Next Article in Journal
Silicone Elastomeric-Based Materials of Soft Pneumatic Actuator for Lower-Limb Rehabilitation: Finite Element Modelling and Prototype Experimental Validation
Previous Article in Journal
Automatic Quantitative Coronary Analysis Based on Deep Learning
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 13
Issue 5
10.3390/app13052976
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Synthesis of New 2D-π-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties

by
Yoke Mooi Ng
and
Carmine Coluccini
*
Institute of New Drug Development, College of Medicine, China Medical University, No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(5), 2976; https://doi.org/10.3390/app13052976
Submission received: 21 December 2022 / Revised: 14 February 2023 / Accepted: 22 February 2023 / Published: 25 February 2023
(This article belongs to the Section Chemical and Molecular Sciences)
Browse Figures
Review Reports Versions Notes

Abstract

:

Featured Application

We designed the synthesized compounds for application as biosensors, the compound 5mT in particular, due to the emission frequency variability in the different media.

Abstract

Emitting organic molecules can find application in Light-Emitting Diodes and as Biosensors. The new generation of organic emitters are full conjugated molecules exhibiting conformational freedom that can gain emission intensity when accumulate. We synthesized new tetraphenyl fulvene (TPF) derivatives by connecting the phenyl rings 3 and 4 with electron donor groups and the fulvene carbon 6 with two electron-withdrawing groups. We analyzed the optical properties, UV–vis absorption, and emission in different solvents with different polarities. The compound with π-donor thiophene and π-acceptor methyl malonate, named 5mT, displays the highest emission intensity compared to unsubstituted TPF, the compounds with a weak electron π-donor group on phenyl rings 3 and 4, and the weak π-withdrawing group on carbon 6 of the fulvene core. The same compound exhibits emission frequencies in solutions that vary from 435 to 495 nm in the different solvents, while the emission frequency is 435 nm independently of the solvent for the other TPF derivatives. We demonstrated that D-π-A TPF derivatives with high dipolar moments display a fluorescence that is strongly influenced by conformational status and intermolecular interactions.

Graphical Abstract

1. Introduction

Small organic emitting molecules are gaining attention for the fabrication of organic light-emitting diodes (OLEDs), due to their easy synthesis and purification, and they are also suitable for device fabrication based on vapor deposition techniques that allow the possibility of building up multiple layers with excellent performances [1]. In biosensing, small organic emitting molecules exhibit the highest selectivity and sensitivity compared to polymers, and they are less invasive, allowing real-time analysis of living systems [2]. The emissive capabilities of molecules with extended π-conjugations are the object of extensive studies due to the HOMO–LUMO energy gap in the UV–visible frequencies, and a need to study different molecular systems has arisen from evidence that the π-conjugated molecular skeleton shape and dynamics strongly affect the electro-optical properties [3,4]. The first excited state presents fluorescence in the majority of organic molecules, and the emission frequency depends on the molecular conformational status in the excited state that, through a radiative process, decays toward the ground state [5,6]. Conjugated polymers exhibit the highest absorption and emission intensities, and small molecules are easier to process [7,8,9,10,11]. Conventional emitting compounds are planar molecules with extended π-conjugation exhibiting high charge mobility [12,13,14]. The molecular packing mode strongly influences the emission by affecting frequencies and intensity [15]. As reported in the literature, traditional fluorophores often suffer emission quenching in an accumulated state (concentrated solutions, solid state, accumulation on the bioanalyte’s surface), which limits their application [16,17]. The phenomenon origins from the π–π 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 [23,26,27,28,29,30]. The molecules are not D-π-A; their helical shape obstructs the π–π 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-π-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–functionalization [33,34]. Substitution of position 6,6′ 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, S0 frontier electrons occupy HOMO orbitals. The Fulvene electronic excited states S1 and S2 correspond to the HOMO → LUMO and HOMO-1 → LUMO electron transitions. According to the Blancfort model, in conditions of free intramolecular motions, the emission (around 430 nm) is generated by S2 state decay, and in conditions of restricted intramolecular rotations, the emission (around 460 nm) is mainly generated by S1 decay. Tetraphenylfulvenes, shown in Figure 1b, with –COOMe and –CN groups in position 6,6′ 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 → LUMO electron transition (S2 state generation). The HOMO → LUMO transition seems to exhibit a signal with low intensity at 380 nm. Tetraphenyl fulvenes with the acceptor groups –C(CN)2 and –C(COOMe)2 in position 6,6′ display similar emission spectra in different solvents. They exhibit maxima at 435 nm, which should be generated from S2 decay, similar to the other fulvene derivatives, which display the same emission peak in solution.
To increase the dipolar moment of the tetraphenyl fulvenes derivatives, we thought to connect electron donor D groups to the phenyl rings 3 and 4, as shown in Figure 1c. We expected this substitution to inject electron charges to the fulvene unit and to induce a molecular dipolar moment. We do not expect the same by substitution on the 2,5 phenyl groups that are forced to lay perpendicular to the pentadienyl ring because of the steric hindrance of 6,6′ large substituents. In this way, the compounds, upon UV–vis light irradiation, could exhibit charge separation, which could enhance and redshift the fluorescence. We synthesized 2D-π-2A chromophores, shown in Figure 2, where the π-Bridge was the tetraphenyl fulvene. As the p electron donor, we selected the 2-thiophene compounds 5mT and 5nT. For understanding the donor group role, we also synthesized the compounds 5mF, 5nF, 5mP, and 5nP, which exhibit 4-fluorophenyl as weak π-electron donor groups and N-bonded phenohiazine as the π-electron donor group with shorter π-conjugations. We analyzed the UV absorption and the emission in different polarity solvents to understand whether the compounds exhibit charge separation under light irradiation and how this charge separation affects the emission.

