Dual amplified spontaneous emission from 7-amino-4-methyl coumarin dye

Journal of Photochemistry

and Photobiology,

DUAL AMPLIFIED 7-AMINO-4-METHYL A. RAMALINGAM,

A: Chemistry, 49 (1989)

SPONTANEOUS EMISSION COUMARIN DYE

P. K. PALANISAMY

89 - 96

89

FROM

and V. MASILAMANI

Department of Physics, Anna University, Madras-600 025 (India) B. M. SIVARAM Department of Physics, Indian Institute of Technology, 600 036 (India) (Received

January

Madras-

10, 1’989)

summary This paper reports on the dual amplified spontaneous emission observed from solutions of 7-amino-4-methyl coumarin dye (coumarin 440) in certain solvents such as n-butyl acetate, dioxane etc. when exposed to high power nitrogen laser excitation. The results suggest that twisted intramolecular charge transfer coumarin photoisomers which form exciplexes with the solvent molecules have enough gain to produce amplified spontaneous emission even where there is apparently no detectable fluorescence.

1. Introduction The phenomenon of dual fluorescence has been studied extensively by various workers [ 1 - 41. For molecules like dimethylaminobenzonitrile (DMABN), the dual fluorescence in certain solvents has been attributed to the photoisomers which exist in dynamic equilibrium in the excited state. Molecules of this type have strong intramolecular charge transfer characteristics. In one of the photoisomers the electron donor and acceptor moieties are in the same plane and this is called the intramolecular charge transfer conformation. In the other photoisomer the two moieties are out of plane and this is known as the twisted intramolecular charge transfer (TICT) conformation. It has been reported that laser action can be achieved from the TICT conformation in coumarin 1 and coumarin 2 which exhibit dual super-radiant bands when exposed to nitkogen laser excitation [ 5, 6 3. This paper reports on the observation of dual amplified spontaneous emission (ASE) from 7-amino-4-methyl coumarin (coumarin 440). Its characteristics are compared with 7_diethylamino-4-methyl coumarin (coumarin 1). lOlO-6030/89/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

90

2. Experimental details The experimental techniques employed in this investigation have been described in ref. 7. The dye 7-amino-4-methyl coumarin was of laser grade and was obtained from the Exciton Company. The molecular structure is given in Fig. 1. The solvents used were all of spectroscopic grade. The dye solution was placed in a quartz cuvette with polished sides which was canted to avoid feedback by Fresnel reflection from the walls of the cuvette. The solution was excited transversely by a nitrogen laser. The ASE from the dye solution was recorded using a constant deviation spectrograph. The absorption spectra of dilute and concentrated solutions of coumarin 440 were recorded using a Kontron spectrophotometer. The corresponding fluorescence spectra were recorded using an Aminco Bowman spectrofluorometer.

CH3 Fig. 1. Molecular

structure of 7-amino-6methyl

coumarin.

3. Results and discussion Table 1 shows the wavelengths of the absorption and fluorescence peaks of coumarin 440 in a few representative solvents. These solvents can be classified as inert, non-polar hydrocarbons (e.g. benzene), moderately polar (e.g. chlorobenzene) and polar with hydrogen bonding (e.g. methanol). Figure 2 shows the Stokes shift as a function of solvent dipole factor defined by Mataga and Tsuno [ 81. The figure shows that the solvent-solute interactions are relatively weak in benzene-like solvents and stronger for alcoholtype solvents and the positive slope of the graph indicates that the dye is more polar in the excited state than in the ground state. This is compared with the Stokes shift for coumarin 1 in these solvents [ 91. It can be noted that the change in dipole moment on excitation to the S1 state is higher for coumarin 1 than for coumarin 440. This is because the ionization potential of the diethylamino donor is lower than that of the amino donor and hence the charge delocalization is greater for coumarin 1. From the absorption and fluorescence spectra alone, we can infer that coumarin 440 undergoes dipole-dipole and hydrogen-bonding interactions only. However, the ASE data obtained on exposure to nitrogen her excitation clearly show the existence of another species. Figure 3 shows the ASE spectra obtained for coumarin 440 in (a) henzene, (b) dioxene, (c) n-butyl acetate and (d) methanol at a concentration of

