The steric-environment sensitivity of fluorescence of 9,10-bis(N,N-dialkylamino)anthracenes (BDAAs) was studied experimentally and theoretically. A new design strategy to tune simple aromatic hydrocarbons as efficient aggregation-induced emission (AIE) luminogens and molecular rotors is proposed. For a variety of BDAAs, prominent Stokes shifts and efficient solid-state fluorescence were observed. Calculations on BDAA-methyl suggested that in the ground state (S0) conformations, the pyramidal amine groups are highly twisted, so that their lone-pair orbitals cannot conjugate with the anthracene π orbitals. Fluorescence takes place from the S1 minima, in which one or both amine groups are planarized. The stability of the S1 excited state minima as well as destabilization of the S0 state is the origin of large Stokes shift. Experimental measurement of the nonadiabatic transition rate suggests that para disubstitution by dialkylamino (or strongly electron-donating) groups is a key for fast internal conversion. Minimum energy conical intersection (MECI) between S1 and S0 states was found to have a Dewar-benzene like structure. Although this can be reached efficiently in liquid phase for fast internal conversion, a large amplitude motion is required to reach this MECI, which is prohibited in the solid state and caused efficient AIE. This strategy is used to find experimentally that naphthalene analogues are also efficient AIE luminogens. The flexibility of alkyl chains on amino groups is also found to be important for allowed charge-transfer transition. Thus, three points [(1) highly twisted N,N-dialkylamines, (2) substitution at the para positions, (3) with flexible alkyl groups] were proposed for activation of small aromatic hydrocarbons.
Twenty years ago, the concept of aggregation‐induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This minireview discusses the basic principles of AIE based on our previous mechanistic study of the photophysical behavior of 9,10‐bis(N,N‐dialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises an anthracene core and small electron donors, which allows the quantum chemical aspects of AIE to be discussed. The key factor for AIE is the control over the non‐radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non‐fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.
Ab initio calculations suggest that the Sb and Bi atoms in group 15 form double bonded compounds, as do the lighter P and As atoms, unlike the heavier Sn and Pb in group 14; dibismuthenes are the heaviest example of double bonding between main group elements.Compounds featuring double bonding between the heavier group 15 elements are of current interest.1 In 1981, the first stable P-P double bonded compound, a diphosphene derivative (RP=PR), was synthesized and isolated.2 Since then, many schemes have been revised and diphosphenes are now commonplace. 1 In addition, compounds with an -As=Aslinkage, diarsenes, have also been successfully synthesized and is0lated.3-~ However, attempts to prepare the heavier analogues, distibenes (RSb=SbR) and dibismuthenes (RBi= BiR), have all been unsuccessful up to now, except for the metal-coordinated distibene complexes, but these lose double bond character because of side-on q2 coordination."9The lighter Si and Ge atoms in group 14 can form isolable ethene analogues ,lo disilenes (R2Si=SiR2)11 and digermenes (R2Ge=GeR2).I2 However, the heavier Sn is reluctant to form double bonds in distannenes ( R2Sn=SnR2) ,13,14 while Pb does not form a diplumbene (R2Pb=PbR2) structure at a11.15-1h From this point of view, it is important to determine if the heavier atoms in group 15 are also incapable of forming double bonded compounds. For this purpose, we have undertaken the first ab initio calculations of the parent compounds, HSb=SbH 3 and HBi=BiH 4.Geometries were fully optimized at the Hartree-Fock (HF) level with relativistic effective core potentials (ECP)17 on Sb and Bi using the double-zeta (DZ) basis sets17 augmented by a set of d-type polarization functions [d exponents18 0.211 (Sb) and 0.185 (Bi)]. The D Z basis set19 for H was scaled by a factor
A new design strategy to activate aggregation-induced emission (AIE) in pyrene chromophores is reported. In a previous report, we demonstrated that highly twisted N,N-dialkylamines of anthracene and naphthalene induce drastic AIE when these donors are introduced at appropriate positions to stabilize the S/S minimum energy conical intersection (MECI). In the present study, this design strategy was applied to pyrene: the introduction of N,N-dimethylamine substituents at the 4,5-positions of pyrene, the so-called K-region, are likely to stabilize MECIs. To examine this hypothesis, four novel pyrene derivatives, which contain highly twisted N,N-dimethylamino groups at the 4- (4-Py), 4,5- (4,5-Py), 1- (1-Py), or 1,6-positions (1,6-Py) were tested. The nonradiative transitions of 4,5-Py are highly efficient (k = 57.1 × 10 s), so that its fluorescence quantum yield in acetonitrile decreases to Φ = 0.04. The solid-state fluorescence of 4,5-Py is efficient (Φ = 0.49). In contrast, 1,6-Py features strong fluorescence (Φ = 0.48) with a slow nonradiative transition (k = 11.0 × 10 s) that is subject to severe quenching (Φ = 0.03) in the solid state. These results underline that the chemistry of the pyrene K-region is intriguing, both from a photophysical perspective and with respect to materials science.
To broaden the application of aggregation‐induced emission (AIE) luminogens (AIEgens), the design of novel small‐molecular dyes that exhibit high fluorescence quantum yield (Φfl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non‐radiative decay pathways, a series of bridged stilbenes was designed, and their non‐radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.
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