Aggregation-induced enhanced emission (AIEE) Myounghee Lee

Aggregation quenching dendritic sidegroups as 3-dimensional barriers to aggregation quenching of conjugated polymer PL Major disadvantage of organic materials in light emitting device applications, low PL quantum yield in most solid state Efficient fluorescent system even in solid State? * Rachel Jakubiak, Zhenan Bao, Lewis Rothberg, Synthitic. Metals 2000, 114, 61 (dotted lines : solution, solid lines : solid state) an example of reducing aggregation quenching

Ph (phenyl) Rotational energy relaxationdeactivate nonradiatively *Junwu Chen, Charles C. W. Law, Jacky W. Y. Lam, Yuping Dong, Samuel M. F. Lo, Ian D. Wiliams, Daoben Zhu, and Ben Zhong Tang, Chem. Mater. 2003, 15, 1535 AIEE chromophore : Hexaphenylsilole*

Hexaphenylsilole in acetone solvent of M/L acetone / water mixture (0~90%) of M/L Nanoparticles formation AIEE chromophore Sample preparations

AIEE phenomenon in molecule with rigid carbon backbones or without flexible side chains, highly fluorescent in solution poorly fluorescent in aggregation state - intermolecular energy transfer deactivation scheme of PV * * a phenylenevinylene derivative (PV) * Dieter Oelkrug, Alfred Tompert, Johannes Gierschener, Hans-Joachim Egelhaaf, Michael Hanack, Michael Hohloch, and Elke Steinhuber, J. Phys. Chem. B 1998, 102, 1902

AIEE phenomenon in AIEE materials such as silole derivatives, intramolecular rotation in good solvent – nonradiative relaxation, weak emission in aggregates - reduce nonradiative deactivation planarizationnonplanarity

experimental evidences of restricted rotational motion induced by aggregates AIEE phenomenon photoluminescence quantum yield (sready-state absorption, emission, viscosity, temperature, dynamic NMR) photoluminescence dynamics (TCSPC) Hexaphenylsilole Absorption maximums of silole solutions silole compounds are not push-pull molecules hardly cause any change in their absorption and emission transitions in solvents with different polarity

Solid-state PL spectra of thin layers of silole absorbed on TLC plate and quartz plate (silole concentration : 0.5 and 4.0 mg/mL) AIEE in solid state PL peak at 496 nm increase of silole concentration - PL quantum yield increases Absorption peak at 366 nm in chloroform PL peak of thin layer on TLC plate – 495nm PL peak of water/acetone mixture – 499nm TLC : thin layer chromatography

PL spectra of silole in acetone and water/acetone mixture (90% water) PL band in silole solutions in pure acetone(good solvent) - non-emissive behavior in acetone/water mixture - intense PL spectrum - AIEE-active silole derivative AIEE in solution state

Quantum yield of silole in water mixtures Emission quantum yield (QE) (using 9, 10-diphenylanthracene as standard) Unchanged up to ~50 vol % water QE : 0.22 for 90 % water fraction times higher than acetone solution QE trajectory above 50% water – size & population of the aggregates increase as water fraction Emission Quantum yield

Size distributions of the nanoparticles of Silole in water/acetone mixture (A : 70%, B : 90%) Nanodimension silole aggregates Size distribution measurement - 30 ~ 72 nm for 70% - 46 ~ 112 nm for 90% - no reliable data for 60% Nanoparticles formation

Origin of AIEE phenomenon - affect emission properties of some donor-acceptor molecules - polarity of solvent alters both ground and excited states of the molecule - no polarity effect by solvents - reduce the likelihood of the involvement of TICT mechanism in AIEE - nonradiatively deactivate excited species in solution - rotations of phenyl rings around the axes of the single bond ( effectively annihilate the excitons --- nonemissive ! ) - in aggregates, the stacking forces involved in the crystal packing ( restrict the intramolecular rotations --- emissive ! ) Twisted intramolecular charge transfer (TICT) Silole compound has no dipole moment Rotational energy relaxation

Viscosity effect using glycerol mixture PL peak intensity of silole in glycerol mixtures Enhanced emission below 50% glycerol fraction - due to viscosity restricting rotational motion of phenyl groups linked to silole center

Temperature effect on AIEE PL spectra of silole solution at various temperatures Spectral profile - hardly change with temperature - still in associated with the radiative decay of the singlet excitons PL intensity - progressively increased at –78 o C - limiting thermally activated intramolecular rotations of phenyl rings - little change below –78 o C - already frozen at that point

Temperature VS PL peak intensity of silole in dioxane and THF solvent Pure temperature effect -using a liquid(THF) with high solvating power but a low melting point(-108 o C) -PL intensity of silole solution increases with decrease of temperature in a nearly linear semilog fashion Temperature effect on AIEE

Steady-state PL spectra obtained by TCSPC results of silole mixture AIEE effect of a silole derivative observed in PL dynamics

Blue-shift of PL peak & Decrease of PL intensity with time Nanoparticles stability at 60% water fraction