Theoretical Simulations of Thermochromic and Aggregation‐Induced Emission Behaviors of a Series of Red‐Light Anthracene‐o‐carborane Derivatives

Abstract

Quantum mechanical and molecular dynamics simulations have been carried out on a series of anthracene‐o‐carborane derivatives (ANT‐H, ANT‐Ph, ANT‐Me and ANT‐TMS) with rare red‐light emission in the solid state. The simulation of the heating process of the crystals and further comparison of the molecular structures and excited‐state properties before and after heating help us to disclose the thermochromic behavior, that is, the red‐shift emission is caused by elongation of the C1−C2 bond in the carborane moiety after heating. Thus, we believe that the molecular structure in the crystal is severely affected by heating. Transformation of the molecular conformation appears in the ANT‐H crystal with increasing temperature. More specifically, the anthracene moiety moves from nearly parallel to the C1−C2 bond to nearly perpendicular, causing the short‐wavelength emission to disappear after heating. As for the aggregation‐induced emission phenomenon, the structures and photophysical properties were investigated comparatively in both the isolated and crystal states; the results suggested that the energy dissipation in crystal surroundings was greatly reduced through hindering structure relaxation from the excited to the ground state. We expect that discussion of the thermochromic behavior will provide a new analysis perspective for the molecular design of o‐carborane derivatives.

Revisiting the Role of Charge Transfer in the Emission Properties of Carborane–Fluorophore Systems: A TDDFT Investigation

“… 52 This TICT emission can be maintained even in aggregated or crystal states due to the presence of sufficient space for rotations as a result of o -CB’s compact spherical structure. 24 , 42 , 52 On the other hand, the photophysical properties of the o -CB– Ant system can be manipulated via functionalization or substitution of the adjacent C atoms on o -carborane. For example, while only a high-energy TICT state with a low quantum yield is observed in solution with methyl or phenyl groups, a substitution with bulky groups (e.g., trimethylsilyl) leads to a significant increase in the quantum yield.…”

“…Among o -carborane–fluorophore adducts studied to date, the o -carborane–anthracene ( o -CB– Ant ) molecule and its derivatives have attracted significant attention for both theoretical and synthetic studies since the seminal works by Chujo and co-workers. , In solution, o -CB– Ant shows dual emissions with low quantum yields, where the high-energy S 1 → S 0 transition has a local excited (LE)-state character on anthracene, whereas the low-energy transition shows twisted ICT (TICT). The TICT state mainly arises from the strong interaction between o -carborane’s “C–C” bond and anthracene’s π-conjugated system, along with the perpendicular rearrangement of the C–C bond with respect to the plane of anthracene . This TICT emission can be maintained even in aggregated or crystal states due to the presence of sufficient space for rotations as a result of o -CB’s compact spherical structure. ,, On the other hand, the photophysical properties of the o -CB– Ant system can be manipulated via functionalization or substitution of the adjacent C atoms on o -carborane.…”

“…The TICT state mainly arises from the strong interaction between o -carborane’s “C–C” bond and anthracene’s π-conjugated system, along with the perpendicular rearrangement of the C–C bond with respect to the plane of anthracene . This TICT emission can be maintained even in aggregated or crystal states due to the presence of sufficient space for rotations as a result of o -CB’s compact spherical structure. ,, On the other hand, the photophysical properties of the o -CB– Ant system can be manipulated via functionalization or substitution of the adjacent C atoms on o -carborane. For example, while only a high-energy TICT state with a low quantum yield is observed in solution with methyl or phenyl groups, a substitution with bulky groups (e.g., trimethylsilyl) leads to a significant increase in the quantum yield .…”

“…More recently, Duan et al. have investigated the o -CB– Ant derivatives using quantum mechanical and molecular dynamics simulations, based on which the elongation of C–C bond is suggested to lead to bathochromically shifted emissions with an increased CT character for the S 1 → S 0 transition …”

“…22 More recently, Duan et al have investigated the o-CB− Ant derivatives using quantum mechanical and molecular dynamics simulations, based on which the elongation of C−C bond is suggested to lead to bathochromically shifted emissions with an increased CT character for the S 1 → S 0 transition. 52 In light of these recent studies, it has been mostly suggested that o-carborane's "C−C" bond elongation and the relative orientations of fluorophore versus o-carborane moieties govern the electronic interaction and, hence, the photophysical properties of o-CB−f luorophore systems. However, a complete theoretical understanding is still lacking in the literature for the interplay of excited-state nature/energetics and potential energy surfaces (PESs) with regard to emissive transitions and quenching mechanisms.…”