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Ground and excited state dipole moments of coumarin laser dyes: Investigation by electro-optical absorption and emission methods
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
71 (1997) 255-263
Ground and excited state dipole moments of coumarin laser dyes: Investigation by electro-optical absorption and emission methods aB.I. Stepanoc
N.A. Nemkovich”,*, H. Reisb, W. Baumannb Institute of Physics, Academy ofSciences of Belarus, F. Skaryna Ave. 70.220072
Minsk, Belarus b Institute qf Physical Chemistry, University ofhlainz, Jakob Welder- Weg 1 I, 55099 Mainz, Germany Received
24 October
1996; accepted
20 January
1997
Abstract Modified electro-optical absorption and emission methods were used to measure the dipole moments of four efficient coumarin laser dyes (CUl, CU4, CU120, CU334) in the equilibrated ground, excited Franck-Condon and equilibrated excited states. The measurements were performed in cyclohexane (CUl, CU120, CU334) and dioxane (all CUs) at room temperature. Our results show that the charge distribution in CU4 differs substantially from that in the other CUs. The equilibrated ground and excited state dipole moments measured by electro-optical methods are compared with those derived from other measurement techniques and from semiempirical calculations. The possible role of twisted intermolecular charge transfer (TICT) state formation is discussed. Keywords:
Coumarins;
Absorption;
Fluorescence; Dipole moments; Electra-optical
1. Introduction The coumarins are a family of substances that have been studied extensively due to their application as an active medium in dye lasers [l-3]. Coumarin derivatives also have many other applications. In non-linear optics, coumarin-doped polymers are widely used as electro-optical materials for modulating of continuous-wave beams [4], frequency doubling of diode laser light [S], harmonic generation [6] and as photorefractive materials [7]. In chemistry some coumarins were used as fluorescence derivatization reagents for
*Corresponding author. Fax: 375 ifanbel%[email protected].
172
393131;
e-mail:
0022-2313/97/$17.00 80 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00098-7
method
high-performance liquid chromatography [8], fluorescence probes for protein studies [9] and as fluorescent ionophores [lo]. In medicine, 4- and 7-hydroxycoumarins are effective anticoagulants [ 111, fluorescence indicators [ 121, etc. Different coumarin derivatives in liquid solution show different absorption and fluorescence properties since their ring structure differs from that of aromatic ring systems like anthracene or pyrene. Hence, they are interesting candidates not only from the view of applications, but also with respect to an understanding of absorption and emission mechanisms. In general, the coumarin laser dyes with an amino group may be divided into two classes, namely, those with a free amino group and those with a rigid amino group. In the first case the coumarins exhibit a pronounced dependence of
256
N.A. Nemkovich
et al. /Journal
their absorption and fluorescence properties on the solvent polarity and have been object of investigations as potential candidates for twisted intermolecular charge transfer (TICT) reaction after optical excitation [lo, 13-191. The rigidized coumarins have other solvent polarity-dependent properties because their amino group does not allow the rotational movements required to stabilize a full charge separation, as is postulated by the model. Intramolecular charge redistribution in molecules after transition to a different electronic state is a fundamental process in chemistry. A very fruitful and widely used approach for studying this process is to determine the electric dipole moments in the related electronic states. A number of papers were published concerning the electronic spectra and the effect of the structure, pH and solvent on the fluorescence and lasing properties of coumarins [20]-[25]. But only very few papers on the determination of dipole moments by different solvent shift methods have been communicated and only few coumarins have been object of determining their dipole moments by electro-optical absorption
of Luminescence
71 (1997) 255-263
and emission methods [26]-[28]. Comparing the electro-optical methods with the solvent shift methods reveals the strong advantage of the former, namely, that the dipole moments are determined in one given solvent. Hence, it is possible to study the influence of the solvent on the process of intramolecular charge redistribution in ground and excited electronic states by determining the dipole moments effective in differently polar solvents. In this paper we report the results from electrooptical absorption and emission measurements on the equilibrated ground state, the excited Frank-Condon state and the equilibrated excited state dipole moments of four highly efficient coumarin laser dyes in cyclohexane and dioxane solution. Three are non-rigid compounds with different substituent groups at the 7th position, while the fourth has a rigid amino group (see Fig. 1).
2. Methods The determination of ground and excited state dipole moments from electro-optical measurements
‘i
H3
HoJ3Clo Coumarin 4 (CU4)
Coumarin 120 (GUI 20)
‘i H3
/ W2
Coumarin 1 (CUl) Fig. 1. Coumarins
Coumarin 334 (CU334) studied
in this work.
N.A. Nemkovich
et al. /Journal
has been given in detail in previous papers [29,30]. So, only the basic principles of electro-optical absorption (EOAM) and integrated emission (IEOEM) measurements in solution are briefly summarized. Using Liptay’s formalism [31] the effect of an external electric field Er on the molar absorption coefficient K(C),where S is the wave number, can be described by a quantity L, which is defined by
Wx) =
KE(C,K) - K(J) K(iqE! ’
(1)
where xE is the compound’s molar absorption coefficient in the presence of an applied field and x is the angle between the direction of Ef and the electric field vector of the incident linearly polarized light. For a homogeneously broadened absorption band, L is given by the following equation [29,32]: L = Dr + Es/6 + Frt + Gst + Hru + Zsu,
(2)
where the parameters r and s are determined by the angle x and the quantities t and u depend on the first and second derivatives of the absorption spectrum: r = (2 - cos2x)/5,
(3)
s = (3co&
(4)
- 1)/5,
t = (l/hc)(~/~)-‘d(~/j)/d~,
(5)
u = ( 1/2h2c2) (K/c)-
(6)
‘d2(rc/;)/dS2,
where h is Planck’s constant and c is the speed of light in the vacuum. For the molecules discussed in this communication, the electric dipole moment terms are dominant and explicit polarizability and transition polarizability effects can be neglected.With this assumption the coefficients D-I are given by D z 0
within the approximation
E = (l/KT)2f,Z[13(V,)2
- &],
F = (lI~Vf,2(~pAaCo,
used here,
(7) (8) (9)
G = (l/~T)f,2(mallg)(ltl~A~~),
(10)
H =.f:(Aap)2>
(11)
I =f?(ltlaAaCo2,
(12)
where k is the Boltzmann constant, T is the temperature, m, is the unit vector in the direction of the
of Luminescence
71 (1997) 255-263
251
transition dipole moment of absorption, pLpis the equilibrated ground state dipole moment vector and A”p is the change of the dipole moment vector upon excitation to the considered Franck-Condon excited state. In nonpolar solvents, it holds in good approximation: b”p = PFC - &>
(13)
