J. Chem. SOC., Faraday Trans. I, 1986, 82, 909-919

Emission Characteristics and Micellization of Cationic 1,4-Bis( P-pyridyl-2-viny1)benzene Laser Dye

El-Zeiny M. Ebeid,* Raafat M. Issa, Mohamad M. Ghoneim and Samy A. EEDaly

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

Cationic species of the recently reported 1,4-bis(P-pyridyl-2-vinyl)benzene (P2VB) blue laser dye has been generated by direct protonation and identified analytically. The steady state emission and the emission quantum yields have been studied both in aqueous and in micellar media. The hydrochloride derivative (P2VB. HCI) shows a substantial increase in fluor- escence quantum yields (q&) as a result of both cationic and anionic micellization. Micellization also induces weak laser action in P2VB. HC1 dye upon pumping with a nitrogen laser.

The photochemical quantum yield of P2VB = HCl (Aex = 337 nm) has been evaluated as q$c = 0.003 and contrasted to that of the parent V2VB dye, for which q4c = 0.004.

Both conductometric and surface tension studies reveal a ground-state aggregation of P2VB.HCl at a critical concentration of ca. 7.5 x mol dm-3. Excited-state molecular aggregation has also been observed in concentrated P2VB * HC1 solutions which show excimeric emission at 500 nm. The enthalpy of excited-state photoassociation has been evaluated as AHa = - 11.0 kJ rno1-l indicating that P2VB .HCl molecules in the excimer configuration are bound less tightly.

In a previous study,l two new blue laser dyes have been reported: 174-bis(P-pyridyl- 2-viny1)benzene (P2VB) and 2,5-distyrylpyrazine (DSP). These dyes are the aza- analogues of distyrylbenzenes.29

P2VB has been tentatively reported1 as a pH sensitive dye regarding its steady state emission. Blue emitting P2VB solutions give greenish emission (Aex = 337 nm) on bubbling HC1 gas.

The nature of the protonated product, together with its emission and photochemical behaviour, needs further investigation. In the present communication, we report the acid-base behaviour of P2VB laser dye and study the steady state and laser emission of the hydrochloride derivative (P2VB * HCl) both in aqueous and in micellar media. Both ground- and excited-state aggregation are also reported.

Experiment a1 P2VB was prepared according to the method of Hasegawa et al.4 and was purified as reported previous1y.l Protonated P2VB (P2VB - HCl) was prepared by flushing a saturated methanolic solution of P2VB with HC1 followed by slow evaporation of the

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910

0 .

Emission and Micellization of a Laser Dye

225 250 275 300 325 350 4 0 0 450 wavelength/ t i i n

Fig. 1. U.V. absorption spectra of a 10 -5 mol dm-3 solution of P2VB; (---), fresh in EtOH and also in ethanolic buffer of pH 6 , (.. s), pH 4; (------), pH 3 and (- x - x - x ), after flushing

with HC1 gas.

solvent. The precipitate was then recrystallized from methanol giving greenish yellow crystals. Unlike the parent P2VB, the hydrochloride is highly soluble in water and gives different X-ray diffraction, i.r., U.V. and emission spectra. Sodium dodecyl sulphate (SDS, Fluka, puriss.) was used to prepare anionic micelles and cetyltrimethylammonium chloride (CTAC, Kodak) to prepare cationic micelles.

The steady-state emission spectra were taken on a Shimadzu RF 510 spectrofluoro- photometer and u.v.-visible absorption spectra on a Unicam SP 8000 spectrophotometer. Photochemical quantum yields were measured using the U.V. excitation light of the spectrofluorophotometer. The pumping nitrogen laser system (of peak pulse I00 kW) is described elsewhere.l?

Fluorescence quantum yields were measured relative to 9,lO-diphenylanthracene as a reference standard.6 Light intensities were measured using ferrioxalate a~t inometry.~ Conductimetric measurements were carried out using a conductivity meter model CM- 1 K and surface tension was measured by the capillary-height method.

Results and Discussion Acid-Base Behaviour of P2VB Fig. 1 shows the changes in u.v.-visible absorption spectra of 9.5 x lop6 mol dmp3 P2VB in 25% ethanolic buffers. As the acidity increases, the spectrum is bathochromically shifted and an isosbetic point is observed at 3, = 365 nm. The same effect is obtained on flushing ethanolic P2VB solutions with HCl gas. The bathochromically shifted spectrum is attributed to protonated P2VB owing to the basicity of the nitrogen heteroatom in the pyridyl ring. The protonated hydrochloride product (P2VB HCl) has been isolated and the chloride ion content was estimated using conductometric titration against AgNO, and NaOH.

