Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

Optics & Laser Technology 43 (2011) 1078–1083

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Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye E.H. El-Mossalamy a,n,1, A.Y. Obaid a, S.A. El-Daly b a b

Chemistry Department, Faculty of Science, King Abdul Aziz University, P.O. Box. 80203, 21589, Saudi Arabia Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 May 2010 Received in revised form 15 January 2011 Accepted 7 February 2011 Available online 21 March 2011

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2  10  3 mol dm  3 of DEAC gives laser emission in the blue region on pumping with nitrogen laser (lex ¼ 337.1 nm). The laser parameters such as tuning range, gain coefficient (a), emission cross section (se), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl4 at a wavelength of 366 nm. The values of photochemical yield (fc) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH3CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex. & 2011 Elsevier Ltd. All rights reserved.

Keywords: DEAC- laser dye TCNQ PA E. absorption Emission Fluorescence and dipolemoment

1. Introduction Derivatives of 1,2-benzopyrone, commonly known as coumarin dyes, are the well-known laser dyes for the blue-green region [1–8]. Among these dyes, those having different amino groups at position 7 (commonly known as 7-aminocoumarins) are of special significance [1–13]. The fluorescence quantum yield (ff) of these dyes is usually very high, often close to unity [1–13]. These dyes undergo substantial changes in their dipole moment on excitation, causing large Stokes shifts between their absorption and fluorescence spectra [1–14]. These Stocks’ shifts are again very sensitive to solvent polarities [1–15]. Because of these properties, the 7-aminocoumarin dyes have widely been used as probes in a variety of investigation, i.e., in the study of solvatochromic properties [9–13], determination of polarities in microenvironments [14,15], and measurements of solvent relaxation times [16–21]. In the literature, it is reported that

coumarin dyes with 7-dialkylamino groups behave unusually in higher polar solvents, showing drastic reduction in their fluorescence quantum yield and fluorescence lifetime values [9–13]. However, the involvement of twisted intramolecular charge-transfer (TICT) states has been invoked to explain these results in high polarity solvents. In the present work, photophysical properties, laser parameters, fluorescence quenching, and photoreactivity of 7-diethyaminocoumarin (DEAC) have been investigated.

N

O

O

2. Experimental details n

Corresponding author. E-mail addresses: [email protected] (E.H. El-Mossalamy), [email protected] (S.A. El-Daly). 1 Permanent Address. Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. 0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.02.002

Laser-grade 7-diethylaminocoumarine (DEAC) dye was obtained from Fluka and used without further purification. The organic solvents used were of spectroscopic grade and were preliminarily

E.H. El-Mossalamy et al. / Optics & Laser Technology 43 (2011) 1078–1083

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checked for the absence of absorbing or fluorescent impurities within the scanned spectral ranges. Picric acid (Aldrich) was recrystallized from ethanol, tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinonedimethane (TCNQ) (Aldrich) was purified by sublimation, sodium dodecyl sulfate (SDS, Fluka) was used to prepare anionic micelles and cetyltrimethyl ammonium chloride (CTAC, Fluka) to prepare cationic micelles. UV–vis electronic absorption spectra were recorded on a Shimadzu UV-160A spectrophotometer and the steady state fluorescence spectra were measured using Shimadzu RF 510 spectrofluorophotometer. The fluorescence spectra were corrected for the machine response using 10  5 mol dm  3 anthracene solution in benzene [22]. The fluorescence quantum yield (ff) was measured relative to 9,10- diphenylanthracene, according to the following equation [23]:

Ff ðsÞ ¼ Ff ðrÞ

Ds Ar n2s Dr As n2r

ð1Þ

where D represents the corrected fluorescence peak area, A is the absorbance at excitation wavelength, and n is the refractive index of solvent used. The subscripts ‘s’ and ‘r’ refer to sample and reference, respectively. All measurements were made at very low concentration of dye (o1  10  5 M) to avoid aggregation and inner-filter effects. The photochemical quantum yields were calculated using the method that was described in detail previously [24]. The light intensity was measured by ferrioxalate actinometry [25]. The laser action of DEAC was determined using GL–302 dye laser, pumped by a nitrogen laser (GL-3300 nitrogen laser, PTI). The pump laser (l ¼337.1 nm) was operated at repetition of 3 Hz with a pulse energy of 1.48 mJ and pulse duration of 800 ps. The narrow-band output of the dye laser was measured with pyroelectric Joule meter (ED 200, GenTec Inc.).

