluorescence from RO-*would not have been observed at pH values well below pK. Some years later Weller4 observed fluorescence from ROH* in ice at temperatures well below 0 OC and concluded that there was a definite activation energy for the quench reaction with water molecules. Henson and Wyatt have recently reported6 the observation of fluorescence from ROH* in aqueous solution. They found a marked increase in the fluorescence intensity of ROH* as the concentration of sodium perchlorate was increased beyond 0.1 M; the fluorescenceenhancement was O022-3654/80/2084-1366$01 .OOlO
attributed to the perchlorate anion, which was thought to disrupt the solvent structure and to probably interfere with the quenching mechanism involving water molecules. Selinger and Weller6 found that the fluorescence of ROH* was the dominant emission from l-naphthol in aqueous solutions of sodium dodecyl sulfate at pH values below pK. It was considered that the rate of deprotonation had been reduced upon incorporation of the naphthol in surfactant. The quenching of ROH* below pH 4 was attributed to diabatic quenching by Ht. In order to clarify the fluorimetric pH titration behavior of l-naphthol in aqueous solution and to investigate whether diabatic quenching by H+is a competing process in the excited state, we have measured relative fluorescence quantum yields and fluorescence lifetimes of l-naphthol at various proton concentrations. Experimental Section l-Naphthol (Fluka puriss.) was recrystallized from ethanol and sublimed twice before use. Solutions for fluorometric titration were prepared in the same way as those for 2-naphthol.’ The emission from quartz or solvents alone was significant only for the determination of the 0 1980 American Chemical Society
