Chemistry Reference
In-Depth Information
Fig. 12 Two-photon absorption spectra of 38c (
circles
), 38d (
triangles
) and 39a (
squares, right
)
[
45
]. Reprinted with permission from [
45
]. Copyright 2007 John Wiley and Sons
b
p
p
to the
positions of 38e, and a similar increase in the
-
distance was also observed
p
p
with increasing temperature. A decreased
distance would improve the intermo-
lecular orbital overlap and strengthen the intermolecular bond interaction; however, it
would also weaken the intramolecular interaction by making unpaired electrons more
localized. As a result, the electronic structure of the 1D chain can be depicted by the
resonating valence bond (RVB) model as a superposition of a resonance balance
between intramolecular bonding (A) and intermolecular bonding (B) (Fig.
11c
)[
38
].
A naphthalene-linked bis(phenalenyl) 39a with even larger biradical character was
synthesized by Kubo et al
.
[
43
]. The HOMO-LUMO gap of this compound
was determined by cyclic voltammetry as 1.04 eV and the singlet-triplet gap was
estimated as 1,900 K by SQUID measurements, both smaller than that of 38d
(1.15 eV and 2,200 K), in agreement with its larger biradical character. Compound
39b without a
tert
-butyl bulky group was prepared to minimize the steric hindrance
and to study the intermolecular interaction in the crystalline phase; the packing of 39b
adopted similar stepped mode with 38d in the 1D chain and the intermolecular
bonding was stronger than the intramolecular one due to the spin-localized nature
on the phenalenyl moieties and can be more adequately described as muticenter
bonding [
44
].
Theoretical studies pointed out many intriguing potentialities of singlet biradical
systems, one of them being a high second hyperpolarizability, a quantity closely
related to two-photon absorption (TPA) capabilities. This prediction was later proved
by experimental results, which showed maximum TPA cross-section values up to
(424
-
130) GM at 1,500 nm for 39a,
which are comparable to similar TPA chromophores with strong donor or accepter
peripheral groups and are among the best for pure hydrocarbons without donor and
acceptor substituents, thus providing new insights into the design criteria of TPA
materials (Fig.
12a
)[
45
]. Moreover, balanced ambipolar charge transport of the thin
films of 38d was reported by Chikamatsu et al., presumably due to the amphoteric
redox properties and strong intermolecular communications. Notably, despite the
strong intermolecular interaction as discussed above, only moderate electron and hole
mobilities of up to 10
3
cm
2
V
1
s
1
were observed as a result of the amorphous
structure and poor crystallinity of the film [
46
]. However, these exciting results still
indicate a promising future for these biradicals in materials science.
64) GM at 1,425 nm for 38d and (890