3. Results and discussion 3.1. Solvent polarity effect on absorption and fluorescence spectra Absorption and fluorescence spectra of DEAC dye are strongly dependent on solvent polarity (Fig. 1). Table 1 lists the photophysical parameters such as labs (max), lem (max), and fluorescence quantum yield along with solvent polarity function (Df). The spectral shifts in absorption are generally found to be smaller than those found in fluorescence emission implying a greater dipole moment (polarity) of the dye in the excited state compared to that in the ground state. This is also in agreement with p, pn nature of the lowest singlet excited state of DEAC. The marginal deviation in data points for alcoholic solvents are attributed to solute–solvent hydrogen bonding interaction, which causes an extra red shift in the observed spectra [26–28]. Analysis of the solvatochromic behavior allows the estimation of the difference in the dipole moment between the excited singlet and the ground state (Dme–Dmg). This was achieved by applying the simplified Lippert–Mataga equation [29–31]:

Dnst ¼ Dno þ Df ¼

2ðme mg Þ2 hca3

e1 n2 1  2e þ2 2n2 þ 1

Df

ð2Þ

ð3Þ

where Dust is the Stokes shift (in cm  1), which increases with increase in solvent polarity pointing to stronger stabilization of excited state in polar solvents, h is Planck’s constant, c is the velocity of light, a is the Onsager cavity radius, and e and n are the dielectric constant and refractive index of the solvent, respectively. The Onsager cavity radius (a) was calculated from the energy minimized structures obtained with the AMI calculation. It

Fig. 1. (a) Electronic absorption spectrum of 3  10  5 mol dm  3 of DEAC n-heptane and MeOH. (b) Emission spectrum of DEAC in (—) MeOH and (y) n-hexane.

Table 1 Photophysical properties of DEAC in different solvents. Solvents

Df

kab (nm)

kem (nm)

Dt (cm  1)

/f

la (D)

n-Hexane n-Heptane Benzene Toluene Diethylether CCl4 CHCl3 DMF n-Propanol Acetone EtOH MeOH Ethylene glycol n-Butanol

0.0 0.0002 0.0052 0.0133 0.166 0.008 0.233 0.274 0.277 0.284 0.290 0.309 0.30 0.263

353 352 355 369 359 360 375 379 379 370 380 381 381 379

392 390 395 418 405 412 435 440 442 430 443 445 445 442

2817 2768 2853 3177 3164 3505 3677 3657 3760 3771 3742 3775 3775 33,760

0.85 0.85 0.83 0.83 0.80 – 0.62 0.56 0.52 0.55 0.39 0.27 0.25 0.47

10.8 10.8 10.4 10.3 10.5 10.2 9.6 9.4 8.85 10.7 8.6 8.62 — —

is defined as the radius of the spherical cavity surrounding the molecular Van der Waals radius. The maximum distance where charge separation can occur across the molecule must be

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considered, rather than the actual molecular axis of the molecule [32]. For DEAC this distance lies between the average coordinates of the nitrogen atoms on one side of the molecule and the oxygen atom on the other side and it was estimated to be 9.0 A˚ , the cavity radius a was taken as half of this distance, that is ˚ Fig. 2 shows the plot of Stokes shift versus the orientation 4.5 A. polarization (Df). The change in dipole moment on excitation that was calculated from slope of the plot and the cavity radius is 4.8 D, indicating the polar nature of the fluorescent states. We assign this state to be of intramolecular charge-transfer (ICT) character. In the earlier works, a similar ICT character was assigned to the fluorescent states of 7-amino coumarin derivatives [8–12]. The fluorescence quantum yield (ff) values of DEAC were estimated in different solvents at room temperature and listed in Table 1. Fig. 3 shows the plot of ff versus Df. The interesting point from this plot is that the ff values are lower in higher polarity solvents, having Df40.2. In lower to medium solvent polarity solvents (Dfo0.2), however, ff is only marginally lower with Df, following apparently a linear correlation, indicating that the dye exists in ICT structure in these solvents. Thus the drastic reduction in the ff values, above Dfffi0.2, clearly indicate that in the higher polarity solvents the excited state of the dye undergoes

Stockes Shift (cm-1).

