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

Abstract

In this study, we performed a detailed investigation of the S 1 potential energy surface (PES) of o -carborane–anthracene ( o -CB– Ant ) with respect to the C–C bond length on o -CB and the dihedral angle between o -CB and Ant moieties. The effects of different substituents (F, Cl, CN, and OH) on carbon- or boron-substituted o -CB, along with a π-extended acene-based fluorophore, pentacene, on the nature and energetics of S 1 → S 0 transitions are evaluated. Our results show the presence of a non-emissive S 1 state with an almost pure charge transfer (CT) character for all systems as a result of significant C–C bond elongation (C–C = 2.50–2.56 Å) on o -CB. In the case of unsubstituted o -CB– Ant , the adiabatic energy of this CT state corresponds to the global minimum on the S 1 PES, which suggests that the CT state could be involved in emission quenching. Despite large deformations on the o -CB geometry, predicted energy barriers are quite reasonable (0.3–0.4 eV), and the C–C bond elongation can even occur without a noticeable energy penalty for certain conformations. With substitution, it is shown that the dark CT state becomes even more energetically favorable when the substituent shows −M effects (e.g., −CN), whereas substituents showing +M effects (e.g., −OH) can result in an energy increase for the CT state, especially for partially stretched C–C bond lengths. It is also shown that the relative energy of the CT state on the PES depends strongly on the LUMO level of the fluorophore as this state is found to be energetically less favorable compared to other conformations when anthracene is replaced with π-extended pentacene. To our knowledge, this study shows a unique example of a detailed theoretical analysis on the PES of the S 1 state in o -CB– fluorophore systems with respect to substituents or fluorophore energy levels. Our findings could guide future experimental work in emissive o -CB– fluorophore systems and their sensing/optoelectronic applications.

Synthesis, Photophysical and Electronic Properties of a D‐π‐A Julolidine‐Like Pyrenyl‐o‐Carborane

“…o ‐Carboranes are also known to influence attached systems via their strong −I inductive effect, stabilizing the frontier orbitals [101–103] . We and others have shown that the coexisting LE and CT excited states can be connected by a low energy barrier and yet show very different geometries and orbital distributions [87,97–98,104–106] . In 1‐(pyrenyl‐2′‐yl)‐ o ‐carborane ( VI ), we were able to simulate and experimentally confirm a thermodynamic equilibrium between the LE and CT excited states by determining the relevant interconversion rates [87] .…”

“…[101][102][103] We and others have shown that the coexisting LE and CT excited states can be connected by a low energy barrier and yet show very different geometries and orbital distributions. [87,[97][98][104][105][106] In 1-(pyrenyl-2'-yl)-o-carborane (VI), we were able to simulate and experimentally confirm a thermodynamic equilibrium between the LE and CT excited states by determining the relevant interconversion rates. [87] Other pyrenyl-o-carboranes have received considerable attention as chromophores and show potential applications in photocatalysis.…”

Tuning of the height of energy barrier between locally-excited and charge transfer states by altering the fusing position ofo-carborane in phenylnaphthalene

“…However, some aryl- o -carboranes possessing little LE/ICT barrier 30 also showed moderate Φ PL values, suggesting there should be an additional factor relating to nonradiative decay from the ICT state to comprehend the overall PL process. 20…”

“…However, some aryl-o-carboranes possessing little LE/ICT barrier 30 also showed moderate F PL values, suggesting there should be an additional factor relating to nonradiative decay from the ICT state to comprehend the overall PL process. 20 Next, temperature-dependent PL spectra in CHCl 3 were collected for further investigation of the excited state (Fig. 5a).…”

“…5 To date, many reports have described that those compounds show significant structural relaxation around the C cage -C cage bond after photoexcitation. [12][13][14][15][16][17][18][19][20][21] Typically, quantum chemical calculations indicate that around 1.66 Å C cage -C cage distance at the S 0 state is elongated to near 2.40 Å at the S 1 state. This unique structural change inspired us to synthesize and investigate the C cage -B cage fused o-carboranes by a biphenyl skeleton.…”