Chemistry Reference
In-Depth Information
indicates highly efficient ET in such dendrimers. The broad absorption spectrum
allows the thiophene dendrimers to be used as active materials in solar cells, as
discussed in the next section. This group also demonstrated the application of these
thiophene dendrimers as entangles photon sensor materials in a later report [94b].
As can be seen from the PA dendrimer, if the thiophene unit could be separated by
an acetylene group, steric crowdedness would be alleviated; and the structure could be
made more extended. In this respect, we have initiated a program of synthesizing
shape-persistent conjugated LH dendrimers using oligo-(thienylethynylene)s (OTEs)
as the building blocks [95]. OTE exhibited several features as ideal branches in LH
dendrimer [96,97]: (1) easily tunable lengths through facile chemical synthesis
approaches; (2) linear and rigid structures with minimum conformational flexibility;
(3) tunable absorption and emission behaviors. As an extension of our work on
truxene-based dendrimers, we have developed a series of dendrimers
29
(
G0
),
30
) (Figures 9.23 and 9.24), using OTEs of different lengths as
branching units [48]. The use of truxene unit allows the molecular weight to increase
rapidly with generation; and the OTE fragments allow the molecular dimension to
expand quickly. The second-generation dendrimer
(
G1
), and
31
(
G2
has the largest diameter (10 nm)
and the highest molecular weight (27 kDa) among all reported second-generation
dendrimers. To understand the photophysical properties of these dendrimer, model
compounds
31
) that represent the largest conjugation
lengths in each generation of dendrimer were also synthesized (Figure 9.25) [98].
The absorption and emission spectra of these compounds are shown in Figure 9.26.
All dendrimers showed two absorption peaks, one around 343 nm, assigned to the
peripheral units (resembling model compound
32
(
G0m
),
33
(
G1m
), and
34
(
G2m
), along with another one in the
longer wavelength region. By comparison with linear model compounds, the latter
peak was assigned to the longest OTE-truxene units in each dendrimer. The
absorptionmaxima of these peaks increased progressively toward longer wavelengths
due to increased conjugation length in higher generation. Upon excitation at 343 nm,
where unit
10
absorbs, nearly quantitative ET to the longest branches was observed.
The ET efficiencies were 96% for
10
, respectively. The
high ETefficiency can be ascribed to the designed energy gradient from the periphery
to the core, and the large overlap between the emission of the donor and absorption of
the acceptor. Significant “antenna effect” was observed, like that in the natural LH
complex; despite similar LQY, the emission intensity (per molecule) of
29
, 97% for
30
, and 98% for
31
31
was several
times higher than that of model compound
when excited at 343 nm.
We went on to address a key issue of shape-persistent conjugated dendrimers that
was seldom tested: towhat extent is the dendrimer “shape-persistent”? And what is its
real conformation? Energy minimization calculations did suggest a relatively flat
conformation; but because the rotation barrier around thiophene-acetylene bond and
truxene-thiophene bond is small, there may nonetheless be considerable freedom for
rotation. To answer these questions, we performed two characterizations. First, we
employed tapping mode atomic force microscopy (AFM) to investigate molecule
34
31
on mica substrates. Figure 9.27 shows the AFM image of
on mica surface at 3 nM
solution, in which many separated and randomly deposited spots were observed. It is
noteworthy that these spots were relatively uniform in width. Further dilution of the
31
