Conjugated polymers with 2,2′-bipyridine and diethinylenebenzene units: Absorption and luminescence properties

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

J. Phys. Org. Chem. 2000; 13: 112–126

Conjugated polymers with 2,2'-bipyridine and

diethinylenebenzene units: absorption and

luminescence properties

U.-W. Grummt,¹* E. Birckner,¹E. Klemm,²D. A. M. Egbe²and B. Heise²

¹Institut fuÈ r Physikalische Chemie der Friedrich-Schiller UniversitaÈt Jena, Lessingstrasse 10, D-07743 Jena, Germany

²Institut fuÈ r Organische und Makromolekulare Chemie der Friedrich-Schiller UniversitaÈt Jena, Humboldtstrasse 10, D-07749 Jena, Germany

Received 22 June 1999; revised 4 October 1999; accepted 8 October 1999

ABSTRACT: Alternating oligomers and polymers consisting of 2,2'-bipyridine and diethinylenebenzene units and

corresponding model compounds were synthesized and investigated in dilute solutions by absorption spectroscopy

and by stationary and time-resolved emission spectroscopy. The strictly linear (rod-like) p-chain oligomers/polymers

were compared with the angularly linked oligomers/polymers and with related model compounds. The model

compounds which already show the essential spectroscopic properties of the oligomers/polymers consist of three

(hetero)aromatics linearly connected by two diethenylene groups. These models exhibit ﬂuorescence quantum yields

close to unity and short ﬂuorescence decay times around 1 ns. Fluorescence anisotropy and rotational relaxation times

are consistent with the Stokes–Einstein equation and the Perrin equation. The absorption and emission spectra of the

polymers and their radiative rate constants determined by ﬂuorescence quantum yield and lifetime and according to

the Strickler/Berg equation show a conjugation length of one to two repetition units. The conjugation along the chain

is stronger in linear than in angular polymers and stronger in alkoxy-substituted than in unsubstituted polymers. In

angular polymers at least two different emitting segments were found. The shortened mean lifetimes and the reduced

ﬂuorescence quantum yields and anisotropies of the oligomers/polymers indicate an additional radiationless

deactivation channel which is opened by energy migration along the chain. Rates of energy transfer calculated for

Ltd.

KEYWORDS: 2,2'-bipyridine; diethinylenebenzene; conjugated polymers; absorption; luminescence

INTRODUCTION

characterization of poly(phenyleneethinylenes) and alter-

nating poly(phenyleneethinylen/heteroarylethinylenes).

Among others, stilbene-4,4'-diyl, anthracene-9,10-diyl,

thiene-2,5-diyl and pyridine-2,5-diyl have been used

as arylenes and heteroarylenes. In order to increase the

solubility of these rigid, linear polymers, alkyl side-

chains have been introduced via ester, ether or other

groups. Kondo et al. reported on a poly(phenylene-

ethinylene) which is well soluble due to its angular

structure.⁶

Essentially, absorption and ﬂuorescence spectra have

been published, and sometimes also ﬂuorescence quan-

tum yields. From a comparison of absorption spectra with

ﬂuorescence excitation spectra, Morgan et al.⁷concluded

that energy transfer occurs along the polymer chain.

Efﬁcient energy transfer has been found by Swager⁸with

anthracene terminal groups. This group exploits energy

transfer to enhance signiﬁcantly the sensitivity of a

ﬂuorescent chemosensor.⁹Picosecond time-resolved

measurements and investigations into the stimulated

emission have been published.^10,11Ley and co-workers

published the synthesis and photophysical data for two

Highly ﬂuorescent conjugated polyarenes have been of

increasing interest as potential materials for electrolumi-

nescence devices and attract particular attention because

of their high non-linear susceptibilities,^1,2which make

them candidates for all optical switching devices.³

From the point of view of more fundamental research,

segmented conjugated polyarenes are useful objects for

investigating energy migration along the polymer chain.⁴

In particular, ﬂuorescent rigid rod oligomers including

suitable model compounds are excellent probes for

studying molecular motions in viscous media by static

and kinetic monitoring of the ﬂuorescence anisotropy. An

additional interesting aspect is their potential use as

molecular wires.⁵

Several papers have been appeared in recent years

which deal with the synthesis and spectroscopic

*Correspondence to: U.-W. Grummt, Institut fu¨r Physikalische

Chemie der Friedrich-Schiller Universita¨t Jena, Lessingstrasse 10,

D-07743 Jena, Germany.

E-mail: cug@rz.uni-jena.de

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

113

Scheme 2

are bromine. Acetylene moieties as terminating groups

could be excluded by NMR spectroscopy.

In order to determine the chromophore unit and to

understand the photophysical behaviour of the oligomers

and the polymers, we compared them with their related

model compounds. Owing to the spectral location of the

long-wavelength absorption band and particularly the

high ﬂuorescence quantum yield, compounds with at

least three aryl units connected by two acetylene groups

should be considered as models.

Scheme 1

model compounds and one polymer containing 2,2'-

bipyridyl in the backbone including their metal com-

plexes.¹²Systematic investigations into the photophysics

have been published for only two compounds.¹¹

In this paper, we report on absorption and emission of

new linear and angular conjugated polymers, oligomers

and their model compounds consisting of 2,2'-bipyridine

and diethinylenebenzene units in the backbone. It was the

aim of this study to identify the chromophore unit and to

unravel the effect of structural variation, particularly the

comparison of a linear or angular backbone, on the

photophysical deactivation and energy transfer along the

chain.

The conjugated alternating oligomers and polymers

investigated contain 2,2'-bipyridine and 1,4-substituted

diethinylenebenzene as building blocks. As shown in

Scheme 1, two different types of linkage are realized.

The abbreviations L and A stand for the linear and

angular linkage, respectively, and P and M designate

polymers (or oligomers) and model compounds, respec-

tively. Owing to the synthetic route the terminating atoms

The model compounds and their abbreviations are

given in Scheme 2.

The polymers and the higher oligomers without alkoxy

substituents are insoluble. Therefore, only oligomers with

chain lengths up to a mean degree of polymerization (DP)

of 4 could be investigated. Alkoxy substitution increases

the solubility of the compounds and opens the possibility

of investigating higher oligomers and polymers.

EXPERIMENTAL

Synthesis

The syntheses of several compounds investigated in this

work have been published previously.¹³All chemicals

were purchased from Fluka and Aldrich. Toluene was

dried and distilled over calcium hydride and, THF,

triethylamine and diisopropylamine were distilled over

potassium hydroxide. All solvents were degassed by

J. Phys. Org. Chem. 2000; 13: 112–126

114

U.-W. GRUMMT ET AL.

bubbling with argon for 1 h prior to use. All reactions

were carried out under an inert gas atmosphere (argon).

di(octyloxy)benzene (M3b) and 1,4-bis(2',2@-bipyridine-

5@-ylethinyl)-2,5-di(dodecyloxy)benzene(M3c)werepre-

pared similarly to 1,4-bis(2',2@-bipyridine-5@-ylethinyl)-

2,5-di(octadecyloxy)benzene (M3d).¹³

Synthesis of reactands

M3b was puriﬁed by column chromatography on silica

gel 60 eluting with n-hexane–THF (2:1, v/v); yield 70%,

m.p. 113–115°C.

1,4-Diethinylbenzene¹⁴and 2,5-di(octyloxy)-, 2,5-di(do-

decyloxy) and 2,5-di(octadecylyloxy)-1,4-diethinylben-

zene^8,15,16were prepared by published procedures. The

syntheses of 5-bromo- and 5,5'-dibromo-2,2'-bipyridine

were performed according to Romero and Ziessel.¹⁷4-

Bromo- and 4,4'-dibromo-2,2'-bipyridine were synthe-

sized as described.^18,19

¹H NMR (CDCl₃): ꢀ = 0.83 (6 H, t, —CH₃), 1.25–1.91

[24 H, m, —(CH₂)₆—], 4.04 (4 H, t, —CH₂O—), 7.04 (2

H, s, H_Phe), 7.30 (2 H, t, H_5@), 7.80 (2 H, t, H_4@), 7.90 (2 H,

dd, H_4'), 8.40 (4 H, d, H_3'and H_3@), 8.67 (2 H, d, H_6@),

8.80 (2 H, d, H_6') ppm. ¹³C NMR (CDCl₃):

ꢀ = 14.05(CH₃—), 22.63, 26.05, 29.29, 29.33, 29.67,

(—C

ꢀ

C—), 113.87 (C_Phe—C

^{31.79 [—(CH}²⁾⁶^{—], 69.66 (—C}ꢀ^H²C^O—^—)⁾,^{, 9}1⁰1^.6⁰.⁷84^an(^dC⁹_P²_he^.03