The hydrochloride product was found to be formed from P2VB and HC1 in molar ratios indicating that protonation occurs on one nitrogen atom, causing a reduction in the basicity of the other pyridyl nitrogen, and no further protonation occurs in the investigated pH range (pH 2-7) under such conditions.

The pyridyl ring is known as a scavenger of protons and the IZ --+ z* transition will disappear from the electronic absorption spectrum of the protonated ring.8 The lone pair ofelectrons on the nitrogen atom of the pyridyl does not contribute to the aromatic sextet. Consequently the pyridyl group has basic properties and forms stable salts with mineral acids.8 The pyridinium ion has a pK, of 5.9

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E-2. M . Ebeid et al. 91 1

' I X

I

I X

600 500 4 00 300 wavelength/ 11 I l l

Fig. 2. Emission spectra (Aex = 337 nm) of 5 x 10P mol dm+ solutions of: (-.-.-.), P2VB in EtOH; ( x - x - x -), P2VB in ethanolic buffer pH 3; (-), P2VB after flushing with HCI

gas ; ( a .), P2VB. HCI recrystallized from EtOH.

There is, however, a probability that diprotonated product is obtained at lower pH values since not all the U.V. absorption curves intersect at a sharp isosbestic point. The emission spectrum of P2VB HCI is also bathochromically shifted compared with parent P2VB as shown in fig. 2 (Amax = 460 nm, Aex = 337 nm).

Since both members of the conjugate pair of P2VB*H+ are fluorescent, then by applying the Forster cycle1* p c is correlated to pK. by the relation p q = pKa-0.4 indicating a slightly higher acidic character in the excited state compared with the ground state.

Micellization of P2VB - HCI

The emission spectrum of P2VB-HC1 is rather weak (q$f = 0.04, hex = 337 nm), but increases substantially upon micellization in cationic CTAC micellar media. Fig. 3 shows the changes in fluorescence [fig. 3(a)] and fluorescence quantum yield bf [fig. 3(b)] as a function of CTAC concentration. Fig. 3(a) shows a hypsochromic shift in emission as CTAC increases. There is also an increase in emission intensities. Fig. 3(b) shows that the fluorescence quantum yield increases as the surfactant concentration increases, with

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912 Emission and Micellization of a Laser Dye

6 00 500 400 w avcle ng t h / n 111

1 I I 0 1.0 2.0

[CTAC]/102 in01 d r K 3

Fig. 3. Effect of increasing CTAC concentration on (a) the emission spectra of 5 x lop5 mol dm-3 P2VB.HCl (Aex = 337 nm), CTAC concentration at increasing intensities are 2, 6, 10 and

18 x mol dm-3; (b) q5f changes of mol dmP3 aqueous P2VB -HC1 (Aex = 337 nm).

a break at a CTAC concentration of 2 x mol dm-3, which is very close to the CTAC critical micelle concentration (c.m.c.).ll* l 2 Despite the partial quenching of P2VB * HC1 emission by C1- (uide infra), it is clear that emission enhancement due to dye solubilization overruns emission quenching by C1-.

An increase in #f values in the presence of micellar aggregates has been reported for many other dyes.ll-ls The mechanism by which P2VB-HCl is solubilized in CTAC micellar aggregates might be anticipated from the general similarity of both molecules in having a positively charged head and a long hydrocarbon chain. P2VB.HCl is suggested to be embedded in the micellar aggregates and involved in micellar-aggregate formation with C1- counter-ions near the polar heads of the molecules.

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E-2. M . Ebeid et al. 913

0 0.5 1 1 .5 2 2.5 [ P2VB IlCl]; 1 O2 11101 diW3

0.01 0.1 1 10 [ P ? V B . H(’1]/102 11101 dnl -’

Fig. 4. The changes in (a) specific conductance 0 (S cm-l) and (b) surface tension y (dyn cm-l) as a function of concentration of P2VB. HC1 in water. The breaks in the curves show the critical

aggregation concentration.

The increase in both emission and absorption intensities in micellar aggregates together with the observed hypsochromic shifts in both emission and absorption spectra is satisfactorily explained by this model since isolation of P2VB HC1 molecules decreases their self aggregation and subsequent emission quenching.

