Luminescence studies in polymers—VII. Exciplex formation in polyvinylnaphthalene and polyacenaphthylene

European Polymer Journal, Vol, 12, pp. 365 to 370. Pergamon Press 1976. Printed in Great Britain.

LUMINESCENCE

S T U D I E S IN P O L Y M E R S - - V I I

EXCIPLEX FORMATION IN POLYVINYLNAPHTHALENE AND POLYACENAPHTHYLENE* C. DAVID, N. PUTMAN-DE LAVARE1LLEand G. GEUSKENS Faculte des Sciences, Universit6 Libre de Bruxelles, Campus Plaine 206-1, 1050 Bruxelles, Belgium

(Received 1 October 1975) Abstract--The reactivities of polymers in exciplexes and CT complex formation were compared with those of model compounds for the following systems in solution: (1) exciplexes between excited diethylaniline as donor and ethylnaphthalene, acenaphtbylene, poly-l-vinylnaphthalene or polyacenaphthylene; (2) exciplexes between excited dicyanoanthracene as acceptor and the same aromatics as donors: (3) CT complexes between chloranil and either poly-l-vinylnaphthalene or polyacenaphthylene. The rate constant for exciplex formation was found to be much larger for the model than for the polymer when diethylaniline is the donor and poly-l-vinylnaphthalene or ethylnaphthalene the acceptor. The equilibrium constant for CT complex formation between chloranil and the same aromatics is also higher in the case of model compounds. This difference is tentatively assigned to entropy terms arising from the lower accessibility of the aromatic groups fixed on a polymer backbone. These conclusions are extended to the other systems. Exciplexes do not form in poly-l-vinylnaphthalene or polyacenaphthylene films containing dicyanoanthracene.

INTRODUCTION The photophysics of polymer systems have been widely developed in recent years [1]. The topics studied include the quantum yield and kinetics of formation of excimer and detailed analysis of energy migration and transfer. The formation of exciplexes involving macromolecules has, however, not been investigated. The purpose of the present work is comparison of the reactivity of a macromolecule and the corresponding low molecular weight model compound in the formation of complexes with the excited state of a low molecular weight compound. The macromolecules and models are polyvinylnaphthalene (PVN), polyacenaphthylene (PAcN), 1-ethylnaphthalene (EtN), naphthalene (N) and acenaphthylene (AcN). The excited low molecular weight compounds are dicyanoanthracene (DCA) or diethylaniline (DEA). These molecules can be brought selectively to the first excited state in the presence of the polymer or the model. Two types of exciplexes are formed. In the first, the excited diethylaniline acts as electron donor and the unexcited aromatic is the acceptor. In the second, the excited dicyanoanthracene is the acceptor that interacts with the donor unexcited aromatic chromophore. Exciplex formation in low molecular weight compounds has been extensively studied by Wellcr and coworkers [2] and reviewed by Birks [3]. There is a major difference between exciplexes and charge transfer (CT) complexes. In exciplexes, indeed, donor and acceptor interact only when one of them is in lhe excited state, whereas in CT complexes donor and acceptor interact in the ground state and in the excited state, ll was thus interesting to compare the reactivity of macromolecules in exciplexes and in CT * Part VI: C. David, D. Baeyens-Volant and G. Geuskens, Europ. Polym. J. 12, 71 (1976). 365

complexes formation. Therefore, the equilibrium constants of charge transfer complexes of chloranil with ethylnaphthalene and PVN were determined. CT complexes of polymer with low molecular weight compounds have been studied by other authors [4, 5].

EXPERIMENTAL

PVN was obtained by spontaneous polymerization of the purified monomer in the refrigerator: PAcN was prepared by free radical polymerization. The polymers were carefully purified by successive dissolutions and precipitations. DEA was freshly distilled under nitrogen. DCA was synthesized from CuCN and dibromoanthracene and purified by crystallization. Toluene and methylene chloride were purified by distillation and their purities tested by u.v. absorption spectroscopy at room temperature and by fluophosphorimetry at 77°K. The emission spectra were obtained either by transmission using an Hitachi-Perkin Elmer MPF 2A spectrofluorometer or by reflection using a laboratory-built device previously described [6]. DEA, toluene and methylene chloride were used as solvent respectively for DEA aromatic exciplexes, DCA-aromatic exciplexes and CT complexes.

