Conjugated Polymers Containing Pendant Terpyridine Complexes as Photoactive Sensors
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Energy Transfer Quenching
"hv
A
D Scheme 1. Representation of energy migration in an extended conjugated polymer system.
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Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
Here we present an alternative approach to constructing new transition metal ion sensitive polymers. Our approach as illustrated in Scheme 2, is to prepare, luminescent, conjugated polymers containing terpyridine receptors in which a quenching center is created upon binding of a transition metal ion. The unique of this design is that pendant terpyridine receptors were connected through conjugated bridges to the polymer backbone. Previous designs generally incorporated the receptor units directly into the conjugated backbone. Terpyridine is an ideal receptor for binding transition metals as demonstrated by the well established coordination chemistry that has been observed [7,8]. In addition, transition metal terpyridine complexes contain a photoactive Metal to Ligand Charge Transfer (MLCT) excited state which has previously been shown to undergo photoinduced electron and energy transfer with conjugated polymers [8]. Based on model complexes, the Metal to Ligand Charge Transfer (MLCT) excited states formed between terpyridine and Ni2+, Fe2", Ru2 +, Re', and Os2" should all act as excitation traps for the exciton generated in conjugated polymers. Thus, binding of transition metal ions in solution would result in quenching of the polymer emission intensity. Furthermore, the terpyridine receptor groups were expected to have no significant effect on the photophysics of the conjugated polymer backbone compared to that of model polymer 4 shown in Scheme 3. Scheme 2. Synthesis of terpyridine-containing conjugated polymer and its coordinate reaction with transition metal ions.
I
/
N N
N--- M,
N .NN
OR OR
OR
MLm Br
/s
Br
R Pd(PPh3 )./CUl
I
-
RO
RO
2
3
Results and Discussion Synthesis of polymer precursor 1 is the key step for preparation of polymer 2. The palladium-catalyzed coupling of 1,4-diethynyl-2,5-dihexadecyloxybenzene and 2,5-dibromo-3terpyridine substituted thiophene 1 leads to polymer 2, as shown in Scheme 2. The polymer 2 is very soluble in common organic solvents such as tetrahydrofuran, chloroform and toluene. The molecular weight was measured by GPC to be Mn = 23,000 against polystyrene. The expected structure of the polymer was confirmed by 'H NMR, IR, UV and fluorescence spectroscopies. Detailed synthetic methods and structure determination of both the polymer 2 and polymer precursor 1 will be described elsewhere [9]. For the comparison, we also synthesized polymer 4 without terpyridine receptor group by using similar palladium-catalyzed coupling reaction.
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Scheme 3. Synthesis of conjugated polymer 4. OC36H33
C 12H-I5
C12H25
+
I-6\-I
OC16H331
/__
Pd(PPh 3)4
THF
S
S
H33S/ -
n
H33 C160
H33CI60
The UV-vis absorption and emission spectra of conjugated polymer 2 in THF solution are shown in Figure 1. Characteristic electronic absorption bands in high energy with max=228 and 334
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