Designing stable open-shell organic materials through the modifications of the π-topology of molecular organic semiconductors has recently attracted considerable attention. However, their uses as an active layer in organic field-effect transistors (OFETs) are very limited, and the obtained hole and electron charge mobilities are around 10-3 cm2 V-1 s-1. Herein, we disclose the synthesis of two peri-fused materials, so-called tetracenotetracene (TT) and pentacenopentacene (PP), which have low band gaps of 1.79 and 1.42 eV, respectively. Their ground state natures have been investigated by different experiments including steady state absorption, electron spin resonance, superconducting quantum interfering device, and variable-temperature NMR along with DFT calculations. TT and PP have closed-shell and singlet open-shell structures in their ground state, respectively, and possess high stability. Their biradical characteristics were found to be 0.50 and 0.64. The origin of the open-shell character of PP is related to the concomitant opening of two tetracenes with the recovering of two extra aromatic sextets and a small HOMO-LUMO energy gap (gap <1.5 eV). Thanks to the high stability, thin film OFET devices could be fabricated. In TG-BC configuration PP shows a remarkably high hole mobility of 1.4 cm2 V-1 s-1, while TT exhibits a hole mobility of 0.77 cm2 V-1 s-1. In the configuration of BG-TC, ambipolar behaviors for both were obtained with hole and electron mobilities of 0.21 and 0.01 cm2 V-1 s-1 for PP and 0.14 and 0.006 cm2 V-1 s-1 for TT.