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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Presence and absence of excited state intramolecular charge transfer with the six isomers of dicyano- N, N -dimethylaniline and dicyano-(N -methyl- N -isopropyl)aniline
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Presence and absence of excited state intramolecular charge transfer with the six isomers of dicyano- N, N -dimethylaniline and dicyano-(N -methyl- N -isopropyl)aniline

机译：双氰基-N，N-二甲基苯胺和双氰基-（N-甲基-N-异丙基）苯胺的六个异构体是否存在激发态分子内电荷转移

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摘要

The excited state behavior of the six m,n-dicyano-N,N-dimethylanilines (mnDCDMA) and m,n-dicyano-(N-methyl-N-isopropyl)anilines (mnDCMIA) is discussed as a function of solvent polarity and temperature. The dicyano moiety in these electron donor (D)/acceptor (A) molecules has a considerably larger electron affinity than the benzonitrile subgroup in 4-(dimethylamino)benzonitrile (DMABN). Nevertheless, the fluorescence spectra of the mnDCDMAs and mnDCMIAs in n-hexane all consist of a single emission originating from the locally excited (LE) state, indicating that a reaction from LE to an intramolecular charge transfer (ICT) state does not take place. The calculated energies E(ICT), obtained by employing the reduction potential of the dicyanobenzene subgroups and the oxidation potential of the amino substituents trimethylamine (N(Me)_3) and isopropyldimethylamine (iPrNMe_2), are lower than E(LE). The absence of an LE → ICT reaction therefore makes clear that the D and A units in the dicyanoanilines are not electronically decoupled. In the polar solvent acetonitrile (MeCN), dual (LE + ICT) fluorescence is found with 24DCDMA and 34DCDMA, as well as with 24DCMIA, 25DCMIA, and 34DCMIA. For all other mnDCDMAs and mnDCMIAs, only LE emission is observed in MeCN. The ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) in MeCN at 25 °C is larger for 24DCDMA (1.2) than for 34DCDMA (0.35). The replacement of methyl by isopropyl in the amino substituent leads to a considerable increase of Φ′(ICT)/Φ(LE), 8.8 for 24DCMIA and 1.4 for 34DCMIA, showing that the LE ? ICT equilibrium has shifted further toward ICT. The appearance of an ICT reaction with the 2,4- and 3,4-dicyanoanilines is caused by a relatively small energy gap ΔE(S_1,S_2) between the two lowest excited singlet states as compared with the other m,n-dicyanoanilines, in accordance with the PICT model. The observation that the ICT reaction is more efficient for 24DCMIA and 34DCMIA than for their mnDCDMA counterparts is mainly caused by the fact that iPrNMe_2 is a better electron donor than N(Me)_3: E(D/D~+) = 0.84 against 1.05 V vs SCE. That ICT also occurs with 25DCMIA, notwithstanding its large ΔE(S_1,S_2), is due to the substantial amino twist angle θ = 42.6°, which leads to partial electronic decoupling of the D and A subgroups. The dipole moments μ_e(ICT) range between 18 D for 34DCMIA and 12 D for 25DCMIA, larger than the corresponding μe(LE) of 16 and 11 D. The difference between μ_e(ICT) and μ_e(LE) is smaller than with DMABN (17 and 10 D) because of the noncollinear arrangement of the amino and cyano substituents (different dipole moment directions).
机译：讨论了六个m，n-二氰基-N，N-二甲基苯胺（mnDCDMA）和m，n-二氰基-（N-甲基-N-异丙基）苯胺（mnDCMIA）的激发态行为与溶剂极性和温度。这些电子供体（D）/受体（A）分子中的二氰基部分比4-（二甲基氨基）苄腈（DMABN）中的苄腈亚基具有更大的电子亲和力。但是，正己烷中mnDCDMA和mnDCMIA的荧光光谱均由源自局部激发（LE）状态的单个发射组成，这表明LE并未发生分子内电荷转移（ICT）状态的反应。利用二氰基苯亚基的还原电势和氨基取代基三甲胺（N（Me）_3）和异丙基二甲基胺（iPrNMe_2）的氧化电势获得的计算能量E（ICT）低于E（LE）。因此，没有LE→ICT反应就可以清楚地表明，二氰基苯胺中的D和A单元不是电子解耦的。在极性溶剂乙腈（MeCN）中，发现24DCDMA和34DCDMA以及24DCMIA，25DCMIA和34DCMIA产生双重（LE + ICT）荧光。对于所有其他mnDCDMA和mnDCMIA，在MeCN中仅观察到LE发射。 MeCN在25°C下的ICT / LE荧光量子产率比Φ'（ICT）/Φ（LE）对于24DCDMA（1.2）大于34DCDMA（0.35）。氨基取代基中的异丙基取代甲基导致Φ'（ICT）/Φ（LE）显着增加，24DCMIA为8.8，34DCMIA为1.4，表明LE？ ICT平衡已进一步转向ICT。与2,4-和3,4-二氰基苯胺发生ICT反应的原因是，与其他m，n-二氰基苯胺相比，两个最低激发单重态之间的能隙ΔE（S_1，S_2）相对较小，按照PICT模型。观察到ICT反应对于24DCMIA和34DCMIA比对mnDCDMA同行更有效，这主要是由于iPrNMe_2是比N（Me）_3更好的电子供体：E（D / D〜+）= 0.84对1.05 V vs SCE。尽管25DCMIA的ΔE（S_1，S_2）较大，但ICT也会发生，这是由于较大的氨基扭转角θ= 42.6°，这导致D和A子组发生部分电子解耦。偶极矩μ_e（ICT）的范围在34DCMIA的18 D和25DCMIA的12 D之间，大于相应的16和11 D的μe（LE）。μ_e（ICT）和μ_e（LE）之差小于DMABN （17和10 D）是因为氨基和氰基取代基的非共线排列（不同的偶极矩方向）。

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