PD Research Report for the 2017 year
Name（Research group） Sunna Jung, 鄭善牙（Hamura Research Group, Graduate School of Science and Technology）
Research Theme

Synthetic Study of High-Ordered Iptycene Derivatives Using Isobenzofuran
as a Reactive Platform

Research PeriodApril 1st, 2017 ~ March 31st, 2018

Research Results
Three dimensional π-conjugated molecules have been drawing substantial attentions among organic
chemists due to their intriguing structures, physical properties and potential applications as organic materials.
Focused on iptycene derivatives, their unique geometrical shapes, derived from the bicyclo[2.2.2]octatriene
system with aromatic rings in three blades, bring to the development of the high ordered iptycene derivatives invarious area. So far, we have been succeeded in three-directional polycyclic iptycene derivatives by developing
iptycene building block, tris-isobenzofuran (2). Due to the high reactivity of 10π-electaron systems with quinoidstructures, [4+2] cycloadditions with dienophiles following several steps of transformations progressedsmoothly to deliver tris-pentacenequinone (3) and tris-TIPS-pentacene (4a). In this report, further synthesis
toward parent pentacene skeleton was examined and physical properties of these novel 3D molecules werestudied.
OR
OO
O

O

R
R

O

O

RO

12

R

4: R =3OSi(i-Pr)3R

1. Synthesis of tris-pentacene
According to the previous report, tris-pentacenequinone 3 and tris-TIPS-pentacene 4 were preparedstarting from triptycene (1). Attempts toward pentacene derivatives with no substituent groups were examined,
therefore, reduction of ketones in tris-pentacenequinone 3 by sodium borohydride and following reductive
aromatization [Tin chloride and concentrated HCl] gave tris-pentacene 5 in 65% yield as purple solid. It is
unstable in air and light and can be stored on bench for several days under dark conditions. Comparing to theparent pentacene 6, which is obtained as blue solid, both pentacene derivatives 5 and 6 shows UV-vis absorption
at 577, 533, 497 nm in chloroform solution. Both 5 and 6 are insoluble in various organic solvents, such as
chloroform, dichloromethane, ethyl acetate, acetone, toluene, and hexane.
OOO1) NaBH 4O3OO2) SnCl2
conc. HClDMF65% (2 steps)

5Purple SolidUV-vis λabs (nm) : 577, 533, 497(in CHCl3)

cf.

Blue SolidUV-vis λabs (nm) : 577, 533, 497 (in CHCl3)

6

2. Physical properties of tris-isobenzofuran and tris-pentacene
The physical properties of tris-isobenzofuran 2 andtris-TIPS-pentacene 4 was investigated. Since both 2 and 4
electronic coupling effect between either isobenzofuran orpentacene subunits can be expected. According to the UV-OO

341 nmNormalized Abs

have rigid three-dimensional structures, through-spaceO346 nm

IBF-trimer (2)OIBF-monomer (7)visible spectra, weak homoconjugation effect was observedin THF, room temperature0

between isobenzofuran subunits in tris-isobenzofuran 2. UV-250

300350
Wavelenghth (nm)400

visible absorption spectra of two isobenzofuran derivatives,
tris-isobenzofuran 2 and isobenzofuran monomer 7, were measured in THF solutions right after the purification
under the Ar atmosphere. The absorption of 2 at 346 nm showed redshift of 5 nm relative to the absorption of 7
at 341 nm, indicating intramolecular electronic coupling between isobenzofuran moieties. According to theB3LYP-6-31G** level of calculations, the energy gap between HOMO and LUMO of isobenzofuran monomer
7 (4.07 eV) became smaller in tris-isobenzofuran 2 (3.81 eV) due to the electronic coupling. Both HOMO andLUMO of 7 split into two stabilized HOMOs/LUMOs and one destabilized HOMO/LUMO when they exist iniptycene derivative 2.
In addition to homoconjugation effect, intramolecular singlet fission in tris-pentacene 4 was studied
using transient absorption spectroscopy. Singlet fission is a spin allowed process that is a conversion of one
singlet excited state into two triplet excited states. This process makes it possible to develop solar cell devices
with better efficiency by generating more high-energy photons. Pentacenes are qualified to show singlet fission,
therefore, they are attractive compounds as potential photovoltaic materials. Since there are three pentacenesubunits in tris-isobenzofuran 4, intramolecular singlet fission was observed by measuring transient absorption
spectroscopy in dilute chloroform solution. After 5.3 picoseconds, triplet states are increasing which has longlife time (~ ns) with ~200 % of triplet yield. This result indicates that intramolecular singlet fission observed intris-pentacene 4 will bring to a new stage of photovoltaic materials or phosphorescent material.
S1

k1
T1
k2

S0
k1: (5.3 ps)-1
k2: long lifetime

Intramolecular Singlet Fission (S1+S0→2T1 )Triplet yield: ~200%

Conference Presentations(1) Sunna Jung and Toshiyuki Hamura, “Synthetic Study of High-Ordered Iptycene Derivatives Using
Isobenzofuran as a Reactive Platform”, 28th Symposium on Physical Organic Chemistry, 1A10 (Presentation),Fukuoka, Japan, 2017, September. (Japanese)(2) Sunna Jung and Toshiyuki Hamura, “Synthetic Study of High-Ordered Iptycene Derivatives Using
Isobenzofuran as a Reactive Platform”, 6th JACI/GSC symposium, E-03 (Poster), Tokyo, Japan, 2017, July.
(Japanese)