3800 3600 3400 3200 3000 2800 2600

0.00

0.05

0.10

0.15

0.20 Δf (ε,n)

0.25

0.30

0.35

ma ¼ 0:0958ðemax Dua1=2 ua Þ0:5

3.2. Laser parameters of DEAC DEAC has high fluorescence quantum yield, high molar absorptivity, and high photostability in most organic solvents. In addition, the dye is free from molecular aggregation either in ground or in the excited state since the emission spectrum suffers no shift as the concentration is raised up to 5  10  3 mol dm  3. Solution of 2  10  3 mol dm  3 in different solvents, namely acetone, dioxane, DMF, and ethanol, gives laser emission on pumping by nitrogen laser (lex ¼337.1 nm). The dye solution was taken in oscillator and amplifier cuvetts of 10 mm path length. The output energy of laser dye was measured as a function of wavelength to determine the lasing range in different solvents. The gain coefficient a(l) of laser emission of DEAC was calculated by measuring the intensity of laser emission from the entire cell length and that from the cell half length (IL and IL/2, respectively) according the following relation [37]:   2 I aðlÞ ¼ ln L 1 ð5Þ L IL=2 The cross section for stimulated laser emission se was calculated at laser emission maximum according to the following

0.9

70

0.8

60

0.7

50

0.6

40

0.5

CTAC SDS

30

0.4

20

0.3 0.2

ð4Þ

where emax is the maximum value of molar absorption coefficient of the absorption band with a maximum at ua and full width at half maximum of Du1/2 the values of ma are listed in Table 1. a The emission spectrum of 1  10  5 mol dm  3 of DEAC has also been measured in cetyltrimethyl ammonium chloride (CTAC) and sodium dodecyl sulfate (SDS) micelles media, as shown in Fig. 4. The emission intensity of dye increases as the concentration of surfactants is increased, an abrupt change in the fluorescence intensity is observed at surfactant concentration of 1  10  4 and 8  10  3 mol dm  3, which is very close to the critical micelle concentration of CTAC and SDS, respectively [35,36].

If

f

Fig. 2. Plot of Stokes shift (Du in cm  1) for DEAC against the solvent polarity function (Df).

a fast non-radiative de-excitation process due to stabilization of the twisting intramolecular charge-transfer (TICT) non-emissive state , which is absent in lower to medium polarity of solvents [31]. The transition dipole moment (ma) of DEAC from ground to excited state was calculated in different solvents from the absorption spectrum using the simplified expression [26,33,34]:

10 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Δf (ε,n) Fig. 3. Plot of fluorescence quantum yield (ff) of DEAC versus solvent polarity (Df).

0.0000

0.0005

0.0010

0.0015

0.0020

[Surfactant] mol dm-3. Fig. 4. Emission intensity of 1  10  5 mol dm  3 DEAC in different concentrations of SDS and CTAC (lex ¼337 nm).

E.H. El-Mossalamy et al. / Optics & Laser Technology 43 (2011) 1078–1083

hydrocarbons such as anthracene and perylene derivatives.