—

Synthesis of the model compounds

H), 120.32 (C_5@), 120.44 (C_5'), 121.32 (C_3@), 123.87 (C_3'),

136.91 (C_4@), 139.19 (C_4'), 149.27 (C_6@), 151.61 (C_6'),

153.78 (C_Phe—OR), 154.83 (C_2@), 155.49 (C_2') ppm. MS

1,4-Bis(3'-pyridylethinyl)benzene (M1). Tetrakis(tri-

phenylphosphine)palladium [Pd(PPh₃)₄] (48 mg, 0.09

mmol, 2.3 mol%) and copper(I) iodide (CuI) (16 mg,

0.08 mmol, 2.0 mol%) were added to a degassed solution

of 1,4-diethinylbenzene (0.5 g, 4 mmol) and 3-bromo-

pyridine (1 ml, 10 mmol) in triethylamine (3 ml) and

toluene (40 ml). The reaction mixture was stirred and

heated under reﬂux for 7.5 h, then ﬁltered hot to remove

the inorganic compounds. After cooling to room

temperature a dark-red solid precipitated. The resulting

suspension was poured into an excess of methanol and

the dark-red precipate collected by ﬁltration. 1,4-Bis(3'-

pyridylethinyl)benzene was isolated from the ﬁltrate. The

solvent was removed under vacuum and the residue

puriﬁed by column chromatography on silica gel 60. The

starting materials 1,4-diethinylbenzene and 3-bromopyr-

idine were reisolated by elution with toluene–ethyl

acetate (1:1, v/v). Byproducts of higher molecular weight

were removed again by column chromatography (silica

gel 60) with toluene–ethyl acetate as eluent (gradient

elution, 9:1–1:9, v/v). Final puriﬁcation by vacuum

(CI): calculated m/z = 690 (M), found m/z = 691

(M 1) , 579 (M

C₈H₁₇ 1) , 466 (M

2

—

C₈H₁₇ 1) g mol ¹. IR (KBr): ꢁ = 3049 (w, C_aryl

H), 2923 and 2854 (vs, CH₂— and CH₃—), 2208 (w,

disubstituted —C

ꢀ

C—), 1390 (m, CH₃—), 1275 (m,

aryl— OR) cm ¹. UV/VIS (CHCl₃): ꢂ_max(log") = 330

(4.67), 387 (4.69) nm. Analysis: C₄₆H₅₀N₄O₂(690.93),

calculated C 79.97, H 7.29, N 8.11; found C 79.83, H

7.59, N 7.94%.

M3c was puriﬁed by column chromatography on silica

gel 60 eluting with n-hexane–THF 2:1, (v/v), yield 60%,

m.p. 100–102°C.

¹H NMR (CDCl₃): ꢀ = 0.83 (6 H, t, —CH₃), 1.21–1.91

[40 H, m, —(CH₂)₁₀—], 4.04 (4 H, t, —CH₂O—), 7.04 (2

H, s, H_Phe), 7.30 (2 H, ddd, H_5@), 7.81 (2 H, ddd, H_4@),

7.90 (2 H, dd, H_4'), 8.40 (4 H, d, H_3'and H_3@), 8.67 (2 H,

d, H_6@), 8.80 (2 H, d, H_6') ppm. ¹³C NMR (CDCl₃):

ꢀ = 13.99 (CH₃—), 22.57, 25.98, 29.25, 29.30, 29.54,

29.57, 31.80 [—(CH₂)₁₀—], 69.59 (—CH₂O—), 91.04

(—C

ꢀ

C—), 113.80 (C_Phe—C

ꢀ

C—), 116.76 (C_Phe—

2

sublimation (135°C, 3 Â 10 mbar) yielded a white

H), 120.25 (C_5@), 120.38 (C_5'), 121.20 (C_3@), 123.82 (C_3'),

136.85 (C_4@), 139.15 (C_4'), 149.20 (C_6@), 151.54 (C_6'),

153.71 (C_Phe-OR), 154.74 (C_2@), 155.39 (C_2') ppm.

MS (CI): calculated m/z = 802 (M), found m/z = 803

powder; yield 30 mg (3%), m.p. 175–177°C.

¹H NMR (CDCl₃): ꢀ = 7.28 (2 H, q, H_5'), 7.53 (4 H, s,

H_Phe

^{), 7.78 (2 H, dt, H ), 8.54 (2 H, dd, H ), 8.76}ꢀ⁽²C^H—^{, d})^,,

(M 1) , 635 (M C₁₂H₂₅ 1) , 466 (M 2C₁₂H₂₅

6'

2'

H_4') ppm. ¹³C NMR (CDCl₃): ꢀ = 88.02 (—C

92.12 (—C

ꢀ

C—), 120.16 (C_3'), 122.91 (C_Phe

123.08 (C_5'), 131.69 (C_Phe—H), 138.43 (C_6'), 148.82

—

ꢀ

—),

1) , 401 g mol ¹. IR (KBr): ꢁ = 3056 (w, C_aryl— H),

2922 and 2851 (vs, CH₂— and CH₃—), 2207 (w,

(C_2'), 152.28 (C_4') ppm. MS (EI): calculated m/z = 280

disubstituted —C

ꢀ

C—), 1389 (m, CH₃—), 1274 (m,

C_aryl— OR) cm ¹. UV/VIS (CHCl₃): ꢂ_max(log") = 330

(4.67), 388 (4.67) nm. Analysis: C₅₄H₆₆N₄O₂(803.14)

calculated C 80.75, H 8.28, N 6.97; found C 80.20, H

8.40, N 6.73%.

(M), found m/z = 280 (M) , 174 (M

ꢀ

Ph) , 140 (1/2

) g mol ¹. IR (KBr): ꢁ = 3070

C—), 1579 and

M) , 126 (

and 3034 (w, C_aryl—H), 2225 (m, —C

ꢀ

Ph

ꢀ

1543 (m, —C = N—, —C = C—) cm ¹. UV/VIS (1,4-

dioxane): ꢂ_max(log") = 319 (4.69), 338 (4.53) nm (" in

l mol ¹cm ¹). Analysis: C₂₀H₁₂N₂(280.33) calculated C

85.71, H 4.29, N 10.00; found C 84.81, H 4.59, N 9.39%.

4,4'-Bis(phenylethinylene)-2,2'-bipyridine

(M4).

Pd(PPh₃)₄(9 mg, 0.00075 mmol, 1.0 mol%) and CuI

(3 mg, 0.0015 mmol, 2.1 mol%) were added to a mixture

of 4,4'-dibromo-2,2'-bipyridine (230 mg, 0.73 mmol),

phenylacetylene (154 mg, 1.47 mmol) and triethylamine

(1 ml) in dried toluene (10 ml). The reaction mixture was

M3 compounds. 1,4-Bis(2',2@-bipyridine-5@-ylethinyl)-

benzene (M3a) was prepared according to El-Ghayoury

and Ziessel.²⁰1,4-Bis(2',2@-bipyridine-5@-ylethinyl)-2,5-

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

115

stirred at reﬂux for 3 h, then poured into methanol

(200 ml). The precipitate was puriﬁed by column

chromatography with n-hexane–THF (3:1, v/v). A

150 mg amount of white crystals was obtained, yield

57.4%, m.p. 203–205°C.

added and the reaction mixture was stirred for 1 h at room

temperature and afterwards heated for 3 h under reﬂux.

After cooling to room temperature, the reaction mixture

was poured into an excess of methanol and the resulting

brownish precipate and inorganic compounds were

collected by ﬁltration. 2,5-Di(octyloxy)-1,4-bis(4'-py-

ridylethinyl)benzene was isolated from the ﬁltrate, which

was concentrated under reduced pressure.

The starting products 4-bromopyridine and 2,5-di

(octyloxy)-1,4-diethinylbenzene were removed by col-

umn chromatography (silica gel 60) with hexane–ethyl

acetate (1:1, v/v) as eluent. Further fractionation of the

residue by column chromatography on alkaline alumi-

nium oxide eluting with hexane–ethyl acetate (gradient

elution, 9:1–1:9, v/v) afforded a fraction of pure 2,5-

di(octyloxy)-1,4-bis(4'-pyridylethinyl)benzene (20 mg,

yield 0.9%) and fractions containing mixtures of 2,5-

di(octyloxy)-1,4-bis(4'-pyridylethinyl)benzene and side-

product 1,4-bis [2',5'-dioctyloxy-1'-4@-pyridylethinyl)-

benzyl]butadiyne (altogether 300 mg, yield 13.9%).

