Karmakar Saheli, Dey Suvendu, Upadhyay Manoj, Ray Debdas
Advanced Photofunctional Materials Laboratory, Department of Chemistry, Shiv Nadar University Delhi NCR, NH-91, Tehsil Dadri, Gautam Buddha Nagar, Uttar Pradesh 201314, India.
ACS Omega. 2022 May 2;7(19):16827-16836. doi: 10.1021/acsomega.2c01909. eCollection 2022 May 17.
Room-temperature phosphorescence (RTP) from organic compounds has attracted increasing attention in the field of data security, sensing, and bioimaging. However, realization of RTP with an aggregate induced phosphorescence (AIP) feature via harvesting supersensitive excited charge transfer triplet (CT) energy under visible light excitation (VLE) in single-component organic systems at ambient conditions remains unfulfilled. Organic donor-acceptor (D-A) based orthogonal structures can therefore be used to harvest the energy of the CT state at ambient conditions under VLE. Here we report three phenoxazine-quinoline conjugates (PQ, PQCl, PQBr), in which D and A parts are held in orthogonal orientation around the C-N single bond; PQCl and PQBr are substituted with halogens (Cl, Br) while PQ has no halogen atom. Spectroscopic studies and quantum chemistry calculations combining reference compounds (Phx, QPP) reveal that all the compounds in film at ambient conditions show fluorescence and green-RTP due to (i) radiative decay of both singlet charge transfer (CT) and triplet CT (CT) states under VLE, (ii) energetic nondegeneracy of CT and CT states (CT- CT, 0.17-0.21 eV), and (iii) spatial separation of highest and lowest unoccupied molecular orbitals. Further, we found in a tetrahydrofuran-water mixture ( = 90%, v/v) that both PQCl (10 M) and PQBr (10 M) show concentration-dependent AIP with phosphorescence quantum yields (ϕ) of ∼25% and ∼28%, respectively, whereas aggregate induced quenching (ACQ) was observed in PQ. The phosphorescence lifetimes (τ) of the PQCl and PQBr aggregates were shown to be ∼22-62 μs and ∼22-59 μs, respectively. The ϕ of the powder samples is found to be 0.03% (PQ), 15.6% (PQCl), and 13.0% (PQBr), which are significantly lower than that of the aggregates (10 M, = 90%, v/v). Film (Zeonex, 0.1 wt %) studies revealed that ϕ of PQ (7.1%) is relatively high, while PQCl and PQBr exhibit relatively low ϕ values (PQCl, 9.7%; PQBr, 8.8%), as compared with that of powder samples. In addition, we found in single-crystal X-ray analysis that multiple noncovalent interactions along with halogen···halogen (Cl···Cl) interactions between the neighboring molecules play an important role to stabilize the CT caused by increased rigidity of the molecular backbone. This design principle reveals a method to understand nondegeneracy of CT and CT states, and RTP with a concentration-dependent AIP effect using halogen substituted twisted donor-acceptor conjugates.
有机化合物的室温磷光（RTP）在数据安全、传感和生物成像领域引起了越来越多的关注。然而，在环境条件下，通过在单组分有机体系中可见光激发（VLE）下捕获超灵敏激发电荷转移三重态（CT）能量来实现具有聚集诱导磷光（AIP）特征的RTP仍未实现。因此，基于有机供体-受体（D-A）的正交结构可用于在VLE下的环境条件下捕获CT态的能量。在此，我们报道了三种吩恶嗪-喹啉共轭物（PQ、PQCl、PQBr），其中D和A部分围绕C-N单键以正交方向排列；PQCl和PQBr被卤素（Cl、Br）取代，而PQ没有卤素原子。结合参考化合物（Phx、QPP）的光谱研究和量子化学计算表明，在环境条件下，薄膜中的所有化合物由于（i）VLE下单重态电荷转移（CT）和三重态CT（CT）态的辐射衰变，（ii）CT和CT态的能量非简并性（CT-CT，0.17-0.21 eV），以及（iii）最高和最低未占据分子轨道的空间分离，而呈现荧光和绿色RTP。此外，我们发现在四氢呋喃-水混合物（=90%，v/v）中，PQCl（10 M）和PQBr（10 M）均显示浓度依赖性AIP，磷光量子产率（ϕ）分别约为25%和约28%，而PQ中观察到聚集诱导猝灭（ACQ）。PQCl和PQBr聚集体的磷光寿命（τ）分别显示为约22-62 μs和约22-59 μs。发现粉末样品的ϕ为0.03%（PQ）、15.6%（PQCl）和13.0%（PQBr），明显低于聚集体（10 M，=90%，v/v）的ϕ。薄膜（Zeonex，0.1 wt%）研究表明，与粉末样品相比，PQ（7.1%）的ϕ相对较高，而PQCl和PQBr的ϕ值相对较低（PQCl，9.7%；PQBr，8.8%）。此外，我们在单晶X射线分析中发现，相邻分子之间的多种非共价相互作用以及卤素···卤素（Cl···Cl）相互作用对稳定由分子主链刚性增加引起的CT起着重要作用。这一设计原则揭示了一种理解CT和CT态非简并性以及使用卤素取代的扭曲供体-受体共轭物实现具有浓度依赖性AIP效应的RTP的方法。