RESULTS AND DISCUSSION

1. Exciplexes of diethylaniline with PVN, PAcN, EtN, AcN and naphthalene in solution The fluorescence spectra of diethylaniline excited in the presence of PVN are given in Fig. 1. The short and long wavelength emissions respectively correspond to the fluorescence of the donor diethylaniline (M) and of the exciplex (E). Similar emission spectra are obtained with the other aromatics as acceptor. The values of 2m.x of the exciplex fluorescence are given in Table 1. Figure 1 clearly shows that, at a

C. DAVID, N. PUTMAN-DELAVAREILLEand G. GEUSKENS

366

I

C, Fig. 2. Ratio tensity as a temperature. tor: EtN(o),

mol/l

of exciplex to diethylaniline fluorescence infunction of acceptor concentration at room Solvent: diethylaniline -LX, : 313 nm. AccepAcN (A), PVN (0) and PAcN (A). Fully corrected values. .

Dissociation of the exciplex into ion pairs and CT complex formation have not to be considered in the media of low dielectric constant used in this work (emA = 5.5 at 18.7”C) [7,8]. Assuming steady-state conditions for the excited states, it can be shown that:

km tz(k, + km)

1

IM = (kM + kEMQ)kE + k,,kM kM =k,,l,+

“In

Fig. 1. Fluorescence spectrum of diethylaniline containing various concentrations of PVN as acceptor at room temperature. A,,, : 313 nm. [PVN]: 1: 0 M; 2: 0.0375 M; 3: 0.075 M; 4: 0.15 M; 5: 0.25 M; 6: 0.6 M. given donor concentration, the ratio of donor to exciplex fluorescence intensity (respectively Ihl and I,) depends on the acceptor concentration (Q). This point is better demonstrated in Figs. 2 and 3 where Ih/I~ and (IO/& are respectively given as a function of aromatic acceptor concentration at constant amine concentration. (lo/& is the ratio of the amine fluorescence intensities in the absence and presence of donor. In both cases, a linear relation is obtained; they are easily explained by the following sequence of photophysical processes [scheme (l)] :

M hv

lM*

I,

‘M* -

M + hv

k FM

iM* M* -

M 3M*

k GM

E*

k EM

E* -

M*+ Q

k ME

E* -

M + Q + hv

kFE

M* + Q -

k TM

k GE

E*-M+Q

k IM

1

k, i

Or r,

kmQ kE

IokmQ IE = kFE (k, + k,,Q)k, + kMEk,

km

kE) = k,,m= =1+

Examination

(2)

km [El*

M2

r,= G(kME+

(3) and (4) are and l/Z, are in Fig. 4 for PVN The relative Fig. 2 can be indicates that

1 Or r,

hdk + h) kdn km Q

kE =kx+ IE

(1)

kFdo(kE + km)

!%CQl k,,

k&Q h,CkE + km]'

(4)

of Figs. 2 and 3 shows that relations verified. The individual values of l/ZM agreement with (1) and (2) as shown in and EtN. values of the slopes of the lines in interpreted in terms of Eqn. (3) that

k&t+i k&kin + ka) >

kE

is larger for the model compounds polymer.

than for the

Table 1. J,,,_ (nm) for exciplex formation Acceptor Donor LIX

EtN DEA 410

AcN DEA 400

(3)

PVN DEA 410

PAcN DEA 400

DCA EtN 515

DCA AcN (540)

DCA‘ PVN 520

DCA PAcN 520

Luminescence studies

367

5

I

0.5

~

I

I

0.50 C,

I

mol/I.

Fig. 3. Ratio of diethylaniline fluorescence intensity in the absence (10) and in the presence (I) of an acceptor at various concentrations at room temperature, 2~x,: 313 nm. Solvent: diethylaniline. Acceptor: same symbols as in Fig. 2. Considering Fig. 3, it appears that the slope of the line corresponding to Eqn. (4) is also larger for the model compound than for the polymer at room temperature. These slopes, however, correspond to the product of different rate constants and are difficult to interpret unequivocally but the problem would be much simpler if kM~ < k,~, i.e. if dissociation of the exciplex is negligible when compared to its other modes of deactivation. Determination of I~/IM as a function of temperature allows a test whether this condition is verified or not.