P2VB * HC1 itself undergoes molecular aggregation in the ground state. Fig. 4(a) shows the changes in the specific conductance D (in Scm-l) as a function of changing the concentration of P2VB - HCl in an aqueous medium. There is a clear break in the resulting curve at a P2VB-HCl concentration of 7.5 x mol dmp3. We assign this break as a critical concentration of P2VB. HCl for ground-state aggregation. Ground-state molecu- lar aggregation has also been revealed by surface tension measurements [see fig. 4(b)]. As the dye concentration increases, the surface tension (7) decreases with a break in the resulting curve at a dye concentration of 7 x lop3 mol dm-:< that is in agreement with the value revealed by conductimetric measurements.

Excited-state aggregation has also been observed in concentrated P2VB HCl in aqueous and non-aqueous media. Fig. 5 shows the changes in emission spectra of

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914 Emission and Micellization of a Laser Dye

n . . =*. I \ I

4

I

I

I

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4 \

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/ * , x

t i. * \ :/ ' . . -, 1 1

.. \ 'L I .

. .'A * \ . - ...

L . . 0 -

'*-A- 1 600 500 400 3 00

wavelength/nm

Fig. 5. Changes in emission spectra of P2VB.HC1 methanolic solutions (Aex = 365 nm) as a function of concentration: 5 x (-+-. - ->, lop4 (.- .) and 5 x mol dmP3 ( - x ~ x - x ). The

scales are multiplied by 1, 2 and 50, respectively.

P2VB - HCl methanolic solutions (Lex = 365 nm) as a function of concentration. At high concentrations (ca. 5 x mol dmP3) the molecular emission is largely bathochromically shifted giving a broad, structureless excimer-like emission with maximum intensity at 500 nm that compares withemission from P2VB - HCl crystals. The excimeric phenomenon in concentrated diolefinic solutions is known for diethyl p-phenylenedia~rylatel~ and p-phenylenediacrylic acid.20 Fig. 6 (a) shows the effect of temperature on the excimeric emission intensity of P2VB.HC1 ( 5 x mol dmP3 in dimethyl sulphoxide). As the temperature increases, the excimeric emission intensity decreases owing to the role of thermal energy in destabilizing excimeric s p e c i e ~ . l ~ - ~ ~ The enthalpy of photoassociation (AH,) has been evaluatedz1 from the slope in fig. 6(b) (slope = A H J R ) as AHa = - 11.0 kJ mol-I.

The molecular emission in fig. 6(a ) is hidden by the excimeric emission band and the molecular emission intensity was taken as unity in the present calculations. The AHa

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E-2. M . Ebeid et al. 915

'* I \

600 5 00 LOO wavelength/nm

3.0 3.2 3 . 4 3 . 6 1 0 3 K I T

Fig. 6. (a) Effect of temperature on the emission intensity of P2VB -HC1 in DMSO (Aex = 365 nm) at 10 (- -----); 30 ( . . a ) ; 40 (---); and 60 "C (-- x - x - x); and (6) a plot of the logarithmic

excimeric emission intensity at 500 nm us. the reciprocal temperature.

value is smaller than those reported for planar aromatic compounds,21 indicating that P2VB * HCl molecules in the excimer configuration are bound less tightly than the planar aromatic compounds. The measured AH, value compares with the AHa value of some other diolefins.

The photochemical quantum yield of P2VB-HC1 in CTAC micelles (Aex = 337 nm) has been determined using a modified method22 based on the loss of dye, summarized as follows. The basic equation for quantum yield determination is given as

where C is the concentration of reactant, t is the time in min, 4c is the quantum yield, 1" is the light intensity of the lamp in mol min-l, D is the absorbance at the irradiation

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916 Emission and Micellization of a Laser Dye

1 .o

0.8

g 0 . 6

$

c ? D

0 . 4

0.2

2

t 1 I I I I

0 275 300 32 5 3 50 400 4 50 wavelength/nm

Fig. 7. Effect of irradiation (Aex = 337 nm and intensity = 4 x spectrum of 2 x lop5 mol dmP3 P2VB.HC1 in

mol min-l) on the absorption mol dmP3 aqueous CTAC solution. Fresh

sample (-.---.), irradiated for 15 (---), 58 (---), 135 (-----.- .-) and 225 min (-).

