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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.


S0 minimum (left), minimum-energy S1/S0 conical intersections for cyclization (middle), and photoisomerization(right) of TPE-4mM. Also shown are some key bond lengths (in Å)and the C3C1C2C6 dihedral angle (in degree). See the Supporting Information for related conical intersections ofTPE-4mM and for the corresponding structures of TPE-4oM.
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fig2: S0 minimum (left), minimum-energy S1/S0 conical intersections for cyclization (middle), and photoisomerization(right) of TPE-4mM. Also shown are some key bond lengths (in Å)and the C3C1C2C6 dihedral angle (in degree). See the Supporting Information for related conical intersections ofTPE-4mM and for the corresponding structures of TPE-4oM.

Mentions: Atthe OM2/MRCI and CASSCF levels, we find two pairs of minimum-energyS1/S0 conical intersection structures for bothTPE-4mM and TPE-4oM, respectively. The two methods give similar optimizedstructures and energies (see Figure S3 and Table S2). The first pair of S1/S0 conicalintersections mediates the photoisomerization around the central C1=C2double bond; they are labeled as S1S0-pyr (C1)and S1S0-pyr (C2); see Figures 2 and S2. In thesestructures, the central double bond is twisted significantly, andthe local environment around either C1 or C2 is strongly pyramidizeddue to sudden polarization effects.58 SimilarS1/S0 conical intersections have previouslybeen found in the stilbene, diphenyldibenzofulvene, and TPE AIEgens.59−62


Excited-State Decay Paths in Tetraphenylethene Derivatives
S0 minimum (left), minimum-energy S1/S0 conical intersections for cyclization (middle), and photoisomerization(right) of TPE-4mM. Also shown are some key bond lengths (in Å)and the C3C1C2C6 dihedral angle (in degree). See the Supporting Information for related conical intersections ofTPE-4mM and for the corresponding structures of TPE-4oM.
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Related In: Results  -  Collection

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

fig2: S0 minimum (left), minimum-energy S1/S0 conical intersections for cyclization (middle), and photoisomerization(right) of TPE-4mM. Also shown are some key bond lengths (in Å)and the C3C1C2C6 dihedral angle (in degree). See the Supporting Information for related conical intersections ofTPE-4mM and for the corresponding structures of TPE-4oM.
Mentions: Atthe OM2/MRCI and CASSCF levels, we find two pairs of minimum-energyS1/S0 conical intersection structures for bothTPE-4mM and TPE-4oM, respectively. The two methods give similar optimizedstructures and energies (see Figure S3 and Table S2). The first pair of S1/S0 conicalintersections mediates the photoisomerization around the central C1=C2double bond; they are labeled as S1S0-pyr (C1)and S1S0-pyr (C2); see Figures 2 and S2. In thesestructures, the central double bond is twisted significantly, andthe local environment around either C1 or C2 is strongly pyramidizeddue to sudden polarization effects.58 SimilarS1/S0 conical intersections have previouslybeen found in the stilbene, diphenyldibenzofulvene, and TPE AIEgens.59−62

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.