Spectroscopic properties and amplified spontaneous emission of fluorescein laser dye in ionic liquids as green media

Optical Materials 47 (2015) 573–581

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Spectroscopic properties and amplified spontaneous emission of fluorescein laser dye in ionic liquids as green media Dalal M. AL-Aqmar a,b, H.I. Abdelkader b, Maram T.H. Abou Kana c,⇑ a

Physics Department, Ibb University, Ibb, Yemen Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt c National Institute of Laser Enhanced Sciences, Cairo University, 12613 Giza, Egypt b

a r t i c l e

i n f o

Article history: Received 26 March 2015 Received in revised form 3 May 2015 Accepted 22 June 2015 Available online 27 June 2015 Keywords: Fluorescein Ionic liquid Amplified spontaneous emission Spectroscopic properties Photostability

a b s t r a c t The use of ionic liquids (ILs) as milieu materials for laser dyes is a promising field and quite competitive with volatile organic solvents and solid state-dye laser systems. This paper investigates some photophysical parameters of fluorescein dye incorporated into ionic liquids; 1-Butyl-3-methylimidazolium chloride (BMIM Cl), 1-Butyl-3-methylimidazolium tetrachloroaluminate (BMIM AlCl4) and 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4) as promising host matrix in addition to ethanol as reference. These parameters are: absorption and emission cross-sections, fluorescence lifetime and quantum yield, in addition to the transition dipole moment, the attenuation length and oscillator strength were also investigated. Lasing characteristics such as amplified spontaneous emission (ASE), the gain, and the photostability of fluorescein laser dye dissolved in different host materials were assessed. The composition and properties of the matrix of ILs were found that it has great interest in optimizing the laser performance and photostability of the investigated laser dye. Under transverse pumping of fluorescein dye by blue laser diode (450 nm) of (400 mW), the initial ASE for dye dissolved in BMIM AlCl4 and ethanol were decreased to 39% and 36% respectively as time progressed 132 min. Relatively high efficiency and high fluorescence quantum yield (11.8% and 0.82% respectively) were obtained with good photostability in case of fluorescein in BMIM BF4 that was decreased to 56% of the initial ASE after continuously pumping with 400 mW for 132 min. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The surrounding medium for laser dye probe plays an important role when defining physical properties and potential hazards for dye laser. The lasing wavelength may be tunable for some dyes by varying the laser cavity parameters, the dye concentration, the solvent (polarity and PH) and also temperature [1]. In dye laser media, an organic solvent such as ethanol which has a good solubility to organic dyes is usually used as a proper solvent. However, a dye laser has many kinds of problems caused by the organic solvent. For example, high volatility of organic solvents results from the dye concentration changes as time progresses. In addition, bubbles generated in the strong excitation, may cause stable oscillation difficulties. Furthermore, most organic solvents are flammable and this makes its use associated with the risk of an explosion and it thus always requires a cooling circulating unit. ⇑ Corresponding author. E-mail addresses: (M.T.H. Abou Kana).

[email protected],

http://dx.doi.org/10.1016/j.optmat.2015.06.045 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

[email protected]

For these reasons, ionic liquids are used as the laser dye solvents. The field of ionic liquid is continuously growing as the practical alternative of conventional organic solvents in dye laser systems. An ideal host matrix should possess good optical transparency in the region of absorption and emission of the dissolvent dye. It should not have chemical and photo-physical interaction with the dye. It should possess thermal and mechanical stability and time durability (longevity). Using of ionic liquids combines thermal stability, lower thermal expansion and better thermal coefficient of refractive index. Ionic liquids are organic salts composed entirely of anions and cations, and remain in the liquid state at ambient conditions. Recently, ionic liquids have been extensively studied as possible ‘‘green substitutes’’ for volatile organic solvents [2]. Due to their unique chemical and physical properties, such as thermal stability, low vapor pressure, and the surface properties [3,4], ionic liquids have also been used for electrochemical applications, solar batteries and biopolymers [5]. Furthermore, many dyes are known to be soluble in different ionic liquids, and the photophysical properties of the dyes in ionic liquids are expected to be interesting. For example, as the solvation of dye molecules in ionic

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liquids is expected to have properties different from those in usual solvents. It is necessary to know and understand the effect of various host matrices on the various properties including, photostability, photophysical properties as well as laser efficiency. Photophysical properties provide basic information about the lasing behavior of the material [6]. Fluorescein dye is well known as a good laser medium in the visible region and is notable for its moderate fluorescence quantum yield and photostability. In this paper a comparison and description of the spectroscopic and photophysical properties of fluorescein dye probe dissolved in different ionic liquids and ethanol as reference were assessed.