wavelength (337 nm), E is the extinction coefficient at irradiation wavelength for the starting material and b is the path length of the cell in cm:

Differentiating with respect to time

(3)

Integrating over the period of time to + t and taking the concentration C,, to be directly proportional to the absorbance A,, and C, to be directly proportional to A , (the Beer-Lambert law is valid in this concentration range), we get

(4)

( 5 )

The absorbance A can be chosen at any wavelength in the absorption spectrum; in

From the spectral data in fig. 7 a plot was made of (1 - 10-D)/D us. t. The areas

(1 - 10-1') dt. ( D )

In- A, = - a l t o ( ( I -10 -D) ) d t

d 1nC = -4,I,Eb

where the constant At0

cx = q$c I , be.

the present study A was followed at 350 nm.

of the resulting trapezia correspond to

Jt: [( 1 - 10-O)/D] dt.

A plot of the cumulative areas of the trapezia us. the corresponding values of ln(A,/A,J gave a straight line of slope -a = - 5 x lop3. From eqn ( 5 ) a = -q$c I , E b = - 5 x In the present study I , = 4 x lop5 mol min-l, E = 4.1 x lo4 dm3 mol--l cm-l, b = 1 cm so is calculated as 0.003. This low 4, value indicates a reasonable photostability of P2VB * HCl in CTAC solutions upon excitation with

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E-2. M . Ebeid et al.

0.6- a u

m 9

m 9 ; 0 . 4 -

0 * 2

917

"*?*

***I :

8 .' : \*

8 .I

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% .'< 'r - +,;

++ ? +

'. * * * *.df ?+ \. H. 8 .

II ,.I*. -,44' ; '* '.

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.-*--

1 I 1 0 1 2

[ SUS]/ 1 o2 I l lO l d1llC3

1 .o

0 2 4 6 8 [ Co2+] / l O3 mol dm-3

Fig. 9. (a) Effect of SDS concentration on the emission intensity (Lex = 337 nm, ,Iern = 455 nm) of P2VB-HCl mol dmP3) in the presence of CoSO, (loP2 mol dmP3). The arrow shows the c.m.c. (b) Stern-Volmer plot for fluorescence quenching of P2VB. HC1 ( I 0-5 mol dmp3) using Co2+

in mol dm-3 SDS (Aex = 337 nm, ,Iern = 450 nm).

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918 Emission and Micellization of a Laser Dye an N, laser. q5c has been calculated for parent P2VB in dimethyl sulphoxide as

= 0.004 (Lex = 337 nm). The emission and absorption spectra of P2VB.HC1 have also been studied in SDS

anionic micelles. Fig. 8 shows a bathochromic shift in the absorption spectrum of P2VBaHCI in the presence of SDS above the c.m.c. There is an increase in fluorescence intensities as the SDS concentration increases. Fig. 9(a) shows the increase in emission intensity at 455 nm (Aex = 337 nm) as the concentration of SDS increases until a limiting value is reached at an SDS concentration of mol dmP3, which is the c.m.c. of SDS.I1 It is obvious that anionic SDS micellar aggregates cause physical separation between P2VB * H+ and C1-. Thus P2VB - H+ will be adsorbed at the micellar surface, while C1- will be repelled. C1- is known to cause slight fluorescence quenching for cations23* 24 cia the heavy atom effect. In a separate experiment, it was observed that the emission intensity (Aex = 337 nm) of aqueous P2VB.HCl is reduced by ca. 13% when using P2VB.HCI in lop2 mol NaCl solution. SDS also plays a role in prohibiting molecular aggregation of P2VB - HCl dye.

It is thus convenient to propose that P2VB.HS cations are solubilized at the anionic micellar interface. This has been further confirmed by studying the role of SDS in enhancing the quenching of P2VB*H+ emission by Co2+ ions. Co2+ ions are known as efficient quenchers of many organic f l u o r e ~ c e r s . ~ ~ ~ 26 In aqueous media, Co2+ does not quench the fluorescence of P2VB.H+ owing to the role of the positive charges on both cations in prohibiting collisional energy transfer. In the SDS micellar system, Co2+ quenches the emission of P2VB H+. Fig. 9 (b) shows a Stern-Volmer plot illustrating the quenching of P2VB.H+ using Co2+ in the presence of 10

The induced quenching arises as both P2VB H+ and Co2+ cations are brought close together as counter-ions at the micellar interface.

mol dmP3 SDS.

