A spectroscopic study of charge transfer complexes of 7-diethylamino-4-methylcoumarine laser dye

Spectrochimica Acta, Vol. 49A, No. 3, pp. 437-442, 1993

0584-8539/93 $6.00+0.00 ~) 1993 Pergamon Press Ltd

Printed in Great Britain

RESEARCH NOTE

A spectroscopic study of charge transfer complexes of 7-diethylamino-4-methylcoumarine laser dye (Received 31 October 1991; in finul form 10 June 1992; accepted 3 July 1992)

THE CHARGE transfer (CT) complexes of 7-diethyiamino-4-methylcoumarine (DMC), with some ~-acceptors such as tetracyanoethylene, 7,7,8,8-tetracyanoquinonedimethane, 2,3-dichloro-5,6dicyano-p-benzoquinone, p-chloranil, picric acid, 2,4-dinitrophenol and 1,3,5-trinitrobenzene were studied in solution by UV-vis and fluorescence spectroscopies. The roles of the solvent, temperature and the use of different acceptors on the efficiency of the fluorescence quenching of DMC were studied. In addition, the solvent effect on the CT complex formed between DMC and tetracyanoethylene was investigated spectrophotometrically. The results show that the CT complex exhibits a solvatochromic behaviour. The ionization potential of DMC and the equilibrium parameters of CT complex of DMC-tetracyanoethylene were also determined. Aminocoumarines are recognized well as laser dyes in the blue-green region [1]. Continuous and quasi-continuous operations of dye lasers have been achieved with these dyes under different conditions [2--4]. A~rrANOV and HAHLA [5] have studied their photostability under excimer laser pumped conditions while FLETCHERand coworkers [6-8] have reported the effects of medium, dye structure and temperature on their laser activity. Under flash lamp pumping, WlwrEgs et ai. [9] have identified two irreversible pathways for the photodegradation of DMC resulting in a total of five photoproducts. JONES and coworkers [10, 11] have proposed a mechanism for the photodegraclarion of DMC dye involving singlet self-quenching. PRIYADARSINIet al. [12] have studied the photodegradation of DMC using monochromatic fight (350 nm) radiation. In the present paper we report the fluorescence quenching of DMC using some organic electron acceptors. The CT complexes of DMC with some ~-electron acceptors have also been studied using spectroscopic methods. The acceptors used in the present paper are tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinonedimethane (TCNQ), 2,3-dichioro-5,6-dicyano-p-benzoquinone (DDQ), p-chloranil (CHL), picric acid (PIC), 2,4-dinitrophenol (DNP) and 1,3,5trinitrobenzene (TNB). The electronic absorption spectra of the (DMC-TCNE) CT complex in solvents of different polarities have been investigated. In addition the equilibrium constant (Kcr) and the molar absorptivity (ecr) of the DMC-TCNE system in chloroform have been determined using Rose and Drago's procedure [13]. The DMC dye was synthesized from 7-diethylaminophenol and ethylacetate by PECHMANN[14] condensation and purified by repeated crystallization from benzene-cyclohexane. The purity was checked by HPLC and TLC; the extinction coefficient, fluorescence and lasing characteristics of the synthesized dye were found to be in agreement with those of an authentic laser grade sample. TCNE and TCNQ (Aldrich) were purified by sublimition; DDQ, CHL, DNP (Aldrich) were recrystallized from benzene; and TNB and PIC were recrystallized twice from ethanol. All solvents used are BDH and Aldrich (spectroscopic grade) and used without further purification. Fluorescence measurements were carried out using a Shimadzu RF 510 spectrofluorophotometer connected to an ultrathermostated Julabo F 10 of temperature precision _0.1°C. Absorption spectra were recorded on a Shimadzu Model UV 240 spectrophotometer using I cm matched silica cells placed in a thermostated cell holder. All measurements were performed using freshly prepared solutions. Fluorescence quenching of DMC laser dye by electron acceptor organic compounds such as TCNE, TCNQ, PIC and DNP was studied in chloroform by steady-state emission measurements. From the values of fluorescence intensities and applying the STERN--VoLMER [15] relation 1o/1 = 1 + Ksv[Q],

(1)

where I0 and I represent the fluorescence intensities in the absence and presence of the quencher, [Q] is the concentration of quencher and Ksv is the second-order quenching rate constant; as shown in Fig. 1, the Stern-Volmer plots in all cases are not linear, indicating a static type mechanism via ground-state complex formation. 437