where p cc is the dipole moment vector in the Franck-Condon excited state. From a set of the five coefficients D-l the values of pg, A”y and the angles between the transition dipole moment m,, ps and A”p can be determined. The cavity field correctionf, is defined according to Onsager’s model [33] as introduced to the electrooptical methods by Liptay [31]: fe = 3~/(2~ + l),
(14)
where E is the relative permittivity of the solvent. The experimental set-up for EOAM has been described in detail previously [29,30]. The quantity L(C J) in the present work was determined for two values of the angle x(x = 0 and x = 7c/2) and for a set of wave numbers within the first absorption band. Then the coefficients D-l and their standard deviations were obtained by multiple linear regression according to Eq. (2). Regrettably, the coefficients H and I often cannot be determined with sufficient accuracy. Nevertheless, the values of the dipole moments pg and 6”~ can be determined, if additional information about the symmetry of the molecules is available. Groenen [34] already used the electro-optical method to show homogeneity of an absorption band for a molecule with C2 symmetry, for which the transition dipole moment is either parallel or perpendicular to pK The same idea was also used by Baumann et al. [35] to show that a field-induced absorption and fluorescence antisotropy can be defined, which will be constant over pure absorption or fluorescence bands, if the solute molecules have C, symmetry, at least approximately. From more general considerations [28] it follows that, if psl(Aa~, the function L(v”,x = 0) =f[L(C,,x = rc/2)] is given by the simple linear relationship L(3,O) = AL(+c/2) + Bf&f/(6k2T2),
(15)
258
N.A. Nemkovich et al. /Journal of Luminescence 71 (1997) 255-263
where
3. Materials
A = (1 + 2cos%)/(2 - cos%),
(16)
B = (3cos% - 1)/(2 - co&Q
(17)
8 is the angle between m, and I(~. Hence, the angle 0 can be determined from the slope of the linear function L(?,O) =f[L(G,x/2)]. In this work the value of the electric dipole moment ,LL~ in the equilibrated excited state has been determined by integral electro-optical emission measurements (IEOEM). IEOEM is presented in detail in Refs. [36, 371 on the basis of the theory given in Ref. [38]. The experimental set-up for IEOEM is described in Ref. [29]. The effect of an external electric field Ef on the total fluorescence photon current q is described by the quantity X, given by x(@)
=
qE(@) - 4(Q) d@)Efz’
(18)
where @is the angle between the direction of Ef and the polarization direction of the emitted fluorescence light as determined by an analysing polarizer. In a typical measurement the quantity X is determined for two polarization angles, namely, Q, = 0 and @ = n/2. If explicit polarizability and transition polarizability terms can be neglected and if further ~(JlA~el(,where Ae”pis the change of the electric dipole moment vector upon transition from the equilibrated excited state to the Franck-Condon ground state, the relationship between X(@ = 0) and X(@ = n/2) is given by a relationship similar to Eq. (15) namely,
Fig. 1 presents the four CU molecules studied in this work. All compounds were purified by repeated liquid chromatography or recrystallization. The purity of the CUs was checked by thin layer chromatography. Cyclohexane and dioxane were purified by methods described in Ref. [39]. The purified solvents were dried prior to use by distillation under reflux conditions over sodium/potassium in an argon atmosphere. The purity of the solvents was checked by UV absorption measurements (1 cm cell; reference air).
4. Experimental results 4.I. Electra-optical absorption measurements (EOAM) The electro-optical absorption spectra of all four CUs are accurately reproducible, and show elements of vibrational structure especially for solutions in cyclohexane. As an example, the experimental data points of the electro-optical absorption spectrum of CU120 in cyclohexane are shown in Fig. 2. In Table 1 we show the results of
X(0) = AX(rc/2) + B[f,ZpU,2/(6PP) - yir,,x = x/2)], (19) and where A = (1 + 2 cos2 f$)/(2 - cos2 4) B = (3 COS”~- 1)/(2 - cos2 4); 4 is the angle between the unit vector of the transition dipole moment of emission m, and the excited state dipole moment I(~. The term L(f,,x = n/2) in Eq. (19) describes the field effect on the excitation process and can often be neglected. Equation (19) offers a simple method to determine the dipole moment in the equilibrated excited state I(~,if the angle 4 between m, and I(~is known in sufficient approximation, e.g. from symmetry [36, 371.
i
0
0
t=l -2 -4’. 28
29
30
31
32
33
Fig. 2. Absorption and electro-optical absorption spectra of CU120 in cyclohexane. The points show the experimental data, the line connecting the points is the approximation by a leastsquares multilinear fit function according to Eq. (2).
259
N.A. Nemkovich et al. 1 Journal of Luminescence 71 (1997) 255-263
Table 1 Results from electro-optical absorption measurements on CUs in cyclohexane and dioxane not be measured since the solubility was too low. For the coefficients H and I, see text Coefficient
cu4
Solvent: dioxane D (lo-“’ V2 m’) E (lo-‘” V-* m’) F ( I 0 _ “I C V ~-’ ml) G (lo- “’ C V ’ mZ) H (lO_“” C* m’) I (l0-h” C’ m’)
76 3174 415 518 183 174
+ * f k + *
2 13 9 9 21 21
the multilinear regression analysis according to Eq. (2) with Eqs. (3)-(6) for all measurements. CU4 in cyclohexane could not be measured because the solubility in this solvent was too low. Unfortunately, the coefficients H and I could not be determined with sufficient accuracy in all cases. In order to calculate the values for cp, A”p and pc’ using Eqs. (7)-(10) additional information about the angles between m,, pg, and A”p is necessary. For CU 1, CU 120 and CU334 in both solvents the plots of L(c,x = 0) versus L(v”,x= 7c/2) yielded straight lines, as shown in Fig. 3 for CU120 in dioxane. In all these cases, the corresponding linear fit resulted in A z 3, at least within the error, where A is the slope defined in Eq. (15). From Eq. (16) it follows that m, )I pg I/ Asp . The same conclusion can be reached by the observation that F z G (see Table 1). Using this symmetry condition, pg was calculated from E - 60 (which is the correct formula to take into account the small effects from the transition polar&ability [40]), then Asp from (F + G)/2 and finally pFc using Eq. (13). The results are shown in Table 2. In the case of CU4 in dioxane the situation is more complicated. The experimental electro-optical data are well approximated by the multilinear fit function (see Fig. 4), but the linear fit of L(v”,x= 0) versus L(<,x = x/2) (see Fig. 5) yields A z 3.7. From Eq. (16) we can see that A must be in the range of 0.5-3 for all possible values of 9. Also,
could
cu334
WI
cu120
Solvent: cyclohexane D (lO_‘” VmZm2) E (lO-z”V-Lmz) F (1O-4” C V ’ m*) G(10-5”CV ‘m2)
at T = 298 K. CU4 in cyclohexane
51 3956 142 780
+ 1 + 10 i 4 +- 6
57 5493 1437 1479
f * i +
7 44 16 16
150 11535 1869 1900
* + + +
63 6340 1521 1607 _ _
+ k & k
43 6504 1922 2030
f + + +
5 21 15 15
178 15129 2732 2774
+ 12 + 71 ) 32 +_ 32
4 25 16 16
14 85 23 23
2500 [‘-‘-r--
Fig. 3. Plot of L(?,x = 0) versus L(c,x = n/2) of CUl20 in dioxane at T = 298 K. The points show the experimental data and the line is their least square approximation by a linear fit.
