C O M M U N I C A T I O N S
population of biradical species of tF-BDPI-2Y (0.4%) as compared
with BDPI-2Y (0.1%) in solution could be confirmed by the
observation of the enhanced ESR signal intensity for the light-
irradiated solution of tF-BDPI-2YD (see Supporting Information).
In conclusion, this study provides conclusive proof that the
thermally excited open-shell state with biradical character is
contributing to the ground state of tF-BDPI-2Y. Of course, the
electronic structure of tF-BDPI-2Y is somewhat different from that
of BDPI-2Y in which no dimerization reaction could be found.
However, a small increase in the population of the biradical species
of tF-BDPI-2Y should be considered to result in the formation of
the dimer form. Controlling the equilibrium between a closed-shell
quinoid state and an open-shell biradical state will also provide
significant progress in the field of biradical chemistry, and especially
in the field of π-conjugated delocalized biradical chemistry.
Moreover, the effects of the intermolecular interactions of the
delocalized biradicals on the electronic wave functions for the
cumulated systems have never been investigated in detail. The study
of their properties will allow for a better understanding of the factors
for spin interactions in π-conjugated delocalized biradicals and will
lead to the rational development of new molecular organic materials.
Figure 1. Perspective view of tF-BDPI-2YD with thermal ellipsoids (30%
probability). The hydrogen atoms and the solvents are omitted.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for the 21st Century COE Program from the Ministry
of Education, Culture, Sports, Science and Technology, Japan. We
also acknowledge Prof. Masahiro Yamashita (Tokyo Metropolitan
University) for the X-ray diffraction facilities.
Supporting Information Available: Details of DFT calculations,
experimental preparations for tF-BDPI-2YD and crystallographic data
(CIF), and ESR studies of tF-BDPI-2YD. This material is available
Figure 2. Absorption spectra of tF-BDPI-2YD in benzene (6.25 × 10-6
M) (a) before irradiation and (b) immediately after irradiation with 360 nm
light at room temperature, and (c) BDPI-2Y in benzene (1.25 × 10-5 M)
(right axis). Inset: Plot of absorbance at 610 nm of tF-BDPI-2YD kept in
the dark at 293 K after irradiation (1 min at 360 nm). All of the
measurements were carried out by using a 10 mm cell.
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