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Fluorescence quenching of phenanthrene and anthacene by maleic
anhydride and n-octadecenylsuccinic snhydride in solution and in bulk
polypropylene
Fang, Haixia; Mighri, Frej; Ajji, Abdellah
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Fluorescence Quenching of Phenanthrene and
Anthracene by Maleic Anhydride and
n-Octadecenylsuccinic Anhydride in Solution
and in Bulk Polypropylene
Haixia Fang,1,2Frej Mighri,1,2Abdellah Ajji3
1
CREPEC, Center for Applied Research on Polymers and Composites, Montreal, QC, Canada
2
Department of Chemical Engineering, Laval University, QC, Canada G1K 7P4
3
Industrial Materials Institute, IMI, NRCC, Boucherville, QC, Canada J4B 6Y4
Fluorescence quenching of phenanthrene (Ph) and an-thracene (An) fluorophores by maleic anhydride (MAH) and n-octadecenylsuccinic anhydride (ODSA) quench-ers in solid polypropylene (PP) films were studied. Results were compared with fluorescence quenching of the same fluorophores by MAH and ODSA quench-ers in chloroform solution. Contrary to the results obtained in solution, it was observed that fluorescence emission of Ph fluorophore in PP films was more effi-ciently quenched by ODSA than by MAH. This was due to the better miscibility of Ph with ODSA than with MAH. When An fluorophore was used instead of Ph, it was observed that its fluorescence intensity in PP films was notably reduced by the addition of MAH. This was mainly due to the Diels–Alder reaction, which con-sumed a part of the An molecule. However, fluores-cence intensity of An strangely increased with the addition of ODSA instead of MAH. Because of short lifetime of An (around 6 ns), ODSA had no quenching effect on An. POLYM. ENG. SCI., 47:192–199, 2007. ª2007 Society of Plastics Engineers
INTRODUCTION
Fluorescence quenching of polyaromatic molecules by electron deficient molecules was extensively studied in liq-uid solutions and both dynamic (collisional) and static fluo-rescence quenching mechanisms were developed [1–3].
The collisional quenching is a function of the fluorophore used, its fluorescence lifetime, and the quencher diffusion coefficients. Collisional quenching efficiency can be esti-mated theoretically according to Smoluchowski and Stokes– Einstein equations [1]. However, there is always a differ-ence between the estimated and experimental values because not all of the collisional encounters are effective in fluorescence quenching. It was commonly reported that steady-state quenching data do not always follow the stand-ard linear Stern–Volmer equation, but often show upwstand-ard (and sometimes downward) deviations [2].
Eftink and Ghiron [3] showed the successful use of flu-orescence quenching technique to study the protein dy-namics. For polymer materials, literature shows that spe-cial attention was given to fluorescence quenching in solid state. It was proven [4–8] that polymer matrices, except some special polymers such as polydimethylsiloxanes (PDMS) [4], severely restrict the diffusion of the dis-solved fluorophore and quencher molecules. Consequently, static quenching mechanism is dominant in these polymer matrices. Perrin, Fo¨rster, and Dexter [9–11] analyzed static quenching data and proposed their specific models. The Perrin model [9] assumes that there is no specific electronic mechanism for energy transfer between the quencher and the polymer and postulates the existence of a quenching sphere around the fluorophore molecule. The Fo¨rster model [10] assumes the existence of a specific Coulombic (dipole–dipole) interaction mechanism for energy transfer and shows that quenching efficiency is re-ciprocal to the sixth power of the distance between fluoro-phore and quencher molecules. The Dexter model [11] considers that energy transfer occurs by electron exchange mechanism. To ensure homogeneous distribution of fluo-rophore molecules, their derivatives (methacrylate or methyl methacrylate) are always copolymerized in the
Correspondence to: F. Mighri; e-mail: frej.mighri@gch.ulaval.ca Contract grant sponsors: Le Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT), Natural Sciences and Engineering Research Council of Canada (NSERC).