438

Research Note

3.4

2.6

1.8

1.0

elo

l;

1'8

[Ol x 105 M

Fig. 1. Fluorescence quenching of DMC (10-5 M) by different acceptors in chloroform solvent (O) DNP, (~) TCNE, and (0) TCNQ (~l== 365 nm). The quenching efficiency increases with the electron affinity of acceptor, i.e. in the order TCNQ (E ^ = 2.2 eV) > TCNE)E ^-- 1.7 eV) > DNP (E A= 0.50 eV). This indicates that the quenching process occurs via charge transfer from donor to acceptor. The emission spectral patterns do not change as the quencher concentration increases despite the observed substantial decrease in emission intensities as shown in Fig. 2. This implies the absence of emitting excited state complexes. The fluorescence quenching of DMC dye using PIC as acceptor has also been studied. Figure 3 shows the effect of medium viscosity and temperature on the fluorescence quenching of DMC dye by PIC. The quenching efficiency increases in glycerol 07= 10.7 poise) more than that in chloroform 07 = 0.005 poise). This is explained in terms of the role of increased viscosities in imposing a cage effect that enhances the static quenching [16]. The static quenching mechanism was substantiated by studying the temperature effect on fluorescence quenching efficiency as shown in Fig. 3. The quenching efficiency decreases as the temperature increases indicating a static-type mechanism. The role of thermal energy in destabilizing the ground state complex leads to a decrease in quenching efficiencies.

/4 C

ii i ii" ",!i

t~ L. v

W r-

e-

i,ip /.\ !

tO

,;,iii"i ',

E

IJJ

/~.//, ii .. 600

,500

400

Wavelength (nm)

Fig. 2. Fluorescence quenching of DMC (10 -s M) by DNP in chloroform (,t~ffi365 nm). The concentrations of DNP at decreasing emission intensities are: 2x 10-5, 5 x 10-5, 10x 10-5, 12 x 10-5, 14 x 10-5, 15 x 10-5 and 20 x 10-5 M.

Research Note

439

4.0

3.C

3 2.0

1.0

1'o

3'o

5'o

I

7.0

[Q] x IO~M

Fig. 3. Effect of temperature and medium viscosity on fluorescence quenching of DMC by PIC in chloroform at 40°C (0), in chloroform at 20°C (C)) and in glycerol at 20°C (~).

Since the CT transition involves ground and excited states with different dipole moments, this suggests that the CT absorption band should exhibit marked solvent polarity effects. For example, the CT absorption band of the acenaphthene-3,5-dinitrophthalic anhydride complex shows a bathochromic shift with increasing solvent polarity [17]. To test this contention, the charge transfer absorption spectra for the D M C - T C N E system have been recorded in cyclohexane, carbon tetrachloride, chloroform, methylenechloride and dichloroethane (Fig. 5). It has been shown that a solvent change from cyclohexane to methylenechloride causes a bathochromic shift of c a 200 nm in electronic absorption spectra (Table 1). Hence the CT complex exhibits a solvatochromic behaviour attributed to the stabilization of the more dipolar excited state than the ground state, thereby reducing the energy requirements for CT electronic transitions [18, 19]. It has been estabfished that the different positions of the CT absorption band in different solvents are found to correlate with

[-:-'l 2n2+1]

0.21

(a)

+

1: 0.2

I

I

(b)

A

+

s'--

,

1= v

0.5

i

+

=

0.1

v

1.0

I

I

1.5

2.0

Fig. 4. The relation between AEcr of the DMC-TCNE system against (a) (n 2- 1)/(2n2+ 1), (b) ( e - 1)/(e+ 2) - (n 2- 1)/(n2 + 2) of the solvents.

1.01

Research Note

go c-

(5)

O teD

I1)

O go .O

I

500

600

700 Wavelength (nm)

91~0

800

Fig. 5. Absorption spectra of CT complexes of DMC-TCNE in different solvents: (1) cyclohexane; (2) carbon tetrachloride; (3) chloroform; (4) methylenechloride; and (5) dichloroethane.

