Page 84 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 61

version of this molecule. The first method for solubilizing pentacene
83
was proposed by Brown et al. The adopted strategy is based on pre-
paring a soluble pentacene precursor that is not a semiconductor but
can be converted to the active form upon heating at 140 to 220°C,
2
84
obtaining a mobility of 0.2 cm /(V · s). Similar synthetic approaches
have been proposed for obtaining soluble pentacene derivates, 85–87
and OTFT based on such molecules showed field-effect mobilities
−3
2
ranging from 8.8 × 10 to 0.89 cm /(V · s).
The preparation of soluble precursors is, however, synthetically
challenging and often a costly annealing process is required to obtain
high-order and semiconducting thin films. Moreover, even though
the thermal conversion processes have been claimed to be quantita-
tive, undetectable amounts of precursors or by-products can intro-
duce trap states in the films, reducing the mobilities. 88
Alternatively, pentacene can be structurally modified to achieve
better solubility through wet techniques, to obtain desirable supra-
molecular order in the solid state and to improve stability toward
oxygen and light. The seminal work of Anthony and coworkers has
shown that all these objectives can be simply achieved in a one-pot
reaction from 6,13-pentacenequinone introducing bulky silyl groups
separated from the acene by alkyne spacers at the 6,13-pentacene
position. 89–92
These novel bis-silylethynylated pentacenes exhibit several
remarkable features compared to the respective parent pentacene
molecule. First, the bis-silylethynylation, apart from increasing the
solubility, lowers the triplet and LUMO (Lowest Unoccupied Molec-
ular Orbital) energy of pentacene and thus enhances the oxidative
stability. Moreover, the sterically demanding substituents prevent
93
the dimerization through Diels-Alder reaction. The improved stabil-
ity of the TIPS pentacene (triisopropylsilylethynyl pentacene) was
demonstrated by its 50 times slower degradation in air saturated tet-
rahydrofurane solution compared to the unsubstituted compound.
In the dark, even oxygen-saturated solutions of TIPS pentacene did
not show considerable decomposition after 24 h. 93
Of all functionalized pentacene only the TIPS pentacene that
exhibits 2D π-stacking interactions yields high-performance thin-film
devices: in OTFT prepared by vacuum deposition a hole mobility of
2
0.4 cm /(V · s) was observed, while in a spin-coated device a mobility
2
6
of 0.17 cm /(V · s) and on/off current ratios of 10 were measured. 91
Notably the electrical measurements were performed in air at room
temperature, proving that the introduction of these substituents
improves the device stability.
The high mobilities observed in the TIPS pentacene were
explained by invoking the two-dimensional self-assembly of the aro-
matic moieties into π-stacked arrays that enhance the intermolecular
overlap. Indeed, TIPS pentacene does not adopt the typical pentacene