i/c ,

I

2.5

t/tool

50

I00

1,50

I

I

I

-/ I

I

I

I

02

04

C,

I

s5

I

4.o

4.5

xlO s

Fig. 5. Diethylaniline and exciplex fluorescence intensity as a function of 1/T. Acceptor: EtN 0.09 M in diethylaniline. 2e,c = 313 nm. IM (O), IF. (A), IM/lt,(©). Therefore, the values of log It, log IM and log IM/I,, as a function of lIT are given in Figs. 5 and 6 for ethylnaphthalene and PVN. The results can be interpreted according to scheme (1) and Eqn. (3) as was done for excimers by Birks and others [3]. In the low temperature range, kM~ ~ k~. is verified and a straight line with a positive slope related to EL~M -Et~ (or to Et~M alone if kt~ is temperature independent) is observed. In the high temperature range, a straight line with a negative slope corresponds to the condition kME > k~ if IM increases with temperature. The slope of this line is thus related to EM~, -- E,:M which is the binding energy of the exciplex. The results are unambiguous for PVN and EtN in the presence of excited DEA. The I~./IM and (Io/1k~ plots at room temperature clearly correspond to the low temperature range, where kMv < kt~. Furthermore, since IM increases in the high temperature range, the condition k~ < ku, is verified there and the binding energy of the exciplex can be deduced from the slope of log IvJlM as a function of 1/T. The various photophysical parameters that can be deduced from Figs. 2 and 3 and Eqns. (3) and (4) in the simplified form are given in Tables 2 and 3 for PVN and EtN. Concerning exciplexes of PAcN and AcN with DEA (Figs. 7 and 8), the minimum of log IM/I~. is situated near room temperature. The simplification

,~

0.~

,/,.

I

3.0

je *'~'~l~°

I

06

mol / I.

Fig. 4. Inverse diethylaniline (1/IM) and exciplex (I//L) fluorescence intensity as a function of acceptor concentration at room temperature. 2~xc:313 nm. Solvent: diethylaniline: same symbols as in Fig. 2. Fully corrected values.

I

2.5

I

I

3.0

I

3.5

I

~-xlO

4-.0

s

Fig. 6. Cf. Fig. 5. Acceptor: PVN 0.142 M in diethylaniline.

C. DAVID,N. PUTMAN-DELAVAREILLEand G. GEUSKENS

368

Table 2. Photophysical parameters for exciplex formation at room temperature Aromatic/M

EtN/DEA

kEM

~-M (1/mole)

1 . kvEkEM(l/mole) kvta

/¢~

59

12.1

17

2.7

k M kFE

0.29

kF M .k E

Aromatic/M

PVN/DEA

0.29

AcN/DEA

PAcN/DEA

-- •- (l/mole) k~ kE -F kME

21

6.2

43

8.8

185

7.8

1 kFEkEM (l/mole) kvM (kE + ku~)

14

3

17.5

(0.4)*

17

(0.6)?

kEM

kE

k~. kvE

0.66

0.48

EtN/DCA

PVN/DCA

0.40

AcN/DCA

(0.045)*

PAcN/DCA

0.09

(0.073)?

kFM kE * These values may be in error owing to the small contribution of IE. t These values may be in error owing to the presence of a small proportion (< 1~) of dehydrogenated units in the chain. Table 3. Photophysical parameters for exciplex formation with excited diethylaniline

Acceptor EtN PVN AcN PAcN

Iso-emissive point range (°C) Low T High T 2-30 4-65 - 3 1 - - 10 - 10-12

Binding energy (EME -- EEM) (kJ/mole)

(kJ/mole)

23.8 20.9 25.1 22.1

15.0 7.1 ---

117-150 125-155 68-110 64-110

kME ~ kE thus cannot be made when IE/IM and (lo/I)M are measured at this temperature. The photophysical parameters for these systems are also given in Tables 2 and 3. Iso-emissive points are observed for all systems in the low temperature range. The significance of these points is ambiguous [9, 10]. If i is the intensity at which an iso-emissive point occurs for the wave number vi:

i = iM q- iE = O~iflpM-b flifll)E,

EEM

where q~ and qSE are the quantum intensities of the monomer and excimer bands. To observe an iso-emissive point, 6i/6T should be zero. According to (5):

6i ~M ~ &PE 6 ~ = ~ i - g f + ~-~- + A,

(6)

where A is associated with the variation with temperature of ~ and fll that are related to the band shape. Rearranging (6) and assuming kuF~ ~ k~, it

(5)

/

I 2.5

~.o

~.~

~.o

~Tx l O -3 Fig. 7. Cf. Fig. 5, Acceptor: PAcN 0.40 M in diethylaniline.

I

2.5

~

3.0

I

5.,5

I

4.0

1

4..5

I_T xlO-S Fig. 8. Cf. Fig. 5. Acceptor: AcN 0.08 M in diethylaniline.

Luminescence studies

369

in the frequency factor that would be lower for the less accessible sites of the polymer side-chain than for EtN. Direct measurement of kL,M as a function of temperature by decay time measurement would allow determination of this parameter. The general trend of the results is the same for PAcN and AcN although exciplex dissociation cannot be neglected at room temperature. Probably the conclusions previously drawn are also valid in this case.