2. Materials and Methods

4,4-Dibromobenzyl, 1,3-diphenylacetone, Pd complexes, methanol, ethanol (CH3CH2OH), tetrahydrofuran (THF), dichloromethane (CH2Cl2), toluene, and acetonitrile (CH3CN) were acquired from commercial sources. The 1H and 13C NMR spectra were recorded for solutions in CDCl3 and DMSO-d6 on a Bruker Avance III 400 MHz BBFO Probe and Bruker AVANCE NEO 400 NMR with the solvent residual proton signal. Flash column chromatography was performed using Silicycle Silica gel 60 (7–230 Mesh, pH 7). UV–vis absorption spectra of all samples in Methanol, DMSO, and H2O were acquired using a Thermo Scientific Multiskan G0 microplate and cuvette spectrophotometer in the range of 200–1000 nm, respectively. Fluorescence spectra were detected with an Edinburgh instruments FS5 Spectrofluorometer equipped with an SC-05 standard cuvette holder and Perkin Elmer LS50B Luminescence Spectrometer.
Synthesis of 3,4-Bis-(4-bromophenyl)-2,5-diphenylcyclopentadienone (3). 4, 4-Dibromobenzil 1 (1.2 g, 3.26 mmol), 1,3-diphenylacetone 2 (600 mg, 2.86 mmol), anhydrous KOH (200 mg, 3.59 mmol), and absolute ethyl alcohol (15 mL) were charged in a 250 mL round bottom flask. The mixture was stirred for 30 min under reflux. The solvent was evaporated under reduced pressure. Column chromatography on silica gel and cyclohexane/ethyl acetate 95:5 as the mobile phase afforded pure product as a red solid (920 mg, 1.70 mmol, yield 59%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.35 (dd, J = 2.0, 8.6 Hz, 4H), 7.30–7.24 (m, 6H), 7.22–7.19 (m, 4H), 6.80 (dd, J = 2.0, 8.6 Hz, 4H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 199.62, 152.59, 131.67, 131.54, 130.97, 130.21, 130.10, 128.26, 127.87, 125.92, 123.16.
Synthesis of 3,4-Bis-(4-bromophenyl)-2,5-diphenyl-1,1′-bis-methylmalonyl-fulvene (5m). To an ice-cooled solution of 3,4-Bis-(4-bromophenyl)-2,5-diphenylcyclopentadienone 3 (0.563 g, 1.039 mmol) and dimethyl malonate 4m (549 mg, 0.5 mL, 4.156 mmol, 4 eq) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 5.195 mmol, 985.39 mg, 0.570 mL) over 5 min and pyridine (24 eq, 1.964 g, 24.83 mmol, 2 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl 0.1 M were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and cyclohexane/ethyl acetate 85:15 as the mobile phase afforded pure product as a red solid (674 mg, 1.027 mmol, 99%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.28–7.22 (m, 6 H), 7.16 (dd, J = 2.0, 8.6 Hz, 4H), 7.15–7.13 (m, 4 H), 6.57 (dd, J = 2.0, 8.6 Hz, 4H), 3.05 (s, 6 H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 164.46, 149.63, 146.16, 134.40, 133.37, 132.57, 132.42, 131.45, 130.77, 130.49, 127.97, 127.17, 121.67, 52.49.
Synthesis of 3,4-Bis-(4-bromophenyl)-2,5-diphenyl-1,1′-bis-malononitril-fulvene (5n). To an ice-cooled solution of 3,4-Bis-(4-bromophenyl)-2,5-diphenylcyclopentadienone 3 (0.563 g, 1.039 mmol) and malononitrile 4n (275 mg, 4.156 mmol) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 5.195 mmol, 985.39 mg, 0.570 mL) over 5 min and pyridine (24 eq, 1.964 g, 24.83 mmol, 2 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl (0.1 M) were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and cyclohexane/ethyl acetate 9:1 as the mobile phase afforded pure product as a dark-green solid (313 mg, 0.530 mmol, 51%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.35–7.29 (m, 6 H), 7.21–7.18 (m, 4H), 7.15 (d, 8.6 Hz, 4H), 6.64 (d, 8.6 Hz, 4H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 167.61, 149.31, 132.33, 131.23, 131.18, 131.05, 130.98, 130.89, 129.39, 128.93, 123.38, 111.05, 87.40.
Synthesis of 3,4-Bis-4-[(5-methyl)-tiofenyl]-2,5-diphenylcyclopentadienone (5T). A solution of 5-Methylthiophene-2-boronic acid pinacol ester 4T (0.58 mg, 2.59 mmol), 3 (364 mg, 0.53 mmol), [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II)*CH2Cl2 (45 mg, 0.053 mmol, 0.1 eq), K2CO3 (366 mg, 2.65 mmol, 5 eq) in dry toluene (20 mL), and dry methanol (20 mL) under nitrogen atmosphere was stirred at reflux for 18 h. At room temperature, H2O was added, and the mixture was extracted with CH2Cl2 (3 × 100 mL). Organic phase was dried with Na2SO4. The crude of the reaction was a red oil. Column chromatography on silica gel and 9/1 hexane: ethyl acetate as mobile phase afforded pure product as a brown solid (194.70 mg, 64%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.39 (d, J = 8.6 Hz, 4H), 7.30–7.27 (m, 10H), 7.12 (d, J = 3.4, 2H), 6.95 (d, J = 8.6 Hz, 4H), 6.73 (d, J = 3.4, 2H), 2.50 (s, 6H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 200.03, 153.71, 141.20, 140.27, 134.80, 131.46, 130.88, 130.21, 130.15, 128.13, 127.51, 126.42, 125.46, 124.67, 123.49, 15.53.
Synthesis of 3,4-Bis-4-[(4-fluoro)-phenyl]-2,5-diphenylcyclopentadienone (5F). A solution of 4-Fluorophenylboronic acid, pinacol ester 4F (573.31 mg, 2.58 mmol, 4 eq), 3 (350 mg, 0.645 mmol), [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II)*CH2Cl2 (52.66 mg, 0.0645 mmol, 0.1 eq), K2CO3 (445.69 mg, 3.225 mmol, 5 eq) in dry toluene (20 mL), and dry methanol (20 mL) under nitrogen atmosphere was stirred at reflux for 18 h. At room temperature, H2O was added, and the mixture was extracted with CH2Cl2 (3 × 100 mL). Organic phase was dried with Na2SO4. The crude of the reaction was a red oil. Column chromatography on silica gel and 9.3/0.7 hexane: ethyl acetate as mobile phase afforded pure product as a brown solid (269 mg, 72%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.58–7.52 (m, 4H), 7.41 (d, J = 8.6 Hz, 4H), 7.32–7.24 (m, 10H), 7.12 (t, J = 8.6 Hz, 4H), 7.05 (d, J = 8.6 Hz, 4H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 200.05, 163.90, 161.44, 153.76, 140.07, 136.22, 131.96, 130.77, 130.21, 130.10, 128.57, 128.49, 128.14, 127.58, 126.36, 125.65, 115.86, 115.64.
Synthesis of 3,4-Bis-4-[(5-methyl)-tiofenyl]-phenyl-2,5-diphenyl-1,1′-bis-methylmalonyl-fulvene (5mT, 2nd procedure). To an ice-cooled solution of 3,4-Bis-4-[(5-methyl)-tiofenyl]-2,5-diphenylcyclopentadienone 5T (0.374 g, 0.65 mmol) and dimethyl malonate 4m (343.49 mg, 0.3 mL, 2.6 mmol, 4 eq) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 3.25 mmol, 616.46 mg, 0.356 mL) over 5 min and pyridine (24 eq, 1.233 g, 15.6 mmol, 1.2 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl 0.1 M were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and hexane/ethyl acetate 9:1 as the mobile phase afforded pure product as a light-brown solid (300 mg, 1.027 mmol, 67%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.30–7.15 (m, 14 H), 7.03 (d, J = 3.4, 2H), 6.72 (d, J = 8.6 Hz, 4H), 6.67 (dd, J = 1.3, 4.1, 2H), 3.07 (s, 6H), 2.48 (s, 6H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 164.80, 147.35, 141.49, 139.67, 134.98, 132.77, 132.44, 131.25, 130.62, 130.49, 127.89, 126.90, 126.21, 124.12, 122.97, 52.40, 15.49. MALDI-TOF mass [M + H]+ calc. 691.1971, exp. 691.1945.
Synthesis of 3,4-Bis-4-[(5-methyl)-tiofenyl]-phenyl-2,5-diphenyl-1,1′-bis-malononitryl-fulvene (5nT). To an ice-cooled solution of 3,4-Bis-4-[(5-methyl)-tiofenyl]-2,5-diphenylcyclopentadienone 5T (0.343 g, 0.595 mmol) and malononitrile 4n (0.157 g, 2.38 mmol, 4 eq) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 2.975 mmol, 0.326 mL, 0.564 g) over 5 min and pyridine (24 eq, 1.129 g, 14.28 mmol, 1.16 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl 0.1 M were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and hexane/ethyl acetate 8.5:1.5 as the mobile phase afforded pure product as a dark-brown solid (94 mg, 25%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.45–7.39 (m, 6H), 7.37–7.32 (m, 4H), 7.26 (d, J = 8.6 Hz, 4H), 7.05 (d, J = 3.4, 2H), 6.88 (d, J = 8.6 Hz, 4H), 6.68 (dd, J = 1.3, 4.1, 2H), 2.47 (s, 6H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 206.97, 150.33, 140.96, 140.51, 134.90, 131.93, 131.61, 131.19, 130.93, 130.61, 129.15, 128.83, 126.43, 124.20, 123.65, 111,48, 85.73, 15.52. MALDI-TOF mass [M + H]+ calc. 625.1767, exp. 625.1782.
Synthesis of 3,4-Bis-4-[(4-fluoro)-phenyl]-phenyl-2,5-diphenyl-1,1′-bis-methylmalonyl-fulvene (5mF). To an ice-cooled solution of 3,4-Bis-4-[(4-fluoro)-phenyl]-2,5-diphenylcyclopentadienone 5F (200.00 mg, 0.349 mmol) and dimethyl malonate 4m (184.57 mg, 0.16 mL, 1.397 mmol, 4 eq) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 1.745 mmol, 331.001 mg, 0.191 mL) over 5 min and pyridine (24 eq, 662.542 g, 8.376 mmol, 0.675 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl 0.1 M were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and hexane/ethyl acetate 8:2 as the mobile phase afforded pure product as a light-brown solid (130 mg, 0.189 mmol, 54%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.49–7.43 (m, 4H), 7.30–7.18 (m, 14H), 7.05 (t, J = 8.6 Hz, 4H), 6.81 (d, J = 8.6 Hz, 4H), 3.08 (s, 6H). 13C NMR (100.61 MHz, CDCl3) ppm, δ: 164.72, 163.69, 150.32, 147.34, 138.58, 136.50, 136.47, 134.94, 132.99, 132.89, 131.58, 130.66, 130.52, 128.44, 128.35, 127.89, 126.95, 125.72, 115.68, 115.46, 52.44. MALDI-TOF mass [M + H]+ calc. 687.2341, exp. 687.2338.
Synthesis of 3,4-Bis-4-[(4-fluoro)-phenyl]-phenyl-2,5-diphenyl-1,1′-bis-malonytril-fulvene (5nF). To an ice-cooled solution of 3,4-Bis-4-[(4-fluoro)-phenyl]-2,5-diphenylcyclopentadienone 5F (200 mg, 0.349 mmol) and malononitrile 4n (92.41 mg, 1.396 mmol, 4 eq) in 20 mL of dichloromethane (dry) under argon atmosphere, TiCl4 (5 eq, 1.745 mmol, 331 mg, 0.91 mL) over 5 min and pyridine (24 eq, 0.662 g, 8.37 mmol, 0.67 mL) were added dropwise. The reaction mixture was allowed to warm at room temperature and stirred overnight. A total of 50 mL of dichloromethane and 50 mL of HCl 0.1 M were added. The mixture was extracted with dichloromethane (3 × 100 mL) and dried with Na2SO4. Column chromatography on silica gel and hexane/ethyl acetate 8.5:1.5 as the mobile phase afforded pure product as a dark-green solid (79 mg, 36%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.50–7.40 (m, 12H), 7.29 (d, J = 8.6 Hz, 4H), 7.07 (t, J = 8.6 Hz, 4H), 6.97 (d, J = 8.6 Hz, 4H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 168.23, 163.92, 161.46, 150.41, 140.16, 136.06, 131.93, 131.83, 131.39, 131.17, 130.53, 129.21, 128.85, 128.54, 128.47, 125.94, 115.83, 115.62, 111.36. MALDI-TOF mass [M + H]+ calc. 621.2137, exp. 621.2113.
Synthesis of 3,4-Bis-4-[1-phenotyazenyl]-phenyl-2,5-diphenyl-1,1′-bis-methylmalonyl-fulvene (5mP). A mixture of 5m (0.24489 g, 0.379 mmol), phenothiazine 4P (151 mg, 0.758 mmol), catalyst palladium diacetate (6.8 mg, 0.030 mmol), tri-tert-butylphosphionium tetrafluoroborate (17.5 mg, 0.0606 mmol), and sodium tert-butoxide (109.55 mg, 1.14 mmol) in toluene (30 mL) was stirred at 110 °C for 72 h. After the reaction cooled to room temperature, 50 mL dichloromethane was added into the reaction solution, which was then washed with 100 mL of brine followed by 100 mL of distilled water. The organic layer was collected, dried over Na2SO4, and evaporated under reduced pressure. Column chromatography on silica gel and ethyl acetate/hexane 2:8 as eluent afforded the pure product (133.83 mg, 44%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.30–7.20 (m, 10H), 7.03 (d, J = 3.4, 4H), 6.96 (t, J = 8.6 Hz, 8H), 6.78–6.68 (m, 8H), 6.08 (d, J = 7.6 Hz, 4H), 3.07 (s, 6H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 164.71, 147.37, 143.96, 140.53, 134.80, 133.75, 133.43, 132.62, 132.27, 130.74, 128.70, 128.07, 127.41, 127.11, 127.08, 122.99, 121.74, 116.95, 52.69. MALDI-TOF mass [M + H]+ calc. 893.250, exp. 893.251.
Synthesis of 3,4-Bis-4-[(4-fluoro)-phenyl]-phenyl-2,5-diphenyl-1,1′-bis-malonytril-fulvene (5nP). A mixture of 5n, 0.256 g, 0.0.434 mmol), phenothiazine 4P (173 mg, 0.868 mmol), catalyst palladium diacetate (7.8 mg, 0.0347 mmol), tri-tert-butylphosphionium tetrafluoroborate (20.15 mg, 0.070 mmol), and sodium tert-butoxide (125 mg, 1.302 mmol) in toluene (30 mL) was stirred at 110 °C for 72 h. After the reaction cooled to room temperature, 50 mL dichloromethane was added into the reaction solution, which was then washed with 100 mL of brine followed by 100 mL of distilled water. The organic layer was collected, dried over Na2SO4, and evaporated under reduced pressure. Column chromatography on silica gel and ethyl acetate/hexane 2:8 as mobile phase afforded pure product (99.1 mg, 32%). 1H-NMR (400 MHz, CDCl3) ppm, δ: 7.45–7.35 (m, 10H), 7.11 (d, J = 7.2, 4H), 6.95–6.85 (m, 8H), 6.87 (m, 8H), 6.44 (d, J = 7.2, 4H). 13C-NMR (100.61 MHz, CDCl3) ppm, δ: 168.09, 150.21, 142.97, 142.87, 131.99, 131.83, 131.58, 131.16, 129.89, 129.30, 128.88, 127.65, 126.99, 125.63, 124.14, 123.93, 119.87, 111.48. MALDI-TOF mass [M + H]+ calc. 827.230, exp. 827.231.