¹H NMR (CDCl₃): ꢀ = 0.85 (6 H, t, —CH₃), 1.53–1.23

[20 H, m, —(CH₂)₅—], 1.83 (4 H, m, —OCH₂CH₂—),

4.01 (4 H, t, —OCH₂—), 7.01 (2 H, s, H_Phe), 7.35 (4 H, d,

H₂), 8.59 (4 H, d, H₃) ppm. ¹³C NMR (CDCl₃): ꢀ = 14.07

(—CH₃), 22.56, 26.05, 29.24, 29.30, 29.68 [— (CH₂)₅—],

¹H NMR (CDCl₃): ꢀ = 7.36–7.41 (8 H, m, H_Phe), 7.53–

7.57 (4 H, m, H_Pheand H₃), 8.52 (2 H, s, H₅), 8.66 (2 H, d,

H₆) ppm. ¹³C NMR (CDCl₃): ꢀ = 86.98 (—C

ꢀ

C—),

94.08 (—C

ꢀ

C—), 122.24 (C_Phe

—

ꢀ

—), 123.27 (C_Phe),

125.51 (C_Phe), 128.49 (C_Phe), 129.16 (C₃), 131.93 (C₅),

132.58 (C₄), 149.23 (C₆), 155.69 (C₂) ppm. MS (CI):

calculated m/z = 356 (M), found m/z = 357 (M 1) ,

1

178(1/2 M) g mol

IR (KBr): ꢁ = 2216 (m, disub-

stituted —C

ꢀ

C—) cm ¹. UV/VIS (CHCl₃): ꢂ_max

(log") = 268 (4.59), 291 (4.70), 306 (4.71) nm. Analysis:

C₂₆H₁₆N₂(356.41) calculated C 87.62, H 4.52, N 7.86;

found C 87.17, H 4.67, N 7.85%.

1,4-Bis(4'-pyridylethinyl)benzene (M5a). 4-Bromo-

pyridine was prepared from 4-bromopyridine hydro-

chloride (3 g, 19 mmol) by treatment with 4 N aqueous

sodium hydroxide (10 ml). The resulting emulsion was

extracted twice with diethyl ether, the ether solutions

were dried over magnesium sulphate and the solvent was

removed immediately before use.

^(—CꢀC^(——^O),^C9^H2.2^C9^H(—^—C⁾ꢀ^,C⁶—^9.5),⁶113.68 (C_Phe

—ꢀ

—),

M5a was prepared and puriﬁed analogously to M1,

replacing the 3-bromopyridine by 4-bromopyridine (2 g,

12 mmol). Vacuum sublimation (150°C, 4 Â 10 ³mbar)

yielded a white substance, yield 150 mg (13%), m.p.

169–171°C.

31.78

(OCH₂—), 90.32

2

116.85 (C_Phe), 125.40 (C_2'), 131.42 (C_1'), 149.77 (C_3'),

153.87 (C_Phe-OR) ppm. MS (CI): calculated m/ z = 536

(M), found m/ z = 537 (M H) , 425 (M H C₈H₁₇) ,

¹H NMR (CDCl₃): ꢀ = 7.36 (4 H, dd, H_2'), 7.54 (4 H, s,

312 (M H 2 C₈H₁₇) g mol ¹. UV/ VIS (1,4-diox-

H_Phe

ꢀ = 88.80 (—C

125.48 (C_2'), 131.01 (C_Phe), 131.91 (C_Phe), 149.85

¹³C NMR (CDCl₃):

ane): ꢂ_max(log") = 237 (4.34), 307 (4.49), 316 (sh, 4.44),

373 (4.36) nm. Analysis: C₃₆H₄₄N₂O₂(536.76) calcu-

lated C 80.60, H 8.21, N 5.22; found C 80.09, H 8.46, N

5.66%.

^{), 8.60 (4}ꢀ^HC^,—^dd),^,9^H3.1⁾5^p(^p—^mC^.

ꢀ

C—), 122.87 (C_1'),

3'

(C_3') ppm. MS (EI): calculated m/ z = 280 (M), found

m/z = 280 (M) , 174 (M

ꢀ

—Ph) , 140 (1/2 M , 126

1

(ꢀ

—Ph—ꢀ

) g mol

1,4- Bis(2',2@-bipyridine-4@-ylethinyl)-2,5-di(octade-

cyloxy)benzene (M6). 4-Bromo-2,2'-bipyridine (640

mg, 2.72 mmol), 1,4-diethynyl-2,5-di(octadecyloxy)ben-

zene (862 mg, 1.3 mmol), Pd(PPh₃)₄(63 mg, 0.053

mmol, 2 mol%) and CuI (12 mg, 0.053 mmol) were

dissolved to a degassed solution of diisopropylamine

(15 ml) and THF (60 ml) under an Ar atmosphere. The

mixture was heated at reﬂux for 24 h. After cooling to

room temperature, it was added dropwise to 400 ml of

vigorously stirred methanol. After stirring for 15 min, the

precipitate was collected by ﬁltration, dissolved in THF

(40 ml), passed through a 3 ml plug of silica gel and then

poured once more into methanol. The collected solid was

puriﬁed through column chromatography on silica gel

eluting with n-hexane–THF (2:1, v/v) and by reprecipita-

tion of its THF solution into methanol. A bright yellow

solid was obtained, yield 1.14g (90%), m.p. 104–106°C.

¹H NMR (CDCl₃): ꢀ = 0.85 (6 H, t, —CH₃), 1.19–1.86

[64 H, m, —(CH₂)₁₆—], 4.04 (4 H, t, —CH₂O—), 7.03 (2

H, s, H_Phe), 7.31 (2 H, t, H_5@), 7.37 (2 H, dd, H_5'), 7.81 (2

H, ddd, H_4@), 8.38 (2 H, d, H_3@), 8.52 (2 H, s, H_3'), 8.64 (2

(w, C_arylH), 2222 (m, —^{. I}C^Rꢀ^(KC^B—^r))^:, ^ꢁ15⁼8³7⁰a⁷n⁵d^a1ⁿ5^d38³⁰(m³⁹,

—C

C—, —C = N) cm ¹. UV/ VIS (1,4-dioxane):

ꢂ_max(log") = 318 (4.61), 336 (4.52) nm. Analysis:

C₂₀H₁₂N₂(280.33): calculated C 85.71, H 4.29, N

10.00; found C 85.35, H 4.77, N 9.54%.

The synthesis of M5a and metal complexes therefrom

has already been reported.²¹M5a was described as a

yellow–orange powder, which leads us to the suspicion

that the product contained impurities with diacetylene

substructures which are generated by oxidative coupling

reactions.

2,5-Bis(octyloxy)-1,4-di(4'-pyridylethinyl)benzene

(M5b). 4-Bromopyridine was liberated from 4-bromo-

pyridine hydrochloride as described before. 4-Bromo-

pyridine (0.5 g, 3 mmol) and 2,5-di(octyloxy)-1,4-

diethinylbenzene (0.4 g, 1 mmol) were dissolved in

degassed diisopropylamine (2 ml) and toluene (10 ml).

Tetrakis(triphenylphosphine)palladium (12 mg, 0.02

mmol) and copper(I) iodide (4 mg, 0.02 mmol) were

J. Phys. Org. Chem. 2000; 13: 112–126

116

U.-W. GRUMMT ET AL.

Table 1. Mean degree of polymerization (DP), number- and weight-average molecular weights (M_n, M_w) and polydispersity

index (M_w/M_n) of polymers obtained from samples prepared with various reaction times

ꢀ

M_n

1

M_w

Code

R

DP

Reaction time (h)

M_n(g mol

)

M_w(g mol )

LP1

LP2

LP3

LP4

LP5

LP6

AP1

AP2

AP3

AP4

H

ꢁ4

10

13

6

11

19

ꢁ4

19

10

25

3

48

24

48

5

48

24

48

1250^a

5332

6907

4660

—

C₈H₁₇

C₁₈H₃₇

H

C₁₂H₂₅

C₁₈H₃₇

13130

13460

23800

23350

30160

—

31750

11940

50570

2.46

1.95

5.11

2.70

1.97

—

2.57

1.45

2.80

8541

15260

1362^a

12310

8181

29790

a

Elemental analysis, calculated with two bromo endgroups.

H, d, H_6@), 8.67 (2 H, H_6') ppm. ¹³C NMR (CDCl₃): 14,09

(CH₃—), 22.67, 26.09, 29.28, 29.35, 29.41, 29.68, 31.91

[—(CH₂)₁₆—], 69.72 (—CH₂O—), 90.43 and 92.67 (—

The preparation procedures of the other polymers have

been published previously.¹³In this work some polymers

of the same type were investigated which vary in the

chain length.