from Table 1 it can be seen that G > F for CU4 in dioxane. These findings show that the vectors pg and A”p are not parallel to each other, and both are not parallel to m,. In this case, Eq. (15) is no more valid. Assuming the three vectors to lie in the plane formed by the coumarin ring and assuming the coefficients H and I to be correct at least
260
N.A. Nemkovich et al. /Journal
of Luminescence
71 (1997) 255-263
Table 2 Dipole moments of CUl, CU120 and CU334 in cyclohexane and dioxane at T = 298 K Molecule
Solvent
pB (lo-s°C
cu120
Cyclobexane Dioxane
CUl cu334
m)
A”p (10m3’ C m)
~c.‘~(10m30C m)
pe (10e30 C m)
14.4 f 0.1 18.4 & 0.1
14.5 + 0.1 23.4 + 0.3
28.9 k 0.2 41.8 + 0.4
28.1 f 0.3 39.0 * 0.4
Cyclohexane Dioxane
17.5 f 0.1 18.8 k 0.1
24.0 t 0.4 28.9 + 0.3
41.5 + 0.5 47.7 _+0.4
41.8 k 0.3 44.8 + 0.6
Cyclohexane Dioxane
25.0 + 0.1 28.2 f. 0.1
21.5 + 0.3 26.9 f 0.4
46.5 f 0.4 55.1 + 0.5
45.5 f 1.1 49.1 + 0.4
800
0
I -4’,““““‘,“‘t”“1-,“““‘1”‘,’ 28
29
30
31
32
33
34
35
Fig. 4. Absorption and electro-optical absorption spectra of CU4 in dioxane. The points show the experimental data, the line connecting the points is the approximation by a least-squares multilinear fit function according to Eq. (2).
Fig. 5. Plot of L(Q = 0) versus L(iQ = a/2) of CU4 in dioxane at T = 298 K. The points show the experimental data and the line is their least-squares approximation by a linear fit.
approximately, p,, Asp and the angles between the three vectors can be evaluated using Eqs. (7)-(12). The results are shown in Table 3.
Table 3 Dipole moments and angles between the different moments of CU4 in dioxane at T = 298 K
4.2. Integral electro-optical emission measurements (IEOEM)
$O‘30Cm)
(10-30Cm)
r;O-30Cm)
19 * 1
11 f 1
27 +2
According to Eq. (18) the quantity X(Q) was determined by measuring the relative-field-induced change of the total fluorescence intensity for the two angles @ = 0 and @ = 7~12.In order to check for possible interferences from superimposed fluorescence bands (impurities, etc.) the measurements
A%
FC
(rpma) (Ova)
(wW
38”
56”
18”
were repeated cutting off parts of the fluorescence band by cut-off filters. The equilibrated excited state dipole moment pe was calculated using Eq. (19). In accordance with 8 = 0 in the case of CUl,
N.A. Nemkovich
et al. 1 Journal of Luminescence
CU120 and CU334 in absorption, the angle d, between the unit vector of the emission transition moment m, and c(~was adopted to be zero, too. This is justified, since then every set of data X(O),X(7c/2)for different cut-off filters yields roughly the same value for the dipole moment pe. It is also in agreement with fluorescence polarization measurements [41], which have shown m, 11m, in the case of the first absorption band of coumarins. In addition, CUs are fairly rigid molecules, thus it might be assumed that the directions of the Franck-Condon (p:‘) and equilibrated (p,) excited state dipole moments are the same. The last column of Table 2 shows the values of 11,determined. As can be seen from the table, the values of the Franck-Condon (aFC) and equilibrated (p,) excited state dipole moments in cyclohexane are roughly equal within the experimental error. In dioxane the agreement is worse since in this medium-polar solvent Eq. (13) is no strictly valid (see Ref. [29]). Before performing the electro-optical measurements on a given solute/solvent system we measured the degree of anisotropy I over the entire fluorescence band, I being defined by
(20) I,, and II are the fluorescence intensities measured through an analyzing polarizer with parallel and perpendicular orientation with respect to the electric field vector of the excitation light. For CUl, CU120 and CU334 in both solvents we found r z 0. For CU4 in dioxane, on the other hand, we found r z 0.2 at room temperature. This indicates a very short lifetime of CU4 in its fluorescent excited state. Therefore, the standard theoritcal model used for the evaluation of IEOEM data cannot be applied and no results for the dipole moment pL,can be reported here.