91 TABLE

1

Spectral characteristics of coumarin 440 (all wavelengths in nanometres) Solvent Cyclohexane Dioxane Benzene Toluene Diethyl ether Chloroform n -Butyl acetate Cyclohexanone Acetone Methanol

D 2.02 2.21 2.28 2.38 4.34 4.81 5.01 18.30 20.70 32.63

n

Ab

Fl

ASE

1.43 1.42 1.50 1.49 1.36 1.44 1.39 1.45 1.36 1.32

340 342 340 342 345 345 345 352 350 355

385 390 385 385 395 395 400 415 415 428

408 408,430 408 408 410,430 410 410,430 430 430 435

-

D, dielectric constant; n, refractive index; Ab, absorption peak; Fl, fluorescence peak; ASE, amplified spontaneous emission.

1 2 3 4 5 6 7 6 9

DIPOLE

Fig. 2. Variation various solutions.

Benzene Toluene 1-4 dioxane Chloroform Dlethyl ether n-butyt acetate Ethyl acetate Cyclohexane Acetone

FACTOR

in Stokes shift of coumarin 440 dye solution with dipole factor of

(b)

Cd)

(e)

-)\

3. Spectrograihic record of ASE from coumarin 440 in (a) benzene, (b) dioxane, (c) n-butyl acetate and (d) methanol. (e) Mercury spectrum as reference.

Fig.

8 mM and a pump power of 100 kW. In inert hydrocarbon solvents (benzene) there is only one ASE band at 408 nm corresponding to the fluorescence peak at 385 nm. Similarly in hydrogen-bonding solvents such as methanol there is only one ASE band at 435 nm corresponding to the fluorescence band at 428 nm. These ASE bands obtained using high power nitrogen laser excitation for which corresponding fluorescence bands exist using continuous wave optical excitation may be termed normal ASE. Figures 3(b) and 3(c) show that there are two ASE bands in n-butyl acetate and dioxane, one around 410 nm and another around 430 nm. Of these, the 410 nm band corresponds to the fluorescence band at 400 nm. There is apparently no detectable fluorescence band which corresponds to the ASE band at 430 nm. The absorption, fluorescence and excitation spectra of coumarin 440 in n-butyl acetate are very similar in concentrated (8 mM) and dilute (0.01 mM) solutions. The spectra are the same before and after strong irradiation with nitrogen laser light. These results indicate that the anomalous ASE is not due to aggregation, dye degradation, protonation or deprotonation. The anomalous ASE is strongly dependent on the solvent environment, dye concentration and pump power. Coumarin 440 gives only one fluorescence band and one ASE band at all concentrations (10 mM) and at all pump powers (maximum 100 kW) in benzene- and alcohol-like solvents. However, it shows a single fluorescence band and dual ASE bands in solvents such as n-butyl acetate, dioxane etc. which all have an oxygen atom for possible solvent-solute exciplex formation in the excited state. We have studied this aspect in more detail by investigating the fluorescence and ASE spectra in a variety of solvents such as chloro compounds, ethers, esters, ketones, etc. and also in solvent mixtures. We obtained dual ASE only in solvents with an oxygen or chlorine atom, which apparently form exciplexes with the dye in the excited state. This is illustrated in Fig. 4 which shows ASE from

93

(b)

(cl

Id) .

(

x

Fig. 4. Spectrographicrecord of ASE from coumarin 440 in (a) benzene, (b) 10% n-butyl acetate and (c) benzene plus 20% n-butyl acetate. (d) Mercury reference.

