Photophysical properties of commercial red dyes in polymer films

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Optical Materials 30 (2008) 1478–1483 www.elsevier.com/locate/optmat

Photophysical properties of commercial red dyes in polymer films Ian B. Burgess a, Paul Rochon b, Nicolas Cunningham a

a,*

Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, P.O. Box 17000, Stn Forces, Canada K7K-7B4 b Department of Physics, Royal Military College of Canada, Kingston, Ontario, P.O. Box 17000, Stn Forces, Canada K7K-7B4 Received 24 May 2007; received in revised form 5 September 2007; accepted 9 September 2007 Available online 26 November 2007

Abstract A fluorescence study was conducted on commercial dyes (squaraines; triphenylmethane dyes; porphyrins and chlorins) in thin films of polyvinyl alcohol (PVA), poly (methyl methacrylate) (PMMA) and Hival 5108, a commercial polystyrene (PS) from Ashland. Fluorescence quantum yields were measured for dye–polymer system and compared to liquid- and solid-phase media values. All dyes were found to have fluorescent yields and stabilities that were dependent on the polymer film material. Squaraine dyes, SQ3 and OH-SQ, exhibited the highest quantum yields (Uf) when miscible in polymer films, reaching values as high as 0.89 in PS and 0.79 in PMMA. Acceptable Uf at wavelengths higher than 650 nm were observed for SQ3, OH-SQ, TPP and PP-IX-DME. Moreover, fluorescence decay was measured after 240 s of CW irradiation for all the dye–polymer films studied. Crown Copyright  2007 Published by Elsevier B.V. All rights reserved. PACS: 78.66.Q; 78.55 Keywords: Commercial red dyes; PVA; PMMA; PS

1. Introduction Dye-doped polymer films have been extensively studied for many applications including optical waveguides [1], polymeric solid-state lasers [2], distributed feedback lasers [3,4], optical amplifiers [5] and optical storage media [6]. The most commonly used polymer as a solid matrix is poly (methyl methacrylate) (PMMA) and a number of studies comparing the properties of different dyes in PMMA [6– 16] or one of its copolymers [17–21] have been reported. Dyes incorporated into PMMA matrices cover the full range of the visible spectrum and in some cases the nearIR [9,16]. Other studied solid matrices are composed of

*

Corresponding author. Tel.: +1 613 541 6000x6610; fax: +1 613 542 9489. E-mail address: [email protected] (N. Cunningham).

organically modified silicate (Ormosil) [8,22] or polystyrene (PS) [23–27]. Polyvinyl alcohol (PVA) [1a,3,28–30] has also been studied but is limited in scope because of the requirement that the dyes be soluble in the same solvent as PVA. However, this has been used advantageously for producing double-layered systems that avoids interlayer mixing during fabrication [3]. In most of the dye–polymer systems studied presented above, the emission wavelength was located in the visible spectrum but few studies have investigated wavelengths above 650 nm [9,14,16,22,27]. Furthermore, comparison between different dye-doped polymer systems is often impossible because critical values such as quantum yield or photostability data are not always accessible and only one polymer matrix system has been studied. In this study, we report the optical properties of a number of traditional and non-traditional dyed polymer thin films, operating close to or above 650 nm, using a simple optical set-up. The short-term stability of the doped-film will also be studied.

0925-3467/$ - see front matter Crown Copyright  2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.09.004

I.B. Burgess et al. / Optical Materials 30 (2008) 1478–1483

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2. Experimental

Table 1 Solvents used to produce dye–PMMA or dye–PS solid films

2.1. Materials and film preparation

Dye

Solvent

Concentration (% w/w)

Cl-e6 Pheo-a PP-IX-DME TPP GV MG R6G S101 OH-SQ SQ3

Tetrahydrofuran (THF) THF THF Toluene Chloroform Chloroform Dichloromethane (DCM) DCM Chloroform Chloroform

0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 2.4 · 10 5 (sat) 0.0010 (sat)

