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Excited-State Decay Paths in Tetraphenylethene Derivatives

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ABSTRACT

Thephotophysical properties of tetraphenylethene (TPE) compoundsmay differ widely depending on the substitution pattern, for example,with regard to the fluorescence quantum yield ϕf andthe propensity to exhibit aggregation-induced emission (AIE). We reportcombined electronic structure calculations and nonadiabatic dynamicssimulations to study the excited-state decay mechanisms of two TPEderivatives with four methyl substituents, either in the meta position(TPE-4mM, ϕf = 0.1%) or in the ortho position (TPE-4oM,ϕf = 64.3%). In both cases, two excited-state decaypathways may be relevant, namely, photoisomerization around the centralethylenic double bond and photocyclization involving two adjacentphenyl rings. In TPE-4mM, the barrierless S1 cyclizationis favored; it is responsible for the ultralow fluorescence quantumyield observed experimentally. In TPE-4oM, both the S1 photocyclizationand photoisomerization paths are blocked by non-negligible barriers,and fluorescence is thus feasible. Nonadiabatic dynamics simulationswith more than 1000 surface hopping trajectories show ultrafast cyclizationupon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactiveduring the 1 ps simulations. We discuss the chances for spectroscopicdetection of the postulated cyclic photoproduct of TPE-4mM and therelevance of our findings for the AIE process.

No MeSH data available.


TwoTPE compounds, that is, tetra(meta-methyl)-tetraphenylethylene(referred to as TPE-4mM) and tetra(ortho-methyl)-tetraphenylethylene(TPE-4oM), studied in this work. Also shown are their fluorescencequantum yields measured in THF solution.
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fig1: TwoTPE compounds, that is, tetra(meta-methyl)-tetraphenylethylene(referred to as TPE-4mM) and tetra(ortho-methyl)-tetraphenylethylene(TPE-4oM), studied in this work. Also shown are their fluorescencequantum yields measured in THF solution.

Mentions: Here we focuson two particular TPE compounds, namely, the tetramethylderivatives TPE-4mM and TPE-4oM (see Figure 1). TPE-4mM is a typical AIEgen because ithas a very low fluorescence quantum yield of 0.1% in THF solution19 and becomes emissive upon aggregation.4 By contrast, TPE-4oM already fluoresces stronglyin THF solution, with a quantum yield of 64% (i.e., an enhancementby a factor of more than 600 compared with TPE-4mM), so that any furtherincrease by aggregation can only be moderate.14 In the RIR model, this is explained by steric hindrance; the ortho-methyl groups in TPE-4oM effectively suppress therotation of the phenyl rings and hence also the proposed radiationlessdecay pathway, so that TPE-4oM is able to fluoresce.14 It is noteworthy that E/Z isomerization was not detectedin TPE-4oM14 and was experimentally foundto play only a minor role in TPE adducts.15


Excited-State Decay Paths in Tetraphenylethene Derivatives
TwoTPE compounds, that is, tetra(meta-methyl)-tetraphenylethylene(referred to as TPE-4mM) and tetra(ortho-methyl)-tetraphenylethylene(TPE-4oM), studied in this work. Also shown are their fluorescencequantum yields measured in THF solution.
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Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385518&req=5

fig1: TwoTPE compounds, that is, tetra(meta-methyl)-tetraphenylethylene(referred to as TPE-4mM) and tetra(ortho-methyl)-tetraphenylethylene(TPE-4oM), studied in this work. Also shown are their fluorescencequantum yields measured in THF solution.
Mentions: Here we focuson two particular TPE compounds, namely, the tetramethylderivatives TPE-4mM and TPE-4oM (see Figure 1). TPE-4mM is a typical AIEgen because ithas a very low fluorescence quantum yield of 0.1% in THF solution19 and becomes emissive upon aggregation.4 By contrast, TPE-4oM already fluoresces stronglyin THF solution, with a quantum yield of 64% (i.e., an enhancementby a factor of more than 600 compared with TPE-4mM), so that any furtherincrease by aggregation can only be moderate.14 In the RIR model, this is explained by steric hindrance; the ortho-methyl groups in TPE-4oM effectively suppress therotation of the phenyl rings and hence also the proposed radiationlessdecay pathway, so that TPE-4oM is able to fluoresce.14 It is noteworthy that E/Z isomerization was not detectedin TPE-4oM14 and was experimentally foundto play only a minor role in TPE adducts.15

View Article: PubMed Central - PubMed

ABSTRACT

Thephotophysical properties of tetraphenylethene (TPE) compoundsmay differ widely depending on the substitution pattern, for example,with regard to the fluorescence quantum yield ϕf andthe propensity to exhibit aggregation-induced emission (AIE). We reportcombined electronic structure calculations and nonadiabatic dynamicssimulations to study the excited-state decay mechanisms of two TPEderivatives with four methyl substituents, either in the meta position(TPE-4mM, ϕf = 0.1%) or in the ortho position (TPE-4oM,ϕf = 64.3%). In both cases, two excited-state decaypathways may be relevant, namely, photoisomerization around the centralethylenic double bond and photocyclization involving two adjacentphenyl rings. In TPE-4mM, the barrierless S1 cyclizationis favored; it is responsible for the ultralow fluorescence quantumyield observed experimentally. In TPE-4oM, both the S1 photocyclizationand photoisomerization paths are blocked by non-negligible barriers,and fluorescence is thus feasible. Nonadiabatic dynamics simulationswith more than 1000 surface hopping trajectories show ultrafast cyclizationupon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactiveduring the 1 ps simulations. We discuss the chances for spectroscopicdetection of the postulated cyclic photoproduct of TPE-4mM and therelevance of our findings for the AIE process.

No MeSH data available.