5. Discussion 5.1. Intramolecular moments
charge redistribution
and dipole
As is well known coumarin itself is non-fluorescent. The intensive fluorescence and high laser
71 (1997) 255-263
261
efficiency of its derivatives with electron-donating groups at the 7-position has been interpreted based on intra-molecular charge-transfer, namely, electron transfer from a substituent (donor) at the 7-position to the lactone carbonyl group (acceptor) in the coumarin ring [2]. As a rule, coumarins intensively fluoresce and show high laser efficiency when electron-donating groups are introduced into electron-deficient positions (4-, 5- and 7-position) and whenever electron-acceptor groups are introduced into the remaining positions (3-, 6- and 8-position). Among the 7-aminocoumarins studied in this work, the values of the dipole moments both in the ground state as well as in the excited state (equilibrated and Franck-Condon) decrease in the series CU334 > CUl > CU120. This parallels the decreasing electron-donating ability of the differently substituted amino groups in the 7-position (diethylamino > amino), in agreement with semiem[13] which show that pirical calculations a diethylamino group in the 7-position leads to a strong charge separation within the coumarin structure and along the N-C axis. In the case of CU334, the COCHJ-group in the 3-position further enhances the electron-accepting power of the lactone structure. The bulk dielectric properties of cyclohexane (dielectric constant E = 2.015 at 25C) and dioxane (E = 2.209) are very similar. However, from solvent shift measurements it is known that the microscopically effective dielectric constant of dioxane is about 6-7 [29]. This is the reason why the dipole moments in dioxane are larger than those in cyclohexane (see Table 2). The spectral properties of coumarins with a free amino group often [lo, 13, 191 were treated in the framework of the twisted intramolecular charge transfer (TICT) model, originally proposed by Grabowski and co-workers [42-441 to explain the dual fluorescence of N,N-dimethylamino-benzonitrile. As can be seen from Table 2, the values of the dipole moments in the Franck-Condon and in the equilibrated excited states are roughly the same. This observation holds for the derivatives with a free amino group (CUl, CU120) as well as for the “rigidized” dye CU334. Hence, our results do not support the idea of fluorescent TICT state
N.A. Nemkovich
262 Table 4 Comparison
of the dipole moments
obtained
et al. /Journal
by electro-optical
measurements
Molecule
Solvent
pg (1O-3o C m)
p. (lo-”
cu4
Dioxane
19 17.3 17.3 13.8
cu120
CUl
Cyclohexane Dioxane
Cyclohexane Dioxane
71 (1997) 255-263
of Luminescence
and other methods Method
Reference
27”
Electra-optical
This work
26.4 19.3 11.7
Bakhshiev correlations KawskiGChamma-Viallet AM1
c451 c451 Cl31
14.4 18.4
28.1 39.0
Electra-optical Electra-optical
This work This work
21.1 21.1 20.1 20.0
30.8 24.5 27.3 29.9 40.4
Bakhshiev correlations Kawski-Chamma-Viallet AM1 AM1 AM1 + Lippert’s method
-
34.5
AM1 + ratio method
c451 1451 Cl31 Cl31 Cl71 Cl71
17.5 18.8
41.8 44.8
Electra-optical Electra-optical
This work This work
22.6 22.6 21.2
27.9 27.9 32.7
Bakhshiev correlations Kawski-ChammaPViallet AM1
c451 c451 Cl31
C m)
“Value of fip”,
formation in the excited state for these coumarins, at least in cyclohexane and dioxane. 5.2. Comparison of dipole moments obtained from electro-optical measurements and from other methods The values of the ground and excited state dipole moments determined in this work may be compared with those obtained from other methods, in particular from experimental techniques [45] such as Bakhshiev correlations [46], from the Kawski-Chamma-Viallet formula [47,48] and from semiempirical calculations using the Austin Model 1 (AMl) alone [ 131 and in combination with the Lippert treatment or a ratio method [17]. Those methods yield the dipole moments of the free molecule. By comparing the results given in Table 4 it should be kept in mind that the dipole moments from electro-optical methods remain solvent-dependent, since they incorporate the effects of the reaction field in the Onsager model. This effect usually enhances the dipole moment of the free molecule somewhat (see e.g. Ref. [29]). As can be seen from Table 4 the dipole moments obtained
from the electro-optical measurements are in reasonable agreement with those derived from other methods, considering the well-known limitations of the solvent shift methods, in particular the uncertainties connected with the estimate of the molecular interaction radius of the solute.
6. Conclusions The permanent dipole moments of the investigated CUs in ground and excited Franck-Condon states in cyclohexane and dioxane have values within the range (14.4-55.1) x 10m3’ C m. Upon optical excitation the dipole moments increase by (14.5-26.9) x 10e3’ C m. CU334 shows the largest values of ,uLgand h,FC. The change of the electric dipole moment on optical excitation is largest for CUl. The angle between the transition moment m, and the vectors pp, A”p, and consequently, rzC is (0 + 100) for CUl, CU120, CU334, while for CU4 the values are in the range (18-56”). Our results do not support the idea of fluorescent TICT formation after optical excitation in the studied coumarins, at least in dioxane and cyclohexane,
N.A. Nemkovich et al. J Journal of Luminescence 71 (1997) 255-263
because non-emissive TICT state formation, proposed e.g on Ref. [ 161, is not ruled out.
as
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft (DFG) for a fellowship (N. A. N.) is gratefully acknowledged. We thank Dr. N. Detzer and Mrs. G. Bormann for the purification of the coumarins and the solvents.
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K.H. Drexhage, in: F.P. Schafer (Ed), Dye Lasers (Springer. New York, 1977). [2] E. Lippert, in: J.B. Birks (Ed.), Organic Molecular Photophysics, Vol. 2, Wiley, New York, 1974. [3] A.N. Rubinov, V.I. Tomin, in: R.G. Mirimanov (Ed.), Electronics: Vol. 9, Quantum Electronics (Radiotehnika, Moscow, 1976). [4] K.D. Singer, S.L. Lalama, J.E. Sohn, R.D. Small, in: D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, Ch. II-8 (Academic Press, Orlando, 1987). c51J.F. Nicoud, R.J. Twieg, in: Ref. [4, Ch. 11-31. C61Ch.R. Moylan, J. Phys. Chem. 98 (1994) 13513. [71 W.E. Moerner, S.M. Silence, Chem. Rev. 94 (1994) 127. T. Masuda, C. Murata, C. Haratake, PI A. Takadate, A. Isobe. M. Irikura, S. Goya, Anal. Sci. 8 (1992) 695. Fluorescent Probes for Study Cells, c91G.E. Dobretsov, Membranes and Proteins (Nauka, Moscow, 1989). T. Masuda, C. Murata, T. Tanaka, M. Cl01A. Takadate, Irikura, S. Goya, Anal. Sci. 11 (1995) 97. Cl11T.L. Gilchrist, Heterocyclic Chemistry (Pittman, London, 1985). [I21 O.S. Wolfbeis, E. Furlinger, H. Kroneis, H.Z. Marsoner, Anal. Chem. 314 (1983) 577. u31 P.K. McCarthy, G.J. Blanchard, J. Phys. Chem. 97 (1993) 12205. Cl41G. Jones II, W.R. Jackson, A.M. Halpern, Chem. Phys. Lett. 72 (1980) 391. Cl51W. Rettig, A. Klock, Can. J. Chem. 63 (1985) 1649. Cl61G. Jones II, W.R. Jackson, C. Choi, W.R. Bergmark, J. Phys. Chem. 89 (1985) 294. Cl71K. Rechthaler, G. Kohler, Chem. Phys. 189 (1994) 99. Cl81L. Taneja, A. Gaur, A.K. Sharma, R.D. Singh, Opt. Commun. 106 (1994) 79. Cl91L. Taneja. A.K. Sharma, R.D. Singh, J. Lumin. 63 (1995) 203. C.A. Gartner, M.G. Miguel, WI R.S. Becker, S. Chakravorti, J. Chem. Sot. Faraday Trans. 89 (1993) 1007.