The dyes 1,3-bis[4-(dimethylamino)phenyl]-2,4-dihydroxycyclobutenediylium dihydroxide, bis(inner salt) squarylium dye III (SQ3), 1,3-bis[4-(dimethylamino)-2hydroxyphenyl]-2,4-dihydroxycyclobutenediylium dihydroxide, bis(inner salt) (OH-SQ), metal-free tetraphenylporphyrin (TPP), sulforhodamine 101 (S101) and gentian violet (GV) were purchased in solid form from Sigma– Aldrich. Gentian violet is a mixture of the hydrochlorides of the more highly methylated pararosanilines [31]. The dyes pheophorbide-a (Pheo-a), chlorin-e6 (Cl-e6), protoporphyrin IX dimethyl ester (PP-IX-DME), were purchased from Porphyrin Products Inc. (Utah, USA). Rhodamine 6G (R6G) and malachite green (MG) were purchased from Kodak Inc. now available at Acr os Organics. In addition metal-free Octaethylporphyrin (OEP) was synthesized according to the literature [32], but is commercially available from Porphyrin Products Inc. Three different polymers were used: Polyvinyl alcohol (PVA, MW 88000, Aldrich), poly

a

O

(methyl methacrylate) (PMMA, inherent viscosity: 0.45, MW 75,000 Scientific Polymer Products Inc.), and polystyrene (PS, Ashland Hival Prime 5108). All chemicals purchased were used as received without further purification. Aqueous solutions of 10% w/w PVA with 0.0125% w/w dye were prepared for each water-miscible dye. All of the dyes that were miscible in the PVA layer were water-soluble (R6G, S101, MG, GV). Solutions of 10% w/w PMMA

b

Squaraines

R1

Sat = solution was saturated at that concentration.

Triphenylmethanes

Rhodamines

-

R

R N

H

N

2+

R

N

-

H3C

+

N

O

CH3

CH3

N

N

CH3

CH3

N

N

+

CH3

H

N

crystal violet malachite green

R6G

OH-SQ: R= CH3 R1 = O H

N

+

O

N

SO3

SO3H

S101

c

d

Porphyrins

N

NH N

NH N

HN

O OEP

CH3

CO2CH2CH3

R1

SQ3: R= CH3 R1 = H

+

H3C

R O

H3C

OCH3

N

NH

HN

H3CO

PP-IX-DME

N

Chlorins

N HN

H3C H

O TPP

NH

N

N

HN H3C H

H O

O OH

OCH3

H

NH

N

N

HN

O

HO

O OH

O

OH

O

Pheo-a

Cl-e6

Fig. 1. Chemical structures for the dye families studied: (a) squaraines; (b) triphenyl-methane dyes; (c) porphyrins; (d) chlorins.

CH3

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and PS were prepared for each miscible dye with 0.0125% w/w dye, unless the solution saturated at a lower concentration in which case a saturated solution was used, with the solvent that were found to maximize dye solubility. The solvents used for each dye are shown in Table 1. Dye-doped polymer films were prepared by spin-coating the solutions onto glass slides (Corning) using a Headway Research Inc. 1-EC101D-R485 spin coater at 2.50 · 103 rpm, and then allowing them to dry in an oven for at least 2 h at 85 C for solutions containing water and toluene and at 60 C for other solvents. This yielded films of 1 ± 0.2 lm in thickness as measured using a Pacific Nanotechnology Nano-R AFM. In addition, thick drop samples were prepared from each solution by depositing a drop onto a glass slide and drying for 16 h in the oven at the above-mentioned temperatures without spin-coating. Absorption spectra were taken using a Shimadzu UV-160 UV–visible spectrometer using a clean glass slide as a reference. 2.2. Fluorescence experiments Fluorescence experiments were conducted using the optical set-up depicted in Fig. 2. The imaging set-up was mounted onto an optical rail that was fastened to the table through the sample holder and angled at 145 from the