3. Results

3.1. Synthesis

We prepared compound 3 of Scheme 1, the TPCP unit with two bromines linked to the phenyl rings 3 and 4, by the condensation reaction of compounds 1 and 2 in an alkaline environment, as reported in the literature [50,51].
For the synthesis of the final products 5mT, 5mF, 5nT, and 5nF, we modified compound 3 by a Suzuki-type reaction with boronic esters to synthesize compounds 5T and 5F of Scheme 2. We converted the compounds 5T and 5F in the final products by reaction with dimethyl malonate and malononitrile in the presence of TiCl4 and pyridine (this kind of reaction is known in the literature) [34,52]. We also functionalized compound 3 with methyl-malonate and malononitrile to synthesize compounds 5m and 5n of Scheme 2, which we converted into the final products 5mP and 5nP by Buchwald Harthwig reaction (Figures S1–S22, NMR spectra of all synthesized compounds). We also prepared compound 5mD from 4m as described in the Supporting Information (Scheme S1). We did not obtain the desired products 5aF, 5nT, or 5bF by the Suzuki reaction on 4m and 4n. The Buchwald Harthwig reactions on compound 3 also failed to functionalize the tetraphenylcyclopentadienone with phenothiazine to obtain 5P as a precursor of 5mP and 5nP (see Supporting Information for the scheme of the failed reactions that induced us to prepare the products according to Scheme 2).

