Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 127

+ -

+e - +e - +e - +e -

-e - -e - -e - -e -

+ + -

-

Kekulé structure singlet biradical
30%

(a)

3.137 Å

(b) (c)

Figure3.50 (a)Amphotericfour-stageredoxbehaviorandsingletbiradicalcharacterintheneutralstateof IDPL
estimated by CASSCF(2,2)/6-31G//RB3LYP/6-31G** and broken symmetry UB3LYP/6-31G** calculations. 1-D
chain with a slipped stack arrangement in the crystal of Ph 2 - IDPL . (b) top view, (c) side view. (b,c Reproduced
withpermissionfrom[74b].CopyrightWiley-VCHVerlagGmbH&Co.KGaA.)

Recently, in the bisphenalenyl neutral system, the singlet biradical state was found to be a significant
resonance form (Figure 3.50a center). 74 The degree of the contribution in IDPL was estimated to be 30 %
by CASSCF(2,2)//6-31G//RB3LYP/6-31G** and broken symmetry UB3LYP/6-31G** calculations. 74b This
large singlet biradical character highly influences the formation of intermolecular interactions and thus crys-
tal packing structure. Ph 2 - IDPL , having two phenyl groups in the central six-membered ring (Figure 3.51a),
forms a 1-D chain in the crystal with a slipped stack arrangement and an average

distance of 3.14 A,
which is substantially shorter than the van der Waals contact of carbon atoms (3.4 A) (Figure 3.50c). 74b
The overlap mode of the phenalenyl rings is quite similar to that of TBPLY in the crystal or the solution
state at low temperature, maximizing the SOMO-SOMO interaction between the molecules (Figures 3.7,
3.10, 3.50b). The band dispersions along the

π - π

stacking direction are estimated to be 0.54 eV (valence
band) and 0.51 eV (conduction band) by EHT. These values reflect the extremely close

π - π

contacts,
which are comparable to the band dispersions of the molecular conductors. The HOMO-LUMO gap of
Ph 2 - IDPL is 1.15 eV by electrochemical methods. The room temperature conductivity measured by the
two-probe method using a compressed pellet is 10 − 5 S/cm with an activation energy of 0.3 eV. This con-
ductivity is the highest value among structurally well defined, single component hydrocarbons in a neutral
state. 74b,67,68
A bisphenalenyl derivative connected with a naphthoquinoid structure, Ph 4 - NDPL , possessing 50 %
singlet biradical character, was also prepared and isolated as air-stable crystals (Figure 3.51a). 74c The
absorption band attributed to intramolecular HOMO-LUMO transition was observed at 865 nm, which is
significantly red-shifted from the corresponding absorption band of Ph 2 - IDPL (746 nm) (Figure 3.51b).
This result indicates that the HOMO-LUMO gap of Ph 4 - NDPL , with larger biradical character, is smaller

π - π