EI IM

2. Exciplexes qJ" DCA with N, EtN, PVN. ACN and PAcN 05 C,

1.0 mol/L

Fig. 9. Ratio of exciplex to dicyanoanthracene fluorescence intensity as a function of donor concentration at room temperature. Solvent: toluene. , ~ : 405 nm. [DCAT: 7.10 -5 M. Donor: EtN (©), AcN (A), PVN (O) and PAcN tA). Fully corrected values.

can be shown that 3i/6T = 0 if:

6qM ~qM ~iqM = ~iql. and OT - 6T - O, (conditions l and2) where qM and q~ are respectively k~M/kM and k~lJkL. E~.M can thus be obtained from the slope of 6(log IM/I~)/6(~,) in the range of the low temperature isoemissive point. The values for PVN and EtN are given in Table 3. They have, however, to be considered cautiously. It has indeed been shown [10] that iso-emissive points can be observed by internal compensation of terms yielding 6i/6T = 0 even if conditions 1 and 2 are not verified. Direct measurements of kM and k~ as a function of temperature by emission decay measurement are presently in progress to verify the validity of these conditions. The various photophysical parameters obtained for the polymers can now be compared with those of the model compounds. From the data in Table 2, it appears that kvtJk~, is of the same order of magnitude for PVN and EtN. The rate constant for exciplex formation on the contrary is much larger for the model than for the polymer. Various factors are responsible for this effect. First of all, exciplex formation is a diffusion-controlled process and its rate constant is proportional to the viscosity of the medium. The viscosity of a 0.6 M PVN solution is about twice that of ethylnaphthalene at the same concentration. The difference in Et:M would reflect the different temperature dependence of the viscosity of (DEA + modell and (DEA + polymer) solutions. The parameter El,M, if correct, would however render the exponential factor e -E' ~ larger for the polymer than for the model. Secondly, the notion of acceptor concentration is not the same for a model compound homogeneously distributed in the solution and a polymer for which the chromophore distribution is imposed by the conformation of the main chain. However, for the range of polymer concentration used in this work, the macromolecules can be considered as interpenetrating coils and the structure of polymer solutions are not so different from that of the low molecular weight analogues. The difference in rate constant k~ ~ must thus be assigned to a difference

The fluorescence of the excited acceptor DCA is quenched in the presence of the aromatic donors. When the experimental values of I~ and IM are plotted according to Eqns. (1)-(4), linearity is obtained (Figs. 9 and 10). The ratio IE/IM was also studied as a function of temperature for these systems, but the obtained results do not indicate that kME can be neglected with respect to kE. The experimental values of the slopes corresponding to Eqns. (3) and (4) are given in Table 2. The difference in kEM between polymer and model can be assigned to the same parameters as for the exciplexes of DEA.

3. Exciplexes in films DCA was introduced as an additive in PVN films to form exciplex sites that could act as energy traps in the polymer. Selective excitation of the additive yields two emissions: normal fluorescence of the additive and a broad emission at 510 nm that could be the exciplex of the naphthalene chromophore with DCA. However, the same two emissions are recorded from polymethylmethacrylate films containing DCA where exciplexes cannot form. Furthermore, in PVN films the ratio of ls~o/IM decreases when the concentration of DCA decreases instead of being constant in agreement with Eqn. (3). The 510 nm emission is thus most probably an excimer of DCA due to aggregation of the additive in either polymethylmethacrylate or PVN films.

o

I

I

05

rO

C,

mol/L

Fig. 10. Ratio of dicyanoanthracene fluorescence intensity in the absence (1o) and in the presence (I) of an acceptor at various concentrations at room temperature. ,~,,.: 405 nm. [DCA]: 7.10-5 M in toluene. Acceptor : same symbols as in Fig. 9.

C. DAVID,N. PUTMAN-DELAVAREILLEand G. GEUSKENS

370

4. Charge transfer complexes of PVN, N and EtN with chloranil The adsorption spectre of the charge transfer complexes of Naphthalene, EtN and PVN with chloranit have 2,,,,x at 480, 512 and 515 nm. The interaction between the aromatics and chloranil can be represented by: kr D+A ~- DA and characterized by the equilibrium constant: K¢-

[DA]

[DA]

[D]EA]

(ED]o - [ D A ] ) ( E A ] o - EDA])

kf kd where [D]o and [A]o are the total (free and uncomplexed) concentration of donor and acceptor. If [D]o >> [A]o then [D]o >> [DA] and [DA] Ke = [D]o([A] ° [DA])' _