3.2. Optical Properties

We investigated the light absorption and emission of all compounds in two solvents of different polarity, dichloromethane and acetonitrile. Table 1 lists the optical properties of the analyzed compounds.

3.2.1. UV–Vis Absorption Spectra

As shown in Figure 3 for 5mT and 5nT in CH3CN (see Figures S23 and S24 of Supporting Information for spectra of all compounds in CH3CN and CH2Cl2), the compounds exhibited three maxima, λ1, λ2, and λ3. The λ1 abs was not visible in CH2Cl2 for all compounds due to the solvent cut-off. The –CN substituent in position 6,6′ moved the λ3 absorption to the highest wavelengths with respect to –COOMe. The compounds with fluorophenyl substituents 5mF and 5nF displayed blue shift of the λ2,3 abs maxima of around 50 nm compared with the corresponding compounds with thiophenyl substituents. The compounds with phenothiazine 5mP and 5nP exhibited the intense signal at 250 nm due to the phenothiazine substituent (Figure S25, Supporting Information, the Phenothiazine UV spectrum) and the λ3 was difficult to visualize due to the low intensity, similar to the corresponding tetraphenyl fulvenes unsubstituted on the phenyl rings in a previous study [41].

3.2.2. Fluorescence Spectra

We determined the fluorescence spectra in dichloromethane and acetonitrile by irradiation at different wavelengths. At a concentration of 10−6 M and under irradiation at λ2 or λ3 abs frequency maxima, as shown in Figure 4 (Figure S26 in Supporting Information, all measured fluorescence spectra in dichloromethane 10−6 M), the spectra in dichloromethane of all compounds exhibited the main maximum at 435 nm and two minor maxima at 410 and 457 nm.
At a higher concentration of 10−5 M (see Figure S27 in Supporting Information for all spectra at this concentration), the emission spectra for compound 5mT, with the maximum at 457 nm, was more intense, and the maximum at 410 nm disappeared under irradiation at λ2. For compound 5nT, the maximum at 456 nm disappeared under irradiation at l2 abs maximum. Compound 5mT displayed the highest fluorescence intensities as compared with 5nT, 5mF, 5mP, 5nP, and 5bF in dichloromethane at both analyzed concentrations (Figure 4 and Figures S26 and S27 in Supporting Information). In acetonitrile, all compounds but 5mT exhibited strongly decreased emission with maximum at 435 nm (see Figures S28–S33 in Supporting Information for all fluorescence spectra in CH2Cl2 and CH3CN). In acetonitrile solutions of 10−6 M and 10−5 M, 5mT exhibited frequency maxima at around 460 and 485 nm, respectively, with a higher intensity in the most concentrated solution (Figure S28 of Supporting Information).
We focused on studying the fluorescence modulation of compound 5mT by detecting spectra at different concentrations and in different solvents. We analyzed the acetonitrile and dichloromethane solutions at around 10−3 M. In dichloromethane, no emission was detected; in acetonitrile, the maximum redshifts at 495 nm had a noticeably increased intensity as compared with the diluted solutions (Figure S34). We measured the emission in THF, ethanol, and THF/H2O 1:1. In these solvents, the UV spectra displayed the same absorption frequencies as in dichloromethane and acetonitrile (Figure S35, see Figures S36–S38 for all fluorescence spectra). In THF solutions with concentrations of around 10−6 M, we observed a very weak emission maxima at around 348 and 463 nm. At concentrations around 10−5 M, depending on the excitation frequency, we observed a major maximum at 432 nm with two minor maxima at 410 and 457 nm, or an emission centered on a maximum at 460 nm. At concentrations of around 10−3 M, no emission was detected. In ethanol solutions, the emission frequency maxima appeared similar to those in acetonitrile solutions. The difference was that the intensity maxima were highest in the 10−5 M solutions. In solution THF/water 1:1, the emission intensity was very weak (maximum at 485 nm) in 10−6 M solutions and was strongest in 10−5 M solutions. We registered the highest intensity with redshift at 495 nm in the most concentrated solutions, the 10−3 M solutions, with noticeably increased intensity. Figure 5a,b presents plots describing the frequency and intensity of emissions that we recorded for the solutions analyzed; Figure 5c presents the most intense registered fluorescence spectra at concentrations of 10−6, 10−5, and 10−3 M.
Under long-wave UV irradiation, the solutions displayed emissions varying from blue to cyan (Figure S39), in agreement with the detected emission spectra. Dichloromethane solutions exhibited blue emission that tended to disappear with increases in concentration. CH3CN, THF/H2O 1:1, and ethanol solutions exhibited blue emission at low concentration but cyan emission at high concentration. Coherently with the emission spectra, in acetonitrile and THF/H2O 1:1, the cyan emission of the concentrated solution was more intense than the blue emission of the diluted solutions. In the less polar ethanol, the cyan emission of the concentrated solution was less intense than the blue emission of the diluted solutions. In THF, we only observed blue emission from the 10−5 M solution.
In Table 2, the emission maxima ranges and intensities are correlated with the solvent polarities. It is important to observe that we observed emissions at all concentrations in the solvents of intermediate polarity CH3CN and CH3CH2OH; we observed no emissions in diluted solutions with the highest polarities (THF/H2O 1:1); and we observed no emissions in concentrated solutions in the solvents with low polarities (THF, CH2Cl2). Furthermore, the high polarity solvents tended to redshift the emission, the low polarity solvents tended to lower the emission frequency, and the intermediate polarity solvents allowed observation of a wide range of emission frequency maxima.
Based on the emissions and absorption maxima in solvents of different polarity, we were able to calculate the dipolar moments of the energy ground and excited states by the method reported in the literature [55,56,57]. We performed the calculations, the results shown in Table 3, based on the data at concentrations of 10−6 and 10−5 M (for details of calculation, see Figure S40 in Supplementary Information). The table shows that the dipolar moment was highest in the excited state and that the difference between the excited and ground states increased with the concentration.
We analyzed the emission of 10−3 M solutions of 5mT in THF/H2O with ratios of 7:3, 4:6, and 1:9 to understand the effect of aggregation on emission (water is not solvent for 5mT, as shown in Figure S41). As shown in Figure 6, the addition of water generated a blue shift in the emission. The emission intensity increased when the THF/H2O fraction changed from 1 to 4:6. In the 1:9 solution, no increased intensity was detected, and the spectral width upon irradiation at 300 nm.

