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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.


Time-dependentphysical variables obtained from typical OM2/MRCItrajectories of TPE-4mM (1 and 2): two key bond lengths (top left);S1–S0 energy gap (top right); six keydihedral angles (bottom)
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fig5: Time-dependentphysical variables obtained from typical OM2/MRCItrajectories of TPE-4mM (1 and 2): two key bond lengths (top left);S1–S0 energy gap (top right); six keydihedral angles (bottom)

Mentions: In this section, we discusstwo typical cyclization trajectories in some detail. Figure 5 shows the time-dependent evolutionof two key distances and six key dihedral angles in a typical trajectoryfor cyclization of TPE-4mM. It starts from a conformation with aninitial C4–C5 distance of about 3.130 Å. Upon cyclization,this distance immediately decreases to about 1.8 Å within ∼200fs, in conjunction with an almost synchronous decrease of the dihedralangles characterizing the phenyl rotations: C1–C2–C6–C5from −55 to −20° and C2–C1–C3–C4from −48 to −15°. By contrast, the C2–C3–C7–C1and C1–C10–C6–C2 dihedral angles merely fluctuateslightly. In this run, the system does not decay to the S0 state when it first approaches the S1/S0 conicalintersection region after 200 fs; instead, it oscillates for morethan 150 fs (ca. 2 vibrational periods) and then hops to the groundstate at 376 fs. Thereafter, the molecule evolves toward the cyclicconformation, accompanied by a decrease of the C2–C3–C7–C1and C1–C10–C6–C2 dihedral angles until the endof the nonadiabatic dynamics simulations. Panel (2) shows a secondtypical cyclization trajectory, in which cyclization occurs at theother side of TPE-4mM (formation of the C8–C9 bond). The hoppingtime and the key geometric parameters show analogous behavior as thatin the first trajectory. For comparison, corresponding time-dependentdata are presented for a typical trajectory of TPE-4oM in the Supporting Information (Figure S7).


Excited-State Decay Paths in Tetraphenylethene Derivatives
Time-dependentphysical variables obtained from typical OM2/MRCItrajectories of TPE-4mM (1 and 2): two key bond lengths (top left);S1–S0 energy gap (top right); six keydihedral angles (bottom)
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Time-dependentphysical variables obtained from typical OM2/MRCItrajectories of TPE-4mM (1 and 2): two key bond lengths (top left);S1–S0 energy gap (top right); six keydihedral angles (bottom)
Mentions: In this section, we discusstwo typical cyclization trajectories in some detail. Figure 5 shows the time-dependent evolutionof two key distances and six key dihedral angles in a typical trajectoryfor cyclization of TPE-4mM. It starts from a conformation with aninitial C4–C5 distance of about 3.130 Å. Upon cyclization,this distance immediately decreases to about 1.8 Å within ∼200fs, in conjunction with an almost synchronous decrease of the dihedralangles characterizing the phenyl rotations: C1–C2–C6–C5from −55 to −20° and C2–C1–C3–C4from −48 to −15°. By contrast, the C2–C3–C7–C1and C1–C10–C6–C2 dihedral angles merely fluctuateslightly. In this run, the system does not decay to the S0 state when it first approaches the S1/S0 conicalintersection region after 200 fs; instead, it oscillates for morethan 150 fs (ca. 2 vibrational periods) and then hops to the groundstate at 376 fs. Thereafter, the molecule evolves toward the cyclicconformation, accompanied by a decrease of the C2–C3–C7–C1and C1–C10–C6–C2 dihedral angles until the endof the nonadiabatic dynamics simulations. Panel (2) shows a secondtypical cyclization trajectory, in which cyclization occurs at theother side of TPE-4mM (formation of the C8–C9 bond). The hoppingtime and the key geometric parameters show analogous behavior as thatin the first trajectory. For comparison, corresponding time-dependentdata are presented for a typical trajectory of TPE-4oM in the Supporting Information (Figure S7).

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.