6,13-BIS-2,6-DIETHYLPHENYLPENTACENE
-0.1
0.1
0.3
0.5
0.7
0.9
1.1
1.3
320 370 420 470 520 570 620
WAVELENGTH (nm)
AB
SO
RB
AN
CE
(A)
NO LIGHT
5 SEC
100 SEC
200 SEC
300 SEC
420 SEC
570 SEC
660 SEC
750 SEC
840 SEC
1065 SEC
1200 SEC
1320 SEC
1440 SEC
1620 SEC
1740 SEC
1860 SEC
2040 SEC
2220 SEC
2400 SEC
ABSTRACTABSTRACT
A convenient, facile synthesis A convenient, facile synthesis of a series of stabilized acenes of a series of stabilized acenes that incorporate several different that incorporate several different aryl substituents located at aryl substituents located at different positions along the different positions along the acene backbone is described. acene backbone is described. These structurally modified These structurally modified acenes show vastly improved acenes show vastly improved solubility (processability) and solubility (processability) and greater photooxidative stability.greater photooxidative stability.
INTRODUCTIONINTRODUCTION
During the last few years, the During the last few years, the development of organic thin film development of organic thin film transistors (OTFTs) has attracted transistors (OTFTs) has attracted much interest because organic much interest because organic molecules can be deposited over molecules can be deposited over large surface areas and are large surface areas and are compatible with flexible plastic compatible with flexible plastic substrates[1].substrates[1].
Pentacene molecules have Pentacene molecules have been deposited on a wide variety been deposited on a wide variety of materials to build flexible of materials to build flexible electronics.electronics. Acenes including pentacene Acenes including pentacene has been utilized in photovoltaics, has been utilized in photovoltaics, organic light emitting diodes organic light emitting diodes (OLEDs) and other electronic (OLEDs) and other electronic applications.applications.Problems encountered include a Problems encountered include a lack of solubility and photo-lack of solubility and photo-oxidative instability.oxidative instability.
OUR APPROACH TO OUR APPROACH TO PERSISTENT PENTACENESPERSISTENT PENTACENES
The most reactive rings on pentacene are those The most reactive rings on pentacene are those indicated with arrows. We have prepared several indicated with arrows. We have prepared several pentacene derivatives in which these sites exhibit pentacene derivatives in which these sites exhibit varying degrees of steric hindrancevarying degrees of steric hindrance
CONCLUSIONSCONCLUSIONS Modified pentacenes show enhanced photo-oxidative stability and improved solubilityModified pentacenes show enhanced photo-oxidative stability and improved solubility Increasing the alkyl chain length at the ortho position of the phenyl substituents Increasing the alkyl chain length at the ortho position of the phenyl substituents increases the kinetic stability of the corresponding aceneincreases the kinetic stability of the corresponding acene 6,13-disubstituted pentacenes show greater photooxidative stability than 5,7,12,14-6,13-disubstituted pentacenes show greater photooxidative stability than 5,7,12,14-tetrasubstituted pentacenes. Thus, the center-most rings are most prone to oxidation tetrasubstituted pentacenes. Thus, the center-most rings are most prone to oxidation and and should be the focus of further synthetic efforts to prepare persistent acenes. should be the focus of further synthetic efforts to prepare persistent acenes.
FORMATION AND STABILITY STUDIES OF PERSISTENT ACENESFORMATION AND STABILITY STUDIES OF PERSISTENT ACENESIrvinder KaurIrvinder Kaur and Glen P. Miller and Glen P. Miller
University of New HampshireUniversity of New Hampshire
This work was supported under the Nanoscale Science and Engineering Centers Program of the National Science Foundation (Award # NSF-0425826)
5,7,12,14-Tetrasubstituted Pentacenes
6,13-Disubstituted Pentacenes
SYNTHETIC DETAILSSYNTHETIC DETAILS6,13-Disubstituted Pentacenes6,13-Disubstituted Pentacenes
5,7,12,14-Tetrasubstituted Pentacenes5,7,12,14-Tetrasubstituted Pentacenes
RESULTSRESULTSUV-Vis Data: IMPROVED PHOTOOXIDATIVE STABILITYUV-Vis Data: IMPROVED PHOTOOXIDATIVE STABILITY
Change in UV-Vis spectrum of structurally modified pentacene upon exposure to light
in ambient atmosphere
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ab
sorb
ance
(a.
u.)
7006005004003002001000Time (min)
(B)(A) (C) (D) (E)
(i) (ii)
4 hr7 min0 min 8 hr(i) (ii) (i) (ii) (i) (ii) (i) (ii)
PICTORIAL EVIDENCEPICTORIAL EVIDENCE
WHY SHOULD STRUCTURALLY MODIFIED WHY SHOULD STRUCTURALLY MODIFIED PENTACENES RESIST PHOTO-OXIDATION?PENTACENES RESIST PHOTO-OXIDATION?
Phenyl rings of the modified acene lie nearly perpendicular to the plane of the acene unit. The alkyl groups are expected to lie above and below the most reactive sites and therefore impede the approach of oxygen.
REFERENCESREFERENCES[1] J. Briggs and G. Miller Comptes Rendus Chimie, 2006, 9, 916.[2] W. Dehaen et al Synlett, 2005, 2, 217.
STABILITYSTABILITY SOLUBILITYSOLUBILITY
Comparison of several different structurally modified pentacene
Absorbance at different max of a single structurally modified pentacene with time
6,13-BIS-2,6-DIETHYLPHENYLPENTACENE
-0.1
0.1
0.3
0.5
0.7
0.9
1.1
1.3
0 5000 10000 15000 20000 25000 30000 35000
TIME (SEC)
AB
SO
RB
AN
CE
(A)
Abs(336)Abs(354)Abs(368)Abs(388)Abs(520)Abs(558)Abs(605)
N2-78 C 0 C,o o
NaI, NaH2PO2
N2, 1.5 hDark, 90-94%
HOOH
OH HO
RR R
R
RRR
R16 h, 52-55%
HOAc
OO
O O
OO
AlCl3,
4 h, reflux
O
PhOH
O
HO
O
Ph
O
O
O
O
O
H2SO4
4-5 h, 100o C 72%
R R
RRR R
RR
R=H, CH3
R
R
Li
O
O
+
Br
Br
Br
Br
NaI, DMF
120 C, 16h 90%
o
O
O
OH
OHR
R
N2 -78 C 0 C,o o
R
R
NaI, NaH2PO2
HOAc, N2, 1.5hDark, 88-92%
16 h, 50-62%
R R
RR
R=H, CH3, C2H5
R
R
L i
PENTACENE
ANTHRACENE
2
2
MECHANISTIC INSIGHTS…
Photo-oxidation of pentacene is believed to require two approaches by oxygen. First, triplet oxygen is converted to singlet oxygen via photoexcited pentacene. Second, singlet oxygen cycloadds across the acene in a [4+2] fashion. Sterically hindered pentacenes can gain kinetic stability by slowing either or both of these steps.
A
B
C
D E