C

ꢀ

C—), 113.90 (C_Phe—C

ꢀ

C—), 117.19 (C_Phe—H),

121.14 (C_3@), 123.11 (C_3'), 123.94 (C_5@), 125.18 (C_5'),

132.48 (C_4@), 136.93 (C_4'), 149.13 (C_6@), 149.22 (C_6'),

153.89 (C_Phe-OR), 155.61 (C_2@), 156.28 (C_2') ppm. MS

(CI): calculated m/z = 971 (M), found. m/z = 972

Mean degree of polymerization. The mean degrees of

polymerization (DP) for the polymers LPx and APx

(Scheme 1) were calculated from the average molecular

weights M_n. The molecular weights of the soluble

polymers were determined by gel permeation chromato-

graphy in THF, using a UV detector, and an RI detector,

and were calibrated with respect to polystyrene standards.

The alkoxy-substituted polymers are completely soluble

in usual organic solvents. The number-average molecular

weights of the poorly soluble polymers LP1 and AP1

were estimated through the bromine content from the

elemental analysis with the assumption that bromine is at

either end of the polymers. These values are approximate

ones; small contents of acetylene end-groups were

observed for both poorly soluble polymers by IR

spectroscopy. For the soluble polymers acetylene end-

groups were excluded by NMR spectroscopy.

(M 1) , 719 (M C₁₈H₃₇) , 485 ( 1/2 M) , 465(M

2 C₁₈H₃₇) , 87 g mol ¹. IR (KBr): v = 3055 (w, C_aryl

—

H), 2918 and 2850 (vs, CH₂— and CH₃—), 2214 (s,

disubstituted —C

ꢀ

C—), 1394 (m, CH₃—), 1279 (m,

C_Aryl—OR) cm ¹. UV/VIS (CHCl₃): ꢂ_max(log") = 286

(4.46), 310 (4.46), 380 (4.48) nm. UV/VIS (1,4-dioxane):

ꢂ_max(log") = 285 (4.59), 310 (4.57), 319 (4.52), 379

(4.49) nm. Analysis: C₆₆H₉₀N₄O₂(971.46) calculated C

81.60, H 9.33, N 5.76; found C 81.40, H 9.62, N 5.66%.

Polycondensation

Poly[5,5'-(1@,4@-diethinylene-phenylene)-2,2'bipy-

ridylene] (LP1). 1,4-Diethinylbenzene (350 mg,

2.78 mmol), 5,5'-dibromo-2,2'-bipyridine (872 mg,

2.78 mmol), Pd(PPh₃)₄(34 mg, 0.029 mmol, 1.0 mol%)

and CuI (13 mg, 0.065 mmol, 2.3 mol%) were added to a

degassed solution of triethylamine (3 ml) and toluene

(75 ml). After stirring the reaction mixture under reﬂux

for 3 h, it was poured into an excess amount of methanol.

The precipitate was collected by ﬁltration and washed

repeatedly with methanol. The resulting yellow solid was

extracted with hot toluene using a Soxhlet extractor to

separate the toluene-soluble fraction, which was used

for the spectroscopic investigations. The insoluble

Mean degrees of polymerization (DP), mean molecular

weights (M_n, M_w) and polydispersity indices (M_w/M_n) are

given in Table 1.

Apparatus, spectroscopic procedures and data

evaluation

Melting-points were measured on a Bu¨chi 530 melting

apparatus. Mass spectrometry was performed using

chemical ionization with H₂O vapour as gas on a Finnigat

Mat SSQ 710 instrument. ¹H and ¹³C NMR spectra were

taken on a Bruker DRX 400 and a Bruker AC 250

spectrometer, respectively. Gel permeation chromato-

graphic molecular weight determinations were made at a

ﬂow-rate of 1 ml min ¹in THF on Jasco UV 975 and RI

part was dried in vacuum. Yield 0.70 g (90.6%), M_n=

1

1250 g mol

.

IR (KBr): v = 3293 (w,

ꢀ

C—H), 2216 (w, —C

ꢀ

C—), 2105(vw, —C

ꢀ

C—H), 1700 (m, —C=N—), 836

(vs, p-disubstituted benzene) cm ¹. Analysis: (C₂₀H₁₀

N₂)_n(278.31)_ncalculated C 86.31, H 3.62, N 10.06;

found C 74.34, H 3.55, N 8.15, Br 12.08%.

930 instruments with SDV columns with 10³, 10⁴and

6

˚

10 A pore sizes and with polystyrene as standard. The "

values were determined in CHCl₃and/or 1,4-dioxane (for

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

117

Table 2. Absorption maxima (ꢂ_a) molar absorptivities (ꢃ), ¯uorescence maxima (ꢂ_f), quantum yields (Φ_f) and lifetimes (t) or mean

lifetimes (<t>) of (a) model compounds and (b) linear and angular polymers in toluene at room temperature

(a)

Φ_f^c

t^c(ns)

1

Code

R

ꢂ_a(nm)

"^b(l mol ¹cm

)

ꢂ_f(nm)

BPB^a

M1

M2

M3a

M3b

M3c

M3d

M4

M5a

M5b

M6

—

322

343

351

389

388

390

305

318

373

381

62000

48900

58000

49400

49000

47000

48000

52000

40700^d

25000

30400

349

377

381

430

431

345

348

419

424

0.79

0.83

0.95

0.76

0.94

0.89

1

0.05

0.49

0.87

0.90

0.68

0.67

0.69

0.55

1.15

1.20

1.12

<0.02

0.51

2.39

2.22

—

OC₈H₁₇

OC₁₂H₂₅

OC₁₈H₃₇

—

OC₈H₁₇

OC₁₈H₃₇

(b)

Φ_f^g

<t>^g(ns)

1

Code

R

DP^e

ꢂ_a(nm)

"^f(l mol ¹cm

)

ꢂ_f(nm)

LP1

LP2

LP3

LP4

LP5

LP6

AP1

AP2

AP4

H

ꢁ4

10

13

6

11

19

ꢁ4

19

25

357

413

406

420

421

329

387

386

—

388

454

456

452

454

456

389

446

445

0.61

0.62

0.67

0.55

0.48

0.45

0.57

0.62

0.38

0.70

0.65

0.70

0.75

0.55

0.42

0.9

OC₈H₁₇

OC₁₈H₃₇

H

(24200)

33000

30000

41000

40000

20900

25000

29000

OC₁₂H₂₅

OC₁₈H₃₇

1.18

a

BPB = 1,4-bisphenylethinylbenzene.

^bSolvent chloroform, Æ10%.

c

Æ10%.

^dIn dioxane.

e

Mean degree of polymerization,

^fPer mole of repeating unit, solvent chloroform.

g

Æ20%.

HPLC, Baker) on a Perkin-Elmer Lambda 19 UV/VIS–

NIR spectrometer. A Nicolet Impact 400 spectrometer

was used for IR measurements. Elemental analysis was

performed on a Leco CHNS-932 Automat.

The solvents toluene and ethanol used for the spectro-

scopic investigations were of spectroscopic grade

(Uvasol, Merck). The ﬂuorescence of ﬂuid parafﬁn oil

(Merck) used for polarization measurements was less

than 1% of the ﬂuorescence of the investigated model

compounds or polymers under the same experimental

conditions.

Absorption spectra at room temperature and at low

temperatures were recorded on a Lambda 16 spectro-

photometer (Perkin-Elmer) equipped with an NKT

Kryostat. PMMA cells were used for low-temperature

absorption measurements (near 77 K). Mostly, the optical

quality of the low-temperature glasses is better in PMMA

cells than in quartz cells, probably owing to the greater

ﬂexibility of PMMA.

Fluorescence and phosphorescence emission and

excitation spectra and steady-state ﬂuorescence polariza-

tion spectra were measured with an LS50B luminescence

spectrometer (Perkin-Elmer).

Fluorescence quantum yields were calculated relative

to quinine sulphate (purum, Fluka) in 0.1 N H₂SO₄(pro

analysi, Laborchemie Apolda) used as a standard

(Φ_f= 0.55) according to Ref. 22. The absorbance at the

excitation wavelength was kept below 0.05 for the

samples and the reference.

Fluorescence kinetics were investigated with a CD900

time correlating single photon counting spectrometer

(Edinburgh Instruments). The excitation source was a

hydrogen-ﬁlled nanosecond ﬂashlamp giving an instru-

ment response pulse of 1.3 ns FWHM. Polarizers with a

vertical orientation on the excitation side and a 55°

(magic angle) orientation on the emission side were used

to avoid polarization effects. Decay curves were

accumulated until 10⁴counts in the maximum with at

least 10³occupied channels. In order to determine

rotational relaxation times directly from kinetic measure-

ments, the decay curves were measured with a parallel

and a perpendicular oriented analyzer. These measure-

ments were performed with the highest possible time

resolution of the multi-channel analyzer (6 ps per

channel).