4. Discussion

That the emission of compound 5mT was the highest compared with the other compounds was expected, considering that it was the compound with the highest D-π-A structure and highest dipolar moment, due to the presence of a strong π-donor group conjugated with a strong π-acceptor group, which should induce more stablecharge separation in the excited state. We can explain the behavior of 5mT in solvents of different polarity in terms of excited state stabilization or intermolecular interactions. Polar solvents can redshift the fluorescence by stabilizing the excited state with charge separation [62]. Molecular emission, or π–π* fluorescence, displays a spectrum with multiple peaks, whilst excimer emission from an excited multimer displays a broad peak at a longer wavelength [63]. Diluted solutions of less polar solvents (CH2Cl2 and THF) may display molecular emission, and the highly concentrated solutions induce π–π aggregation in the excited state, which decreases the fluorescence. Polar solvents (CH3CN, ethanol) can stabilize the excited state and redshift the fluorescence in diluted solutions by stabilizing the excited state. Enhancing the concentration further leads to redshift, and we can hypothesize that intermolecular interactions generate excimers with increased dipolar moments. In the most polar solvent, acetonitrile, the most polar excimer formation is favored, as shown by the emission intensities in concentrated solutions. The increased polarity induced by a non-solvent-like water in THF produces complex aggregation, and a dual behavior manifests at low and high THF/H2O ratios, similarly to unsymmetrical tetraphenylethene substituted donor–acceptor benzothiadiazoles reported in 2015 [64]. At high THF/H2O ratios, it is possible that excited state stabilization and formation of high polar aggregates is predominant, and a red shifted fluorescence is observed. The blue shift at low THF/H2O ratios can be explained by further enhanced aggregation, which induces the restriction of intramolecular rotation.

5. Conclusions

In this research work, we investigated the optical properties of the 2D-π-2A molecular design, with tetraphenylfulvene as a π-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 (l2) that was not visible (or difficult to detect) in the analogue compounds without the donor group. Compounds with stronger π-donor and π-acceptor groups display more intense fluorescence, which is highly affected by the environment. Compound 5mT 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 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 5mT 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.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app13052976/s1, Figures S1–S22: 1H and 13C NMR spectra of compounds 5mT, 5nT, 5mF, 5nF, 5mP, 5nP; Scheme S1: Alternative procedure for synthesis of 5mT. Schemes S2 and S3. Failed syntheses; Figures S23–S25: UV–vis spectra in CH2Cl2 and CH3CN 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 5mT, 5nT, 5mF, 5nF, 5mP, 5nP; Figure S34: Fluorescence spectra of 5mT in Acetonitrile solutions 10−6 M, 10−5 M and 10−3 M; Figure S35: UV–vis spectra of 5mT in Ethanol, THF, and THF/H2O 1:1; Figures S36–S38: Fluorescence spectra of 5mT in Ethanol, THF, and THF/H2O 1:1; Figure S39: Solutions of 5mT under UV lamp long-wave; Figure S40: Calculations, of the dipolar moments in the ground and excited states of compound 5mT by using Lippert-Megata (a), Bakhshiev (b), KawsKi-Chamma (c), McRae (d), Reichard (e) equations; Figure S41: Suspension of compound 5mT in water. References [65,66] are cited in the Supplementary Materials.

Author Contributions

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.

