Conformational Planarization versus Singlet Fission: Distinct Excited‐State Dynamics of Cyclooctatetraene‐Fused Acene Dimers

Abstract

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Consideration of Molecular Structure in the Excited State to Design New Luminogens with Aggregation‐Induced Emission

“…This understanding will allow the design of new AIEgens, because AIE is complementary to solution‐state quenching. Furthermore, the consideration of π twist and distortion in the excited state has successfully produced new functional molecules, including twisted intramolecular charge‐transfer (TICT) molecules, thermally activated delayed fluorescence (TADF) molecules, or flapping (FLAP) molecules . On another front, the consideration of molecular twist and distortion in the excited state will explain some traditional understandings for organic luminogens such as “loose bolt” or “free rotor” effects in photochemistry, as well as up‐to‐date introduced concept, “suppression of Kasha's rule” .…”

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…Compounds of this type are found in a wide range of applications, for example in antipsychotic drugs such as quetiapine and chlorpromazine ( Figure 1), and in photofunctional molecular materials for usage as viscosity probes and photoresponsive liquid crystals. [9,10] Dibenzo[b,f] [1,4]oxazepin is a very strong lachrymatory agent known as CR gas, and dibenzodioxin is the core in some of the most environmentally hazardous polychlorinated chemicals known. Finally, oxepin units situated at the edges of graphene nanosheets as cyclic ethers, effectively benzannelated oxepins, have been proposed as the cause for exciton self-trapping observed in graphene quantum dots and carbon dots.…”

Flapping Peryleneimide as a Fluorogenic Dye with High Photostability and Strong Visible‐Light Absorption

“…Flapping molecules (FLAP) have rapidly emerged as a new photofunctional system, bearing a molecular structure in which rigid aromatics are fused with a flexible 8p ring, such as p-expanded cyclooctatetraene (COT), [1][2][3][4][5] dihydrophenazine, [6] phenothiazine, [7] dibenzo-oxepin, [8] and dibenzoarsepins. [9] A variety of functions can be created by the conformationally flexible motion, demonstrated in environment-sensitive fluorophores, [2,[6][7][8][9] singlet fission chromophores, [3] photoresponsive liquid crystals, [4] and a flexible mechanophore. [5] In most cases, FLAP takes a bent conformation in equilibrium between V-and L-shaped structures in S 0 (singlet ground state), and, by photoexcitation, it shows a fast planarization in S 1 (lowest singlet excited state), often emitting FL with a large Stokes shift.…”

“…Different dynamics from conventional twisted biaryls [10] (flapping vs. twisting) is a current hot topic in photochemistry and related science. [1][2][3][4][5][6]11] However, applications of FLAP for physics and biology are still limited mainly because 1) almost all of FLAPs reported so far exhibit little or no absorption in the visible range, 2) degradation by continuous photoirradiation cannot be ignored, and 3) absence of FL lifetime probe (rather than ratiometric FL probe), suitable for FL lifetime imaging microscopy (FLIM). [12] Here we report an unprecedented FLAP, "flapping peryleneimide" that overcomes these shortcomings ( Figure 1).…”