were attached to the phenol groups to give compounds 1i
and 1j in 28% and 15% yield, respectively. By incorporation
of dendrons, the undesired intramolecular rotation can be
minimized with the retention of the electronic factor in the
conjugate core. The detailed synthetic procedures and
increase the distance of the neighboring molecules and could
reduce the ACQ effect. Meanwhile, the long or branched
side chains may also induce significant disorder in the
packing arrangement, which could weaken the emission.
Therefore, an optimum size of the side chain is expected to
give the most favored molecular packing arrangement for
solid-state emission. In this work, the n-C6H13 chain was
found to have an optimal effect on the solid-state emission,
as molecule 1e shows sharp and strong emission with the
highest quantum yield of 60.3%. It is interesting to note that
compounds 1i and 1j, which have branched alkyl dendrons
as side chains, show very weak emission in the series. The
ΦF of 1i and 1j in the solid state are only slightly higher
than those in solution, indicating that though the aggregation
restricts IMR they are probably in an oil-like state because
of the enwrapping of the dendritic alkyl substituents, which
have better motions, vibrations, and rotations than 1a-1h
in the solid state. In fact, the melting point of 1i is only at
64-68 °C, and 1j is an oil at room temperature.
1
characterization data of H and 13C NMR, HRMS, or MS
and elemental analysis are shown in the Supporting Informa-
tion.
To assess the influence of side chains on the fluorescence
of the aromatic core, the synthesized molecules were
characterized by UV-vis spectroscopy and fluorescence
spectroscopy (Table 1). Dilute THF solution of 1a-j revealed
absorption peaks (λab), all at approximately 369 nm, and
emission peaks at nearly 476 nm, which suggests that the
incorporation of various side chains does not bring about
changes to the electronic structure of the conjugated core in
the solution. All compounds emit weakly in the diluted
solutions with the fluorescence quantum yields (ΦF) around
0.5-1.3%, but the ΦF are somewhat higher than the classical
hexaphenylsilole (HPS) (0.1%).5b
In an effort to understand the effect of the side chains on
the molecular conformation and packing arrangements, the
crystal structures of compounds 1a-f and 1h were deter-
mined. As can be seen from the single-crystal structures
shown in Figure 2 (the large packing arrangement images
of 1a-f and 1h are shown in Supporting Information), a
little structural change, the same core with different side
chains, brings obvious variety of molecular conformation and
packing arrangement in the solid state. When the chains are
short (1a and 1b), the conformations tend to be cis-geometric
structure (the two benzene rings are on the same side of the
nonplanar conjugated structure as the chromophore which
consists of three benzene rings and two double bonds). As
the length of the side chains is beyond three carbons, the
molecular conformations exhibit anti-geometric structures
and tend to be centrosymmetric structures. The packing
arrangements of these compounds are also different. The
molecules of 1e have strong head-to-face interactions
between the conjugated structures, which give so-called
J-aggregates11 extending on the long axis direction of the
molecule, which are generally considered to be good emitters.
The molecules of compounds 1a, 1b, and 1h have one
conformational molecule in the crystals like 1e, but they
exhibit different packing arrangements. The molecules of 1a
exhibit interlaced H-aggregates. The conjugation system of
1b is almost coplanar, and the plane of one molecule is
almost vertical on that of the neighboring molecule (the
dihedral angel θ ) 73.9°). In the crystal of 1h, the molecules
are packed in the form of two neighboring back to back
molecules as a unit. There are two conformational molecules
in the crystals of compounds 1c, 1d, and 1f, which exhibit
J-aggregates in more than one extending direction. In
addition, the CH/π hydrogen bond plays a crucial role in
the packing arrangement in the crystals and is also important
in terms of the conformations of these compounds. As an
example shown in Figure 2, a CH/π hydrogen bond (d1 )
2.63 Å) is formed between two phenyl rings in neighboring
molecules of 1e. A CH/π hydrogen bond (d2 ) 3.00 Å) is
Table 1. Absorption and Emission Characteristics of the
Samples in Solutiona and Amorphousb
λab [nm]
λem [nm]
ΦF [%]c
samples solution solution amorphous solution amorphous
1a
1b
1c
1d
1e
1f
1g
1h
1i
369
369
369
369
369
370
368
370
368
360
476
476
477
476
475
475
474
475
475
483
477
463
499
492
490
500
499
485
485
478d
1.0
1.2
0.8
1.1
1.2
1.1
1.3
1.1
0.5
0.9
29.9
21.8
44.8
13.0
60.3
19.6
19.2
15.7
1.4
1j
2.5d
a Measured in THF (10 µM). b Measured using a powder. c Absolute
quantum yield. d Measured using a thin film.
Compared to the weak fluorescence of all the compounds
in THF solution, the solid-state fluorescence spectra are
distinctly different. The amorphous power of 1c, 1e, and 1g
fluoresces at 499, 490, and 499 nm, which exhibits an
obvious red-shift compared to 1a (λem at 477 nm). The
emission peak of 1b shows an exceptional 14 nm blue-shift
compared to 1a. The emission efficiencies of 1a-j are all
significantly enhanced in the solid state because the aggrega-
tion positively restricts IMR, and the degree of enhancement
is strongly dependent on the structure of the side chains.
The conjugated core and the alkyl side chains play
different roles in the solid-state packing arrangement of these
molecules. Due to its highly conjugated structure, the π-π
stacking of the core could facilitate ordered stacking with
strong intermolecular interaction, and ACQ is generally
expected. The steric hindrance of the side chains could
(10) Shi, Z. F.; Chai, W. Y.; An, P.; Cao, X. P. Org. Lett. 2009, 11,
4394.
(11) Spano, F. C. Annu. ReV. Phys. Chem. 2006, 57, 217.
Org. Lett., Vol. 12, No. 19, 2010
4366
An, Peng