fluorescence signal registration with a fast phototube (Hamamatsu R1328U-03) through optical fiber. The fast phototube (+H.V) powered by power supply at 750 V and connected to the 300 MHz eZ-digital oscilloscope (DS-1530) attached to the computer processing unit for processing the spectrum. The dye samples were contained in 1 cm optical-path quartz cells that were transversely pumped by blue laser diode (450 nm). The exciting beam was directed toward the surface of cell sample with a combination of concave lens (f = 10 cm) and a cylindrical lens forming a line shape of 1 cm. The pumping energy (input energy) was measured via a beam splitter (4%) and the Gentec power meter (ModelQE50) detector head. The ASE output was focused by convex lens (f = 15 cm) onto Oplenic spectrophotometer which was connected to a computer unit for processing the spectrum. The samples were transversely pumped and were allowed to emit in the super radiant mode without employing a cavity mirror, since optical feedback were provided by reflection at host material air interface.

2. Experimental parts 2.1. Materials The chemical materials which are used in this work were; fluorescein, 1-Butyl-3-methylimidazolium chloride (BMIM Cl), 1-Butyl-3-methylimidazolium tetrachloroaluminate (BMIM AlCl4), 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), (cf. Fig. 1) and ethanol. All chemicals were used as received without further purification. Fluorescein laser dye dissolved into three types of ILs (BMIM Cl, BMIM AlCl4, BMIM BF4) and ethanol as reference with different dye concentrations. Pre-cleaned amber measuring flasks were used to prepare the required amounts of appropriate different dye concentrations. Fluorescein dissolved in (BMIM AlCl4), with concentrations of (1  103, 7  104, 3.5  104, 1  104, 7  105, 2  105) M, in (BMIM BF4), with concentrations of (1  103, 7  104, 3.5  104, 1  104, 7  105) and in (BMIM Cl, ethanol), with concentrations of (1  102, 8  103, 6  103, 2  103, 1  103, 7  104, 3.5  104, 1  104, 7  105, 2  105) M. Above 1  102 M of fluorescein, we noticed the aggregation of dye in ethanol.

3. Results and discussion The study of the photophysics of fluorescein dye in different environments provides useful informations on the potential of the different ionic liquids herein studied as laser media. Absorption spectra of (1  104 M) of fluorescein dissolved in different solvents with 2 mm path length cuvette are presented in Fig. 2. The absorption profile of fluorescein was dependent on the host material and their PH value since fluorescein dye has different 2.5

Absorbance (a.u)

2.0

2.2. Measurements Absorption and excitation–emission spectra were measured by Camspec M501 UV–Vis Spectrophotometer and PF-6300 Spectrofluorometer respectively. Fluorescence quantum yields uf were obtained by applying a comparative method [7]. Optimum concentration of the dye as linear fluorescence in ethanol was detected from its absorption and emission spectra. The fluorescence lifetime (sf), was measured by using Nitrogen laser (laser photonics LN1000) of pulse duration of 800 ps and wavelength 337.1 nm. The maximum energy per pulse was 2 mJ. The

HO

-4

1X10 M Fluorescein in BMIM Cl (a) (b) BMIM AlCl

(b)

4

(c)

BMIM BF

(d)

EtOH

1.5 (c)

1.0

0.5 (d)

(a)

0.0 350

400

450

500

550

Fig. 2. Absorption spectra of fluorescein in different host materials with 2 mm path length cuvette.

O

O

Fluorescein

N

+

+

AlCl 4

N

BMIM AlCl4

N BMIM Cl

+

N

600

Wavelength (nm)

COOH

N

4

N

BF4

BMIM BF4 Fig. 1. Molecular structure of laser dye (fluorescein) and ionic liquids.

Cl

-

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prototropic forms [8,9]. These spectral characteristics depend both on intra-molecular interactions of the dye molecules and on intermolecular interactions between dye molecules and host material. Due to the different polarity structures of ILs, there were different interactions between the laser dye and ILs. The absorbance value of fluorescein in BMIM Cl is not large; this may be attributed to the high viscosity of BMIM Cl compared to the other ionic liquids. The molar absorption coefficients (extinction coefficients) ‘‘e’’ were calculated from the absorption measurements and Beer– Lambert law; which is defined as the following; within the absorption spectra, the intensity is expressed as molar extinction coefficient (e). The amount of light absorbed depends on the extinction coefficient and the number of molecules in the light path. The latter depended on the concentration of the dye and the path length of the absorption sample. The amount of light that passes through the sample (transmittance) is given by BEER’s law:

log I0 =I ¼ e  c  d

ð1Þ

where I0 is the intensity of the light before it encounters the sample. I is the intensity of the light emerging from the sample, c is the concentration in moles per liter, and d is the path length in centimeters. It was found that e is of the order of 103–104 which indicates that p–p* transition takes place via a singlet–singlet transition and not via a triplet transition because in the latter case e is much smaller (e  101–102). The absorption and emission cross sections ra, re (cm2) are then respectively calculated according to formula [10].

