A geometric key for enhancing the radiative efficiency of ortho‐carboranyl luminophores: Indolocarbazole‐ and m‐terphenyl‐ortho‐carboranes

Abstract

In this study, we introduce two ortho‐carboranyl compounds, namely ICzCB and mTPCB, which exhibit different planarity in their appended π‐conjugated aromatic groups, to investigate the influence of geometric effects on the regulation of the radiative efficiency. Both the compounds are fully characterized, revealing distinct geometric differences (planarity) in their single‐crystal structures. While ICzCB presents strong ICT‐based emission in the solid (film) state, mTPCB displays dual emission in the high‐ and low‐energy regions that originates from locally excited (LE)‐ and ICT‐based processes, respectively. Quantitative analysis of the photophysical characteristics of the compounds shows that the ICT‐based radiative efficiency in ICzCB is higher than that in mTPCB. Consequently, these findings demonstrate that the planarity of the π‐conjugated aryl group appended on o‐carborane, which is represented by planar ICzCB and distorted mTPCB, is an important key to control the ICT‐based radiative decay efficiency.

Influence of Intermolecular Structural Effects on Radiative Efficiency in Xanthene-Based Carboranyl Luminophores

“…38−45 For example, a geometry restricting the rotation of the aromatic group relative to the carborane cage in an ocarboranyl compound with a dimethylfluorene D group was shown to not only promote high-efficiency ICT-based emission but also enhance thermally activated delayed fluorescence. 34 Furthermore, compounds featuring o-carborane units linked to various π-aromatic fluorescent donors, such as naphthyl, 36,46,47 anthracenyl, 30,35,39,43,44,48 tetraphenylethenyl, 49,50 pyrenyl, 14,40,42 chrysenyl, 51 and indolocarbazole 52,53 moieties, can exhibit multiple photoluminescence and specific emissive features (e.g., mechanochromism, thermochromism, photosalient effect, etc.) in the TICT state.…”

“…Consequently, they are highly sought after as functional groups in optoelectronic materials and hold great potential as electron acceptors. − The emission efficiency of D–A-type luminophores with aromatic D groups and o -carboranyl A groups is strongly influenced by their molecular geometry. − In particular, the efficiency of ICT-based emission is significantly affected by the rotation of the o -carborane moiety relative to the aromatic ring plane, a phenomenon known as twisted intramolecular charge transfer (TICT). − This behavior is due to the fact that the ICT-based transition occurs through an exo -π-interaction between π-delocalized region of the aromatic group and the tangential p -orbital on the carbon of the appended o -carborane, with the magnitude of this interaction depending on the degree of orthogonality between the o -carborane C–C bond and aromatic plane. − For example, a geometry restricting the rotation of the aromatic group relative to the carborane cage in an o -carboranyl compound with a dimethylfluorene D group was shown to not only promote high-efficiency ICT-based emission but also enhance thermally activated delayed fluorescence . Furthermore, compounds featuring o -carborane units linked to various π-aromatic fluorescent donors, such as naphthyl, ,, anthracenyl, ,,,,, tetraphenylethenyl, , pyrenyl, ,, chrysenyl, and indolocarbazole , moieties, can exhibit multiple photoluminescence and specific emissive features (e.g., mechanochromism, thermochromism, photosalient effect, etc.) in the TICT state.…”