Table 1. The maximum absorption of CT complexes of DMC-TCNE in different solvents, the dielectric constant (e) and the refractive index of the solvents (n) Solvent Cyclohexane Carbon tetrachloride Chloroform Dichloroethane Methylenechloride

~cr(nm)

r*

n*

AEcT(eV)

580 590 725 790 795

2.00 2.20 4.70 10.36 8.90

1.4262 1.4601 1.4459 1.4448 1.4241

2.14 2.10 1.71 1.57 1.56

* Reference [25]. and

-

1/]

of the solvent, where n is the refractive index and e is the dielectric constant [211]. Thus when the energies of the ~ band (AEcr) are plotted against these functions, a linear relatiomhip is obtained as shown in Fig. 4(a) and (b). It would be reasonable to conclude that the refractive index and/or the dielectric constant of the solvent should be considered as an important factor for the shift of the CT absorption bands and the results obtained satisfy the prediction. The absorption spectra for a mixed solution of DMC with different ~r-acceptors such as TCNE, TCNO, D D Q , CHL, PIC, DNP and TNB in chloroform at 20°C were measured. The spectra were characterized by the appearance of a new CT band (Act) at different wavelengths. The energy of the absorption (AEcr) for each complex is in agreement with the electron donor and electron 3.0

~

e

1.0

2'.o

31o

E^

Fig. 6, The relation between AEcr of CT complexes against E A of the acceptors.

441

Research Note

Table 2. The absorption and dissociation energies (AE and W) of the CT complexes of DMC dye with different electron acceptor in chloroform and electron affinities (E*) of the acceptors Acceptor TCNE TCNO DDQ CHL PIC DNP TNB

EA(eV)

AEcr(eV)

W(eV)

1.70" 2.20* 1.90" 1.37" 0.701, 0.501' 0.70*

1.720 1.564 1.540 1.730 2.830 2.440 2.490

4.579 4.395 4.709 5.059 4.729 5.219 4.969

* Reference [21]. 1,Reference [26]. acceptor characters. It has been found that the plot of AECT vs electron affinity (E ^) is linear for complexes of pyrene, hexamethylbenzene and tetramethyl-p-phenylene diamine [21] for a number of acceptors. It can be shown that the plots of AEcr of DMC CT complexes with various E ^ acceptors is linear with a correlation of 0.92 (Fig. 6). This linear relationship may perhaps be employed to determine the electron affinity of acceptors. The ionization potential (Ip) of DMC may be obtained from an empirical linear relationship between lp and OCT[22, 23]: Ip = 5.76 + 1.52 × 10-4vcT(DDQ)

(2)

lp = 5.21 + 1.65 x 10-4Ocr(TCNE),

(3)

where OcT is the wavenumber (in reciprocal centimetres) corresponding to the first CT band formed between the donor and each acceptor of D D Q or TCNE, respectively. Since, as described earlier, the polarity of the solvent has a pronounced effect on the position of 2cT, and to obtain a more justified lp value, the m a ~ m u m ~.cr for the CT complex formed between DMC and D D Q and TCNE in the inert cyclohexane solvent was recorded. These are 607 and 580 nm for the D D Q and TCNE acceptor, respectively. Using Eqns (2) and (3), the ionization potentials are calculated and found to equal 8.26 and 8.05 eV. The average value is equal to 8.159 eV. The relation between AEcr and the electron affinity (E ^) of the acceptors with a certain donor is given by the following equation [23] A Ecr = Ip - E ^ - W,

(4)

where W is the dissociation energy of the CT excited state. Using this relationship, the dissociation energies of the CT excited state were calculated and are given in Table 2. The absorption spectra of mixed solutions with a fixed concentration of TCNE (5.927 x 10-3 M) and varying the concentration of D M C (9.77 x 10-3-44.6 x 10-3 M) in chloroform at 20°C are 1,C

-

II}

0.5.

I I

0 w ¢,1

Donlr

<

~,

400

I

I

I

I

I

500

600

700

800

900

Wavelength (nm) Fig. 7. The absorption spectra of DMC-TCNE in cidoroform at 20°C (1 cm cell). The concentration of TC'~E was 5.927 x 10-JM and the concentration of DMC (1) 0.0, (2) 0.977 x 10-3, (3) 1.76x 10-2, (4) 2.59x 10-2, (5) 3.54x 10-2 and (6) 4.46x 10-2 M. The broken curve represents the spectra of DMC of concentration 4.46 x 10-2 M.