ra ¼ 0:385  1020 e

ð2Þ

dye in ethanol as reference) care was taken to keep the concentration of all the samples at levels low enough to avoid re-absorption of the emitted photons [12,13]. The calculated values of e, ra, re and uf are tabulated in Tables 1 and 2. From these calculated values it is obvious that the photo-physical properties of fluorescein dye molecules depend on the molecular environment around it (acidic, basic, polar, non-polar, protic, non-protic) which is being created by solvent molecules. The p* value reflects the dipolarity/polarizability of the solvent; while in case of ILs, the possible ion-dye coulombic interactions should be taken into consideration. This explains the common high value of p* of the ILs (1.047, 1.17 for BMIM BF4, BMIM Cl, respectively) compared to the organic molecular solvents (0.54 for ethanol) [14]. The spectral profiles of the absorption and fluorescence spectra of fluorescein dissolved in different solvents (BMIM Cl, BMIM AlCl4, BMIM BF4 and EtOH) with optimum concentration of (1  102 M) in case of BMIM Cl and concentration of (7  105 M) in case of the other solvents, were shown in Figs. 3a–3d. This optimum concentration showed the highest peak intensity. The emission intensity decreased up concentrations higher than optimum concentration. This related to the effect of re-absorption/re-emission phenomena because the possibility of exciting molecules by absorption of a photon previously emitted by another molecule in the medium that depending on the overlapping between absorption and fluorescence spectra, which is affected by the dye concentration [15], as shown in Figs. 4a–4d. The selective excitation wavelengths were (462, 446, 440 and 483 nm) for (BMIM Cl, BMIM AlCl4, BMIM BF4

and

re ¼

uf k4Re EðkÞ 8pcsf n2 EðkÞdðkÞ

0.30

ð3Þ

Absorbance (a.u)

12

b 10

0.20

8 6

0.15

4 0.10 2

u S hx ¼ u x h Sx

ð4Þ

0

0.05 400

where h is the absorption peak height; S is the area enclosed by the emission curve and wavelength axis. In determination of fluorescence quantum yields uf(s) (determined relative to the R6G laser

Fluorescence intensity (a.u)

a 0.25

where ke is the emission wavelength, n the refractive index of the solvent, c is the velocity of light, sf is the florescence life time, R E(k) dk is the normalized fluorescence spectrum and uf is the quantum yield of the dyes (defined as the ratio of the number of photons emitted to the photons absorbed). Quantum yield uf was estimated by comparing the absorption and emission spectra to those of a known standard ux according to the relation [11].

14

Fluorescein in BMIM Cl (a) A bsorbance (b) Fluorescence

450

500

550

600

650

700

Wavelength (nm) Fig. 3a. Absorption and emission spectra of (1  102 M) fluorescein in BMIM Cl.

Table 1 Absorption properties of fluorescein dissolved in different solvents. (ka) maximum absorption peak, (e) molecular extinction coefficient, (ra) absorption cross section, (f) oscillator strength, (K) the attenuation length, l12(D) the transition dipole moment. Host

ka (nm)

e L ML1 cm1 (104)

ra (1017) cm2

f

K (cm)

l12(D)

BMIM Cl BMIM AlCl4 BMIM BF4 EtOH

462 446 440 457

0.12 4.4 4.02 0.5

0.5 17 15.5 2

0.04 0.78 0.95 0.33

0.32 0.13 0.09 0.43

0.6 8.6 9.4 6.3

Table 2 Fluorescence properties of fluorescein dissolved in different solvents. (ke) maximum emission peak, (dk) Stokes shift,(Ef) energy yield of fluorescence, (Kr) the radiative decay rate, (Kisc) the intersystem crossing rate, (uf) fluorescence quantum yield, (scall) calculated fluorescence life time, (sme) measured fluorescence life time. Solvent

ke (nm)

dk (nm)

Lasing range

re (1017) cm2

Ef

Kr 108 s1

Kisc 108 s1

uf

scall (ns)

sme (ns)

BMIM Cl BMIM AlCl4 BMIM BF4 Ethanol

521.5 472.5 477 517

59.5 26.5 37 60

507–555 461–493 465–506 502–555

2.4 11.2 10.5 4.7

0.11 0.71 0.76 0.45

0.4 1.8 2.2 0.7

3.3 0.6 0.5 0.7

0.12 0.75 0.82 0.51

3 3.9 3.6 5.3

2.7 4.2 3.7 7

D.M. AL-Aqmar et al. / Optical Materials 47 (2015) 573–581 1.6

14

Fluorescein in BMIM AlCl 4

12

Absorbance (a.u)