benzene plus spectrum

as

coumarin 440 in a solvent mixture of benzene and n-butyl acetate. Figure 4(a) shows a single ASE band for coumarin 440 in benzene at a concentration of 4 mM and a nitrogen laser pump power of 100 kW. Figure 4(b) shows the emergence of a new ASE band in a mixture of benzene and 10% n-butyl acetate. Figure 4(c) shows the presence of two distinct ASE bands in a mixture of benzene and 20% n-butyl acetate. It thus appears that the presence of an oxygen or chlorine atom in the solvent is apparently required for the stabilization of the TICT conformation probably by the formation of an exciplex between the excited TICT conformation of coumarin 440 and an electronegative atom such as oxygen. The exciplex is formed between the nitrogen atom of coumarin 440 and the oxygen or chlorine atom of the solvent molecules. The anomalous ASE strongly depends on the concentration of the dye as shown in Fig. 5. Figure 5(a) shows the ASE for coumarin 440 in n-butyl acetate at a concentration of 4 mM with a nitrogen laser pump power of 100 kW. Figure 5(b) shows the ASE at a concentration of 8 mM with all other parameters as in Fig. 5(a). This figure shows that the anomalous ASE at 435 nm occurs only at high concentrations. However, the fluorescence spectrum at this concentration does not show any peak or shoulder around 435 nm. Figure 6(a) shows the superfluorescence from coumarin 440 at a concentration of 8 mM in n-butyl acetate solution at a pump power of 50 kW. Figure 6(b) shows the superfluorescence from the same solution at the same concentration but at a pump power of 100 kW. This figure shows that for a lower pump power the anomalous ASE decreases in intensity. We can compare the dual emission characteristics of coumarin 440 with those of coumarin 1. Both have very similar characteristics with one major

(b)

Fig. 5. Spectrographic record of ASE from coumarin 440 in n-butyl acetate of (a) 4 mM and (b) 8 mM at a constant pump power. (c) Mercury spectrum as reference.

Fig. 6. Spectrographic record of ASE from coumarin 440 in n-butyl acetate for a pump power of (a) 50 kW and (b) 100 kW at a constant concentration. (c) Mercury spectrum as reference.

difference: with an identical nitrogen laser pump power (100 kW), identical solvent (n-butyl acetate) and identical concentration (5 mM), the normal ASE band is stronger than the anomalous ASE band in coumarin 440, whereas it is just the reverse in coumarin 1. This is shown in Figs. 7(a) and 7(b) respectively. This may be due to two facts. (i) In coumarin 1 the electron donor is the diethylamino group and in coumarin 440 it is the amino group. The heavy substitution of the ethyl group may facilitate the internal rotation of the donor group and hence photoisomers with the TICT conformation will be greater in number in coumarin 1 than in coumarin 440. (ii) The change in dipole moment of coumarin 440 in the excited state is less

95

(a)

(b)

Fig. 7. Spectrographic record of ASE from (a) coumarin butyl acetate. (c) Mercury spectrum as reference.

440

and (b) coumarin

1 in IZ-

than that for coumarin 1. This will mean a reduction in charge delocalization and hence polarization compared with coumarin 1 and this may reduce the strength of bonding between the solvent and solute. In an earlier paper [6] the ASE characteristics of coumarin I have been compared with those of coumarin 2 (a coumarin dye with an electron donor group which is capable of internal rotation). It has been hypothesized that the anomalous ASE in coumarin 1 is due to the TICT conformation which forms a complex with the solvent in the excited state. Since the characteristics of coumarin 440 are very similar to those of coumarin 1 we can conclude that the anomalous ASE in coumarin 440 in certain solvents is due to the formation of exciplexes between TICT photoisomers and the solvent. 4. Conclusions The results of the spectral and ASE characteristics of 7-amino-4-methyl coumarin show that this dye behaves in a similar manner to coumarin 1 and produces dual ASE bands (one around 408 nm and another around 435 nm) in certain complex-forming solvents at fairly high concentrations and pump power.

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3 R. 3. Visser and C. A. G. 0. Varma, J. Chem. Sot., Faraday Trans. II, 76 ( 1980) 453. 4 Y. Wang and K. B. Eisenthal, J. Chem. Phys., 77 (1982) 6076. 5 V. Masilamani, V. Chandrasekar, B. M. Sivaram, B. Sivasankar and S. Natarajan, Opt. Commun., 59 (1986) 203. 6 V. Masilamani, D. Sastikumar, S. Natarajan and P. Natarajan, Opt. Commun., 62 (1987) 389. 137. 7 V. Masilamani and B. M. Sivaram, J. Lumin., 27 (1982) 8 N. Mataga and S. Tsuno, Bull. Chem. Sot. Jpn., 29 (1956) 373,465; 30 (1957) 368,

771. 9 V. Masilamani and B. M. Sivaram, J. Lumin.,

27 (1982)

147.