442

Research Note

shown in Fig. 7. These spectrophotometric data were used to calculate the equilibrium constant (Kcr) and the molar extinction coefficient (ecT) of the CT complex. Under the assumption of 1:1 (donor:acceptor) complex formation and since the complex absorbs at a wavelength where neither the donor nor the acceptor absorbs, the linear form of R o s e and DRAGO'S [24] equation can be used [Alo[Dlo 1 1 A = Kcr'eCT + ~ ([D]0 + [A]0),

(5)

where [A]0 and [D]0 are the initial concentrations of the acceptor and donor, respectively, and A is the absorbance of the complex. The data were processed using the least-squares method at different wavelengths of the charge transfer band (600, 700, 800 and 900 nm) to calculate KCT and tcT. The average values are 7.76 + 0.681 m o l - t and 228 + 21 1mol -t cm -1, respectively. From the foregoing data, it has been shown that the laser dye DMC forms CT complexes with a set of :~ electron acceptors. The plots of A ECT of the CT complexes with E ^ of the acceptor were found to be linear. Further, the solvent effect on the D M C - T C N E system was studied and revealed that the CT complex exhibits a solvatochromic behaviour. In addition, the quenching efficiencies of DMC by various acceptors was investigated. This indicated that the mechanism of the quenching was a static type which: (i) increases as the E ^ of the acceptor increases; (ii) increases as the medium viscosity increases; and (iii) decreases as the temperature increases.

Department of Chemistry Faculty of Science University of Tanta Tanta, Egypt

M . M . AYAD* S . A . EL-DALv S . A . AZIM

REFERENCES [1] K. H. Drexhage, Dye Laser (Edited by F. D. Schafer), p. 144. Springer, New York (1973). [2] G. A. Reynolds and K. H. Drexhage, Opt. Commun. 13, 222 (1975). [3] S. A. Tuccio, K. H. Drexhage and G. A. Reynolds, Opt. Commun. 7, 248 (1973). [4] V. Masilamani and B. M. Sly^ram, J. Luminesc. 27, 137 (1982). [5] V. S. Antonov and K. L. Hahla, Appi. Phys. B32, 9 (1983). [6] A. N. Fletcher and R. H. Knipr, Appl. Phys. B27, 93 (1982). [7] A. N. Fletcher, Appl. Phys. B31, 19 (1983). [8] A. N. Fletcher and D. E. Bliss, AppL Phys. B167, 289 (1978). [9] W. H. Winters, H. Mandelberg and W. Mohr, Appl. Phys. Left. 25, 723 (1974). [10] G. Jones, II, W. R. Bergmark and W. R. Jackson, Opt. Commun. 30, 320 (1984). [11] G. Jones, II and W. R. Bergmark, J. Photochem. 26, 1979 (1984). [12] K. Priyadarsini, J. Tkunjappu and P. N. Moorthy, Indian J. Chem. 26A, 899 (1987). [13] F. Foster, Organic Charge- Transfer Complexes. Academic Press, New York (1969). [14] V. Pechmann, Bet. Bunsenges Phys. Chem. 17, 929 (1884); Chem. Abstr. 98, 107,121 (1983). [15] O. Stern and M. Volmer, Phys. Z. 20, 183 (1919). [16] E. M. Ebeid, M. Gaber, A. M. Habib, R. M. Issa and S. A. Azim, J. Ch/m. Phys. 86, 2015 (1989). [17] C. Reichardt, Solvents and Solvents Effects in Organic Chemistry, p. 293. VCH, FRG (1988). [18] J. N. Marrel, Q. Rev. Lond. 15, 191 (1961). [19] S. F. Masou,Q. Rev. Lond. 15, 287 (1961). [20] E. G. McKae, J. Phys. Chem. 61, 61 (1957). [21] G. Briegleb, Angew. Chem. 8, 326 (1964). [22] R. Foster, Nature, Lond. 183, 1253 (1959). [23] H. M. McConnel, J. J. Ham and J. R. Platt, J. Chem. Phys. 21, 66 (1964). [24] N. J. Rose and R. S. Dr^go, J. Am. Chem. Soc. 81, 6138 (1959). [25] S. L. Murove, Handbook of Photochemistry, p. 85. Marcel Dekker, New York (1973). [26] G. Briegleb, Electron-Donor-Acceptor-Complex, Springer, Berlin (1961).

* Present address: T. T. Junior College, AI-Baha, Kingdom of Saudi Arabia.