1.4

5

(b)

(a) 1.2

4 3

1.0 2

Fluorescence Intensity (a.u)

6

Fluorescein in BMIM Cl

(a)

7

Absorbance Fluorescence

(a) (b)

(b)

10

Intensity (a.u)

576

(c)

8

-2

(a)

1x10 M

(b)

8x10 M

(c)

6x10 M

(d)

2x10 M

(e)

1x10 M

-3

-3

-3 -3

-4

(f)

6

7x10 M

(d)

4 (e) 2 (f)

1 0.8

0 0 400

450

500

550

600

650

500

550

700

600

650

700

wavelength (nm)

wavelength (nm) Fig. 3b. Absorption and emission spectra of (7  105 M) fluorescein in BMIM AlCl4.

Absorbance (a.u)

14 0.8

12

a

b 10

0.6 8 6

0.4

4

3

500

600

(c)

3.5x10 M

(d)

1x10 M

(e)

7*10 M

-4

-4

-4

-5

(c)

(b)

(a)

400

700

450

500

550

600

650

700

wavelength (nm)

Fluorescein in EtOH a Absorbance Fluorescence b

b

16

4

0.16 3

0.14 0.12

2

0.10 1

0.08

Fluorescein in BMIM BF 4

(e)

14

5

Fluorescence Intensity (a.u)

0.20

Fig. 4b. Emission spectra of fluorescein in BMIM AlCl4 as a function of dye concentration.

12

Intensity (a.u)

0.22

Absorbance (a.u)

7x10 M

0

Fig. 3c. Absorption and emission spectra of (7  105 M) fluorescein in BMIM BF4.

a

1x10 M

(d)

wavelength (nm)

0.18

-3

(a) (b)

2

1

0 400

(e)

4

2

0.2

Fluorescein BMIM AlCl4

excited wavelength

16

Intensity (a.u)

1.0

5

18

Fluorescence Intensity (a.u)

Fluorescein in BMIM BF4 Absorbance a Fluorescence b

Fig. 4a. Emission spectra of fluorescein in BMIM Cl as a function of dye concentration.

10 (d)

8

0

1x10 M 7x10 M

(c)

3.5x10 M

(d)

1x10 M

(e)

7x10 M

-4

-4

-4 -5

6 4 (c)

2 (b)

(a)

0 400

0.06

-3

(a) (b)

450

500

550

600

650

700

wavelength (nm)

400 425 450 475 500 525 550 575 600 625 650 675 700

wavelength (nm)

Fig. 4c. Emission spectra of fluorescein in BMIM BF4 as a function of dye concentration.

Fig. 3d. Absorption and emission spectra of (7  105 M) of fluorescein in ethanol.

and EtOH), respectively, which represent the maximum absorption wavelength for the dye in different solvents. These solvent molecules may interact with the dye molecules (dipole–dipole, hydrogen bonding, charge transfer etc.) resulting in change in their spectroscopic properties. We noted from Figs. 3a–3d that, the absorption and the emission profiles of fluorescein in BMIM Cl and EtOH solvents had similarity in behavior at higher wavelengths compared to the optical profiles of fluorescein in BMIM

AlCl4 and BMIM BF4 which had other similarity. These higher wavelengths of optical spectra indicated that fluorescein molecules were electronically in more stable energy states in case of BMIM Cl and ethanol media. This red-shifting of emission arising from the increasing of dye concentration or from the host material could be useful for extending the tuning rang of some dyes. The values of stokes shifts are tabulated in Table 2. Larger magnitude of the Stokes shift indicates that the excited state geometry may be more different from that of ground state, which shows that there is an

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D.M. AL-Aqmar et al. / Optical Materials 47 (2015) 573–581

20

1x10 M

(e)

(b)

8x10 M

(f)

(c)

6x10 M

(d)

2x10 M

(g)

12

(c)

8

7 6

-3 -3

1x10 M -4

(f)

7x10 M

(g)

3.5x10

(h)

1x10 M

-4

M

-4

-5

7x10 M

5

c

BMIM BF4

d

EtOH

(d)

4 3 2

(b)

1

0 500

-4

(1×10 M) Fluorescein in BMIM Cl a BMIM AlCl4 b

-3

(i)

(b) (i) (h) (a)

4

-3

(e)

16

(c)

8

-2

(a)

Intensity (a. u)

24

Intensity (a.u)

Fluorescein in EtOH

(d)

28

550

600

650

700

(a)

0

wavelength (nm)