DOI 10.1002/pen.20691
Published online in Wiley InterScience (www.interscience.wiley.com). V
polymer chain. The effects of labeling techniques on fluo-rescence quenching are largely reported in literature [5, 7, 12–15], which also reported the difference between fluo-rescence quenching of the labeled fluorophores in solid state and in liquid solution. Zhao and Swager [12] com-pared fluorescence quenching of poly(iptycenebutadiyny-lene) by electron deficient aromatic compounds in solu-tion and in solid state. They showed that fluorescence emission is more quenched by trinitrotoluene (TNT) than by dinitrotoluene (DNT) in chloroform solution. However, no measurable quenching difference was obtained in solid state because the hydrodynamic volume of the polymer restricts the close approach of the quenchers [13]. Ng and Guillet [14] also observed that quenching efficiency in so-lution is largely higher than that in solid state. This large difference is due to the enhanced segmental diffusion in solution. Clements and Webber [15, 16] studied, using monovalent Tlþions, quenching efficiency of poly(metha-crylic acid) (PMA) labeled by An fluorophore at mid-chain and end-mid-chain. They found that the mid-mid-chain la-beled PMA was much more quenched than the end-chain labeled PMA and attributed the low quenching efficiency of the latter to the relatively low density of condensed Tlþions near the PMA chain ends.
To the best of our knowledge, only few studies focused on fluorescence quenching of doped-free fluorophore mol-ecules by electron deficient molmol-ecules in polymer matrices [4, 17]. Chu and Thomas [4] measured the quenching of pyrene fluorescence by phthalic anhydride (PA) at room temperature in a series of PDMS matrices having different molecular weights. The following relationship between the quenching rate constant (Kq) and the viscosity (Z) was
found: Kq ¼ Z 0.04 for Z > 50 cP. Johnson [17] studied
the quenching of N-isopropylcarbazole (NIPC) fluophore by dimethylterephalate (DMT) quencher in polystyrene films. He reported that NIPC fluorescence lifetime decreases not only with increasing the DMT concentration, but also with increasing the concentration of NIPC fluoro-phore. The present study focuses on fluorescence quench-ing of An and Ph by MAH and ODSA in solid polypropyl-ene (PP) films. In favor of elucidating experimental con-clusions, the chemical structures of MAH, ODSA, An, and Ph are shown below. For comparison purpose between flu-orescence quenching in solid PP film and in liquid solu-tion, An and Ph quenching by MAH and ODSA are stud-ied first in chloroform solution. The effect of alkyl chain in ODSA on fluorescence quenching efficiency in both liq-uid solution and polymer matrix are compared.
EXPERIMENTAL
Materials Used, Solutions and Film Preparation
Chloroform was a HPLC grade purchased from Fisher Scientific. Ph, An, and MAH were purchased from Sigma-Aldrich Inc., and ODSA from Alfa Aesar Company. PP (PP1052 gade) was a commercial product, supplied in pel-let form by ExxonMobil Corp. All the samples were used as received.
For fluorescence quenching in solution, a series of chloroform solutions with constant Ph and An concentra-tions and different MAH and ODSA quencher’s concen-trations were prepared as follows: 8.5 mg of An and 5.0 mg of Ph were first dissolved in 200 mL chloroform at room temperature to obtain two solutions with An and Ph concentrations of 28.7 and 16.9 ppm, respectively. Differ-ent amounts of MAH and ODSA quenchers were then added separately to each solution. The concentrations of ODSA were lower than those of MAH because the solu-bility of ODSA in chloroform is much lower than that of MAH. Concerning polymer film preparation, different samples, in which the concentrations of Ph and An were maintained constant (20 and 50 ppm, respectively) but
with different MAH or ODSA concentrations, were firstly prepared using a Haake Rheomix batch mixer. All mix-tures were prepared under the same temperature, rota-tional speed, and mixing time conditions (1808C, 30 rpm, and 7min, respectively). Since Ph and An concentrations in PP are very low, the following mixing procedure was used: (i) a mixture of PP/Ph or PP/An (both in a weight proportion of 99/1) were first prepared in batch mixer; (ii) a small quantity (0.1 g) of these mixtures were then added to 49.9 g of PP/MAH or PP/ODSA blends. The total quantity (50 g) corresponds to the maximal loading of the internal mixer used in this study. Polymer films of about 100 mm in thickness were obtained from the prepared mixtures by compression molding during 2 min at 1808C using a Carver hydraulic compression machine under a force of around 4 tons.
