Packing directed beneficial role of 3-D rigid alicyclic arms on the templated molecular aggregation problem

Abstract

Subtle changes in molecular structure have been used to alter the molecular packing and optical properties of organic luminophores. Thus it is important to study simple and advantageous structural modification to overcome limitations of aggregation quenching of fluorescence. Planar conjugated organic compounds are unlikely to be used in OLED devices as they suffer with luminescence weakening due to p-p cofacial stacking and excimer formation in the solid state. To avoid such critical issues, doped device architecture for OLED devices has widely been adopted which actually complicates the device fabrication process. Thus an approach for delimiting these drawbacks using simple methodologies is highly desirable for cost effective OLED device fabrication. In this context, two 3-dimensional rigid arms have been introduced into a planar benzo[h]chromen-2-one core, which suffers from aggregation caused quenching, as peripheral substituents to tune the molecular packing and subsequently their optical properties. The resultant compounds 1 and 2 have been tested for non-doped and doped OLED device application. Compound 1 was found to be highly thermally stable with a decomposition temperature of 344 C. Single crystal XRD studies helped to elucidate that voluminous ring substituents are capable of eradicating co-facial p-p stacking by increasing the intermolecular distance and hence the luminescence in the solid state could be regenerated. This tuning of intermolecular distance between molecules helped to mitigate the close packed arrangement at a molecular level and also provided the bulk with the enhanced emission property. Electroluminescence from a pristine layer of compound 1 was possible. Thus an approach enabling planar luminophores to be used in OLED applications utilizing thin films of structurally engineered luminophores has been presented.

Role of Voluminous Substituents in Controlling the Optical Properties of Disc/Planar-Like Small Organic Molecules: Toward Molecular Emission in Solid State

“…The introduction of alkyl long chains and three-dimensional (3D) rigid cyclic systems into the flat molecular design has been utilized to prevent pi–pi stacking and to generate solid-state emission. 13,15 The alkyl substitution provides solubility to the molecular structure, and thus the spin-coating technique becomes feasible. Moreover, such a modification does not alter the optical properties of the central emitting core because of the noninvolvement nature of the sp 3 -hybridized aliphatic system into the conjugated emitting core.…”

“…There has been many design strategies introduced to prevent the face-to-face pi–pi stacking interactions, so that the solid-state emission could be generated in the flat luminophores. ,,− As far as the modern fluorophore design is concerned, the strategies capable of understanding the photophysical processes and developing real world applications from the conventional fluorophore are equally important as the twisted molecular design is not reliable for every application. The introduction of alkyl long chains and three-dimensional (3D) rigid cyclic systems into the flat molecular design has been utilized to prevent pi–pi stacking and to generate solid-state emission. , The alkyl substitution provides solubility to the molecular structure, and thus the spin-coating technique becomes feasible. Moreover, such a modification does not alter the optical properties of the central emitting core because of the noninvolvement nature of the sp 3 -hybridized aliphatic system into the conjugated emitting core.…”

Exploring an Emissive Charge Transfer Process in Zero-Twist Donor–Acceptor Molecular Design as a Dual-State Emitter

“…Many planar organic emitters suffer from an aggregation-caused quenching (ACQ) problem largely due to the face-to-face stacking and intermolecular interactions. In the last few decades, remarkable strategies have been developed and demonstrated by many researchers to deter close packing which include twist geometry, ,, enhanced intermolecular charge transfer transitions, and bulky substitutions. − However, so far there exists only limited reports where planar ACQ systems have been successfully tethered with solid-state emission property through innovative molecular parameter tuning strategies. − It is quite evident in the literature that structural planarity, conformational rigidity, and stacking modulator are the key requirements for a small molecule to be a dual-state (solution and aggregate) emitter . In this context, fundamental studies of new donor–acceptor (D–A) systems are also necessary to develop efficient organic emitters. − Organic systems developed using D–A design strategy are known for their interesting photophysical properties but have the drawback of the dark TICT (twist intramolecular charge transfer) state. − Twist conformation of D–A pairs is accompanied by the twist intramolecular charge transfer (TICT) states which limit their solution-state applications.…”

“…Out of these attracting properties, independency to print OLEDs on flexible substrates and on large area is a primary factor for growth and a big selling point of this technology . The extensive research since the first report of OLED devices in 1987 has led researchers to develop a huge library of organic emitters for OLED application, and efforts are still on to explore more emitter systems and design strategies. − To make OLEDs more energy efficient, both material design strategies and device technology have to be developed to a maximum extent.…”