450

500

increase in the dipole moment on excitation [16]. In our study, the general observation is that there is an increase in the Stokes shifts values, (59.5 nm and 60 nm in case of BMIM Cl and EtOH respectively), because of increasing their solvent polarities. However, BMIM Cl and EtOH solvents have higher electronegativity due to the presence of Cl anion and O-atom (3.16 and 3.44 respectively)  compared to electronegativity of AlCl 4 anion and BF4 anion (electronegativity  zero) due to their Lewis structures. In Lewis struc tures of AlCl 4 and BF4 , the least electronegativity atoms (Al = 1.61 and B = 2.04) were in the central and surrounded by ligand (outer) atoms of higher electronegativity, having covalent character compound. The effect of the variation of the dye concentration in different host materials (BMIM Cl, BMIM AlCl4, BMIM BF4, and EtOH) on the position of the peak wavelength of the ASE was plotted in Fig. 5. There is a spectral shift (red shift) in the peak wavelength arising from increasing the dye concentrations. The tuning curve for fluorescein dissolved in BMIM AlCl4 was found to extend from 476 nm at concentration 7  105 mol/L to 502 nm at concentration 1  103 mol/L (26 nm). In case of BMIM BF4 the tuning curve extended from 479 nm at concentration 7  105 mol/L to 495 nm at concentration 1  103 mol/L (24 nm). Also in case of EtOH the tuning curve extended from 514 nm at concentration 7  105 mol/L to 525 nm at concentration 1  102 mol/L (11 nm). On the other hand in case of BMIM Cl, the tuning curve extended from 517 nm at concentration 7  105 mol/L to 523 nm at concentration 1  102 mol/L (7 nm). This may be attributed to very low probability of unexcited dye molecules to reabsorb emitted photons of other excited molecules due to its high restriction. In general the red shift observed from increasing the dye concentration dissolved in different host materials arises from

550

600

650

700

wavelength (nm)

Fig. 4d. Emission spectra of fluorescein in EtOH as a function of dye concentration.

Fig. 6. ASE of fluorescein in different solvents transversely pumped by blue diode laser (k = 450 nm) with 150 mW. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

increases in the number of unexcited dye molecules. These unexcited dye molecules will absorb the radiation emitted by the excited dye molecules (re-absorption effect) [17]. Besides the red shift observed due to increasing the dye concentration, there is also shift arising from host materials, which appear from the tuning curves in Fig. 6. The effect of the variation of the host material (Ionic Liquids and ethanol) on the position and the peak intensity of the (ASE) was plotted in Fig. 6. The samples were transversely pumped by blue diode laser (450 nm) with 150 mW and were allowed to emit in the super radiant mode without employing a cavity mirror, since optical feedback was provided by reflection at host material air interface. BMIM BF4 host has the highest peak intensity of the mirrorless ASE (a.u) compared with the other hosts (BMIM Cl, BMIM AlCl4, EtOH). Different structures of active centers in host materials have been known to cause shifting of emission peak position [18]. The change in the fluorescence peak position and intensity was closely correlated to the acidity scale of the C2–H of imidazolium RTILs. The acidity scale of C2–H is highly dependent on the hydrogen bonding strength (C2–H  X) between the hydrogen on the electron-deficient C2 carbon atom of the imidazolium ring and the counter anion (X) [19–22]. The oscillator strength (f) is an important characteristic of the dyes which shows the effective number of electrons whose transition from ground to excited state gives the absorption area in the electron spectrum [23]. Values of oscillator strength are calculated by using Eq. (5) [24]:

f ¼ 4:32  109

Z

eðmÞdm

ð5Þ

530 525 520

Wavelength (nm)

515 510 505 500 495 490

Fluorescein in BMIM Cl BMIM AlCl4

485 480

BMIM BF4

475

EtOH

470 1*10-4

7*10-4

2*10-3

8*10-3

Concentration (mol/L) Fig. 5. Peak wavelength emission spectra of fluorescein in different hosts as a function of dye concentration.

A value of 1 represents a strong transition while a quantum-mechanically forbidden transition might have f  0.001. The oscillator strength values of fluorescein in BMIM AlCl4 and BMIM BF4 are higher than those in EtOH host which have attenuation length (0.43 cm); where the attenuation length K(k): (the distance at which the original light intensity I0 reduced to (I = I0/e) given by the Eq. (6) where (e) Euler’s number (e  2.7) [25,26]:

KðkÞ ¼

1

eðkÞc ln 10

ð6Þ

where e(k) is the molar extinction coefficient, (c) the molar concentration. Hence, the effective number of electrons transferred from the ground to excited states in BMIM AlCl4 and BMIM BF4 is higher

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D.M. AL-Aqmar et al. / Optical Materials 47 (2015) 573–581

than that in EtOH. While, BMIM Cl has the lowest value of oscillator strength (0.04). The transition dipole moment (l12) from ground to excited state was calculated by using the Eq. (7), f is related to the transition moment and the Einstein coefficient by the following expressions:

4pmme l212 3he2

Or

l212 ¼ 2:36  1051  f  k

ð7Þ

where v is the optical frequency at the maximum absorption, me and e are the mass and electrical charge of an electron, all in the International System of Units (SI). A Debye (D) is the traditional non-SI unit of dipole moment. The conversion between D and SI units is 1Dp = 2.36  1030 (coulomb.meter). The polarity and solvation properties of the RTILs are addressed in several recent works [27–33]. A recent study suggests that the change in the dipole moment on excitation of the pure ionic liquid systems is rather low (around 4.4-5D) and the molecule emits from a locally excited state [34]. In our study, we found that the change in the dipole moment on excitation of the laser dye (fluorescein) in different hosts (BMIM AlCl4, BMIM BF4 and EtOH) were rather high (8.6, 9.4 and 6.3D, respectively). While in BMIM Cl solvent, abnormal very low change in dipole moment on excitation (l12(D) = 0.6) was recorded. This may be also attributed to restriction of dye molecules in BMIM Cl. This restriction may be due to the electrostatic attraction (H-bonding) between H-atoms of hydroxyl and carboxylic groups in fluorescein molecule with high electronegativity of Cl anion in BMIM Cl. Also, the fluorescence quantum yield of fluorescein dissolved in BMIM Cl was very low compared of the other solvents, see Table 2. The energy yield of fluorescence (Ef) was calculated by the following equation:

Ef ¼ uf kA =kf

ð8Þ

where uf is the fluorescence quantum yield, kA, kf are the maximum absorption and fluorescence wavelengths. The energy yield of fluorescence values of the laser dye in different hosts were evaluated and tabulated in Table 2. Fluorescein in BMIM BF4 gave the highest quantum yield (0.82), so it has the highest energy yield and radiative decay rate with the lowest intersystem crossing rate kisc. The intersystem crossing rate constant (kisc) and the radiative decay rate constant (kr) are related to the quantum fluorescence yield uf for (uf  1) by the approximate relationship [35].

kisc ¼ ð1  uf Þ=sf

ð9Þ

kr ¼ uf =sf

  GðkÞ ¼ 2=L ln I1 =I1  1

ð12Þ

2

where L is the full length of the sample, I1 and I1/2 are ASE intensities of the full and half pumped length respectively. Fig. 7 shows the computed gain for L = 10 mm for different concentrations of fluorescein dissolved in Ionic Liquids (BMIM Cl, BMIM AlCl4, BMIM BF4) and ethanol when pumped by blue diode laser (450 nm) at (260 mW). The gain depends on the dye concentration where the optimum gains per unit length were (0.8, 3 and 2.4) cm1 for fluorescein in different host materials (BMIM AlCl4, BMIM BF4 and EtOH) respectively. The values of gain initially increased to a maximum and then decreased with concentration increased as shown in Fig. 7. This decrease in the gain with increasing in dye concentration beyond optimum concentrations (1  104, 1  104, 2 8  10 mol/L for fluorescein in BMIM AlCl4, BMIM BF4 and EtOH) respectively because of the dye molecules quenching and aggregation [41–43]. In case of BMIM Cl, fluorescence intensity of fluorescein dye increased gradually with increasing dye concentration till 1  102 mol/L without getting optimum concentration so, its optimum gain may be at concentration higher than 1  102 mol/L. The lasing efficiencies of dye in the different host materials under study are defined as the ratio between the energy of the

ð10Þ

Since excited-state lifetime gives significant information about kinetics of the intermolecular interactions such as excimer formation, energy transfer and molecular distances [36]. From the quantum yield, absorption and emission spectra, fluorescence lifetime s were calculated and tabulated using the following [37]:

R

FðmÞdm R 1=s ¼ 2:88  10 n u1 f m3 FðmÞdm 9 2

Z

eðmÞ dm m

ð11Þ

where m is wavenumber (in m1), F(m) is the emitted fluorescence intensity, and n is sample refractive index. Based on absorption and emission spectra, in which the values of wavelength (k) were converted in wavenumber (m: k1). Chapman and Maroncelli, while studying the solvation dynamics of several probes in solutions containing various electrolytes, found that the ionic solvation dynamics is rather slow, taking place on a 1–10 ns time scale, and is