Fluorescence Measurements
Fluorescence quenching measurements were carried out at room temperature using a Cary Eclipse spectropho-tofluorometer (from Varian Inc.) equipped with a xenon flash lamp. For fluorescence measurements in chloroform solutions, the liquid samples were excited inside a cell
made from quartz. Excitation wave lengths were 297 nm and 365 nm for solutions with Ph and An, respectively. These values correspond to UV highest absorption peaks of Ph and An. For fluorescence emission measurements on solid films, film samples were fixed on a solid sample holder. The fluorescence intensity was then measured at four different positions and only the average intensity value was calculated.
RESULTS AND DISCUSSION Fluorescence Quenching of An and Ph in Chloroform Solution
According to Stern–Volmer [1], the ratio between the fluorescence intensity of unquenched emission,F0, to that
of quenched emission,F, follows the following relation: F0=F¼ 1 þ KsvQ¼ 1 þ Kqt0Q (1) whereQ (mol/L) is the quencher molar concentration and Ksv(L/mol) is the Stern–Volmer constant. Kq(L/mol s) is
the diffusion controlled bimolecular quenching constant and t0 (ns) is the lifetime of the fluorophore in the
ab-sence of the quencher.
Fluorescence quenching effect (F0/F) of An and Ph by
MAH and ODSA in chloroform solution are respectively presented in Fig. 1a and 1b as a function of MAH and ODSA quencher concentrations. The smaller graph in Fig. 1a shows the fluorescence emission spectra of An with and without ODSA quencher. Higher value of F0/F
corre-sponds to higher quenching efficiency of the correspond-ing MAH or ODSA quencher. For both An (Fig. 1a) and Ph (Fig. 1b), the corresponding F0/F ratio increases with
increase in MAH quencher concentration. However, this increase is much lower for ODSA quencher. As shown in the inset of Fig. 1a, the overlapped fluorescence emission spectra in the absence and in the presence of ODSA indi-cate that An fluorescence is not influenced by addition of ODSA. Both MAH and ODSA are more efficient (i.e., higher values of F0/F) to quench the fluorescence
emis-sion of Ph than that of An. Our experimental results for An quenching by MAH in chloroform solution were com-pared with those in literature [18, 19] and no big differ-ence was observed. For MAH molar concentration of 0.1 mol/L, F0/F was about 8.5 (see Fig. 1a) whereas the
reported literature values varied between 7 and 9, depend-ing on the experimental conditions, such as the type of solvent used and the fluorophore concentration [20].
Analysis of Fluorescence Quenching of An in Chloroform Solution
As shown in Fig. 1a, measured fluorescence intensities of An fluorophore in chloroform solution in the presence of MAH are well presented by the linear Stern–Volmer
equation (Eq. 1) with Ksv¼ 87 L/mol, which corresponds
to the slope of the straight line. Since the lifetime t0 of
An is around 6.2 ns [19, 21], the bimolecular quenching constantKq¼ 1.4 1010L/(mol s) was directly obtained
from Eq. 1. This constant can also be obtained from the following Smoluchowski equation:
Kq¼ 4pN
1000ðRfþ RqÞðDfþ DqÞ
(2) whereRiandDi are the collision radii and diffusion
coef-ficients of the fluorophore (i¼ f ) and the quencher (i ¼ q), respectively, and N is the Avogadro number. The fluoro-phore and quencher molecular radii are determined from LeBas volume theory [22]. The diffusion coefficients can be obtained by the following Stokes–Einstein equation [23, 24]:
D¼ KBT=f pZR (3)
FIG. 1. (a) Fluorescence quenching of An (28.7 ppm) by MAH (n) and ODSA (&). (b) Fluorescence quenching of Ph (16.9 ppm) by MAH (n) and ODSA (&). The acronym ‘‘a.u.’’ in the smaller graph of Figure 1(a) means ‘‘arbitrary unit’’ of fluorescence intensity.
