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Excited-State Decay Paths in Tetraphenylethene Derivatives

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.


(Left) Distribution ofthe C4–C5 and C8–C9 distancesat all S1 → S0 hopping points of TPE-4mM.(Right) Distribution of the S1 → S0 hoppingtimes via the two S1/S0 conical intersectionregions related to the cyclization (blue and green bars). Averagehopping times ⟨t⟩ are given for thetwo channels (blue and green) and for all hopping events (black).Also shown in the right panel are the time-dependent state populationsof the S1 (dashed line) and S0 (solid line)electronic states. See the text for discussion.
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fig4: (Left) Distribution ofthe C4–C5 and C8–C9 distancesat all S1 → S0 hopping points of TPE-4mM.(Right) Distribution of the S1 → S0 hoppingtimes via the two S1/S0 conical intersectionregions related to the cyclization (blue and green bars). Averagehopping times ⟨t⟩ are given for thetwo channels (blue and green) and for all hopping events (black).Also shown in the right panel are the time-dependent state populationsof the S1 (dashed line) and S0 (solid line)electronic states. See the text for discussion.

Mentions: The left panel of Figure 4 shows the distribution of two key geometrical parameters,that is, the C4–C5 and C8–C9 distances, at all S1 → S0 hopping points of TPE-4mM. It wasfound that 50.6% of the hops to the S0 state occur in theneighborhood of the S1S0-cyc (C4C5) conicalintersection, and the remaining 49.4% occur in the vicinity of theS1S0-cyc (C8C9) intersection. The S1 → S0 hopping-time distribution of TPE-4mM is shownin the right panel of Figure 4. During the first 200 fs, there are no S1 →S0 hops. This time period corresponds to the initial S1 relaxation from the Franck–Condon region via the S1 minima to the S1S0-cyc conical intersections.After 200 fs, the trajectories start to hop to the S0 statewhen they get close to the S1/S0-cyc conicalintersections. We see an extended hopping-time distribution, whichimplies that some trajectories do not hop to the S0 stateduring their first approach to the S1/S0-cycconical intersections, but only at a later encounter.


Excited-State Decay Paths in Tetraphenylethene Derivatives
(Left) Distribution ofthe C4–C5 and C8–C9 distancesat all S1 → S0 hopping points of TPE-4mM.(Right) Distribution of the S1 → S0 hoppingtimes via the two S1/S0 conical intersectionregions related to the cyclization (blue and green bars). Averagehopping times ⟨t⟩ are given for thetwo channels (blue and green) and for all hopping events (black).Also shown in the right panel are the time-dependent state populationsof the S1 (dashed line) and S0 (solid line)electronic states. See the text for discussion.
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Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC5385518&req=5

fig4: (Left) Distribution ofthe C4–C5 and C8–C9 distancesat all S1 → S0 hopping points of TPE-4mM.(Right) Distribution of the S1 → S0 hoppingtimes via the two S1/S0 conical intersectionregions related to the cyclization (blue and green bars). Averagehopping times ⟨t⟩ are given for thetwo channels (blue and green) and for all hopping events (black).Also shown in the right panel are the time-dependent state populationsof the S1 (dashed line) and S0 (solid line)electronic states. See the text for discussion.
Mentions: The left panel of Figure 4 shows the distribution of two key geometrical parameters,that is, the C4–C5 and C8–C9 distances, at all S1 → S0 hopping points of TPE-4mM. It wasfound that 50.6% of the hops to the S0 state occur in theneighborhood of the S1S0-cyc (C4C5) conicalintersection, and the remaining 49.4% occur in the vicinity of theS1S0-cyc (C8C9) intersection. The S1 → S0 hopping-time distribution of TPE-4mM is shownin the right panel of Figure 4. During the first 200 fs, there are no S1 →S0 hops. This time period corresponds to the initial S1 relaxation from the Franck–Condon region via the S1 minima to the S1S0-cyc conical intersections.After 200 fs, the trajectories start to hop to the S0 statewhen they get close to the S1/S0-cyc conicalintersections. We see an extended hopping-time distribution, whichimplies that some trajectories do not hop to the S0 stateduring their first approach to the S1/S0-cycconical intersections, but only at a later encounter.

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.