Laser Action of PZVB-HCl

Upon pumping aqueous P2VB HCl solutions ( lop3 mol dm-3) using an N, laser, no laser action has been observed, but in lop2 mol dmp3 SDS solutions, loF3 mol dm-3 P2VB HCl gives weak laser emission that can be seen visually. The resulting laser beam, however, was insufficiently intense to give reliable energy dispersion measurements using the equipment available to us.

In concentrated P2VB * HC1 solutions it seems that molecular aggregation (accompanied by a substantial decrease in q5f values) prohibits the laser action. In micellar media, assuming the aggregation number is ca. 50, there are ca. five dye molecules in every micelle, yet the dye molecules are expected to be separated by the surfactant molecules. Aqueous dye solutions of concentrations ca. (typical of that used in laser generation) show a substantial increase in emission intensities (&, = 337 nm) as we increase SDS concentrations. This confirms the role of SDS in prohibiting molecular aggregation of P2VB - HCl dye.

mol dm

References 1 E. M. Ebeid, M. M. F. Sabry and S. A. El-Daly, Laser Chem., 1985, 5, 223. 2 T. E. Bush and G. W. Scott, J . Phys. Chem., 1981, 85, 144, and references therein. 3 H. Telle, U. Brinkmann and R. Raue, Opt. Commun., 1978, 24, 248. 4 M. Hasegawa, Y. Suzuki, F. Suzuki and M. Nakanishi, J . Polyrn. Sei., Part A-1, 1969, 7, 743. 5 M. M. F. Sabry, A. Hassan and M. Ewaida, J . Phys. E, 1984, 17, 103. 6 J. V. Morris, M. A. Mahaney and J. R. Huber, J. Phys. Chem., 1976,80, 969. 7 C. G. Hatchard and C. A. Parker, Proc. R . Soc. London, Ser. A , 1956, 235, 518. 8 Fluorescence Spectroscopy, ed. A. J. Pesce, C. G. Rosen and T. L. Pasby (Marcel Dekker, New York,

9 N. L. Allinger, M. P. Cava, D. C. De Jongh, C. R. Johnson, N. A. Lebel and C. L. Stevens, Urganir 1971), p. 33.

Chemistry (Worth, New York. 1974), p. 249.

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E-2. M . Ebeid et al. 919 10 S. G. Schulman, Modern Fluorescence Spectroscopy, ed. E. L. Wehry (Plenum Press, New York, 1976),

vol. 2, chap. 6. 1 1 C. A. Bunton, Prog. Solid State Chem., 1973, 8, 239. 12 H. F. Eicke, Top. Curr. Chem., 1980, 87, 1 ; 86. 13 M. L. Corrin and W. D. Harkins, J . Am. Chem. Soc., 1947, 69, 679. 14 L. Arkin and C. R. Singleterry, J . Am. Chem. Soc., 1948, 70, 3965; J . Colloid Sci., 1947, 4, 537. 15 C. R. Singleterry and L. A. Weinberger, J . Am. Chem. Soc., 1951, 73, 4574. 16 M. Gratzel and J. K . Thomas, in Modern Fluorescence Spectroscop-y, ed. E. L. Wehry (Plenum Press,

17 M. I . Snegov and A. S. Cherkasov, Opt. Spectrosc. (USSR), 1980, 49, 35. 18 R. R. Alfano, S. L. Shapiro and W. Yu, Opt. Commun., 1973, 7, 191. 19 M. Sakamoto, S. Huy, H. Nakanishi, F. Nakanishi, T. Yurugi and M. Hasegawa, Chem. Lett., 1981,

99. 20 E. M. Ebeid, S. A. El-Daly and M. Hasegawa, Laser Chem, 1985, 5, in press. 21 B. Stevens, Adz,. Photochem., 1971, 8, 161. 22 A. J . Lees, personal communication. 23 J. A. Kemlo and T. M. Shepherd, Chem. Phys. Lett., 1977, 47, 158. 24 A. R. Watkins. J . Phys. Chem., 1974, 78, 2555, and references thercin. 25 E. Keh and B. Valeur, J . Colloid Interface Sci., 1981, 79, 465. 26 E. M. Ebeid, J . Chem. Educ., 1985, 62, 165.

New York, 1976), vol. 2 , chap. 4.

Paper 51849; Receiivd 20th May, 1985

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