Fluorescein in BMIM Cl BMIM AlCl4

4

BMIM BF4 EtOH

3

Gain (cm-1)

f ¼

dependent on the viscosity of the media [38]. Slower dynamics in the latter solvents is expected on the basis of the consideration of the viscosity values (BMIM Cl has the highest viscosity value). The solvation in ionic liquids is distinctly different from that in other polar solvents. In polar molecular solvents such as water or alcohol, the solvent molecules reorient themselves around the photoexcited molecule. On the contrary, the time-dependence of the fluorescence spectrum of the probe in ionic liquids arises mainly from the motion of the ions. In view of this situation, the solvation dynamics in a room-temperature ionic liquid is likely to be different from that in conventional molecular solvents. Even though no studies on the solvation dynamics have been undertaken thus far in any room-temperature ionic liquid such as BMIM BF4, dynamical results are available in molten salts as well as in ionic solutions [39]. From our study, we noticed that the fluorescence lifetime of fluorescein in ethanol (7 ns) is very higher than its fluorescence life time in the other liquid solutions, which is between 2.7 and 4.2 ns. So, fluorescein dissolved in ethanol had higher nonradiative rate than the other ionic solvents, see Table 2. A single pass gain was measured by the adoption of the amplified spontaneous emission (ASE) method proposed by Shank et al. [40]. The ASE gain was defined as the increase in the ratio of the emitted to the incident light intensity per unit length of the pumped material and calculated according to Eq. (12):

2 1 0 -1 7*10-5

3.5*10-4

1*10-3

6*10-3

1*10-2

concentration (mol/L) Fig. 7. The gain as a function of concentration of fluorescein in different solvents.

579

D.M. AL-Aqmar et al. / Optical Materials 47 (2015) 573–581

Fluorescein in EtOH

4500

peak intensity of ASE (a.u)

dye laser output and the energy of the pump laser. The dependence of the output energy on the dye concentration, over a range of input energies, has been investigated and represented in Figs. 8a–8d. The samples were transversely pumped and were allowed to emit in the highest peak intensity of the mirrorless ASE (a.u). The pump pulse energy was controlled and varied between 70 and 400 mW. In general the output energy increased with increasing the pumping energy for all samples. This may be attributed to the increasing of the number of excited molecules

-2

1*10 M -3

8*10 M

4000

8.91%

-3

6*10 M

3500

-3

2*10 M -3

6.11%

1*10 M

3000

-4

7*10 M

2500

-4

11.62% 8.02%

3.5*10 M -4

1*10 M

2000

-5

7*10 M

3.3% 2.74% 2.86% 2.14%

1500 1000

1.72%

500 1.86%

peak intensity of ASE (a.u)

800

100

Fluorescein in BMIM Cl 1×10 M

700

-3

1.16%

6×10 M 2×10 M

300

350

Fig. 8d. Peak intensity of ASE of fluorescein in EtOH as a function of pumping power for different dye concentrations.

0.49% 500 0.71% 400 300

100

150

200

250

300

350

Input power (mW) Fig. 8a. Peak intensity of ASE of fluorescein in BMIM Cl as a function of pumping power for different dye concentrations.

720

peak intensity of ASE (a.u)

250

8×10 M -3

600

200

input power (mW)

-3

200

640 560 480

Fluorescein in BMIM AlCl 4 1×10-3M 7×10-4M 3.5×10-4M 1×10-4M 7×10-5M 2×10-5M

1.14% 0.60% 0.85%

1%

400 320 0.52% 0.4%

240 160 80 100

150

200

250

300

input power (mW) Fig. 8b. Peak intensity of ASE of fluorescein in BMIM AlCl4 as a function of pumping power for different dye concentrations.

which yields more emitted photons. Relatively high percentage of energy conversion were observed for fluorescein in BMIM BF4 (11.8%) with concentration (7  104 mol/L) compared with EtOH (2.86%) at the same concentration. The relation between ASE slope efficiencies and the dye concentrations are presented in Fig. 9. Photo-physics and photochemistry of laser dye depend not only on the solvent polarity but also on the solvent viscosity [44] by the rotational motion of the molecules of the dye. Viscosity of ILs is also controlled by number of hydrogen bonding between cation and anion, as well as Vander Waals interactions present in it. The results show that the ASE efficiency of fluorescein dye is clearly dependent on the solvent. Improving the photostability of laser dyes within Ionic liquid hosts had been the focus of this study. The evolution of the lasing output as a function of the number of pump, in the same position of the samples, is plotted in Fig. 10. This study was carried out for a sample of fluorescein dye dissolved in (BMIM Cl, BMIM AlCl4, BMIM BF4 and EtOH) at optimum concentrations of (8  103, 3.5  104, 7  104 and 1  102 mol/L) respectively which examined by blue diode laser (450 nm) of 400 mW, which showed the highest energy conversion. The time exposure was exactly 132 min. A gradual decrease in the output energy due to the progressive photodegradation and thermodegradation of the dye molecules was observed for all the samples except in case of fluorescein dissolved in BMIM Cl. This decreasing occurred at a faster rate for the fluorescein in ethanol, and after 132 min, the peak ASE dropped to 36% of its initial value compared to a drop to only 56% in the case of the fluorescein in BMIM BF4. The presence of