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1211 CF. Chapman. R.S. Fee, M. Maroncelli, J. Phys. Chem. 99 (1995) 4811. [22] M. Maroncelli, G.R. Fleming. J. Chem. Phys. 86 (1987) 6221. 1231 J. Seixas de Melo, A.L. Macanita, Chem. Phys. Lett. 204 (1993) 556. [24] J. Seixas de Melo, R.S. Becker, A.L. Macamta, J. Phys. Chem. 98 (1994) 6054. [25] A.N. Fletcher, Appl. Phys. 14 (1977) 295. [26] W. Baumann, Z. Nagy, A.K. Maiti, H. Reis, S.V. Rodrigues. N. Detzer, in: N. Mataga, T. Okada. H. Masuhara (Eds.), Dynamics and Mechanisms of Photoinduced Transfer and Related Phenomena, (Elsevier. Amsterdam, 1992). ~271W. Baumann, Z. Nagy, Pure. Appl. Chem. 65 (1993) 1729. WI H. Reis, Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” am Fachbereich Chemie und Pharmazie der Johhannes Gutenberg-Universitat in Mainz, Mainz, 1995. in: B.W. Rossiter, J.F. Hamilton (Eds.), 1291 W. Baumann, Physical Methods of Chemistry. Vol. 3B (Wiley, New York, 1989). c301 W. Rettig, W. Baumann, in: J.F. Ralek (Ed.), Progress in Photochemistry and Photophysics, Vol. VI (CRC Press, Boca Raton, 1992). c311 W. Liptay, in: E.C. Lim (Ed.), Excited States. Vol. 1 (Academic Press. New York, 1974). H. SchaIfrin. 0. Burkhard, ~321 W. Liptay, R. Wortmann, W. Reitinger, N. Detzer, Chem. Phys. 120 (1988) 429. c331 L. Onsager, J. Amer. Chem. Sot. 58 (1936) 1486. c341 E.J.J. Groenen, Mol. Phys. 36 (1978) 1555. c351 W. Baumann, 2. Nagy, H. Reis, N. Detzer, Chem. Phys. Lett. 224 (1994) 517. 1361 W. Baumann, H. Bishof, J. Mol. Struct. 84 (1982) 181. c371 W. Baumann, H. Bishof, J. Mol. Struct. 129 (1985) 125. C381 W. Baumann, H. Deckers, Ber. Bunsenges. Phys. Chem. 81 (1977) 786. Solc391 J.A. Riddicu, W.B. Bunger, T.U. Sakano, Organic vents, 4th ed. Wiley, New York 1986. P. Kramer, C. Glama. N. Detzer, Chem. c401 R. Wortmann, Phys. 173 (1993) 99. and Emisc411 A.M. Sarzshevski, A.N. Sevchenko, Absorption sion Anisotropy of Molecules (Izdatelstvo BGU, Minsk, 1971). K.H. Grellman. Z.R. Grabowski, Chem. c421 K. Rotkiewicz, Phys. Lett. 19 (1973) 315. K. Rotkiewicz, A. Siemiarczuk. D.L. c431 Z.R. Grabowski, Cowley. W. Baumann, Nouv. J. Chim. 3 (1979) 443. J. Dobkowski, Pure Appl. Chem. 55 c441 Z.R. Grabowski, (1983) 245. M.Sh. Antonius, J.J. Aaron. M. Buna, c451 C. Parkanyi, A. Tine, L. Cisse, Spectr. Lett. 27 (1994) 439. 1461 N.G. Bakhshiev, Opt. Spectrosk. 16 (1964) 821. c471 A. Kawski. L. Bilot, Acta Phys. Polon. 26 (1964) 41. C481 A. Chamma, P. Viallet. CR. Acad. Sci. Ser. C 270 (1970) 1901.
LUMINESCENCE ELSEVIER
Journal
of Luminescence
71 (1997) 255-263
Ground and excited state dipole moments of coumarin laser dyes: Investigation by electro-optical absorption and emission methods aB.I. Stepanoc
N.A. Nemkovich”,*, H. Reisb, W. Baumannb Institute of Physics, Academy ofSciences of Belarus, F. Skaryna Ave. 70.220072
Minsk, Belarus b Institute qf Physical Chemistry, University ofhlainz, Jakob Welder- Weg 1 I, 55099 Mainz, Germany Received
24 October
1996; accepted
20 January
1997
Abstract Modified electro-optical absorption and emission methods were used to measure the dipole moments of four efficient coumarin laser dyes (CUl, CU4, CU120, CU334) in the equilibrated ground, excited Franck-Condon and equilibrated excited states. The measurements were performed in cyclohexane (CUl, CU120, CU334) and dioxane (all CUs) at room temperature. Our results show that the charge distribution in CU4 differs substantially from that in the other CUs. The equilibrated ground and excited state dipole moments measured by electro-optical methods are compared with those derived from other measurement techniques and from semiempirical calculations. The possible role of twisted intermolecular charge transfer (TICT) state formation is discussed. Keywords:
Coumarins;
Absorption;
Fluorescence; Dipole moments; Electra-optical
1. Introduction The coumarins are a family of substances that have been studied extensively due to their application as an active medium in dye lasers [l-3]. Coumarin derivatives also have many other applications. In non-linear optics, coumarin-doped polymers are widely used as electro-optical materials for modulating of continuous-wave beams [4], frequency doubling of diode laser light [S], harmonic generation [6] and as photorefractive materials [7]. In chemistry some coumarins were used as fluorescence derivatization reagents for
*Corresponding author. Fax: 375 ifanbel%[email protected].
172
393131;
e-mail:
0022-2313/97/$17.00 80 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00098-7
method
high-performance liquid chromatography [8], fluorescence probes for protein studies [9] and as fluorescent ionophores [lo]. In medicine, 4- and 7-hydroxycoumarins are effective anticoagulants [ 111, fluorescence indicators [ 121, etc. Different coumarin derivatives in liquid solution show different absorption and fluorescence properties since their ring structure differs from that of aromatic ring systems like anthracene or pyrene. Hence, they are interesting candidates not only from the view of applications, but also with respect to an understanding of absorption and emission mechanisms. In general, the coumarin laser dyes with an amino group may be divided into two classes, namely, those with a free amino group and those with a rigid amino group. In the first case the coumarins exhibit a pronounced dependence of
256
N.A. Nemkovich
et al. /Journal
their absorption and fluorescence properties on the solvent polarity and have been object of investigations as potential candidates for twisted intermolecular charge transfer (TICT) reaction after optical excitation [lo, 13-191. The rigidized coumarins have other solvent polarity-dependent properties because their amino group does not allow the rotational movements required to stabilize a full charge separation, as is postulated by the model. Intramolecular charge redistribution in molecules after transition to a different electronic state is a fundamental process in chemistry. A very fruitful and widely used approach for studying this process is to determine the electric dipole moments in the related electronic states. A number of papers were published concerning the electronic spectra and the effect of the structure, pH and solvent on the fluorescence and lasing properties of coumarins [20]-[25]. But only very few papers on the determination of dipole moments by different solvent shift methods have been communicated and only few coumarins have been object of determining their dipole moments by electro-optical absorption
of Luminescence
71 (1997) 255-263
and emission methods [26]-[28]. Comparing the electro-optical methods with the solvent shift methods reveals the strong advantage of the former, namely, that the dipole moments are determined in one given solvent. Hence, it is possible to study the influence of the solvent on the process of intramolecular charge redistribution in ground and excited electronic states by determining the dipole moments effective in differently polar solvents. In this paper we report the results from electrooptical absorption and emission measurements on the equilibrated ground state, the excited Frank-Condon state and the equilibrated excited state dipole moments of four highly efficient coumarin laser dyes in cyclohexane and dioxane solution. Three are non-rigid compounds with different substituent groups at the 7th position, while the fourth has a rigid amino group (see Fig. 1).
