## 5 Conclusions  
In this research work, we investigated the optical properties of the 2D-\(\pi\)-2A molecular design, with tetraphenylfulvene as a \(\pi\)-bridge. The introduction of the donor groups on the phenyl rings 3 and 4 induces charge transfer upon excitation, as proved by the appearance of additional UV absorption (I2) that was not visible (or difficult to detect) in the analogue compounds without the donor group. Compounds with stronger \(\pi\)-donor and \(\pi\)-acceptor groups display more intense fluorescence, which is highly affected by the environment. Compound 5**mT** is fully conjugated, displaying donor and acceptor groups, and it also exhibits high rotational freedom with several possible conformational states that can generate emission. As a result, we can finely adjust the fluorescence solutions by varying the solvent polarity, which affects the aggregation and also the conformational status. The complex emissions of the compounds can be further investigated in the future by functionalization  
Figure 6: Fluorescence spectra in different mixtures of THF and H\({}_{2}\)O.  
with different donor or acceptor groups, and also with computational studies, to increase understanding of the emission mechanism. We believe that the capability of an emitter to display different fluorescence frequencies in different solutions can be useful for developing new molecules as biosensors. Compounds such as **5 mT** could yield different emission frequencies for each different biological tissue on which it accumulates or for each different biological receptor with which it interacts due to the different possible conformations or aggregates.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13052976/s1](https://www.mdpi.com/article/10.3390/app13052976/s1)]([https://www.mdpi.com/article/10.3390/app13052976/s1](https://www.mdpi.com/article/10.3390/app13052976/s1)), Figures S1-S22: 1H and 13C NMR spectra of compounds **5 mT**, **5 nT**, **5 mF**, **5 nF**, **5 mP**, **5 nP**; Scheme S1: Alternative procedure for synthesis of 5 mT. Schemes S2 and S3. Failed syntheses; Figures S23-S25: UV-vis spectra in CH\({}_{2}\)Cl\({}_{2}\) and CH\({}_{3}\)CN of all synthesized compounds; Figure S26: Fluorescence spectra in Dichloromethane solutions \(10^{-6}\) M; Figure S27: Fluorescence spectra in Dichloromethane solutions \(10^{-5}\) M; Figures S28-S33: Fluorescence spectra in Dichloromethane and Acetonitrile solutions \(10^{-6}\) M and \(10^{-5}\) M of **5 mT**, **5 nT**, **5 mF**, **5 nF**, **5 mP**, **5 nP**; Figure S34: Fluorescence spectra of **5 mT** in Acetonitrile solutions \(10^{-6}\) M, \(10^{-5}\) M and \(10^{-3}\) M; Figure S35: UV-vis spectra of **5 mT** in Ethanol, THF, and THF/H\({}_{2}\)O 1:1; Figures S36-S38: Fluorescence spectra of **5 mT** in Ethanol, THF, and THF/H\({}_{2}\)O 1:1; Figure S39: Solutions of **5 mT** under UV lamp long-wave; Figure S40: Calculations, of the dipolar moments in the ground and excited states of compound **5 mT** by using Lippert-Megata (a), Bakhshiev (b), KawsKi-Chamma (c), McRae (d), Reichard (e) equations; Figure S41: Suspension of compound **5 mT** in water. References [65; 66] are cited in the Supplementary Materials.  
C.C.: research design, manuscript organization, and writing. Y.M.N.: syntheses and spectroscopy analyses. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Technology in Taiwan, grant numbers MOST 110-2113-M-039-001 and MOST 111-2221-E-039-009.  
Not applicable.  
All data and analyses are available in the manuscript and in the Supplementary Materials.  
The authors declare no conflict of interest.