These results show that the Concentration of CT complexes at equilibrium is larger for the models than for the polymer at room temperature but we cannot assign it to an increase in ki, to a decrease in k~ or a combination of both effects. It must be remembered that the equilibrium constant K¢ has a meaning different from the ratio of the rate constants in Eqn. (3). The true equilibrium constant for the exciplex is kLm/ kMv. but it can be measured only in the high temperature range where kML ~> ke, provided kvA and kvE are known. Nevertheless, the aromatic groups of the polymer are probably less accessible to the low molecular weight additive than the models to form CT complexes as well as exciplexes. Charge transfer complexes studies in which a polymer is involved have been performed in few cases [4, 5]. An increase in K~ is usually reported when a low molecular weight donor is replaced by the corresponding polymer but when the carbazole group is the donor, a decrease in Kc is observed as in the present work. Much remains to be done in that field before general conclusions can be drawn.

(8)

Acknowledgements--We are very grateful to the Fonds At a wavelength at which only D A absorbs: log Io ~- = ECTI[ D A ] ,

(9)

where l is the cell thickness. Then, from (8) and (9): /[A]o

loglo(lo/I)

[D]o + [A]o EcT[D]o

+

1 ~cvKe[D]o"

(10)

If the absorption spectre are recorded as functions of donor concentration [Do] at constant acceptor concentration [Ao], with the condition [Ao] '~ [Do], the results can be analysed by the method of Benesi-Hildebrand based on the equation: 1 [Ao]

1

logto Io/I

¢CT

+

1

(11)

EcTKe[D]o

ECT and K~, can be obtained respectively from the intercept and slope. This linear relation is verified when chloranil (4- l0 3 M in CHECI2) interacts with N, EtN or PVN (0.062-1 M). The values of K~ are respectively 1.04, 1.2 and 0.36, ~ being 625 1/mole.cm in all cases.

National de la Recherche Scientifique and to the Fonds de la Recherche Fondamentale Collective for financial support to the laboratory. Thanks are also due to the Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture for a grant to one of us (N.P.d.L.). REFERENCES 1. ref. cited in Part VI, (1976). 2. D. Rehm and A. Weller. Z. phys. Chem. N.F. 69, 183 (1970). 3. J. B. Birks, Photophysics of Aromatic Molecules, WileyInterscience, New York (1970). 4. A. Renbaum, A. M. Hermann and R. Haack, J. Polym. Sci. A1, 6, 1955 (1968), and ref. cited. 5. S. Iwatsuki and K. Inukai, J. Polym. Sci. Polym. Chem. 12, 1443 (1974) and ref. cited. 6. C. David, N. Putman-de Lavareille and G. Geuskens, Europ. Polym. J. 10, 617 (1974). 7. N. Nakashima and N. Mataga, Z. phys. Chem. N.F. 79, 150 (1972). 8. M. Itoh, J. Am. chem. Soc. 96, 7390 (1974). 9. T. D. S. Hamilton and K. Razi Naqvi, Chem. Phys. Lett. 2, 374 (1968). 10. A. J. H. Al-Wattar and M. D. Lumb, Chem. Phys. Lett. 8, 331 (1971).

Resum~-La r6activit6 des polym6res a 6t6 compar6e h celle des compos6s modules lors de la formation d'exciplexes ou de complexes de transfert de charge en solution. Les syst~mes 6tudi6s sont: (1) les exciplexes entre la di6thylaniline excit6e jouant le r61e de donneur et l'6thylnaphtal6ne, l'ac6naphthylene, le poly-1vinylnaphtal6ne ou le polyac6naphthyl6ne; (2) les exciplexes entre le dicyanoanthrac6ne jouant le r61e d'accepteur et les m~mes d6riv6s aromatiques; (3) les complexes de transfert de charge entre le chloranileet le poly-1-vinylnaphtal6ne, le naphtal6ne ou l'6thylnaphtal6ne. On a montr6 que la constante de vitesse de formation de l'exciplexe est plus grande pour le mod61e que pour le polym6re et attribu6 cette diff6rence au facteur entropique, ces groupements aromatiques 6tant moins accessibles dans le polym&e que dans le mod61e. Ceci est vrai 6galement pour la constante d'6quilibre caract6risant la formation de complexes de transfert de charge de ces d6riv6s avec le ehloranile. Ces conclusions ont 6t6 6tendues aux autres syst~mes. Les exciplexesse ne forment pas darts des films de poly-l-vinylnaphtal~ne ou de polyac6naphtyl~ne contenant du dicyanoanthrac6ne.