equation [37]:

se ¼

l4e EðlÞFf 8 cn2 f

p

1081

ð6Þ

t

where se is the emission wavelength, n is the refractive index of solution, c is the velocity of light, and E(l) is the normalized fluorescence line-shape function. This is correlated with fluorescence quantum yield by the following equation [38]: Z ð7Þ EðlÞdl ¼ Ff E(l) is obtained from solution whose absorbance is low to avoid reabsorption processes (optical density at absorption maximumo0.1). The values of a(l) and se at maximum laser emission are listed in Table 2. The photochemical stability of DEAC laser dye was determined as half-life energy (E½) [39], which is the amount of total absorbed pump energy until the dye laser energy has dropped to 50% of its output initial value (Table 2). 3.3. Photoreactivity of DEAC in carbon tetrachloride solvent The photoreactivity of DEAC has been studied in carbon tetrachloride (CCl4) on irradiation of 2  10  5 mol dm  3 solution of DEAC by 366 nm light (Io ¼ 3.6  10  6 Ein/min). The absorbance of DEAC decreases with increase in the radiation time and two isobestic points appear at 325 and 385 nm, indicating the presence of two photoproducts. The two new absorption peaks appear: one at 410 nm corresponds to the absorption of contact ion pair, which has a large dipole moment and the second absorption peak at 280 nm corresponding to the absorption of trichloromethyl radical, as shown in Fig. 5. The net photochemical quantum yield (fc) of the underlying reaction was calculated as 0.054. The formation of photoproduct is a one-photon process and is represented by the following scheme as reported earlier [40–46] for some aliphatic and aromatic amines and for aromatic Table 2 Laser parameters of DEAC laser dye. Solvent

Lasing range

l (max) (nm)

a (cm  1)

se  1016 cm2

E1/2 (J/L)

Acetone Dioxane EtOH DMF

415–451 405–450 425–465 420–455

430 425 440 438

2.2 1.8 1.74 2.4

1.6 1.45 1.5 1.65

7400 13,333 14,800 15,564



1:

DEAC þhn-1 ðDEACÞ

2:

1

ðDEACÞ -DEAC þ hn fluorescence

3:

1

ðDEACÞ þCCl4 -1 ½DEAC þ d . . .d CCl4  exciplex

4:

1

½DEAC þ d . . .d CCl4  -DEAC d þ  d Cl þ d CCl3

absorption of light

 



It was proposed that the electron transfer from excited singlet DEACn to CCl4 within transient excited charge-transfer complex is the main primary photochemical process (step 3). It leads to the DEAC radical cation, a chloride ion, and a trichloromethyl radical in solvent cage (step 4). The formation of a contact ion pair [DEAC þ d y  Cl] usually occurs by electron transfer from excited donor molecule to acceptor. In order to form such a compound a low ionization potential of donor and high electron affinity of acceptor are necessary. The rate constant of photodecomposition of DEAC was calculated by applying the simple first order rate equation as follows: ln

Ao A1 ¼ kt At A1

ð8Þ

where Ao, A, and AN are the initial absorbance, absorbance at time (t), and at infinity, respectively. k is the rate constant. The rate constant was found to be 0.067 min  1. 3.4. Fluorescence quenching by organic acceptors The fluorescence quenching of 1  10  5 mol dm  3 DEAC is also studied in acetonitrile solvent using organic acceptors such as picric acid (PA, EA ¼0.7 eV), tetracyanoethylene (TCNE, EA ¼1.7 eV), and 7,7,8,8-tetracyanoquinonedimethane (TCNQ, EA ¼2.2 eV). It was observed that the absorption spectrum when taken with increasing concentration of acceptor does not show any shift in wavelength. There is no additional peak appearing; hence there could not be any ground-state interaction between DEAC and acceptors. The emission spectrum of DEAC shows (i) no change in fluorescence spectra other than diminishing fluorescence intensity with increase in quencher concentration, (ii) no appearances of any new band at longer wavelength, as shown in Fig. 6, which shows the absence of emissive exciplex, and (iii) the Stern–Volmer plot for the fluorescence quenching of DEAC by organic acceptor is linear (Fig. 7). The Stern–Volmer equation is given by Io ¼ 1 þksv ½Q  I

