The Influence of Molecular Packing on the Emissive Behavior of Pyrene Derivatives: Mechanoluminescence and Mechanochromism

Abstract

Although some bright, organic mechanoluminescence (ML) luminogens with aggregation‐induced emission properties and twisted molecular structures are recently reported, those with planar structures and bright ML and their related effective design strategies are not yet explored, partially due to aggregation‐caused quenching in luminogens with planar structures. Herein, using the unique solid‐state packing style of pyrene, bright dual monomer‐excimer‐ML and excimer‐ML from two pyrene derivatives (Py‐Bpin and Py‐Br) with a simple, planar molecular structure are reported for the first time, and two analogs of pyrene and Py‐H are used for comparison. These four luminogens display similar optical properties in dilute solution but different luminescent properties in the solid state, mainly due to their different molecular packing. Interestingly, pyrene is mechanochromism (MC)‐ and ML‐inactive, while Py‐H is MC‐active, Py‐Bpin is MC‐ and ML‐active, and Py‐Br is ML‐active. The relationship between molecular packing and ML/MC properties is confirmed by analyzing single‐crystal structures and related experimental results, providing important information to further understand the mysterious ML process and opening a new way to design efficient luminogens with both MC and ML.

Molecular Conformation‐Dependent Mechanoluminescence: Same Mechanical Stimulus but Different Emissive Color over Time

“…This could effectively restrict non-radiative transitions by decreasing the molecular slippage under the mechanical stimulation, then contribute much to the ML effect. [7,8,10] Only after being ground heavily,c ould it transform to the nearly amorphous state without obvious peaks.I nc onsideration of the two remained emissive peaks after heavy grinding,t he mechanochromism effect of FCO-CzS could not be simply ascribed to the transition of packing mode,such as crystal to amorphous state.I tw as considered that the changes of molecular conformation from quasi-axial to quasi-equatorial one of phenothiazine group under mechanical stimulus might have played the key role.…”

“…Accordingly,b ased on our experience on ML luminogens,p heno- thiazine (electron donor) and 4-fluorobenzophenone (electron acceptor) groups were integrated together to yield FCO-CzS,i nw hich, the D-A structure could further reinforce the dynamic electronic character and related emissive property in response to the dynamic molecular conformation, while the fluorine atom could enhance the intermolecular interactions and restrict the nonradiative transition, thus to promote the emission, both for PL and ML. [7,8,10] Excitedly FCO-CzS exhibited changeable mechanoluminescence from blue to white and yellow under continuous mechanical stimulus.T othe best of our knowledge,this is the first example of dynamic mechanoluminescent luminogens. Careful analyses of experimental results,c oupled with wellunderstood PL theory,showed that the molecular conformation transition from quasi-axial to quasi-equatorial should be responsible for the interesting dynamic ML effect.…”

Molecular Conformation‐Dependent Mechanoluminescence: Same Mechanical Stimulus but Different Emissive Color over Time

“…[9a] This told us that the conformation of phenothiazine derivatives could be changed upon the mechanical stimuli. [7,8,10] Excitedly FCO-CzS exhibited changeable mechanoluminescence from blue to white and yellow under continuous mechanical stimulus.T othe best of our knowledge,this is the first example of dynamic mechanoluminescent luminogens. Accordingly,b ased on our experience on ML luminogens,p heno- thiazine (electron donor) and 4-fluorobenzophenone (electron acceptor) groups were integrated together to yield FCO-CzS,i nw hich, the D-A structure could further reinforce the dynamic electronic character and related emissive property in response to the dynamic molecular conformation, while the fluorine atom could enhance the intermolecular interactions and restrict the nonradiative transition, thus to promote the emission, both for PL and ML.…”

AIE Fluorescent Gelators with Thermo‐, Mechano‐, and Vapochromic Properties

“…The molecular packing should play ak ey role in these stimuli-responsive chromatic properties because this packing dictates the energy gaps between the HOMO and LUMO of excimers, as shownb yother researchers. [15] Further investigationw ith the aid of theoretical calculations is in progress in understand the precise reason for the differentemission properties in amorphous and crystalline phases.…”