2. Methods The determination of ground and excited state dipole moments from electro-optical measurements
‘i
H3
HoJ3Clo Coumarin 4 (CU4)
Coumarin 120 (GUI 20)
‘i H3
/ W2
Coumarin 1 (CUl) Fig. 1. Coumarins
Coumarin 334 (CU334) studied
in this work.
N.A. Nemkovich
et al. /Journal
has been given in detail in previous papers [29,30]. So, only the basic principles of electro-optical absorption (EOAM) and integrated emission (IEOEM) measurements in solution are briefly summarized. Using Liptay’s formalism [31] the effect of an external electric field Er on the molar absorption coefficient K(C),where S is the wave number, can be described by a quantity L, which is defined by
Wx) =
KE(C,K) - K(J) K(iqE! ’
(1)
where xE is the compound’s molar absorption coefficient in the presence of an applied field and x is the angle between the direction of Ef and the electric field vector of the incident linearly polarized light. For a homogeneously broadened absorption band, L is given by the following equation [29,32]: L = Dr + Es/6 + Frt + Gst + Hru + Zsu,
(2)
where the parameters r and s are determined by the angle x and the quantities t and u depend on the first and second derivatives of the absorption spectrum: r = (2 - cos2x)/5,
(3)
s = (3co&
(4)
- 1)/5,
t = (l/hc)(~/~)-‘d(~/j)/d~,
(5)
u = ( 1/2h2c2) (K/c)-
(6)
‘d2(rc/;)/dS2,
where h is Planck’s constant and c is the speed of light in the vacuum. For the molecules discussed in this communication, the electric dipole moment terms are dominant and explicit polarizability and transition polarizability effects can be neglected.With this assumption the coefficients D-I are given by D z 0
within the approximation
E = (l/KT)2f,Z[13(V,)2
- &],
F = (lI~Vf,2(~pAaCo,
used here,
(7) (8) (9)
G = (l/~T)f,2(mallg)(ltl~A~~),
(10)
H =.f:(Aap)2>
(11)
I =f?(ltlaAaCo2,
(12)
where k is the Boltzmann constant, T is the temperature, m, is the unit vector in the direction of the
of Luminescence
71 (1997) 255-263
251
transition dipole moment of absorption, pLpis the equilibrated ground state dipole moment vector and A”p is the change of the dipole moment vector upon excitation to the considered Franck-Condon excited state. In nonpolar solvents, it holds in good approximation: b”p = PFC - &>
(13)
where p cc is the dipole moment vector in the Franck-Condon excited state. From a set of the five coefficients D-l the values of pg, A”y and the angles between the transition dipole moment m,, ps and A”p can be determined. The cavity field correctionf, is defined according to Onsager’s model [33] as introduced to the electrooptical methods by Liptay [31]: fe = 3~/(2~ + l),
(14)
where E is the relative permittivity of the solvent. The experimental set-up for EOAM has been described in detail previously [29,30]. The quantity L(C J) in the present work was determined for two values of the angle x(x = 0 and x = 7c/2) and for a set of wave numbers within the first absorption band. Then the coefficients D-l and their standard deviations were obtained by multiple linear regression according to Eq. (2). Regrettably, the coefficients H and I often cannot be determined with sufficient accuracy. Nevertheless, the values of the dipole moments pg and 6”~ can be determined, if additional information about the symmetry of the molecules is available. Groenen [34] already used the electro-optical method to show homogeneity of an absorption band for a molecule with C2 symmetry, for which the transition dipole moment is either parallel or perpendicular to pK The same idea was also used by Baumann et al. [35] to show that a field-induced absorption and fluorescence antisotropy can be defined, which will be constant over pure absorption or fluorescence bands, if the solute molecules have C, symmetry, at least approximately. From more general considerations [28] it follows that, if psl(Aa~, the function L(v”,x = 0) =f[L(C,,x = rc/2)] is given by the simple linear relationship L(3,O) = AL(+c/2) + Bf&f/(6k2T2),
(15)
258
N.A. Nemkovich et al. /Journal of Luminescence 71 (1997) 255-263
where
3. Materials
A = (1 + 2cos%)/(2 - cos%),
(16)
B = (3cos% - 1)/(2 - co&Q
(17)
8 is the angle between m, and I(~. Hence, the angle 0 can be determined from the slope of the linear function L(?,O) =f[L(G,x/2)]. In this work the value of the electric dipole moment ,LL~ in the equilibrated excited state has been determined by integral electro-optical emission measurements (IEOEM). IEOEM is presented in detail in Refs. [36, 371 on the basis of the theory given in Ref. [38]. The experimental set-up for IEOEM is described in Ref. [29]. The effect of an external electric field Ef on the total fluorescence photon current q is described by the quantity X, given by x(@)
=
qE(@) - 4(Q) d@)Efz’
(18)
where @is the angle between the direction of Ef and the polarization direction of the emitted fluorescence light as determined by an analysing polarizer. In a typical measurement the quantity X is determined for two polarization angles, namely, Q, = 0 and @ = n/2. If explicit polarizability and transition polarizability terms can be neglected and if further ~(JlA~el(,where Ae”pis the change of the electric dipole moment vector upon transition from the equilibrated excited state to the Franck-Condon ground state, the relationship between X(@ = 0) and X(@ = n/2) is given by a relationship similar to Eq. (15) namely,
Fig. 1 presents the four CU molecules studied in this work. All compounds were purified by repeated liquid chromatography or recrystallization. The purity of the CUs was checked by thin layer chromatography. Cyclohexane and dioxane were purified by methods described in Ref. [39]. The purified solvents were dried prior to use by distillation under reflux conditions over sodium/potassium in an argon atmosphere. The purity of the solvents was checked by UV absorption measurements (1 cm cell; reference air).
