Molecular Engineering of the Band Gap of π‐Conjugated Systems: Facing Technological Applications

Abstract

IntroductionThe control of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap of p-conjugated systems and hence of the band gap of the corresponding materials has been in the centre of the synthetic chemistry of functional p-conjugated systems for more than twenty years. Following the discovery of the metallic conductivity of doped polyacetylene, [1] conjugated polymers derived from heteroaromatic units such as polypyrrole [2] or polythiophene (PT) [3] emerged in the early eighties as an answer to the problems posed by the lack of stability of polyacetylene in atmospheric conditions. Owing to a unique combination of environmental stability, conductivity, moderate band gap and structural versatility, thiophenebased p-conjugated systems have progressively supplanted other classes of systems in both fundamental and technologically-oriented research.[4]Whereas band gap control has been a target already at an early stage of research on conjugated polymers, during the past two decades the field has undergone major changes in both its finalities and synthetic approaches. During the 1980-1990 period conducting polymers were essentially considered as possible alternatives for metals or metal oxides in bulk applications such as electrode Feature ArticleFor almost two decades, the search of an intrinsically-conductive organic metal has represented the major driving force for research on control of the band gap of extended p-conjugated systems. However, the emergence of the application of p-conjugated oligomers and polymers in field-effect transistors, light-emitting diodes, electrochromic devices and solar cells has introduced major changes in the chemistry of gap engineering. Besides controlled band gap, active materials for electronic and photonic applications must present appropriate absorption and/or emission properties, highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energy levels and charge-transport properties. The aim of this short review is to present an overview of the recent trends in this area in order to identify possible directions for future research. Consequently, while considerable research effort was invested in the optimization of the conductivity of doped conjugated polymers, the search for a zero-band-gap polymer capable of forming an intrinsically conductive material, namely a true organic metal, represented the ''Holy Grail'' for the chemistry of conjugated systems.[5]The turn of the nineties has been marked by a progressive decline of research on the bulk applications of conducting polymers, with the parallel emergence of research focused on the applications of conjugated systems in electronic and photonic devices such as field-effect transistors (FETs), light-emitting diodes (LEDs) and solar cells. [6][7][8] These applications, based on the electronic properties of the neutral semiconducting form of conjugated systems, have had a profound impact on the chemistry of these systems by providing new finalities. This new situation ...

Effect of the length of alkyl side chains in the electronic structure of conjugated polymers

“…Among the factors that can influence these properties, withdrawing or donating electrons from/to the main chain by a substituent is one the most important 14,19 . These effects can move the position of HDMD and LUMD levels, depending on the amount of charge in the main chain 19 . The alkyl groups can donate electrons by both induction and resonance 45 , thus causing changes in the electronic levels of the polymer.…”

“…These side chains are replaced by a methyl group 17,18 , so that the conformational search becomes easier. However, it is known from literature that the size of the alkyl side chains can influence the structural conformation and electronic properties of polymers 8,19 . This suggests that replacing an long alkyl side chain with a methyl group can be a risky approach, which could compromise the results obtained for theoretical studies of conjugated polymers.…”

Photoluminescent energy transfer from poly(phenyleneethynylene)s to near‐infrared emitting fluorophores

“…The highly efficient energy transfer 1 and exciton migration processes 2 in conjugated polymers can be exploited in various electronic applications [3][4][5][6][7][8][9][10][11] and in amplifying sensor responses. [12][13][14][15][16][17][18][19] Highly sensitive, amplified quenching of polymer emission has been accomplished with various quenchers in solution as well as in the solid state.…”

Improving the leakage current of polyimide-based resistive memory by tuning the molecular chain stack of the polyimide film

“…25) Five essential factors are known to affect the energy gap of a PI film: the resonance energy required for PI to transform from being aromatic to being quinoid (E res ), the energy of θ, the angle between the units of electrons along the conjugated backbone (E θ ), the energy from the introduction of electron-donating and electron withdrawing units into the conjugated main chain (E sub ), the energy of the difference in length between adjacent carbon-carbon bonds in a chain (E BLA ), and the energy of intermolecular interactions (E int ). [26][27][28] Among these factors, E int affects the microstructure but not the molecular structure and hence tunes the energy gap of the PI film. E int can be controlled simply through the PI film density using the dilution technique.…”