Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 128

(a) (b)

Figure3.51 (a)Chemicalstructuresand(b)solutionabsorptionspectraofPh 4 - NDPL (blackline)andPh 2 - IDPL
(grayline).(Reprintedwithpermissionfrom[74c].Copyright2007AmericanChemicalSociety.)

than that of Ph 2 - IDPL . The high stability of such hydrocarbons possessing contributions of singlet biradical
characters makes a complete contrast to most singlet biradicaloids, which can only be detected in matrixes
at low temperature due to their extremely high reactivity.
A phenyl-substituted derivative of BPLE (Figure 3.49), Ph 2 - BPLE , possesses a contribution of a singlet
biradical structure (14 %) and gives a charge transfer (CT) complex with F 4 -TCNQ (Figure 3.52). 74d Ph 2 -
BPLE exists in the charge transfer complex as a radical cationic state, Ph 2 - BPLE • + , which is represented
by two canonical structures (Figure 3.52b). Interestingly,

overlap of the phenalenyl moieties of
Ph 2 - BPLE • + is large, with an average distance of 3.34 A in a staggered stacking motif, indicating a
maximizing SOMO-SOMO interaction similar to PLY (Figures 3.52c,d). The band structure calculation
with EHT gives a relatively large dispersion (0.42 eV) for a half-filled band along the

π - π

π - π stacking

direction. The room temperature (290 K) resistivity cm with metallic-like behavior

down to 280 K. Below 280 K, the charge transfer complex shows a transition into a semiconductive state
as a result of bond formation between the phenalenyl and F 4 -TCNQ carbon atoms. 74d
The bisphenalenyl neutral hydrocarbon with singlet biradical character shows strong two-photon absorp-
tion (TPA) properties. The measurement of the TPA cross-section of Ph 2 - IDPL by the open-aperture
Z -scan method with a femtosecond optical parametric amplifier shows the large TPA cross-sectional value
of 5490 GM (Figure 3.53) utilized by resonance enhancement and sequential TPA process. 75 The widely
accepted guiding principles of the molecular design for large TPA is the introduction of electron donor
and acceptor groups to the ends of a

ρ RT is about 0 . 7

π -conjugated system as well as the extension of electronic commu-

nication through conjugation. Thus, the TPA experiment of the singlet biradical hydrocarbons based on

the bisphenalenyl system provides new insights into design criteria.
The electronic properties, including amphoteric multistage redox ability and strong intermolecular elec-
tronic communication, of Ph 2 - IDPL lead to ambipolar carrier transport properties similar to graphene. 4a,76
Organic field-effect transistors (OFETs) showing ambipolar properties attract considerable interest from the
viewpoint of fundamental science for organic semiconductors and also with respect to device applications
for light-emitting transistors and logic circuits. The OFET characteristics of Ph 2 - IDPL were measured in

π