Synthesis of New 2D-\(\pi\)-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties  
[PERSON] and [PERSON]  
1 Institute of New Drug Development, College of Medicine, China Medical University, No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan  
Correspondence: [EMAIL_ADDRESS]; Tel. : +4-2205-3366-8200  
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accumulated state (concentrated solutions, solid state, accumulation on the bioanalyte's surface), which limits their application [16; 17]. The phenomenon origins from the \(\pi\)-\(\pi\) intermolecular interactions, which induce the excited to ground state decay through non-radiative mechanisms. A significant breakthrough was the synthesis of 1-methyl-1,2,3,4,5-pentaphenylsilole (PPS), a compound that exhibits enhanced fluorescence when aggregated [18]. Due to the greater amount of accumulation-induced emission (AIE), modified PPS and tetraphenyl-ethylene (TPE) are widely studied for applications such as biosensors and OLEDs [19; 20; 21; 22; 23; 24; 25]. The preparation and the study of HPS and TPE derivatives allow the study of the AIE phenomenon, particularly the mechanistic aspects [26; 23; 27; 28; 29; 30]. The molecules are not D-\(\pi\)-A; their helical shape obstructs the \(\pi\)-\(\pi\) interactions in the aggregates, and the increased emission is due to the restricted intramolecular rotations or formation of J-aggregates. In addition, helical-shaped D-\(\pi\)-A molecules (in these compounds TPE unit bridges an electron donor and an electron acceptor functional group) exhibiting emission enhancement in the accumulated state have also been synthesized and studied [31]. A wide variety of molecular designs for emissive molecules are possible because a wide variety of mechanisms can restore the radiative ground state. The fulvene group is an organic scaffold that emits light, as shown in a recent study about the application of dimethyl amino fulvene as a fingermark detector [32]. Fulvene derivatives have long been overlooked due to their tendency to oligomerize if not suitably substituted and their difficulty in selective substitution-functionalized [33; 34]. Substitution of position 6,6\({}^{\prime}\) with an electron acceptor group can furnish a HOMO-LUMO energy gap suitable for utilization as n-type additives in bulk heterojunction solar cells [35]. Substitution along the pentadienyl unit can render the fulvene core emissive, as reported for 1,3-diphenyl fulvenes, and mainly for dibenzofulvenes, which also display AIE [36; 37; 38; 39; 40; 41]. The emissions of dibenzofulvenes was also explained in theoretical studies that attempted to elucidate the emission mechanism in the fulvene derivatives [42; 43]. The description was based on previous theoretical studies on the fulvene molecular orbitals in which HOMO-1, HOMO, and LUMO are drowned, as reported in the literature and shown in Figure 1a [44; 45; 46; 47; 48]. In the electronic ground state, S\({}_{0}\) frontier electrons occupy HOMO orbitals. The Fulvene electronic excited states S\({}_{1}\) and S\({}_{2}\) correspond to the HOMO \(\rightarrow\) LUMO and HOMO-1 \(\rightarrow\) LUMO electron transitions. According to the Blanc-fort model, in conditions of free intramolecular motions, the emission (around 430 nm) is generated by S\({}_{2}\) state decay, and in conditions of restricted intramolecular rotations, the emission (around 460 nm) is mainly generated by S\({}_{1}\) decay. Tetraphenylfulvenes, shown in Figure 1b, with -COOMe and -CN groups in position 6,6\({}^{\prime}\) were investigated through computational and experimental analysis [49]. The synthesized compounds exhibit the main UV-vis absorption at 320 nm, which was assigned as HOMO-1 \(\rightarrow\) LUMO electron transition (S\({}_{2}\) state generation). The HOMO \(\rightarrow\) LUMO transition seems to exhibit a signal with low intensity at 380 nm.