path of the incoming Ar laser beam. The path of light from the 635 and 670 nm diode lasers crossed the sample at an angle normal to the Ar laser beam. The beams from all three sources were circular with a diameter of approximately 1 mm at the sample, which was maintained constant throughout the experiments. The power outputs from the Ar (514 line) and diode lasers at 635 and 670 nm at the sample location were found to be 32.6, 2.01, and 7.42 mW, respectively, as measured by a Metrologic 45– 540 power meter. Emission spectra were collected using an Edmund Industrial Optics BRC111A-USB-VIS/NIR spectrometer connected to a computer using BWTEK Inc. BWSpec 3.23 software. Fluorescence in solution was verified by replacing flat sample by a circular based 2.2 mL vial (Fisher Scientific) containing 1 mL of the solution. For the fluorescence measurements in both thick drop and thin film samples, the glass slides containing the samples were placed perpendicular to the path of the imaging set-up as shown in Fig. 1. The alignment of the imaging set-up was ascertained using a fresh thin PVA film containing S101 prior to each set of experiments. Absorption and emission peak wavelengths were measured using thick drop samples. Fluorescence stabilities and quantum yields (Uf) were measured using spectra from the thin film samples. Total emission as a fraction of total absorption of the pumped radiation was calculated

Fig. 2. Optical assembly for fluorescence experiments using three different light sources: (1) lines from an Innova (Coherent) multiline Ar gas laser (a) were separated through a prism (b) A pinhole (c) selected the line at 514 nm to irradiate the sample (d); (2) collimated light from a diode laser (k = 635 nm, CalPac Lasers Inc.) (e) was focused onto the sample through a plano-convex lens (fl = 5 cm) (f); (3) light from a diode laser (k = 670 nm, Power Technology Inc., PPM10/3804) (g) was collimated through a planoconvex lens (fl = 10 cm) (h) and by a removable mirror (i) before being focused onto the sample by (f). Fluorescence was imaged by two imaging lenses, (k) and (l) (fl = 10 cm and 5 cm, respectively), onto an optical fiber connected to a spectrometer. A low-pass 530 nm filter (j) was used to remove any scattered laser light from the fluorescence spectrum when the Ar laser was used and was left in place when the other light sources were used so as to maintain the calibration of the imaging setup.

430s

Absorption and emission maxima were consistent in all polymers unless otherwise indicated. V = PVA, M = PMMA, S = PS. s = secondary absorption or emission peak; t = tertiary absorption or emission peak; I = insufficient dye solubility to measure fluorescence in thick drop sample; U = fluorescence in film samples below detection limit; * = excitation laser line removed from peak integration through a linear interpolation.

Uf in PS (±10%) Uf in PMMA (±10%)

U U 0.04 0.009 0.10 U 0.008 0.70 U 0.72 0.79 I U I I I U 0.16 0.38 0.16 I I

Uf in PVA (±10%) (solution) lit

Uf,

0.16 [34] 0.32 [35] 0.13 [36] 0.06 [37] 0.11 [38] (0.019) [39] 0 (<10 4) [40] viscosity dependent 0.95 [41] 0.90 [33] 0.86 [23] and references therein 0.65 [23] and references therein 514 670 514 514 514 514 635 514 514 635 635

kpump (nm) kmax, f (nm)

682 675 621, 690s 674, 634s 650,715s 610 (V), 645 (M, S) 670 560 610 (V), 592 (M, S) 657 648

kmax, a (nm)