whereKBis the Boltzmann constant,T (K) is the absolute
temperature, Z (cP) is the solvent viscosity (for HCCl3,
Z¼ 0.59 cP at T ¼ 293 K), R is the gas constant, and f is the Stokes–Einstein number. It was established that f¼ 3 for self-diffusion and f ¼ 6 for diffusion of large mole-cules in a liquid of small molemole-cules [24]. For our study, the radius of MAH molecule is smaller than that of the HCCl3solvent molecule (so,f¼ 3). However, the radii of
ODSA, Ph, and An are larger (so, f ¼ 6). Using these values, the following bimolecular quenching constant,Kq¼
1.83 1010 L/(mol s) was directly obtained from Eq. 2. This high value ofKqindicates that not all the collisional
encounters are effective in quenching. Quenching effi-ciency, g, of An by MAH is: g ¼ (1.4 1010
)/ (1.83 1010
)¼ 0.77.
Analysis of Fluorescence Quenching of Ph in Chloroform Solution
In comparison with the linear Stern–Volmer equation (Eq. 1), Fig. 1b shows that fluorescence quenching of Ph with the addition of MAH and ODSA presents a positive deviation, which is also reported in the literature [12, 25– 29]. This positive deviation is due to the formation of a ground state complex or static quenching processes. In our case, the formation of a ground state complex is not possible since the maximum fluorescence emission band was not changed and no additional band appeared in the presence of MAH or ODSA quenchers. Therefore, the sphere of action model equation presented below (Eq. 4) is more adequate than the linear Stern–Volmer equation. This model [29] assumes that, if the quencher is situated inside a spherical volume V adjacent to the fluorophore, an instantaneous quenching of the excited fluorophore by a quencher takes place without the need for diffusion-controlled collisional interaction. The probability for the quencher to be inside this volume at the time of excitation depends on the volume itself and on quencher concentration. The probability of static quenching, defined by the Poisson distribution, exp (VQ) [24], is introduced into the linear Stern–Volmer equa-tion to give the following equaequa-tion that describes both MAH and ODSA quenching processes:
F0=F¼ ð1 þ KsvQÞ expðVQÞ (4) or
F0=ðF expðVQÞÞ ¼ ð1 þ KsvQÞ: (5) By using Eq. 5 to fit the experimental data presented in Fig. 1b, the static and collisional quenching constants for Ph quenched by MAH and ODSA can be estimated. The corresponding optimized fitting curves are presented in Fig. 2a and 2b, and the estimated values of the volume V and those of the collisional quenching constantKsvare: (i)
V¼ 7.2 L/mol and Ksv¼ 300 L/mol for Ph quenching by
MAH; and (ii)V ¼ 2.0 L/mol and Ksv¼ 24 L/mol for Ph
quenching by ODSA. It can be concluded from the calcu-lated quenching constants that the quenching effect F0/F
of Ph by MAH is around 3.5 times higher than that of An by the same MAH quencher [(Ksv)MAH,Ph/(Ksv)MAH,An ¼
300/87 ¼ 3.47]. Since the molecular radii of An and Ph calculated by using LeBas volume method [22] are the same, their bimolecular quenching constants are also iden-tical. For Ph lifetime t0of around 60 ns [30], Ph
bimolec-ular quenching constantKqcan be determined from Eq. 2
Kq¼ 5.0 109L/(mol s), which leads to a quenching
effi-ciency g¼ (5.0 109)/(1.83 1010)¼ 0.28.