12 6.37%

Fluorescein in BMIM BF4

11.8%

-3

3500

1×10 M -4

3000

3.5 ×10 M -4

1×10 M

2500

10

-4

7×10 M

-5

7×10 M

2000

4.06%

1500 2.92% 1.6%

1000

ASE efficiency (%)

4000

peak intensity of ASE (a.u)

150

-2

Fluorescein in BMIM Cl BMIM AlCl 4 BMIM BF 4 EtOH

8 6 4 2

500 0 100

150

200

250

300

350

input power (mW) Fig. 8c. Peak intensity of ASE fluorescein in BMIM BF4 as a function of pumping power for different dye concentrations.

7*10 -5

3.5*10-4

1*10 -3

6*10 -3

1*10 -2

concentration (mol/L) Fig. 9. Slope efficiency as a function of dye concentration for fluorescein in different solvents.

D.M. AL-Aqmar et al. / Optical Materials 47 (2015) 573–581

Normalized intensity of ASE (a.u)

580

160 140

Fluorescein in BMIM Cl BMIM AlCl4

144%

BMIM BF4 EtOH

120 100 80 60

56%

40

39% 36%

20

40

60

80

100

120

140

Time (min) Fig. 10. Normalized intensity ASE as a function of time. Pumping power (400 mW) for fluorescein in different solvents.

4000

Intensity of ASE (a.u)

3500 3000 2500

-3

8x10 M Fluorescein in BMIM Cl (a) 2min (b) 10min (c) 26min (d) 94min (e) 118min

(e) (d) (c) (b) (a)

consequently lower transition dipole moment was in case of fluorescein dissolved in BMIM Cl. This may be also attributed to restriction of dye molecules in BMIM Cl. This restriction may be due to the electrostatic attraction (H-bonding) between H-atoms of hydroxyl and carboxylic groups in fluorescein molecule with high electronegative of chloride anion (3.16) in BMIM Cl. The calculated absorption and emission cross-sections showed superiority values of fluorescein dissolved in ILs (BMIM AlCl4 and BMIM BF4). The oscillator strength values of fluorescein in BMIM AlCl4 and BMIM BF4 are higher than those in EtOH and BMIM Cl hosts. This behavior is attributed to the crucial role of H-bonding. Fluorescein laser dye in BMIM BF4 showed relatively high value of the fluorescence quantum yield (82%) and increased photostability compared to the other hosts. As progressed time, exposure to laser beam (450 nm) of high photon energy in case of laser dye dissolved in BMIM Cl overcame H-bonding with rearranging of energy states of dye molecules to higher energy. So, it showed new abnormal behavior (increase of ASE intensity peak with time till 144% of its initial ASE intensity and slight blue shift in the wavelength) after 132 min. Finally the results offer a promising efficient ionic liquid as benign green solvent for dye laser systems, which could replace conventional volatile toxic organic solvents and solid state -dye laser systems in applications. In some cases some dye properties were degraded, so the choice of suitable ionic liquid is essential.

2000

References

1500 1000 500

300

350

400

450

500

550

600

650

700

750

wavelength (nm) Fig. 11. The ASE of fluorescein in BMIM Cl as a function of the exposure time to blue diode laser (450 nm) of 400 mW.

atomic fluorine in or along the backbone of ionic liquid possesses many desirable physical properties, mainly due to high thermal stability, high optical damage threshold, and enhanced chemical resistance compared to their non-fluorinated analogues [45]. On the other hand, fluorescein in BMIM Cl exhibited a new non-classical behavior of ASE intensity with some oscillations during its monitoring as a function of exposure time of diode laser (450 nm) at 400 mW pumping power for 132 min. It increased with time till 144% of its initial ASE intensity. This new abnormal behavior may be attributed to the high restriction of unexcited dye molecules by H-bonding with Cl anions in medium. This H-bonding (5–30 kJ/mol) produced interatomic distance shorter with higher stability (low energy) of fluorescein dye in BMIM Cl. Exposure to laser beam (450 nm) of high photon energy may overcome of H-bonding with rearranging of energy states of dye molecules (high energy). This expectation was confirmed by a slight hypsochromic shift in the wavelength of the peak of the ASE with increasing its intensity of dye with continuous pumping as shown in Fig. 11.

4. Conclusion Investigation was carried out for some photo-physical properties of fluorescein dissolved in three types of ionic liquids and ethanol as reference. The absorbance value of fluorescein in BMIM Cl is small, due to the high viscosity of BMIM Cl compared to the other ionic liquids. Lower magnitude of tuning range, oscillator strength

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