665, 670, 400, 425, 420, 590 630, 540 590 635 635

Dye

Cl-e6* Pheo-a* OEP PP-IX-DME TPP GV MG* R6G S101 OH-SQ* SQ3*

Table 2 Absorption and fluorescence data for all dyes studied

Table 2 summarizes the fluorescence properties of each dye in each polymer matrix. When a dye is not soluble in a solvent that can also dissolve the PVA matrix, the polymer–dye system is identified as insoluble (I). When the dye was soluble but the fluorescence was below the detection level of our set-up, the polymer–dye systems were identified as undetected (U). It should be noted that the quantum yield of the dyes varies depending on the solvent used and interested readers should consult the appropriate referenced paper to obtain further information. Although signal from droplets was always more intense than those of the thin film samples, accurate quantum yield (Uf) and fluorescence stability measurements could not be made in the droplet samples due to their non-uniform thickness and the possible presence of trapped solvent. These factors also made it impossible to obtain accurate stability and intensity measurements. The droplet samples were therefore only used to find the absorbance and fluorescence peak wavelength. Error in Uf measurements was estimated through the deviation in results from five different SQ3 thin film samples in PMMA. The estimated error in the quantum yield measurement was 10%. The Uf obtained using this set-up are comparable to those obtained in previously published studies (see Table 3 for further details). The only significant difference is the case of SQ3 in PMMA. The value of the quantum yield measured for SQ3-PMMA in the present study, 0.79 ± 0.08, is just within the experimental error of the values reported elsewhere, 0.57 ± 0.15 [23]. It must be noted that the SQ3PMMA system exhibited one of the least reproducible behaviour in previously studied squaraine polymer systems [23] and that a different molecular weight for PMMA MW = 30,000 [23] was used compare to 75,000 in the present study. The solvent choice is another possible factor that could influence the observed quantum yield and this is still being investigated. However, it should be noted that the OH–SQ in PMMA exhibits a better quantum yield and should be favoured even if it has a much lower solubility in the studied solvent. Two squaraine dyes (OH-SQ, SQ3) in PMMA and PS films and MG and PP-IX-DME in PS films exhibited an increase in Uf with respect to solution. For MG, this has been observed previously and its improved quantum yield as a function of solvent viscosity is reported elsewhere [40,42,43]. Similar increases in the quantum yields upon transitioning from the solution-state to polymer films have been reported for squaraines [23]. Unlike the aforementioned dyes, the quantum yield of TPP in PMMA was measured to be the same as in solution. This dye–polymer system was also the only combination that showed an acceptable quantum yield with emission above 700 nm.

420t 510t 530t 540t 550t

3. Discussion

500s, 420s, 500s, 505s, 515s,

in arbitrary units and then normalized against a standard (0.014% w/w solution of S101 in ethanol Uf = 0.90 [33]).

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0.02 0.02 0.04 0.20 0.002 0.002 U 0.04 0.007 0.89 0.77

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Table 3 Comparison between literature and experimentally determined quantum yields in select dye–polymer system Dye

PPMA literature

PPMA this study

PS literature

PS this study

R6G OH-SQ SQ-3

0.7 [31] 0.68 ± 0.12 [23] 0.57 ± 0.15 [23]

0.70 ± 0.07 0.72 ± 0.07 0.79 ± 0.08

0.87 ± 0.09 [23] 0.79 ± 0.13 [23]

0.89 ± 0.09 0.77 ± 0.08

For all other dyes, a significant decrease in Uf was observed in the polymer films relative to the solution values. Although the fluorescence yields of squaraine dyes are lower than those of the rhodamines (R6G, S101) dyes in solution, SQ3 and OH-SQ exhibited the strongest fluorescence in our cast polymer films. It is worth noting that both squaraine dyes have strong absorptions at 635 nm, where many of the cheapest and most widely available diode lasers emit. When considering squaraine dyes for solidstate fluorescence applications, one must remember that their solubility is lower than most other dyes in solvent and this ultimately limits their concentration in dye–polymer systems that are fabricated via spin-coating methods. Table 1 gives their saturation solubilities in chloroform, which was highest for both SQ3 and OH-SQ among the many solvents tried. This low solubility is partly counteracted by their very high extinction coefficients (180,000 cm 1 M 1 for SQ3 in PMMA [23]). Photobleaching [8,30,45–48] has been found to be a limiting factor that plagues the dye-doped polymer systems and is of primary importance in the development of polymer lasers. Table 4 presents the intensity remaining after 240 s (4 min) of continuous exposure to the pump light. Even if they do not exhibit high Uf, the PP-IX-DME– PMMA and TPP–PMMA, MG–PMMA systems were found to be some of the most photostable among those presented here while emitting fluorescence above 650 nm. It is interesting to note that the three most stable dye/polymer systems (TPP, S-101, OH-SQ) were recorded in PS. It is well known that additives can play an important role in the stability of dye–polymer systems. Since the PS used was a commercial blend, its exact composition is unknown and it is impossible to explain why the stability was much Table 4 Photostability of dyes in polymer thin films Dye