Collisional quenching constant, Ksv, of Ph by ODSA,
calculated usingEq. 2, is 3.7 108M 1s 1and the the-oretically estimated value of Kq is 1.15 1010 M 1s 1.
It can be seen that ODSA has a very low quenching effi-ciency, g ¼ (3.7 108
)/(1.15 1010
) ¼ 0.032. The n-octadecenyl group on ODSA not only decreased the diffu-sion coefficient of ODSA but also acted as a shield that
FIG. 2. Fluorescence quenching curve analysis according to Eq. 5, MAH (n) and ODSA (&).
was unfavorable to the energy transfer from the aromatic molecule to the succinic anhydride group. According to Eftink and Ghiron [3], the volumeV is situated in the range of 1–3 L/mol, which gives an acceptable radius (<10 A˚ , i.e., smaller than van der Waals radii) for the diffusion inter-action. However, for MAH quenching, the value of V is much larger than 3 L/mol. Similar results were also reported by Sadlej-Sosnowska and Siemiarczuk [28].
Fluorescence Quenching of Ph and An by MAH and ODSA in PP Bulk Film
Normalized fluorescence emission, F/t, of Ph and An fluorophores in PP films are presented in Fig. 3(a) and 3(b), respectively, as a function of the corresponding fluo-rophore weight concentration. For each film, the
fluores-cence intensity is divided (normalized) by the film thick-ness, t, in order to eliminate the thickness effect. The results show a perfect proportionality between F/t and both Ph and An weight concentrations. The corresponding high correlation coefficients, R, are also presented in Fig. 3a and 3b. In the present study, fluorescence quenching measurements were carried out with An and Ph concen-trations of 50 and 20 ppm, respectively. These concentra-tions are in the linear range.
Fluorescence Quenching of Ph by MAH and ODSA in PP Bulk Film
The quenching effect,F0/F, of Ph by MAH and ODSA
in PP films is shown in Fig. 4a and 4b, as a function of MAH and ODSA molar concentration, respectively. It is clear to conclude from Fig. 4a that no quenching effect was observed (F0/F 1) at low MAH concentrations FIG. 3. Normalized fluorescence emission of Ph (a) and An (b) in PP
film, as a function of their weight concentration. F is the fluorescence intensity andt is the film thickness (mm).
FIG. 4. Fluorescence quenching effect of Ph (20 ppm) in PP: (a) by MAH and (b) by ODSA.
(<0.1 mol/L). For MAH concentrations higher than 0.1 mol/L, the quenching effect increases rapidly. Fig. 5a and 5b present the fitting curves of the experimental results shown in Fig. 4a and 4b, together with their correspond-ing experimental error bars, accordcorrespond-ing to the Perrin static quenching model [9] given by the following equation:
lnðF0=FÞ ¼ VQ ¼ 4 3pR 3 sN 8 > : 9 > ;Q (6)
whereRsis the radius of the quenching sphere. This model
describes the static quenching between randomly distributed and immobile fluorophores and quenchers, which are in proximity inside a spherical volume, V. It assumes an in-stantaneous quenching of an excited fluorophore by the quencher if this latter is inside the same volume, V. How-ever, there is no quenching if the quencher is outside the volumeV. Compared to the sphere of action model (Eq. 5), the difference is the lack of the collisional quenching
com-ponent,Ksv. This is due to (i) the restricted diffusion of
flu-orophore molecules inside the solid polymer film or (ii) the short diffusion distance before the decaying of the excited fluorophore. For these reasons, the collisional quenching is negligible [4–7]. At very low MAH concentrations, the probability for MAH molecules to be very close to fluoro-phore molecules is so small that it has no effect on the fluo-rophore emission. The radii, Rs, of MAH and ODSA
quenching spheres are obtained directly from the slopes of the fitting curves presented in Fig. 5a and 5b, which corre-spond to the term 43pR3
sN 8 :
9
; in Eq. 6. Optimized fitting curves lead toRs¼ 15.2 A˚ for MAH and Rs¼ 28.7 A˚ for
ODSA. As mentioned in Fluorescence Quenching of An and Ph in Chloroform Solution section, both collisional and static quenching effects of ODSA on An and Ph in so-lution were much lower than those of MAH on the same fluorophores. However, the contrary was observed in solid films. It is not reliable to say that this is due to the larger radius of the quenching sphere of ODSA but a reasonable cause could be the better miscibility between ODSA and Ph than between MAH and Ph. In solution, the long alkyl chain of ODSA hinders its diffusion and consequently decreases its quenching efficiency. However, in PP films, it decreases the polarity of the succinic anhydride and increases ODSA miscibility with Ph. So ODSA is more likely to be adjacent to Ph molecules in the PP film, which is propitious to the quenching process. This is in concord-ance with literature results [31] in which the reported solu-bility parameters for PP, Ph, and MAH are d¼ 19.2, 20.0, and 27.8 MPa1/2, respectively. The ODSA solubility pa-rameter, calculated from the group molar attraction con-stants [32], is 21.4 MPa1/2. These results clearly show that the ODSA solubility parameter is closer to the solubility parameter of Ph than that of MAH.
