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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.


OM2/MRCI-computed LIIC paths connecting the Franck–Condonpoints and the S1/S0 conical intersections relatedto the cyclization and photoisomerization of TPE-4mM (left) and TPE-4oM(right). See the Supporting Information for the corresponding CASPT2 energy profiles.
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fig3: OM2/MRCI-computed LIIC paths connecting the Franck–Condonpoints and the S1/S0 conical intersections relatedto the cyclization and photoisomerization of TPE-4mM (left) and TPE-4oM(right). See the Supporting Information for the corresponding CASPT2 energy profiles.

Mentions: To answer the question, we first examinethe pathways to these S1/S0 conical intersections.Thus, we present linearly interpolated internal coordinate (LIIC)paths connecting the Franck–Condon points and the two pairsof S1/S0 conical intersections. Figure 3 shows the OM2/MRCI-computedLIIC paths for TPE-4mM and TPE-4oM (see Figure S6 for corresponding paths from single-point CASPT2 calculations).In the case of TPE-4mM, the S1 path toward S1S0-cyc (C4C5) is barrierless at both the OM2/MRCI andCASPT2 levels so that cyclization should be facile; by contrast, theS1 path toward S1S0-pyr (C1) encounterssmall barriers of 1.8 (OM2/MRCI) and 8.4 (CASPT2) kcal/mol. Qualitativelysimilar energy profiles are obtained for the S1 paths ofTPE- 4 mM toward S1S0-cyc (C4C5) and S1S0-pyr (C1) (see Figures 3 and S6). These findingsare in line with the known experimental facts for TPE-4mM. First,the observed ultralow fluorescence quantum yield of ∼0.1%19 is caused by the access to the barrierless S1 cyclization path, which quickly dissipates excess photonenergy so that fluorescence emission is very weak. Second, the experimentalevidence that photoisomerization plays no role or only a minor rolein photoexcited TPE compounds14,15 is consistent withthe presence of a barrier on the computed S1 photoisomerizationdecay path.


Excited-State Decay Paths in Tetraphenylethene Derivatives
OM2/MRCI-computed LIIC paths connecting the Franck–Condonpoints and the S1/S0 conical intersections relatedto the cyclization and photoisomerization of TPE-4mM (left) and TPE-4oM(right). See the Supporting Information for the corresponding CASPT2 energy profiles.
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Related In: Results  -  Collection

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

fig3: OM2/MRCI-computed LIIC paths connecting the Franck–Condonpoints and the S1/S0 conical intersections relatedto the cyclization and photoisomerization of TPE-4mM (left) and TPE-4oM(right). See the Supporting Information for the corresponding CASPT2 energy profiles.
Mentions: To answer the question, we first examinethe pathways to these S1/S0 conical intersections.Thus, we present linearly interpolated internal coordinate (LIIC)paths connecting the Franck–Condon points and the two pairsof S1/S0 conical intersections. Figure 3 shows the OM2/MRCI-computedLIIC paths for TPE-4mM and TPE-4oM (see Figure S6 for corresponding paths from single-point CASPT2 calculations).In the case of TPE-4mM, the S1 path toward S1S0-cyc (C4C5) is barrierless at both the OM2/MRCI andCASPT2 levels so that cyclization should be facile; by contrast, theS1 path toward S1S0-pyr (C1) encounterssmall barriers of 1.8 (OM2/MRCI) and 8.4 (CASPT2) kcal/mol. Qualitativelysimilar energy profiles are obtained for the S1 paths ofTPE- 4 mM toward S1S0-cyc (C4C5) and S1S0-pyr (C1) (see Figures 3 and S6). These findingsare in line with the known experimental facts for TPE-4mM. First,the observed ultralow fluorescence quantum yield of ∼0.1%19 is caused by the access to the barrierless S1 cyclization path, which quickly dissipates excess photonenergy so that fluorescence emission is very weak. Second, the experimentalevidence that photoisomerization plays no role or only a minor rolein photoexcited TPE compounds14,15 is consistent withthe presence of a barrier on the computed S1 photoisomerizationdecay path.

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.