Synthesis, excited state dynamics, and optical characteristics of oligophenyl-based swivel cruciforms in solution and...

Oligophenyl-based swivel cruciforms are an amorphous class of materials for potential use in display applications. In this work, we describe the design, synthesis, and structural and optical properties of a group of chromophores with varying degrees of π-conjugation that produce blue emission and high photoluminescence quantum yields (PLQYs). The swivel cruciforms are branched complexes consisting of two arms, and the relative rotation of these arms is an important factor that determines the optical properties of these systems. The studies reveal that the physical size of the system, that is, the arms, and the presence and position of functional groups results in different degrees of sterical effects. This has a dramatic impact on the interplay between nonradiative and radiative decay of the excited state. For the least π-extended system, control of the excited state decay can be achieved by modifying the properties of the medium through an increase in the viscosity, which is demonstrated accordingly. The data shows that an intramolecular excited state is responsible for the swivel cruciform emission in solution. The character of this "aggregate" and its emission is not dissimilar to an excimer, and we therefore attribute it to an intramolecular excimer. In the solid state, a combination of intra- and intermolecular excimers are the most likely source of the emission. The data also shows that intra molecular excimers can produce a surprisingly high PLQY, when either properties of the medium facilitate this or when functional groups introduce steric hindrance, which subsequently prevents the nonraditive decay through conformational change.