Fluorescence Quenching of An by MAH and ODSA in PP Bulk Film
The fluorescence intensity, F, of An in PP film in the absence and in the presence of MAH and ODSA quenchers are presented in Fig. 6a and 6b, respectively. Fluorescence emission of An decreases rapidly with increasing MAH concentration up to around 0.20 mol/L. For MAH concen-trations higher than 0.225 mol/L, fluorescence emission of An was completely quenched by MAH (F 0). The very high quenching effect of MAH on An in PP film was con-trary to the corresponding results in the solution. During the blending step in the batch mixer at 1808C, the Diels– Alder reaction [33, 34] consumed a part of the An mole-cule, which was the main cause of the observed fluores-cence intensity decrease. However, Fig. 6b shows that ODSA did not quench the fluorescence emission of An, which strangely increased with increasing ODSA molar concentration up to around 0.04 mol/L. This is dramatically different from the quenching behavior of Ph by ODSA, although the miscibility parameters of Ph and An are approximately the same. The main reason that explains this
FIG. 5. Fitting curve of fluorescence quenching of Ph in PP: (a) by MAH at high concentration and (b) by ODSA.
quenching behavior is the shorter lifetime of An (6 ns) compared to that of Ph (60 ns). Due to their short lifetime, An molecules must be very close to ODSA molecules in order to quench their fluorescence emission, which should not be achieved in PP film. The similar phenomenon was observed by Chu and Thomas [4]. They reported that, although fluorescence emission of pyrene was quenched by PA in solution, its quenching by PA in epoxy and polysty-rene films was not possible for PA concentrations up to 0.1 mol/L.
CONCLUSIONS
This experimental study focused on fluorescence quenching of both Ph and An fluorophores by MAH and ODSA in chloroform solution and in PP films. It was observed that fluorescence quenching of An in chloroform solution was well presented by the linear Stern–Volmer
equation and that of Ph showed a positive deviation from this equation but was adequately predicted by the sphere of action model. Both collisional and static quenching effects of ODSA were much lower than those of MAH. The long alkyl chain on ODSA not only slows down ODSA diffusion in solution but also shields the succinic anhydride from accepting the energy transfer from the excited fluorophore. Therefore, it is less efficient to quench the fluorescence of An and Ph.
In solid PP film, it was observed that the diffusion of MAH and ODSA molecules was restricted and the static quenching mechanism was dominant. Fluorescence quenching efficiency of Ph by ODSA was higher than that by MAH. This was due to the better miscibility between ODSA and Ph than between MAH and Ph. The dramatic decrease of fluorescence emission of An in the presence of MAH was mainly due to the Diels–Alder reaction dur-ing sample preparation process, and not to the energy transfer mechanism. So, fluorescence quenching in solid polymer matrix is a very complex process and quite dif-ferent from that in liquid solution.
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