Shaik Md Abdus Salam, Samanta Dipanjan, Sharma Ankit Kumar, Shaw Manisha, Prodhan Sayan, Basu Rajarshi, Mondal Imran, Singh Shailab, Dutta Prasanta Kumar, Pathak Amita
Department of Chemistry, Indian Institute of Technology Kharagpur, West Bengal, 721302, India.
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West Bengal 741246, India.
Nanoscale. 2023 Dec 7;15(47):19238-19254. doi: 10.1039/d3nr04802k.
White light emission (WLE), particularly from heteroatom free carbon dots (CDs), is unusual. Besides, deciphering the origin of WLE from a H-aggregated molecular fluorophore in such kinds of CDs is a challenging task due to their non-fluorescent character resulting from a forbidden transition from a lower-energy excitonic state. Therefore, rigorous investigation on their elusive excited state photophysical properties along with their steady-state optical phenomena has to be carried out to shed light on the nature of distinct emissive states formed in the CDs. Herein, for the first time, we report WLE from imperfect H-aggregates of co-facially π-π stacked humin-like structures comprising furfural monomer units as a unique molecular fluorophore in CDs, as revealed from combined spectroscopic and microscopic studies, synthesized through hydrothermal treatment of the single precursor, dextrose. H-aggregates in CDs show a broad range of excitation-dependent emission spectra with color coordinates close to pure white light, , CIE (0.35, 0.37) and a color temperature of 6000 K. Imperfect orientation between the transition dipole moments of adjacent monomer units in the H-aggregate's molecular arrangement is expected to cause ground state symmetry breaking, as confirmed by Circular Dichroism (CD) studies, which established helically stacked nature in molecular aggregates and produced significant oscillatory strength at lower energy excitonic states to enable fluorescence. TRES and TAS investigations have been performed to minimise the intricacies associated with excited state photophysics, which is regarded as an essential step in gaining a grasp on emissive states. Based on the observation of two isoemissive spots in the time-resolved area normalized emission spectra (TRANES), the existence of three oligomeric species in the excited state equilibrium of the pure/hybrid H-aggregates has been established. The exciton dynamics through electron relaxation from the higher to the lower excitonic states, charge transfer (CT) states, and surface trap mediated emission in excimer states of H-aggregates have also been endorsed as three distinct emissive states from femtosecond transient absorption spectroscopy (TAS) studies corroborating with their steady-state absorption and emission behavior. The results would demonstrate the usage of CDs as a cutting-edge fluorescent material for creating aggregate-induced white light emission.
白光发射（WLE），尤其是来自无杂原子碳点（CDs）的白光发射，是不寻常的。此外，由于此类碳点中由低能激子态的禁戒跃迁导致的非荧光特性，解读来自H聚集分子荧光团的白光发射起源是一项具有挑战性的任务。因此，必须对其难以捉摸的激发态光物理性质以及稳态光学现象进行严格研究，以阐明碳点中形成的不同发射态的本质。在此，我们首次报道了通过对单一前驱体葡萄糖进行水热处理合成的碳点中，由共面π-π堆积的类腐殖质结构的不完美H聚集体发出的白光发射，该结构包含糠醛单体单元作为碳点中独特的分子荧光团，这是通过光谱和显微镜联合研究揭示的。碳点中的H聚集体显示出广泛的依赖激发的发射光谱，其色坐标接近纯白光，CIE（0.35, 0.37），色温为6000 K。圆二色性（CD）研究证实，H聚集体分子排列中相邻单体单元的跃迁偶极矩之间的不完美取向预计会导致基态对称性破缺，该研究确定了分子聚集体中的螺旋堆积性质，并在低能激子态产生了显著的振荡强度以实现荧光。已经进行了时间分辨发射光谱（TRES）和时间分辨吸收光谱（TAS）研究，以尽量减少与激发态光物理相关的复杂性，这被视为理解发射态的关键步骤。基于在时间分辨区域归一化发射光谱（TRANES）中观察到的两个等发射点，确定了纯/混合H聚集体的激发态平衡中存在三种寡聚体物种。飞秒瞬态吸收光谱（TAS）研究证实了H聚集体在激子态中通过从高能激子态到低能激子态的电子弛豫、电荷转移（CT）态以及表面陷阱介导的发射等激子动力学过程，这三种过程作为三种不同的发射态与其稳态吸收和发射行为相佐证。结果将证明碳点作为一种前沿荧光材料用于产生聚集诱导白光发射的用途。