ð9Þ

where Io and I are the fluorescence intensities in the absence and presence of the quencher [Q], respectively, and Ksv is the Stern– Volmer rate constant obtained from the slope. For dynamic quenching Ksv ¼kqtf, where kq is the bimolecular quenching rate constant (M  1 s  1) and tf is the excited singlet lifetime of fluorophore in the absence of quencher. For DEAC tf is 2.25 ns, the bimolecular quenching rate constants kq were calculated to be 5.3  1012, 8.8  1012, and 1.5  1013 M  1 s  1 for PA, TCNE, and TCNQ, respectively, indicating that the fluorescence quenching rate constant increases with increase in the electron affinity of acceptor. The effect of temperature of fluorescence quenching of DEAC by picric acid was also studied in the range of 25–60 1C. The activation energy Ea associated with kq was calculated assuming the Arrhenius equation to be valid for kq: kq ¼ A expðEa =RTÞ

ð10Þ 1

Fig. 5. (a) Effect of irradiation on electronic absorption spectrum of 2  10  5 mol dm  3 of DEAC (lex ¼365 nm and Io ¼3.6  10  6 Ein/min). The irradiation times at decreasing absorption are 0.0, 3, 7, 10, 15, 20, 26, 26, 32, 40, 45, and 50 min.

The value obtained is 9.8 kJ mol , which is very close to that of the activation energy associated with solvent viscosity (EZ ¼10–13 kJ mol  1), indicating a diffusion mechanism for fluorescence quenching (dynamic quenching mechanism). Thus, we

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especially in polar solvents. In the extreme case, an exciplex is more accurately described as a contact ion pair. Emission from exciplexes proceeds by a vertical, Franck–Condon allowed transition from a minimum on an excited-state surface to a low-lying repulsive ground-state surface. If the transition to the low-lying repulsive ground state involves a significant nuclear change, then emission may not be observed. Other decay processes, such as ionic dissociation into solvent-separated radical ions or chemical reactions, may prevail [47]. For c3 5c2, exciplex formation leads to an energy transfer. In such a case, exciplexes tend to emit light or undergo separation to form the excited state of the acceptor and ground state of donor. The absence of static quenching of DAEC by organic acceptor is indicated from the following observations: (i) the absorption spectrum of the fluorophore shows no change in the presence of the quencher. (ii) No excitation wavelength dependence of kq was observed, and (iii) Stern–Volmer plot is linear and (iv) the quenching rate constant increases with increase in the temperature.

4. Conclusion

Fig. 6. Emission spectrum of 1  10  5 mol dm  3 of DEAC in absence (—) and presence (y) of 1.4  10  4 mol dm  3 of picric acid (lex ¼ 337 nm).

EDAC is well known as a good laser dye, it has high fluorescence quantum in non-polar solvents, high molar absorptivity, and its singlet excited state is more polar than the ground state, and it is well highly photostable in non-chlorinated solvents. DEAC undergoes solubilization in cationic and anionic micelles. DEAC displays fluorescence quenching by organic acceptors in acetonitrile via formation of non-emissive exciplex.

PA

2.8

TCNE

References

TCNQ

Io /I

2.4 2.0

[1] [2] [3] [4] [5]

1.6

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

1.2 0.8 0.00000

0.00002

0.00004

0.00006

[Q] mol

0.00008

0.00010

dm-3.

Fig. 7. Stern–Volmer plots for fluorescence quenching of 1  10 DEAC in CH3CN using PA, TCNE, and TCNQ (lex ¼337 nm).

[16] [17] 5

mol dm

3

of

infer that quenching of DEAC by organic acceptor proceeds predominantly via a non-emissive exciplex with charge transfer occurring from the excited fluorophore to acceptor according to the following scheme [47]: D . . .A 2 D A - ½DA -D þ d A d - D þ d . . .A d -D þ d þ A d Encounter Complex

collision complex

exciplex

contact ion pair

solvent separated ion pair

free ions

A molecular orbital wave function representation is used to express the electronic interaction in an exciplex a summation of possible states [47] (when ground-state interactions are neglected):

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

C ¼ c1 cðD AÞ þ c2 cðDAÞ þ c3 cðD þ d A d Þ

[33]

If c3 bc2, the exciplex has pronounced charge-transfer character and will have a tendency to dissociate into radical ion pairs,

[34] [35]

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