Remaining intensity after 240 s (%) PVA

PMMA

PS

Cl-e6 OEP PP-IX-DME TPP GV MG R6G S101 OH-SQ SQ3

I I I I U <70 [49] 31 87 I I

U 48 86 82 U 83 91 U 75 68

71 50 54 95 74 U 77 94 98 74

Fluorescence intensity measurements were acquired after 240 s of continual irradiation and compared to the original intensity values.

higher in this polymer. On the other hand, some of the least photostable dye/polymer systems were observed in PVA (R6G and MG). In the case of MG in PVA, the signal fell below the detection limit of the set-up and could not be differentiated from the noise. OEP had generally the worst stability of all the dyes studied with a signal that was reduced by 50% in both PMMA and PS. 4. Conclusions A fluorescence study was conducted on commercial dyes in thin films of PVA, PMMA and PS. Fluorescence quantum yields were measured for each dye incorporated into films of these polymeric materials and were consistent with values previously reported. Although the rhodamines dyes, R6G and S101 had the highest Uf in solution from the literature, squaraine dyes SQ3 and OH-SQ exhibited higher Uf in polymer films that both were miscible. If one wants to maximize quantum efficiency and concomitantly reduce photobleaching, the optimal system studied was OH-SQ in PS. If one is interested in near-IR emission, the best dye/ polymer system was observed to be a porphyrin-based thin film, either TPP–PMMA or PP-IX-DME–PS. Finally, if the desired final product is a multi-layer system, then one has to work with PVA, and S101 is the most stable dye in this matrix, even if the Uf is originally low. Further investigations will be focussed on finding dyes that are soluble and stable in PVA and to understand why all the more stable systems produced were made using the commercial polystyrene (Ashland Hival 5108). References [1] (a) R. Kumar, A.P. Singh, A. Kapoor, K.N. Tripathi, J. Mod. Opt. 52 (2005) 1471; (b) M.N. Weiss, R. Srivastava, R.R.B. Correia, J.F. Martins-Filho, C.B. de Araujo, Appl. Phys. Lett. 69 (1996) 3563; (c) R. Kumar, A.P. Singh, A. Kapoor, K.N. Tripathi, J. Mod. Opt. 53 (2006) 2657. [2] S. Singh, V.R. Kanetkar, G. Sridhar, V. Muthuswamy, K. Raja, J. Lumin. 101 (2003) 285. [3] T. Ubukata, T. Isoshima, M. Hara, Adv. Mater. 17 (2005) 1630. [4] T. Isoshima, E. Ito, T. Ubukata, M. Hara, Mol. Cryst. Liq. Cryst. 444 (2006) 81. [5] M.A. Reilly, B. Coleman, E.Y.B. Pun, R.V. Penty, I.H. White, M. Ramon, R. Xia, D.D.C. Bradley, Appl. Phys. Lett. 69 (2005) 231116. [6] G. Wang, L. Hou, F. Gan, Mater. Sci. Eng. B 65 (1999) 75. [7] N.J. Cherepy, R.D. Sanner, Opt. Mater. 28 (2006) 1350. [8] E. Yariv, S. Schultheiss, T. Saraidarov, R. Reisfeld, Opt. Mater. 16 (2001) 29. [9] J. Thompson, M. Anni, S. Lattante, D. Pisignano, R.I.R. Blyth, G. Gigli, R. Cingolani, Synthetic Met. 143 (2004) 305.

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