To calculate the ﬂuorescence lifetime and the rota-

J. Phys. Org. Chem. 2000; 13: 112–126

118

U.-W. GRUMMT ET AL.

Values of w²larger than 1.3 were not accepted. The

ﬂuorescence kinetics of the model compounds measured

at the magic angle is always singly exponential and

independent of the emission wavelength.

The ﬂuorescence kinetics of the oligomers and

polymers (magic angle) show deviations from ﬁrst-order

decay and, in some cases, a dependence on the

wavelength of the ﬂuorescence detection. In these cases

the mean ﬂuorescence lifetime <t> is calculated as

P

<t> = a_it_i/ a_i, where a_irepresents the amplitude of

the ith decay component or, in the case of a time-resolved

emission scan, the intensity in the steady-state ﬂuores-

cence spectrum.

Figure 1. Absorption and ¯uorescence spectra of the

unsubstituted oligomer LP1 (dashed line) and the related

model M3a (solid line) in toluene at room temperature

RESULTS AND DISCUSSION

Linear models and oligomers without alkoxy

substitution

tional relaxation time from the decay curves, the Level 1

(up to four exponentials) and Level 2 (spherical rotor

anisotropy) packages implemented in the Edinburgh

Instruments software were used. (The analysis makes use

of the iterative reconvolution technique and the Mar-

quardt ﬁiting algorithm.) Plots of weighted residuals and

of the autocorrelation function and values of reduced

residuals w²were used to judge the quality of the ﬁt.

The absorption of the unsubstituted linear model

compounds (M1, M2, M3a) is characterized by longest

wavelength absorption maxima in the region ꢂ_a= 320–

1

350 nm and molar absorptivities " > 40000 l mol ¹cm

[Table 2(a)].

The bathochromic shift of the spectra increases with

Table 3. Rate constants of ¯uorescence, determined by Φ_f/t and by the Strickler/Berg equation [k_f(SB)] and rate constants

[k_nr= (1 Φ_f)/t] of radiationless deactivation

(a)

1

Code

R

Φ_f/t (ns

)

k_f(SB)^b(ns

)

k_nr(ns )

BPB^a

M1

M2

M3a

M3b

M3c

M3d

M4

M5a

M5b

M6

—

1.2

1.4

1.0

0.31

0.25

0.07

0.44

0.05

0.09

<0.09

>4.5

1.0

0.83

0.91

0.89

0.52

0.50

0.52

0.8^c

0.68

0.27

0.34

—

1.4

OC₈H₁₇

OC₁₂H₂₅

OC₁₈H₃₇

—

OC₈H₁₇

OC₁₈H₃₇

0.82

0.74

0.89

>0.25

0.96

0.36

0.41

0.05

(b)

1

Code

R

DP

<Φ_f/t> (ns

)

k_f(SB)^b(ns

)

k_nr(ns )

LP1

LP2

LP3

LP4

LP5

LP6

AP1

AP2

AP4

H

ꢁ4

10

13

6

11

19

ꢁ4

19

25

1.6

0.89

1.0

0.78

0.64

0.82

1.1

0.63

0.53

—

1.0

OC₈H₁₇

OC₁₈H₃₇

H

0.26

0.38

0.34

0.46

0.45

0.49

0.24

0.27

0.54

0.51

0.64

0.69

1.0

1.3

0.48

0.32

OC₁₂H₂₅

OC₁₈H₃₇

a

1,4-bis-phenylethinyl-benzen.

^bWith " from solvent chloroform.

c

Measured in EtOH.

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

119

Figure 3. Spectra of the angular model compound M4 in

EtOH: absorption and ¯uorescence (F) spectra at room

temperature and absorption, phosphorescence (P) and

phosphorescence excitation (PE) spectra at 77 K

Figure 2. Absorption and ¯uorescence spectra of the model

compound M2 in EtOH at 293 and 80 K

the number of the arylene units, i.e. with the extension of

the aromatic system, as expected. The transitions are

dipole allowed (pp*) and polarized essentially parallel to

the long molecular axis as revealed by semi-empirical

calculations.

The ﬂuorescence of these compounds is characterized

by high quantum yields Φ_fand lifetimes t less than 1 ns.

Figure 1 shows the absorption and the ﬂuorescence

spectra of the linear model compound M3a and of the

short-chain (soluble) oligomer LP1, dissolved in toluene

at room temperature. The coincidence of the absorption

and the ﬂuorescence excitation spectra of the model

compounds M1 and M3a is perfect and still satisfactory

for the oligomer LP1.

The ﬂuorescence spectra, especially of the models but

to a lesser extent also with the oligomers, show a more

pronounced vibrational structure than the absorption

spectra. Such spectral behaviour is known for di- and

higher aryleneethylenes and aryleneethinylenes. It is

explained by the more rigid structure of the molecules in

the S₁state with respect to torsional motion around the

formal single bonds. According to the Imamura–Hoff-

mann rules,²³and also the NEER principle,²⁴aromatic

rings connected by a single bond become coplanar in the

excited state with a considerably increased bond order.

The deactivation rate constants at room temperature are

given in Table 3. The ﬂuorescence rate constants k_f

calculated by the Strickler/Berg equation are smaller than

the k_fvalue, calculated by Φ_f/t in all cases. Obviously,

the transition moments of the emission exceeds those of

the absorption, and the molar absorptivity " which is used

in the Strickler/Berg equation for the description of the

emission process gives too small an oscillator strength for

the emission. This is in full accord with shallower

torsional minima in the S₁state. The same observation

was made with distyrylstilbene and PPV derivatives.²⁵

A decrease in temperature should lead to an increasing

population of the lower torsional modes of the most

stable S₀and S₁geometries. Consequently, the absorp-

tion spectra show a strong temperature dependence with

the model compound M2 in EtOH as an example (Fig. 2).

While the structure of the ﬂuorescence spectra at room

temperature becomes clearer at low temperatures, the

absorption spectra show a pronounced change. There is a

bathochromic shift of the absorption maximum of 30 nm,

and the structure shows mirror symmetry to the

ﬂuorescence. At low temperature the Stokes shift is very

small, which indicates that the Franck–Condon states and

the equilibrated states have essentially the same geometry

in both the ground and the S₁states.

The short-chain soluble linear unsubstituted oligomer

LP1 shows nearly the same absorption and ﬂuorescence

maxima as the corresponding model compound M3a

(Fig. 1). Obviously, the relevant chromophore system of

the unsubstituted oligomer is already realized in this

model compound. We assume that the coplanar and

perpendicular orientation of the rotating arylene units

have nearly the same probability, which causes an

effective segmentation. Rotation of the aromatic rings

limits the number of coplanar rings to three as a statistical

mean, thus preventing stronger long-wavelength absorp-

Figure 4. Fluorescence and ¯uorescence excitation spectra

of the unsubstituted angular oligomer AP1 (solid line) and

the related model M5a (dashed line) in EtOH at 77 K

J. Phys. Org. Chem. 2000; 13: 112–126

120

U.-W. GRUMMT ET AL.

The ﬂuorescence and the ﬂuorescence excitation

spectra of AP1 and those of the model M5a in EtOH at

77 K are shown in Fig. 4. One may conclude from a

comparison of the spectra that the linear model M5a is an

appropriate model at least for the angular oligomer. This

means that conjugation in the oligomer chain may be

distorted mainly by torsion around the s-bond connecting

the pyridine rings of the bipyridyl moiety.

Semi-empirical calculations (INDO1/s after AM1

geometry optimization) of differently deﬁned repetition

units as a function of the number of repetition units are in

full accord with M1 and M5a being the most reasonable

model compounds for the linear and angular polymers,

respectively. Because of known shortcomings of these

methods, and because a more detailed ab initio study is in

progress (A. Go¨ller and U.-W. Grummt, to be published),

we will not go into details of these results. The absorption

and emission spectra and the ﬂuorescence rate constant,

k_f, of the linear model M1 closely resemble those of 1,4-

bisphenylethinylbenzene (BPB) and M5a [cf. Table

3(a)]. Aza-substitution in the 4,4'-position of the aromatic

rings of M5a merely leads to a faster radiationless

deactivation process.

Figure 5. Absorption and ¯uorescence spectra of the

substituted linear polymer LP4 (dashed line), the linear

oligomer LP5 (dotted line) and the related model compound

M3d (solid line) in toluene at room temperature

tion. Obviously, there is no geometrical relaxation in the

S₁state which would lead to a more extended and

coplanar conjugated system because this should result in

a signiﬁcantly larger Stokes shift.

The oligomer shows a smaller ﬂuorescence quantum

yield and a shorter decay time in comparison with the

model. Again, the rate constant of ﬂuorescence, deter-

mined by Φ_f/t (Table 3), is nearly the same as for M3a,

supporting the assumption that M3a can be regarded as

model for the oligomer. The enhanced radiationless

deactivation can at least in part be due to intersystem

crossing caused by the terminal bromine substituent. At

77 a weak phosphorescence was found.

Alkoxy-substituted linear models, oligomers and

polymers

Alkoxy substitution at the phenylene ring of the model

compounds and polymers has a pronounced effect on the

electronic structure of all compounds. The alkoxy

substituents are electron donors which enhance the

electron density in the ethinylene groups and hence

increase the conjugation along the molecule.²This leads

to a bathochromic shift of the longest wavelength

absorption band of about 40 nm and a decrease of the

absorptivity. The ﬂuorescence spectra are concomitantly

red shifted. An additional absorption band appears at ca

330 nm. Essentially the same effects of alkoxy substitu-

tion were found with PPV and distyrylbenzene deriva-

tives.

Absorption and ﬂuorescence data of three differently

substituted linear models M3b–d are given in Table 2(a).

The photophysical data and the spectra are essentially

identical. They show that the length of the alkyl chain, at

least for longer chains in solutions, has no inﬂuence on

the photophysical properties in solution, as expected.

The absorption coefﬁcient of the longest wavelength

band is smaller in comparison with the unsubstituted

model M3a. This indicates a smaller transition moment

which also leads to the larger natural ﬂuorescence lifetime

1/k_f. Again the ﬂuorescence quantum yield remains high.

Thus, alkoxy substitution introduces no additional deac-

tivation processes. The k_fvalues calculated from the

spectra (according to the Strickler/Berg equation) and

quantum yield and lifetime (Φ_f/t) are given in Table 3(a).

Angular model compounds and oligomers with-

out alkoxy substitution

The photophysical data of the angularly linked compound

M4, the model M1 and the angular oligomer AP1 are also

given in Table 2. A comparison of M4 with the oligomers

AP1 reveals that M4 cannot be regarded as model

chromophore for AP1. AP1 shows a remarkable batho-

chromic shift of both the absorption and the emission

maxima and exhibits a higher ﬂuorescence quantum yield

and a longer decay time. The absorption of M4 may be

regarded as a superposition of two tolan units [tolan:

ꢂ_a= 295 nm, " = 29 000 l mol ¹cm ¹, "(M4) ꢂ 2 Â "

(tolan)]. Absorption and emission spectra of M4,

recorded at room temperature and at 77 K, are shown in

Fig. 3. While the ﬂuorescence is still weak even at 77 K,

an intense phosphorescence could be detected, indicating

an efﬁcient intersystem crossing deactivation channel of

the S₁state.

The ﬂuorescence spectrum of AP1 overlaps consider-

ably with the absorption spectrum. It is not independent

of the excitation wavelength. Similar effects were found

for the substituted angular polymers and will be discussed

below.

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

121

absent in the model compounds. Hence, the explanation by

impurities or products is neither supported nor excluded.

(Puriﬁcation of the polymers from trace amounts of side-

products is much more difﬁcult! It cannot be excluded that

traces of diethinylidene moieties are present in the

polymer backbone owing to oxidative coupling during

synthesis.) Further investigations to clarify this question

are necessary.

The ﬂuorescence quantum yields and lifetimes of the

oligomers and polymers are reduced by ca 30% relative to

the model compounds as a result of faster non-radiative

deactivation of the polymers (cf. Tables 2 and 3). The

ﬂuorescence rate constants Φ_f/t are essentially the same as

those of the corresponding model compounds. The lower

absorptivities of the polymers are at variance with this

result because they should also lead to smaller Strickler/

Berg radiative rate constants k_f(SB) (Table 3). The molar

absorptivities are determined per unit weight of the

repetition unit as usual. Probably, determination of the

absorption coefﬁcients per unit weight of the correspond-

ing model compound would be more reasonable.

Figure 6. Absorption and ¯uorescence spectra of the

substituted angular oligomer AP2 (dashed line), angular

polymer AP4 (dash-dotted line) and related models M5b

(solid line) and M6 (dotted line) in toluene at room

temperature

The relation between the values and their explanation are

the same as in the case of unsubstituted models.

The absorption and the ﬂuorescence spectra of the

substituted linear model compound M3d, the linear

substituted oligomer LP4 and the polymer LP5 are

presented in Fig. 5. Evidently, there is a moderate

bathochromic shift of the ﬂuorescence between the model

compound and the oligomer (DP ꢂ 4). A larger degree of

polymerization (DP = 20) leaves the emission wave-

length nearly unchanged. The p-system is somewhat

larger than that of the model compound M3d, by

constrast with the unsubstituted M3a and LP1. Elec-

tron-donating alkoxy substituents increase the conjuga-

tion length. A second effect is the weakening of the

330 nm transition relative to the long wavelength

absorption with increasing degree of polymerization.

The ﬂuorescence spectra of the oligomers and polymers

exhibit smaller bandwidths at room temperature than the

absorption spectra, but they are essentially unstructured.

In general, the ﬂuorescence excitation spectra coincide

with the absorption spectra, thus the most common

criterion of the purity of ﬂuorescent substances is fulﬁlled.

Small differences do exist in the long-wavelength tail of

the spectra. Frequently, the absorption spectra show a less

steep edge and sometimes a long weak tail, which is absent

in the excitation spectra. The origin of this tail is not yet

clear. Usually, such ﬁndings indicate non-ﬂuorescent

impurities. With the bipyridyl-containing polymers var-

ious reaction side-products are conceivable,²⁶some of

which absorb at longer wavelengths: complexes with

traces of metals, oxidation products, protonated bipyridyls

and radicals. Metal complexes of bipyridylethinylenes

have been described.¹²We measured the UV–visible

spectra of new Ru complexes of models and polymers

Alkoxy-substituted models and angular polymers

Figure 6 shows the absorption and ﬂuorescence spectra of

the angular polymers AP2 and AP4 and of the

corresponding model compounds M5b and M6.

The spectra of these model compounds are not very

different from those of the linear model compound M3d.

They show the essential spectral features of the angular

polymers. In comparison with the linear polymers,

increasing chain length affects the spectra in a different

way. The absorption spectra of the angular polymers are

less red shifted with respect to those of their models and

the short-wavelength transition retains the same intensity

as the long-wavelength band. Both effects hint at a more

effective hindrance of the p-conjugation in the angular

polymers because of non-planar geometries present

already in the s skeleton due to rotations around the

pyridyl–pyridyl single bond. This should also be the

reason for the unusual ﬂuorescence behaviour of the

angular oligomers and polymers. The ﬂuorescence and

ﬂuorescence excitation spectra depend on the excitation

and emission wavelengths, respectively. This becomes

more evident at lower temperature. The angular polymers

should be better described as a multi-chromophore

system rather than a molecule showing multiple ﬂuores-

cence. In the case of segmented polymers with consider-

able conformational freedom, the existence of different

absorbing and emitting units is readily conceivable.

Especially the angularly linked polymers may exist in a

large manifold of conformers by contrast with their linear

counterparts which might be a tentative explanation. In

particular, helical structures may be formed which might

give rise to pp-stacking interaction in sandwiched

showing broad absorption bands in the region of 450 nm

2 27

and emission quantum yields of the order of 10

.

Although all these side-products may also be formed with

the model compounds, such long-wavelength tails are

chromophors.²⁸

However, this interpretation has to be

J. Phys. Org. Chem. 2000; 13: 112–126

122

U.-W. GRUMMT ET AL.

Figure 8. The vv and vh traces of the ¯uorescence kinetics of

M3a in toluene at room temperature together with the

excitation pulse pro®le (I); channel width = 6 ps

Figure 7. Absorption (solid line), ¯uorescence (dashed line)

and ¯uorescence excitation (dotted line) spectra, excitation

(dotted line) and emission (dashed line) anisotropy spectra

(solid, dashed lines) and ¯uorescence lifetime (diamonds) of

the linear polymer LP6 in toluene at room temperature

polymers were measured at room temperature using the

polarizer and analyzer of the instrument. The ﬂuores-

cence kinetics of the model compounds measured at the

magic angle are always singly exponential and indepen-

dent of the emission wavelength. The ﬂuorescence

kinetics of the oligomers and polymers (magic angle)

show deviations from the ﬁrst-order decay and depend in

some cases a on the wavelength of the ﬂuorescence

detection. An example (LP6) is given in Fig. 7. The

emission decay of the angular LP2, for instance, can be

supported by further investigations. From the comparison

of the low-temperature spectra, we conclude that M5b

represents one subchromophore unit of the angular

polymers.

Fluorescence kinetics

Fluorescence kinetics of the model compounds and the

Table 4. Lifetimes (t), rotational relaxation times (r) and emission anisotropies (r) of polymers and model compounds in toluene

and paraf®n at room temperature

c

d

e

f

Code

DP a^a

Solvent

t (ns)

ꢄ_calc^b(ns)

ꢄ_exp(ns)

r_calc

r_exp

LP1

ꢁ4

34.6

11

130

19

164

ꢁ4

32.2

10

64.4

19

153

25

201

—

11.3

—

13.6

—

Toluene

Parafﬁn

Toluene

Parafﬁn

Toluene

0.38

0.37

0.75

0.62/0.95

0.55

ꢁ1.62

ꢁ238

12.3

1800

36.6

ꢁ0.276

ꢁ0.339

0.320

0.340

0.335

0.17

0.32–0.29

0.27–0.23

0.33–0.30

0.23–0.20

LP5

LP6

AP1

AP3

AP2

AP4

M2

Toluene

Parafﬁn

Toluene

Parafﬁn

Toluene

Parafﬁn

Toluene

0.42

0.44

0.72

ꢁ2.45

ꢁ360

15.3

2250

55.3

8110

96

0.290

0.340

0.325

0.340

0.335

0.340

0.336

0.16

0.34–0.18

0.15

0.30

0.16

0.27

0.9

0.65/1.35

1.18

0.15–0.12

Toluene

Parafﬁn

Toluene

0.69

0.65

0.55

0.17

25

0.25

0.17

0.26

0.34

0.068

0.332

0.106

0.067

0.11

0.068

0.325

0.13

M3a

M3b

M3d

Toluene

1.15

0.25

0.061

0.063

0.079

0.087

13.6

—

13.6

Toluene

Parafﬁn

1.12

1.11

0.25

37

0.063

0.33

0.09

0.33

a

˚

Long molecular half-axis in A (second values).

^bCalculated by r_calc= 4Zpa²b/3kT, Z(toluene) = 0.6cP, Z(parafﬁn) = 88cP, b = short half-axis.

c

Calculated from polarized kinetic measurements.

^dCalculated by r_calc= 0.34/(1 t/ꢄ_calc).

e

Calculated by r_calc= 0.34/(1 t/ꢄ_exp).

^fCalculated by r_exp= (I_vvGI_vh)/(I_vv 2GI_vh) from steady-state ﬂuorescence spectra.

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

123

described by two exponentials with 0.71 Æ 0.08 and

1.46 Æ 0.18 ns with relative yields of the faster compo-

nent increasing smoothly from 62 to 83% in the wave-

length region from 350 to 420 nm.

r

_exp(1 t/ꢄ_calc) = 0.34 Æ 0.02. Similar values have been

found by us for substituted distyrylbenzenes and have

also been reported for a number of prolate stilbenes³⁰and

phenyloxazoles.³¹

Reasons why the the limiting anisotropy may deviate

from the theoretical value r₀= 0.4 (fundamental aniso-

tropy of the S₀–S₁transition) have been discussed by

Kawski.³²One possibility is a different orientation of the

absorption and emission transition moments in the

molecule. We determined the angle a between the

absorption and the emission transition moments as 18°

according to the equation cos²a = (3r₀1)/5.³³Although

the stilbene and phenyloxazole derivatives investigated

by Kawski and the arylene ethinylenes in this work have

different structures (sp²-and sp-hybridisized carbon

atoms in the chain, respectively), they do show the same

fundamental anisotropies. Possibly the arylene ethiny-

lenes with the bipyridyl units in the chain lose the linear

structure of the ground state upon excitation to the S₁

state. Concerning the geometry of the excited subchro-

mophores, contradictory results have been reported. For

acetylene a transoid structure has been calculated for the

S₁state.³⁴Hirata et al.³⁵have presented strong experi-

mental evidence in favour of a linear and coplanar

diphenylacetylene in the ﬁrst excited state (see also Ref.

36).

The other subchromophore 2,2'-bipyridine becomes

asymmetric in the S₁state according to time-resolved

Raman experiments and ab initio calculations.³⁷Detailed

quantum chemical calculations for our systems are in

progress (A. Go¨ller and U.-W. Grummt, to be published).

For all model compounds in toluene and parafﬁn the

emission anisotropies calculated from the Perrin equation

with r₀= 0.34 and with the different rotational times

agree well with the values from steady-state measure-

ments. In the case of the oligomers and polymers, the

situation is more complicated: the measured emission

anisotropies are mostly smaller than the calculated values

and they are not constant over the ﬂuorescence spectrum

(see Fig. 7).

One reason for the smaller anisotropies could be the

inadequate determination of the rotational volumes,

especially of the alkoxy-substituted oligomers and of

the cisoid angular polymers. Other reasons are depolar-

ization mechanisms due to additional molecular motions

which are not described by the simple rotational model

used above and due to energy transfer processes. Energy

transfer should take place in both the linear polymers and

the angular polymers as calculated below (see Table 5).

With rigid linear polymers intramolecular energy transfer

along the chain alone should not alter the direction of the

transition moment and hence should not diminish the

emission anisotropy. However, energy transfer together

with transversal or torsional vibrations might cause the

smaller anisotropies observed in the low-viscosity

solvent toluene. Torsional vibrations are assumed to be

responsible for the devation of prolate molecules in low-

Emission anisotropy

The emission anisotropy gives valuable information

about the directions of the transition moment of

absorption and emission, rotational relaxation of mol-

ecules and energy transfer among molecules. The steady-

state ﬂuorescence polarization of two heteroarylene

ethinylene polymers has been reported.²⁹We performed

polarized steady-state measurements with all models and

polymers in solvents of different viscosity at room

temperature and also time-resolved anisotropy measure-

ments with selected compounds. An example of experi-

mental vv and vh traces is given in Fig. 8 together with

the excitation pulse.

The ﬂuorescence anisotropy of all compounds is non-

zero even in the low-viscosity solvent toluene owing to

the rod-like shape of the molecules with the orientation of

the transition moments parallel to the the long axis and

the short ﬂuorescence lifetimes. Table 4 gives experi-

mental (exp) and calculated (calc) rotational relaxation

times and emission anisotropies of polymers and model

compounds in toluene and parafﬁn oil. Rotational

relaxation times were measured only for the model

compounds in toluene. Rotational relaxation times r_calc

were calculated using the Stokes–Einstein equation for

all compounds. The effective rotational volumes were

assumed to be represented by a rotational ellipsoid with

the long axis equal to the length of the rod-like models or

polymers. The diameter of the linear or angular bipyridyl

unit in the aromatic plane was taken as the short axis

because semi-empirical calculations predict the possibi-

lity of nearly free rotation around the acetylene–

(hetero)arene single bonds. The molecular dimensions

of the models and of the repetition units of the polymers

were obtained from simple molecular mechanics calcula-

tions (MM3) neglecting the alkoxy substituents. To

obtain the dimensions of the polymers the corresponding

repetition units were multiplied by the degree of

polymerization. The results are presented in Table 4.

It is obvious that the rotational relaxation time of the

model compounds in toluene, calculated from the mol-

ecular dimensions with the Stokes–Einstein equation, are

in very good agreement with those determined from the

polarized ﬂuorescence kinetics. From this we conclude

that the simple geometric model is suitable for a

reasonable description of the depolarization of rod-like

molecules.

From the calculated rotational relaxation times and the

experimental steady-state emission anisotropies of the

model compounds in the high-viscosity solvent parafﬁn,

we determined the limiting emission anisotropy r₀=

J. Phys. Org. Chem. 2000; 13: 112–126

124

U.-W. GRUMMT ET AL.

Table 5. (a) Data for FoÈ rster transfer along polymers in toluene and radiationless deactivation and (b) data for monomer `donors'

[cf. Table 2(a)] and `acceptors' in toluene at room temperature

(a)

a

1

c

4d

1

I_DA(10 ¹¹cm⁶mol

)

k^2b

R₀(A)

r (A)

k_et^e(10¹¹

s

)

k_nr^f(10⁹s

)

˚

Polymer

Model

LP1

LP6

AP1

M3a

M1

M3d

M5a

2.2

4.6

2.4

4

47.6

48.2

56.3

43.3

22.4

17.5

15.5

10.5

15.5

10.5

15.5

10.5

7.4

6.5

9.9

9.4

1.9

1.4

3.0

1.0

2.0

1.0

1.3

3.24 (t)

0.0625 (c)

3.24 (t)

0.0625 (c)

3.24 (t)

0.0625 (c)

AP2

AP4

M5b

1.0

0.48

0.32

0.69

38.7

29.4

(b)

^g(l mol ¹cm

)

1

Model compound

R

Φ_f,D

t

_D(ns)

"

A

M1

—

OC₁₈H₃₇

—

0.83

0.76

1

0.49

0.87

0.67

0.55

1.12

0.51

2.39

48900

49400

48000

40500

25000

M3a

M3d

M5a

M5b

OC₈H₁₇

a

I_DA= [ F_D(ꢂ)"_A(ꢂ)ꢂ²dꢂ]/[ F_D(ꢂ)/ꢂ²dꢂ] with absorption spectrum "_A(ꢂ) and ﬂuorescence spectrum F_D(ꢂ) from the polymers.

^bk²= (cosΦ_DA3cosΦ_DRcosΦ_AR)²; (t) transoid conformation, (c) cisoid conformation.

c

R₀⁶= 8.8Â10 ²⁸k²Φ_f,D/n⁴I_DA

.

^dEstimated from model calculations (MM3).

k_et= 1/t_D(R₀/r)⁶; energy transfer in one direction.

e

^fData from Table 3.

^gFrom chloroform as a solvent.

viscosity solvents from the Perrin equation, resulting in

smaller rotational relaxation times. In highly viscous

solvents the prolate molecules behave according to the

Perrin equation.³⁸Thus, in the high-viscosity parafﬁn oil

the torsional vibrations are suppressed and the theoretical

anisotropy approaches the experimental value.

A different situation is met with the angular polymers,

which may exist in various geometries. In the limiting

case of all-transoid geometry of the bipyridine moieties

the transition moments of the chromophores in the chain

are parallel to each other and the long molecular axis is

large, resulting in high rotational relaxation times and

high anisotropies. This case seems to be approximately

realized in parafﬁn oil. The long aliphatic chains of the

high viscous parafﬁn possibly promote all-transoid

structures of the polymers.

In the limiting case of all-cisoid geometries, the

transition moments of the chromophores in the chain

are randomly oriented and the rotational relaxation time

of the globular polymer is not so large. As a consequence,

both molecular rotation and energy transfer should

contribute to the depolarization of the ﬂuorescence. This

case seems to be realized to a larger extent in the low-

viscosity solvent toluene.

length dependence of the ﬂuorescence anisotropy is

observed, as expected for one-component systems.

However, the polymers are characterized by a distribu-

tion of the chain length. In contrast to the models, the

ﬂuorescence kinetics are not singly exponential and vary

within the ﬂuorescence spectrum. From the observed

increase in the mean decay times with the emission

wavelength, a decrease in the emission anisotropy has to

be expected. However, this effect alone cannot explain

the measured values.

Hennecke et al.³⁹reported a decrease in the anisotropy

with the emission wavelength for poly(p-phenyleneviny-

lene) in a polystyrene ﬁlm, which they explained by the

existence of various emitting species characterized by

different lengths of excited segments. We observed this

effect in diluted liquid solution where energy transfer

along the polymer chain is much more probable than

intermolecular transfer between different chains. The

emission wavelength dependence should be also a

consequence of this energy transfer. The larger the

number of energy transfer steps becomes between

absorption and emission of a photon, the larger should

be the depolarization. From energetic reasons the most

probable number of energy transfer steps should be larger

for red than for blue photons, and hence the anisotropy

becomes lower at the long wavelength tail of the

ﬂuorescence spectrum.

The decrease of the anisotropy with the emission

wavelength is primarily observed with the linear poly-

mers. With all model compounds no emission wave-

J. Phys. Org. Chem. 2000; 13: 112–126

CONJUGATED POLYMERS OF BIPYRIDINE AND DIETHINYLENEBENZENE

125

Energy transfer

compounds 1,4-bis(3'-pyridylethinyl)benzene and 1,4-

bis(4'-pyridylethinyl)benzene, respectively, including

their alkoxy-substituted analogues. Whereas the absorp-

tion maximum is still slightly bathochromically shifted

with an increase in the mean degree of polymerization

from 4 to 20, the ﬂuorescence band remains essentially

unchanged. Possibly the emissive excited state wave-

function is less delocalized than that of the ground state.

The model compounds exhibit ﬂuorescence quantum

yields close to unity and short ﬂuorescence decay times

of around 1 ns. Fluorescence anisotropy and rotational

relaxation times are consistent with the Stokes–Einstein

equation and the Perrin equation. The absorption and

emission spectra of the polymers and their radiative rate

constants determined by ﬂuorescence quantum yield and

lifetime and according to the Strickler/Berg equation

show a conjugation length of one to two repetition units.

The conjugation along the chain is stronger in linear than

in angular polymers and stronger in alkoxy-substituted

than in unsubstituted polymers. In angular polymers at

least two different emitting segments were found.

The shortened mean lifetimes and the reduced ﬂuores-

cence quantum yields of the oligomers/polymers indicate

an additional radiationless deactivation channel which is

attributed to energy migration along the chain. Rates of

energy transfer calculated for linear and angular polymers

correlate with rates of radiationless deactivation.

The experimental limiting emission anisotropy of the

model compounds agrees well with that obtained from

the Stokes–Einstein equation and the molecular dimen-

sion in low-viscosity solvents. Deviations are observed in

high-viscosity solvents and with the polymers in all

solvents. The emission anisotropy becomes wavelength

dependent with the polymers.

As a result of model calculations, energy migration

along the polymer chain occurs in both the linear and

angular polymers. Evidently, this contributes to the

decrease in the emission anisotropy. In the case of the

linear polymers, additional effects are essential to explain

the anisotropy reduction also with the linear polymers,

e.g. geometry changes in the excited states and low-

frequency bending vibrations which render the perpendi-

cular exciton component weakly allowed even with the

rigid linear polymers.

Energy migration along phenyleneethinylene polymer

chains and transfer to incorporated acceptors were

investigated by ﬂuorescence quenching experiments by

Swager et al.⁸The same group investigated two polymers

with related poly(phenyleneethinylene) structures,⁹and

the all-para system was found to exhibit more efﬁcient

energy migration than the more electronically localized

analogue which contains meta linkages. Energy migra-

tion along rigid-rod polymer chains has also been

discussed by Liu.⁴It was described by an energy hopping

model with a rate derived from the dipole–dipole energy

transfer mechanism.

The rates of energy transfer from the initially excited

segment to the ﬁrst neighbouring segment in our

polymers estimated by the Fo¨rster equation for some

oligomers and polymers are given in Table 5.

The overlap integral was calculated using the shapes of

the absorption and ﬂuorescence spectra of the polymers;

however, the more realistic molar absorptivities of the

corresponding models were used instead of those of the

polymers. Donor quantum yields and lifetimes were also

taken from the corresponding model compounds, which

were regarded as the donor parts in the polymer without

energy transfer deactivation. Whereas the values of the

overlap integrals are of the same order of magnitude for

the different polymers, the parameter k²depends strongly

on the mutual orientation of the donor and acceptor

transition moments.

In the linear polymers the maximum value k²= 4 is

assumed (neglecting the result of an angle of 18°). For the

angular polymers only the two limiting cases of a transoid

and a cisoid conformation of the donor–acceptor pairs in

the same plane were investigated. Interestingly, the small

k²values of the cisoid conformations are compensated by

shorter distances between donor and acceptor, which

results in comparable energy transfer constants k_etof

transoid and cisoid conformations. Generally, the rates of

energy transfer are two to three orders of magnitude higher

than the ﬂuorescence rate constants, and many energy

transfer steps can procede before a photon is emitted. A

comparison with the rate constant of radiationless

deactivation k_nrshows a correlation with k_et. A high k_et

increases the probability of reaching a trap in the polymer

chain which leads to radiationless deactivation.

A picosecond time-resolved anisotropy study is in

progress together with theoretical investigations in order

to obtain a more detailed quantitative interpretation of the

emission anisotropy.

CONCLUSIONS

Alternating linear rigid-rod and angular oligomers and

polymers consisting of 2,2'-bipyridine and diethinylene-

benzene units, although fully conjugated according to

their structural formulae, are segmented to p-systems

which comprise three (hetero)aromatic rings and two

ethinylene units. Their spectroscopic and photophysical

behaviour can be reasonably described by the model

Acknowledgements

This work was funded in part by the Deutsche For-

schungsgemeinschaft via the ‘Sonderforschungsbereich

196, Physik und Chemie Optischer Schichten.’ U.-W.G.

acknowledges ﬁnancial support by the Fonds der

Chemischen Industrie.

J. Phys. Org. Chem. 2000; 13: 112–126

126

U.-W. GRUMMT ET AL.

Trans. 2 1984; 1293–1301; (c) Polin J, Schmohle E, Balzani V.

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Article Doi

Conjugated polymers with 2,2′-bipyridine and diethinylenebenzene units: Absorption and luminescence properties

DOI: 10.1002/(SICI)1099-1395(200002)13:2<112::AID-POC218>3.0.CO;2-0

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