Funding

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.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data and analyses are available in the manuscript and in the Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Duan, L.; Hou, L.; Lee, T.W.; Qiao, J.; Zhang, D.; Dong, G.; Wang, L.; Qiu, Y. Solution processable small molecules for organic light-emitting diodes. J. Mater. Chem. 2010, 20, 6392–6407. [Google Scholar] [CrossRef] [Green Version]
  2. Tian, X.; Murfin, L.C.; Wu, L.; Lewis, S.E.; James, T.D. Fluorescent small organic probes for biosensing. Chem. Sci. 2021, 12, 3406–3426. [Google Scholar] [CrossRef]
  3. Yamaguchi, Y.; Matsubara, Y.; Ochi, T.; Wakamiya, T.; Yoshida, Z.I. How the π conjugation length affects the fluorescence emission efficiency. J. Am. Chem. Soc. 2008, 130, 16442. [Google Scholar] [CrossRef] [Green Version]
  4. Milián-Medina, B.; Gierschner, J. π-Conjugation. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 513–524. [Google Scholar] [CrossRef]
  5. Itoh, T. Fluorescence and phosphorescence from higher excited states of organic molecules. Chem. Rev. 2012, 112, 4541–4568. [Google Scholar] [CrossRef]
  6. Lichtman, J.W.; Conchello, J.A. Fluorescence microscopy. Nat. Methods 2005, 2, 910–919. [Google Scholar] [CrossRef]
  7. Forrest, S.R.; Thompson, M.E. Introduction: Organic electronics and optoelectronics. Chem. Rev. 2007, 107, 923–925. [Google Scholar] [CrossRef]
  8. Mitschke, U.; Bäuerle, P. The electroluminescence of organic materials. J. Mater. Chem. 2000, 10, 1471–1507. [Google Scholar] [CrossRef]
  9. Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent progresses on materials for electrophosphorescent organic light-emitting devices. Adv. Mater. 2011, 23, 926–952. [Google Scholar] [CrossRef] [PubMed]
  10. Thejo Kalyani, N.; Dhoble, S.J. Organic light emitting diodes: Energy saving lighting technology—A review. Renew. Sustain. Energy Rev. 2012, 16, 2696–2723. [Google Scholar] [CrossRef]
  11. Yu, T.; Liu, L.; Xie, Z.; Ma, Y. Progress in small-molecule luminescent materials for organic light-emitting diodes. Sci. China Chem. 2015, 58, 907–915. [Google Scholar] [CrossRef]
  12. Tang, C.W.; Vanslyke, S.A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913–915. [Google Scholar] [CrossRef]
  13. Ma, D.; Wang, G.; Hu, Y.; Zhang, Y.; Wang, L.; Jing, X.; Wang, F.; Lee, C.S.; Lee, S.T. A dinuclear aluminum 8-hydroxyquinoline complex with high electron mobility for organic light-emitting diodes. Appl. Phys. Lett. 2003, 82, 1296–1298. [Google Scholar] [CrossRef]
  14. Burroughes, J.H.; Bradley, D.D.C.; Brown, A.R.; Marks, R.N.; Mackay, K.; Friend, R.H.; Burns, P.L.; Holmes, A.B. Light-emitting diodes based on conjugated polymers. Nature 1990, 347, 539–541. [Google Scholar] [CrossRef]
  15. Wang, C.; Li, Z. Molecular conformation and packing: Their critical roles in the emission performance of mechanochromic fluorescence materials. Mater. Chem. Front. 2017, 1, 2174–2194. [Google Scholar] [CrossRef]
  16. Qian, G.; Zhong, Z.; Luo, M.; Yu, D.; Zhang, Z.; Wang, Z.Y.; Ma, D. Simple and efficient near-infrared organic chromophores for light-emitting diodes with single electroluminescent emission above 1000 nm. Adv. Mater. 2009, 21, 111–116. [Google Scholar] [CrossRef]
  17. Liu, J.; Zhang, H.; Dong, H.; Meng, L.; Jiang, L.; Jiang, L.; Wang, Y.; Yu, J.; Sun, Y.; Hu, W.; et al. High mobility emissive organic semiconductor. Nat. Commun. 2015, 6, 10032. [Google Scholar] [CrossRef] [Green Version]
  18. Luo, J.; Xie, Z.; Lam, J.W.Y.; Cheng, L.; Tang, B.Z.; Chen, H.; Qiu, C.; Kwok, H.S.; Zhan, X.; Liu, Y.; et al. Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem. Commun. 2001, 381, 1740–1741. [Google Scholar] [CrossRef] [PubMed]
  19. Ding, D.; Li, K.; Liu, B.; Tang, B.Z. Bioprobes based on AIE fluorogens. Acc. Chem. Res. 2013, 46, 2441–2453. [Google Scholar] [CrossRef]
  20. La, D.D.; Bhosale, S.V.; Jones, L.A.; Bhosale, S.V. Tetraphenylethylene-Based AIE-Active Probes for Sensing Applications. ACS Appl. Mater. Interfaces 2018, 10, 12189–12216. [Google Scholar] [CrossRef]
  21. Mei, J.; Leung, N.L.C.; Kwok, R.T.K.; Lam, J.W.Y.; Tang, B.Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 11718–11940. [Google Scholar] [CrossRef] [PubMed]
  22. Chen, J.; Law, C.C.W.; Lam, J.W.Y.; Dong, Y.; Lo, S.M.F.; Williams, I.D.; Zhu, D.; Tang, B.Z. Restricted Intramolecular Rotation of 1, 1-Substituted 2, 3, 4, 5-Tetraphenylsiloles. Chem. Mater. 2003, 79, 1535–1546. [Google Scholar] [CrossRef]
  23. Ren, Y.; Dong, Y.; Lam, J.W.Y.; Tang, B.Z.; Wong, K.S. Studies on the aggregation-induced emission of silole film and crystal by time-resolved fluorescence technique. Chem. Phys. Lett. 2005, 402, 468–473. [Google Scholar] [CrossRef] [Green Version]
  24. Tang, B.Z.; Zhan, X.; Yu, G.; Sze Lee, P.P.; Liu, Y.; Zhu, D. Efficient blue emission from siloles. J. Mater. Chem. 2001, 11, 2974–2978. [Google Scholar] [CrossRef]
  25. Yu, M.; Huang, R.; Guo, J.; Zhao, Z.; Tang, B.Z. Promising applications of aggregation-induced emission luminogens in organic optoelectronic devices. Photonix 2020, 1, 11. [Google Scholar] [CrossRef]
  26. Hong, Y.; Lam, J.W.Y.; Tang, B.Z. Aggregation-induced emission. Chem. Soc. Rev. 2011, 40, 5361. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  27. Hong, Y.; Lam, J.W.Y.; Tang, B.Z. Aggregation-induced emission: Phenomenon, mechanism and applications. Chem. Commun. 2009, 29, 4332–4353. [Google Scholar] [CrossRef]
  28. Nitti, A.; Villafiorita-Monteleone, F.; Pacini, A.; Botta, C.; Virgili, T.; Forni, A.; Cariati, E.; Boiocchi, M.; Pasini, D. Structure–activity relationship for the solid state emission of a new family of “push–pull” π-extended chromophores. Faraday Discuss. 2017, 196, 143–161. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  29. Liu, B.; Zhang, R. Aggregation induced emission: Concluding Remarks. Faraday Discuss. 2017, 196, 461–472. [Google Scholar] [CrossRef] [PubMed]
  30. Ren, Y.; Lam, J.W.Y.; Dong, Y.; Tang, B.Z.; Wong, K.S. Enhanced Emission Efficiency and Excited State Lifetime Due to Restricted Intramolecular Motion in Silole Aggregates. J. Phys. Chem. B 2005, 109, 1135–1140. [Google Scholar] [CrossRef] [PubMed]
  31. Yuan, W.Z.; Gong, Y.; Chen, S.; Shen, X.Y.; Lam, J.W.Y.; Lu, P.; Lu, Y.; Wang, Z.; Hu, R.; Xie, N.; et al. Efficient solid emitters with aggregation-induced emission and intramolecular charge transfer characteristics: Molecular design, synthesis, photophysical behaviors, and OLED application. Chem. Mater. 2012, 24, 1518–1528. [Google Scholar] [CrossRef]
  32. Wells, J.M.; Parsons, Z.; Jelly, R.; Payne, A.D.; Lewis, S.W. Negative result: 6-(N,N-dimethylamino)fulvene as a reagent for the detection of latent fingermarks on paper surfaces. Forensic Sci. Int. Rep. 2019, 1, 100005. [Google Scholar] [CrossRef]
  33. Finke, A.D.; Diederich, F. 6,6-Dicyanopentafulvenes: Teaching an old dog new tricks. Chem. Rec. 2015, 15, 19–30. [Google Scholar] [CrossRef] [PubMed]
  34. Finke, A.D.; Dumele, O.; Zalibera, M.; Confortin, D.; Cias, P.; Jayamurugan, G.; Gisselbrecht, J.-P.; Boudon, C.; Schweizer, W.B.; Gescheidt, G.; et al. 6,6-Dicyanopentafulvenes: Electronic structure and regioselectivity in [2 + 2] cycloaddition-retroelectrocyclization reactions. J. Am. Chem. Soc. 2012, 134, 18139–18146. [Google Scholar] [CrossRef] [PubMed]
