Direct Evidence of the Effect of Water Molecules Position in the Spectroscopy, Dynamics, and Lighting Performance of an Eco-Friendly Mn-Based Organic-Inorganic Metal Halide Material for High-Performance LEDs and Solvent Vapor Sensing.

Luminescent Mn(II)-based organic-inorganic hybrid halides have drawn attention as potential materials for sensing and photonics applications. Here, the synthesis and characterization of methylammonium (MA) manganese bromide ((MA)nBrxMn(H2O)2, (n = 1, 4 and x = 3, 6)) with different stoichiometries of the organic cation and inorganic counterpart, are reported. While the Mn2+ centers have an octahedral conformation, the two coordinating water molecules are found either in cis (1) or in trans (2) positions. The photophysical behavior of 1 reflects the luminescence of Mn2+ in an octahedral environment. Although Mn2+ in 2 also has octahedral coordination, at room temperature dual emission bands at ≈530 and ≈660 nm are observed, explained in terms of emission from Mn2+ in tetragonally compressed octahedra and self-trapped excitons (STEs), respectively. Above the room temperature, 2 shows quasi-tetrahedral behavior with intense green emission, while at temperatures below 140 K, another STE band emerges at 570 nm. Time-resolved experiments (77-360 K) provide a clear picture of the excited dynamics. 2 shows rising components due to STEs formation equilibrated at room temperature with their precursors. Finally, the potential of these materials for the fabrication of color-tunable down-converted light-emitting diode (LED) and for detecting polar solvent vapors is shown.

Photocyclization reaction and related photodynamics in the photoproducts of a tetraphenylethylene derivative with bulky substituents: unexpected solvent viscosity effect.

Tetraphenylethylene (TPE) derivatives are ones of the most versatile building blocks showing aggregation-induced emission (AIE). However, their applications are limited by the photophysical and photochemical processes that occur in their excited state. Herein, we report a detailed study of the photochemical behaviour of a new TPE derivative with bulky terphenyl groups (TTECOOBu) in solvents of different viscosities and in a PMMA film. UV light irradiation shows an efficient photocyclization reaction, which produces a 9,10-diphenylphenanthrene (DPP) derivative photoproduct. The emission spectra of the irradiated samples show intermediate (∼420 nm) and final (∼380 nm) species. The photocyclization events are more efficient in environments of higher viscosities or rigidity. We show that in a photoirradiated PMMA film containing TTECOOBu, it is possible to etch a message for more than 1 year. The kinetics is dictated by the motions of the phenyl rings and is faster when their motions are precluded or inhibited. We also elucidated the femto- to millisecond photodynamics of the intermediate and final photoproducts and provide a full picture of their relaxation, with the latter in ∼1 ns at S1 and ∼1 µs at T1. We also demonstrate that the kinetics of the bulky TTECOOBu is much slower than that of the TPE core. Our results also show that both photoevents are not reversible contrary to the case of TPE kinetics. We believe that these results will shed more light on the photochemical behaviour of TPE derivatives and should help in the development of novel TPE-based materials with improved photostability and photo-properties.

Deciphering the ultrafast dynamics of a new tetraphenylethylene derivative in solutions: charge separation, phenyl ring rotation and CC bond twisting.

Tetraphenylethylene (TPE) derivatives are one of the fundamental units for developing aggregation induced emission (AIE) scaffolds. However, the underlying mechanisms implicated in the relaxation of the excited TPE remain a topic of ongoing discussion, while the effect of bulky substituents on its photobehaviour is still under scrutiny. Here, we report a detailed study of the photophysical properties of a new symmetrical and bulky TPE derivative with terphenyl groups (TTECOOBu) in solvents of different polarities and viscosities. Using femto- to nanosecond (fs-ns) time-resolved absorption and emission techniques, we elucidated the role of the phenyl group rotations and core ethylene bond twisting in its behaviour. We demonstrate that TTECOOBu in DCM solutions undergoes a 600 fs charge separation along the ethylene bond leading to a resonance structure with a lifetime of ∼1 ns. The latter relaxes via two consecutive events: a twisting of the ethylene bond (∼ 9 ps) and a rotation of the phenyl rings (∼ 30 ps) leading to conformationally-relaxed species with a largely Stokes-shifted emission (∼ 12 500 cm-1). The formation of the red-emitting species clearly depends on the solvent viscosity and rigidity of the medium. Contrary to the photobehavior in the highly viscous triacetin or rigid polymer matrix of PMMA, a reversible mechanism was observed in DCM and DMF solutions. These results provide new findings on the ultrafast mechanisms of excited TPE derivatives and should help in the development of new molecular rotors with interesting AIE properties for photonic applications.