4. Experimental results 4.I. Electra-optical absorption measurements (EOAM) The electro-optical absorption spectra of all four CUs are accurately reproducible, and show elements of vibrational structure especially for solutions in cyclohexane. As an example, the experimental data points of the electro-optical absorption spectrum of CU120 in cyclohexane are shown in Fig. 2. In Table 1 we show the results of
X(0) = AX(rc/2) + B[f,ZpU,2/(6PP) - yir,,x = x/2)], (19) and where A = (1 + 2 cos2 f$)/(2 - cos2 4) B = (3 COS”~- 1)/(2 - cos2 4); 4 is the angle between the unit vector of the transition dipole moment of emission m, and the excited state dipole moment I(~. The term L(f,,x = n/2) in Eq. (19) describes the field effect on the excitation process and can often be neglected. Equation (19) offers a simple method to determine the dipole moment in the equilibrated excited state I(~,if the angle 4 between m, and I(~is known in sufficient approximation, e.g. from symmetry [36, 371.
i
0
0
t=l -2 -4’. 28
29
30
31
32
33
Fig. 2. Absorption and electro-optical absorption spectra of CU120 in cyclohexane. The points show the experimental data, the line connecting the points is the approximation by a leastsquares multilinear fit function according to Eq. (2).
259
N.A. Nemkovich et al. 1 Journal of Luminescence 71 (1997) 255-263
Table 1 Results from electro-optical absorption measurements on CUs in cyclohexane and dioxane not be measured since the solubility was too low. For the coefficients H and I, see text Coefficient
cu4
Solvent: dioxane D (lo-“’ V2 m’) E (lo-‘” V-* m’) F ( I 0 _ “I C V ~-’ ml) G (lo- “’ C V ’ mZ) H (lO_“” C* m’) I (l0-h” C’ m’)
76 3174 415 518 183 174
+ * f k + *
2 13 9 9 21 21
the multilinear regression analysis according to Eq. (2) with Eqs. (3)-(6) for all measurements. CU4 in cyclohexane could not be measured because the solubility in this solvent was too low. Unfortunately, the coefficients H and I could not be determined with sufficient accuracy in all cases. In order to calculate the values for cp, A”p and pc’ using Eqs. (7)-(10) additional information about the angles between m,, pg, and A”p is necessary. For CU 1, CU 120 and CU334 in both solvents the plots of L(c,x = 0) versus L(v”,x= 7c/2) yielded straight lines, as shown in Fig. 3 for CU120 in dioxane. In all these cases, the corresponding linear fit resulted in A z 3, at least within the error, where A is the slope defined in Eq. (15). From Eq. (16) it follows that m, )I pg I/ Asp . The same conclusion can be reached by the observation that F z G (see Table 1). Using this symmetry condition, pg was calculated from E - 60 (which is the correct formula to take into account the small effects from the transition polar&ability [40]), then Asp from (F + G)/2 and finally pFc using Eq. (13). The results are shown in Table 2. In the case of CU4 in dioxane the situation is more complicated. The experimental electro-optical data are well approximated by the multilinear fit function (see Fig. 4), but the linear fit of L(v”,x= 0) versus L(<,x = x/2) (see Fig. 5) yields A z 3.7. From Eq. (16) we can see that A must be in the range of 0.5-3 for all possible values of 9. Also,
could
cu334
WI
cu120
Solvent: cyclohexane D (lO_‘” VmZm2) E (lO-z”V-Lmz) F (1O-4” C V ’ m*) G(10-5”CV ‘m2)
at T = 298 K. CU4 in cyclohexane
51 3956 142 780
+ 1 + 10 i 4 +- 6
57 5493 1437 1479
f * i +
7 44 16 16
150 11535 1869 1900
* + + +
63 6340 1521 1607 _ _
+ k & k
43 6504 1922 2030
f + + +
5 21 15 15
178 15129 2732 2774
+ 12 + 71 ) 32 +_ 32
4 25 16 16
14 85 23 23
2500 [‘-‘-r--
Fig. 3. Plot of L(?,x = 0) versus L(c,x = n/2) of CUl20 in dioxane at T = 298 K. The points show the experimental data and the line is their least square approximation by a linear fit.
from Table 1 it can be seen that G > F for CU4 in dioxane. These findings show that the vectors pg and A”p are not parallel to each other, and both are not parallel to m,. In this case, Eq. (15) is no more valid. Assuming the three vectors to lie in the plane formed by the coumarin ring and assuming the coefficients H and I to be correct at least
260
N.A. Nemkovich et al. /Journal
of Luminescence
71 (1997) 255-263
Table 2 Dipole moments of CUl, CU120 and CU334 in cyclohexane and dioxane at T = 298 K Molecule
Solvent
pB (lo-s°C
cu120
Cyclobexane Dioxane
CUl cu334
m)
A”p (10m3’ C m)
~c.‘~(10m30C m)
pe (10e30 C m)
14.4 f 0.1 18.4 & 0.1
14.5 + 0.1 23.4 + 0.3
28.9 k 0.2 41.8 + 0.4
28.1 f 0.3 39.0 * 0.4
Cyclohexane Dioxane
17.5 f 0.1 18.8 k 0.1
24.0 t 0.4 28.9 + 0.3
41.5 + 0.5 47.7 _+0.4
41.8 k 0.3 44.8 + 0.6
Cyclohexane Dioxane
25.0 + 0.1 28.2 f. 0.1
21.5 + 0.3 26.9 f 0.4
46.5 f 0.4 55.1 + 0.5
45.5 f 1.1 49.1 + 0.4
800
0
I -4’,““““‘,“‘t”“1-,“““‘1”‘,’ 28
29
30
31
32
33
34
35
Fig. 4. Absorption and electro-optical absorption spectra of CU4 in dioxane. The points show the experimental data, the line connecting the points is the approximation by a least-squares multilinear fit function according to Eq. (2).
Fig. 5. Plot of L(Q = 0) versus L(iQ = a/2) of CU4 in dioxane at T = 298 K. The points show the experimental data and the line is their least-squares approximation by a linear fit.
approximately, p,, Asp and the angles between the three vectors can be evaluated using Eqs. (7)-(12). The results are shown in Table 3.