  35. Finke, A.D.; Jahn, B.O.; Saithalavi, A.; Dahlstrand, C.; Nauroozi, D.; Haberland, S.; Gisselbrecht, J.P.; Boudon, C.; Mijangos, E.; Schweizer, W.B.; et al. The 6,6-dicyanopentafulvene core: A template for the design of electron-acceptor compounds. Chem.—A Eur. J. 2015, 21, 8168–8176. [Google Scholar] [CrossRef] [PubMed]
  36. Choi, S.K.; Rho, J.; Yoon, S.E.; Seok, J.H.; Kim, H.; Min, J.; Yoon, W.; Lee, S.; Yun, H.; Kwon, O.P.; et al. Full Color Tunable Aggregation-Induced Emission Luminogen for Bioimaging Based on an Indolizine Molecular Framework. Bioconjug. Chem. 2020, 31, 2522–2532. [Google Scholar] [CrossRef] [PubMed]
  37. Peloquin, A.J.; Stone, R.L.; Avila, S.E.; Rudico, E.R.; Horn, C.B.; Gardner, K.A.; Ball, D.W.; Johnson, J.E.B.; Iacono, S.T.; Balaich, G.J. Synthesis of 1, 3-diphenyl-6-alkyl/aryl-substituted fulvene chromophores: Observation of π–π interactions in a 6-pyrene-substituted 1, 3-diphenylfulvene. J. Org. Chem. 2012, 77, 6371–6376. [Google Scholar] [CrossRef]
  38. Wang, Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. Dibenzosuberenylidene-Ended Fluorophores: Rapid and Efficient Synthesis, Characterization, and Aggregation-Induced Emissions. J. Phys. Chem. B 2005, 109, 19627–19633. [Google Scholar] [CrossRef] [PubMed]
  39. Ooyama, Y.; Sagisaka, R.; Enoki, T.; Tsunoji, N.; Ohshita, J. Tetraphenylethene-and diphenyldibenzofulvene-anthracene-based fluorescence sensors possessing photo-induced electron transfer and aggregation-induced emission enhancement characteristics for detection of water. New J. Chem. 2018, 42, 13339–13350. [Google Scholar] [CrossRef]
  40. Gopikrishna, P.; Iyer, P.K. Monosubstituted dibenzofulvene-based luminogens: Aggregation-induced emission enhancement and dual-state emission. J. Phys. Chem. C 2016, 120, 26556–26568. [Google Scholar] [CrossRef]
  41. Crocker, R.D.; Zhang, B.; Pace, D.P.; Wong, W.W.H.; Nguyen, T.V. Tetrabenzo[5.7]fulvalene: A forgotten aggregation induced-emission luminogen. Chem. Commun. 2019, 55, 11591–11594. [Google Scholar] [CrossRef]
  42. Li, Q.; Blancafort, L. A conical intersection model to explain aggregation induced emission in diphenyl dibenzofulvene. Chem. Commun. 2013, 49, 5966–5968. [Google Scholar] [CrossRef]
  43. Tran, T.; Prlj, A.; Lin, K.H.; Hollas, D.; Corminboeuf, C. Mechanisms of fluorescence quenching in prototypical aggregation-induced emission systems: Excited state dynamics with TD-DFTB. Phys. Chem. Chem. Phys. 2019, 21, 9026–9035. [Google Scholar] [CrossRef]
  44. Kleinpeter, E.; Fettke, A. Quantification of the (anti)aromaticity of fulvenes subject to ring size. Tetrahedron Lett. 2008, 49, 2776–2781. [Google Scholar] [CrossRef]
  45. Deeb, O.; Cogan, S.; Zilberg, S. The nature of the S1/S0conical intersection of fulvene. Chem. Phys. 2006, 325, 251–256. [Google Scholar] [CrossRef]
  46. Najafian, K.; Schleyer, P.V.R.; Tidwell, T.T. Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes. Org. Biomol. Chem. 2003, 1, 3410–3417. [Google Scholar] [CrossRef]
  47. Krygowski, T.M.; Oziminski, W.P.; Palusiak, M.; Fowler, P.W.; McKenzie, A.D. Aromaticity of substituted fulvene derivatives: Substituent-dependent ring currents. Phys. Chem. Chem. Phys. 2010, 12, 10740–10745. [Google Scholar] [CrossRef] [PubMed]
  48. Rosenberg, M.; Ottosson, H.; Kilså, K. Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives. Phys. Chem. Chem. Phys. 2011, 13, 12912–12919. [Google Scholar] [CrossRef]
  49. Coluccini, C.; Anusha, P.T.; Chen, H.-Y.T.; Liao, S.-L.; Ko, Y.K.; Yabushita, A.; Luo, C.W.; Ng, Y.M.; Khung, Y.L. Tuning of the Electro-Optical Properties of Tetraphenylcyclopentadienone via Substitution of Oxygen with Sterically-Hindered Electron Withdrawing Groups. Sci. Rep. 2019, 9, 12762. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  50. Ren, H.; Tao, Q.; Gao, Z.; Liu, D. Synthesis and properties of novel spirobifluorene-cored dendrimers. Dye. Pigment. 2012, 94, 136–142. [Google Scholar] [CrossRef]
  51. Boiani, M.; Baschieri, A.; Cesari, C.; Mazzoni, R.; Stagni, S.; Zacchini, S.; Sambri, L. A new tetraarylcyclopentadienone based low molecular weight gelator: Synthesis, self-assembly properties and anion recognition. New J. Chem. 2012, 36, 1469–1478. [Google Scholar] [CrossRef]
  52. Andrew, T.L.; Cox, J.R.; Swager, T.M. Synthesis, Reactivity and Electronic Properties of 6, 6-Dicyanofulvenes. Org. Lett. 2010, 12, 5302–5305. [Google Scholar]
  53. Nath, S.; Pal, H.; Palit, D.K.; Sapre, A.V.; Mittal, J.P. Steady-state and time-resolved studies on photoinduced interaction of phenothiazine and 10-methylphenothiazine with chloroalkanes. J. Phys. Chem. A 1998, 102, 5822–5830. [Google Scholar] [CrossRef]
  54. Ohki, S. Dielectric constant and refractive index. J. Theor. Biol. 1968, 19, 97–115. [Google Scholar] [CrossRef] [PubMed]
  55. Manohara, S.R.; Kumar, V.U.; Shivakumaraiah; Gerward, L. Estimation of ground and excited-state dipole moments of 1, 2-diazines by solvatochromic method and quantum-chemical calculation. J. Mol. Liq. 2013, 181, 97–104. [Google Scholar] [CrossRef]
  56. Mathapati, G.B.; Ingalagondi, P.K.; Patil, O.; Hanagodimath, S.M.; Scholars, R. Estimation of ground and excited state dipole moments of newly synthesized coumarin molecule. Int. J. Res. Anal. Rev. 2018, 5, 1061–1065. [Google Scholar]
  57. Espinar-Barranco, L.; Meazza, M.; Linares-Perez, A.; Rios, R.; Paredes, J.M.; Crovetto, L. Synthesis, photophysics, and solvatochromic studies of an aggregated-induced-emission luminogen useful in bioimaging. Sensors 2019, 19, 4932. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  58. Mukhopadhyay, A.; Maka, V.K.; Moorthy, J.N. Remarkable influence of ‘phane effect’ on the excited-state properties of cofacially oriented coumarins. Phys. Chem. Chem. Phys. 2017, 19, 4758–4767. [Google Scholar] [CrossRef]
  59. Divac, V.M.; Šakić, D.; Weitner, T.; Gabričević, M. Solvent effects on the absorption and fluorescence spectra of Zaleplon: Determination of ground and excited state dipole moments. Spectrochim. Acta—Part A Mol. Biomol. Spectrosc. 2019, 212, 356–362. [Google Scholar] [CrossRef]
  60. Duereh, A.; Anantpinijwatna, A.; Latcharote, P. Prediction of solvatochromic polarity parameters for aqueous mixed-solvent systems. Appl. Sci. 2020, 10, 8480. [Google Scholar] [CrossRef]
  61. Marcus, Y. The use of chemical probes for the characterization of solvent mixtures. Part 2. Aqueous mixtures. J. Chem. Soc. Perkin Trans. 2 1994, 8, 1751–1758. [Google Scholar] [CrossRef]
  62. Zhu, H.; Li, M.; Hu, J.; Wang, X.; Jie, J.; Guo, Q.; Chen, C.; Xia, A. Ultrafast Investigation of Intramolecular Charge Transfer and Solvation Dynamics of Tetrahydro[5]-helicene-Based Imide Derivatives. Sci. Rep. 2016, 6, 24313. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  63. Chanmungkalakul, S.; Ervithayasuporn, V.; Boonkitti, P.; Phuekphong, A.; Prigyai, N.; Kladsomboon, S.; Kiatkamjornwong, S. Anion identification using silsesquioxane cages. Chem. Sci. 2018, 9, 7753–7765. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  64. Jadhav, T.; Dhokale, B.; Misra, R. Effect of the cyano group on solid state photophysical behavior of tetraphenylethene substituted benzothiadiazoles. J. Mater. Chem. C 2015, 3, 9063–9068. [Google Scholar] [CrossRef]
  65. Loginov, S.V. Electron loss in microsecond megaampere plasma opening switches. IEEE Trans. Plasma Sci. 2011, 39, 3386–3390. [Google Scholar] [CrossRef]
  66. Fulmer, G.R.; Miller, A.J.M.; Sherden, N.H.; Gottlieb, H.E.; Nudelman, A.; Stoltz, B.M.; Bercaw, J.E.; Goldberg, K.I. NMR chemical shifts of trace impurities: Common laboratory solvents, organics, and gases in deuterated solvents relevant to the organometallic chemist. Organometallics 2010, 29, 2176–2179. [Google Scholar] [CrossRef] [Green Version]
