Optical Limiting Characteristics of Dendrons and Dendrimers Incorporating Dithienylpolyene Moieties
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has an alkylthio substituent terminated with an OH group for attachment to 3,5-dihydroxybenzyl alcohol. The synthetic scheme for chromophore synthesis is illustrated in Figure 1. 1. BuLi
s
POC13
2.u
S
S
D~HOC"
3:BuI
I
2
1. NaBH 4. 2. PBr3 3. Bu3P
Br
1. BuLi
1. Ac20 2. POCI3 / DM •F HOC f
3.S 4
sý-S
_OH
- OH HOC0
PBU 'Br 3
NaOEt
j
Bus BuS
--
IS
OH
S""ý
NaOEt
2. THF / H30+
NaOEt 6 + 3--"O
Bus
iS~
HC
,S
-
OH
8
Figure 1. Synthesis of functionalized dithienylpolyene chromophores. SYNTHESIS OF DENDRONS AND DENDRIMERS The two chromophores (n=l, 7, and n=2, 8) whose syntheses are outlined in the previous section, were converted to dendrons for convergent dendrimer synthesis 2 by first converting the alcohol functionality to a bromide followed by condensation with 3,5-dihydroxybenzyl alcohol according to the methodology developed by the Frechet group 34' . The conversions were carried out in good yield (n=l dendron, 65%; n=2 dendron, 77%), and the products were quite soluble in common organic solvents and could be purified by standard column chromatography. The absorption characteristics of the dendrons compared to the precursor chromophores will be discussed below. The dendron syntheses are illustrated in Figure 2. Convergent approaches to dendrimer synthesis require that for each generation, the appropriate generation dendron be coupled to central core molecules with two or more functionalities. In our case to form model dendrimers, in order to examine the photonic properties as one progresses through the chromophore to dendron to dendrimer, we synthesized G-0 dendrimers by coupling the dendrons to bisphenol A. This yields model dendrimers containing four photonic-active chromophores. As in the dendron case, following Frechet
390
methodology, we were able to successfully couple both the n=l (87%) and n=2 (88%) dendrons to bisphenol A. Both dendrimers were readily processible, soluble in common solvents and could be purified by column chromatography. Structures were confirmed by MALDI. The dendrimer syntheses are illustrated in Figure 3. r
OH
S S BuS
HO n-
OH
1,2
SK 2C0 3 18-C.6
OH
Q
BusS
SBU
9: nal; 10:n,2
Figure 2. Synthesis of dendrons containing dithienylpolyene chromophores. POTENTIAL APPLICATIONS: OPTICAL POWER LIMITING VIA REVERSE SATURABLE ABSORPTION FROM PHOTOGENERATED CHARGE STATES As discussed above, preliminary investigation of the possibility of polaronic radical-cations or bipolaronic dications functioning as highly absorbing photogenerated charge species in optical limiting applications were inconclusive'. However, we have recently reviewed the possibilities of dithienylpolyene, as well as other substituted polyenes, as optical limiting materials operating by both reverse saturable absorption as well as two-photon absorption5 . Previously the photonicactive chromophore had been attached to a PMMA backbone (10% incorporation). It is unlikely that we could achieve percentage incorporation above 30% without losing processibility or observing chromophore-chromop
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