Table 3 Dipole moments and angles between the different moments of CU4 in dioxane at T = 298 K
4.2. Integral electro-optical emission measurements (IEOEM)
$O‘30Cm)
(10-30Cm)
r;O-30Cm)
19 * 1
11 f 1
27 +2
According to Eq. (18) the quantity X(Q) was determined by measuring the relative-field-induced change of the total fluorescence intensity for the two angles @ = 0 and @ = 7~12.In order to check for possible interferences from superimposed fluorescence bands (impurities, etc.) the measurements
A%
FC
(rpma) (Ova)
(wW
38”
56”
18”
were repeated cutting off parts of the fluorescence band by cut-off filters. The equilibrated excited state dipole moment pe was calculated using Eq. (19). In accordance with 8 = 0 in the case of CUl,
N.A. Nemkovich
et al. 1 Journal of Luminescence
CU120 and CU334 in absorption, the angle d, between the unit vector of the emission transition moment m, and c(~was adopted to be zero, too. This is justified, since then every set of data X(O),X(7c/2)for different cut-off filters yields roughly the same value for the dipole moment pe. It is also in agreement with fluorescence polarization measurements [41], which have shown m, 11m, in the case of the first absorption band of coumarins. In addition, CUs are fairly rigid molecules, thus it might be assumed that the directions of the Franck-Condon (p:‘) and equilibrated (p,) excited state dipole moments are the same. The last column of Table 2 shows the values of 11,determined. As can be seen from the table, the values of the Franck-Condon (aFC) and equilibrated (p,) excited state dipole moments in cyclohexane are roughly equal within the experimental error. In dioxane the agreement is worse since in this medium-polar solvent Eq. (13) is no strictly valid (see Ref. [29]). Before performing the electro-optical measurements on a given solute/solvent system we measured the degree of anisotropy I over the entire fluorescence band, I being defined by
(20) I,, and II are the fluorescence intensities measured through an analyzing polarizer with parallel and perpendicular orientation with respect to the electric field vector of the excitation light. For CUl, CU120 and CU334 in both solvents we found r z 0. For CU4 in dioxane, on the other hand, we found r z 0.2 at room temperature. This indicates a very short lifetime of CU4 in its fluorescent excited state. Therefore, the standard theoritcal model used for the evaluation of IEOEM data cannot be applied and no results for the dipole moment pL,can be reported here.
5. Discussion 5.1. Intramolecular moments
charge redistribution
and dipole
As is well known coumarin itself is non-fluorescent. The intensive fluorescence and high laser
71 (1997) 255-263
261
efficiency of its derivatives with electron-donating groups at the 7-position has been interpreted based on intra-molecular charge-transfer, namely, electron transfer from a substituent (donor) at the 7-position to the lactone carbonyl group (acceptor) in the coumarin ring [2]. As a rule, coumarins intensively fluoresce and show high laser efficiency when electron-donating groups are introduced into electron-deficient positions (4-, 5- and 7-position) and whenever electron-acceptor groups are introduced into the remaining positions (3-, 6- and 8-position). Among the 7-aminocoumarins studied in this work, the values of the dipole moments both in the ground state as well as in the excited state (equilibrated and Franck-Condon) decrease in the series CU334 > CUl > CU120. This parallels the decreasing electron-donating ability of the differently substituted amino groups in the 7-position (diethylamino > amino), in agreement with semiem[13] which show that pirical calculations a diethylamino group in the 7-position leads to a strong charge separation within the coumarin structure and along the N-C axis. In the case of CU334, the COCHJ-group in the 3-position further enhances the electron-accepting power of the lactone structure. The bulk dielectric properties of cyclohexane (dielectric constant E = 2.015 at 25C) and dioxane (E = 2.209) are very similar. However, from solvent shift measurements it is known that the microscopically effective dielectric constant of dioxane is about 6-7 [29]. This is the reason why the dipole moments in dioxane are larger than those in cyclohexane (see Table 2). The spectral properties of coumarins with a free amino group often [lo, 13, 191 were treated in the framework of the twisted intramolecular charge transfer (TICT) model, originally proposed by Grabowski and co-workers [42-441 to explain the dual fluorescence of N,N-dimethylamino-benzonitrile. As can be seen from Table 2, the values of the dipole moments in the Franck-Condon and in the equilibrated excited states are roughly the same. This observation holds for the derivatives with a free amino group (CUl, CU120) as well as for the “rigidized” dye CU334. Hence, our results do not support the idea of fluorescent TICT state
N.A. Nemkovich
262 Table 4 Comparison
of the dipole moments
obtained
et al. /Journal
by electro-optical
measurements
Molecule
Solvent
pg (1O-3o C m)
p. (lo-”
cu4
Dioxane
19 17.3 17.3 13.8
cu120
CUl
Cyclohexane Dioxane
Cyclohexane Dioxane
71 (1997) 255-263
of Luminescence
and other methods Method
Reference
27”
Electra-optical
This work
26.4 19.3 11.7
Bakhshiev correlations KawskiGChamma-Viallet AM1
c451 c451 Cl31
14.4 18.4
28.1 39.0
Electra-optical Electra-optical
This work This work
21.1 21.1 20.1 20.0
30.8 24.5 27.3 29.9 40.4
Bakhshiev correlations Kawski-Chamma-Viallet AM1 AM1 AM1 + Lippert’s method
-
34.5
AM1 + ratio method
c451 1451 Cl31 Cl31 Cl71 Cl71
17.5 18.8
41.8 44.8
Electra-optical Electra-optical
This work This work
22.6 22.6 21.2
27.9 27.9 32.7
Bakhshiev correlations Kawski-ChammaPViallet AM1
c451 c451 Cl31
C m)
“Value of fip”,
formation in the excited state for these coumarins, at least in cyclohexane and dioxane. 5.2. Comparison of dipole moments obtained from electro-optical measurements and from other methods The values of the ground and excited state dipole moments determined in this work may be compared with those obtained from other methods, in particular from experimental techniques [45] such as Bakhshiev correlations [46], from the Kawski-Chamma-Viallet formula [47,48] and from semiempirical calculations using the Austin Model 1 (AMl) alone [ 131 and in combination with the Lippert treatment or a ratio method [17]. Those methods yield the dipole moments of the free molecule. By comparing the results given in Table 4 it should be kept in mind that the dipole moments from electro-optical methods remain solvent-dependent, since they incorporate the effects of the reaction field in the Onsager model. This effect usually enhances the dipole moment of the free molecule somewhat (see e.g. Ref. [29]). As can be seen from Table 4 the dipole moments obtained
from the electro-optical measurements are in reasonable agreement with those derived from other methods, considering the well-known limitations of the solvent shift methods, in particular the uncertainties connected with the estimate of the molecular interaction radius of the solute.
6. Conclusions The permanent dipole moments of the investigated CUs in ground and excited Franck-Condon states in cyclohexane and dioxane have values within the range (14.4-55.1) x 10m3’ C m. Upon optical excitation the dipole moments increase by (14.5-26.9) x 10e3’ C m. CU334 shows the largest values of ,uLgand h,FC. The change of the electric dipole moment on optical excitation is largest for CUl. The angle between the transition moment m, and the vectors pp, A”p, and consequently, rzC is (0 + 100) for CUl, CU120, CU334, while for CU4 the values are in the range (18-56”). Our results do not support the idea of fluorescent TICT formation after optical excitation in the studied coumarins, at least in dioxane and cyclohexane,
N.A. Nemkovich et al. J Journal of Luminescence 71 (1997) 255-263
because non-emissive TICT state formation, proposed e.g on Ref. [ 161, is not ruled out.
as
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft (DFG) for a fellowship (N. A. N.) is gratefully acknowledged. We thank Dr. N. Detzer and Mrs. G. Bormann for the purification of the coumarins and the solvents.
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