Applsci 13 02976 g001 550
Figure 1. (a) Molecular structure of fulvene with description of frontier orbitals involved in the S1 and S2 excited states (S1: HOMO → LUMO. S2: HOMO-1 → LUMO) reported in the literature [44,45,46,47,48]. (b) Tetraphenyl fulvene with electron acceptor groups linked to the carbon 6 of fulvene reported by Coluccini et al. [49]. (c) Connection of TPF with electron donor on the phenyl rings 3 and 4.
Figure 1. (a) Molecular structure of fulvene with description of frontier orbitals involved in the S1 and S2 excited states (S1: HOMO → LUMO. S2: HOMO-1 → LUMO) reported in the literature [44,45,46,47,48]. (b) Tetraphenyl fulvene with electron acceptor groups linked to the carbon 6 of fulvene reported by Coluccini et al. [49]. (c) Connection of TPF with electron donor on the phenyl rings 3 and 4.
Applsci 13 02976 g001
Applsci 13 02976 g002 550
Figure 2. Structure of synthesized compounds. Minuscule letters indicate the acceptor functional groups linked to the carbon 6 of the fulvene (m is methylester, n is nitrile). Majuscules indicate the donor functional groups linked to the phenyl groups (T is thiophene, F is fluorophenyl, P is phenothiazine).
Figure 2. Structure of synthesized compounds. Minuscule letters indicate the acceptor functional groups linked to the carbon 6 of the fulvene (m is methylester, n is nitrile). Majuscules indicate the donor functional groups linked to the phenyl groups (T is thiophene, F is fluorophenyl, P is phenothiazine).
Applsci 13 02976 g002
Applsci 13 02976 sch001 550
Scheme 1. Synthesis of compound 3.
Scheme 1. Synthesis of compound 3.
Applsci 13 02976 sch001
Applsci 13 02976 sch002 550
Scheme 2. Successful synthesis of the products 5mT, 5mF, 5nT, 5nF, 5mP, 5nP.
Scheme 2. Successful synthesis of the products 5mT, 5mF, 5nT, 5nF, 5mP, 5nP.
Applsci 13 02976 sch002
Applsci 13 02976 g003 550
Figure 3. UV spectra of compounds 5mT and 5nT in CH3CN with indication of the electronic transitions responsible for the absorption maxima.
Figure 3. UV spectra of compounds 5mT and 5nT in CH3CN with indication of the electronic transitions responsible for the absorption maxima.
Applsci 13 02976 g003
Applsci 13 02976 g004 550
Figure 4. More intense fluorescence spectra in dichloromethane, concentration 10−6 M, of all compounds.
Figure 4. More intense fluorescence spectra in dichloromethane, concentration 10−6 M, of all compounds.
Applsci 13 02976 g004
Applsci 13 02976 g005 550
Figure 5. (a) Emission maxima frequencies vs. concentrations. (b) Emission intensity as ratio between area under the emission plot of solution and area under emission plot of phenothiazine 10−6 M detected on the same instrument and under the same conditions vs. concentration. (c) Normalized fluorescence spectra with highest intensities. More intense fluorescence spectra in dichloromethane, concentration 10−6 M, of all compounds.
Figure 5. (a) Emission maxima frequencies vs. concentrations. (b) Emission intensity as ratio between area under the emission plot of solution and area under emission plot of phenothiazine 10−6 M detected on the same instrument and under the same conditions vs. concentration. (c) Normalized fluorescence spectra with highest intensities. More intense fluorescence spectra in dichloromethane, concentration 10−6 M, of all compounds.
Applsci 13 02976 g005
Applsci 13 02976 g006 550
Figure 6. Fluorescence spectra in different mixtures of THF and H2O.
Figure 6. Fluorescence spectra in different mixtures of THF and H2O.
Applsci 13 02976 g006
Table
Table 1. Optical properties of compounds in diluted solutions.
Table 1. Optical properties of compounds in diluted solutions.
λabs = nm (e) 1λex = nm 2λem = nm (Φdil.) 3
SolventCH2Cl2CH3CNCH2Cl2CH3CNCH2Cl2CH3CN
5mT305 (30)
385 (10)		300 (20)
365 (7)		385365 435 (0.53)465 (0.0024)
5nT310 (37)
480 (13)		305 (32)
460 (10)		310305 435 (0.018)435 (0.0010)
5mF260 (28)
350 (9)		260 (27)
335 (9)		350 365 435 (0.076)435 (0.0012)
5nF275 (19)
425 (9)		270 (24)
410 (11)		----
5mP254 (210)
328 (50)		254 (98)
328 (20)		 328 328 435 (0.010)435 (0.0020)
5nP254 (110)
348 (20)
500 (9)		254 (66)
390 (10)
476 (5)		348 390 435 (0.056)435 (0.0018)
1 Average values of absorption wavelengths (nm) λ2 and λ3 (5mP and 5nP also l1) at concentrations around 10−6 M. The intensities are reported in brackets in units of M−1cm−1 × 103. Tables may have a footer. 2 Irradiation produces the highest fluorescence at a concentration around 10−6 M. 3 Quantum yield measured at concentrations ~5 × 10−6 M, with phenothiazine used for reference; the value of Φ is 0.01 [53].
Table
Table 2. Optical properties of 5mT in different polarity solvents.
Table 2. Optical properties of 5mT in different polarity solvents.
Solventd1n1λabs nm 2λem nm 3I 4
CH2Cl29.081.424305 (30)
385 (10)		435–4370–31
THF7.601.407300 (25)
376 (10)		431–4630–3.6
CH3CH2OH24.551.363305 (23)
359 (10)		461–4931.4–6.9
CH3CN37.51.344300 (20)
360 (7)		460–4860.08–3.80
THF/H2O 1:141.211.38300 (25)
376 (10)		490–4960–2.5
1d and n refer to the dielectric constant and refraction index at 20 °C, respectively. In THF/H2O 1:1, the values reported are from the literature [54]. 2 Absorption wavelengths (nm) λ2 and λ3 measured at concentrations around 10−6 M. The intensities are reported in brackets in units of M−1cm−1 × 103. 3 Emission maxima ranges measured at different concentrations. 4 Ranges of relative intensity of emission by measuring the area under the emission curve and dividing for the area of phenothiazine 10−6 M emission measured in the same conditions (same instrument and environment).
Table
Table 3. Dipolar moment of 5mT calculated from absorption and emission spectra data.
Table 3. Dipolar moment of 5mT calculated from absorption and emission spectra data.
Conc. 1a (Å) 2μg 3μe 3Δμ 4Δμ 5Δμ 6Δμ 7
10−6 M5.336.676.9123.8713.585.646.09
10−5 M5.336.778.1026.1514.873.986.22
1 Concentrations that compound display emission in the three solvents. 2 Onsager cavity radius calculated as reported in the literature [58]. 3 Dipolar moments of ground (μg) and excited (μe) states by integration of Bakhshiev and Kawski–Chamma models. 4 Dipolar moment variation calculated from Lipert–Megata model. 5 Dipolar moment calculated from Bakshiev model. 6 Dipolar moment calculated from McRae model. 7 Dipolar moment calculated from molecular–microscopic solvent polarity parameter ETN that can be found in the literature [59,60,61].
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Ng, Y.M.; Coluccini, C. Synthesis of New 2D-π-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties. Appl. Sci. 2023, 13, 2976. https://doi.org/10.3390/app13052976

AMA Style

Ng YM, Coluccini C. Synthesis of New 2D-π-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties. Applied Sciences. 2023; 13(5):2976. https://doi.org/10.3390/app13052976

Chicago/Turabian Style

Ng, Yoke Mooi, and Carmine Coluccini. 2023. "Synthesis of New 2D-π-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties" Applied Sciences 13, no. 5: 2976. https://doi.org/10.3390/app13052976

APA Style

Ng, Y. M., & Coluccini, C. (2023). Synthesis of New 2D-π-2A Chromophores Based on Tetraphenyl Fulvene and Investigation of Their Optical Properties. Applied Sciences, 13(5), 2976. https://doi.org/10.3390/app13052976

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop