PREPARATION, SPECTROSCOPIC, CYCLIC VOLTAMMETRY AND DFT/TD-DFT STUDIES ON FLUORESCEIN CHARGE TRANSFER COMPLEX FOR PHOTONIC APPLICATIONS

. The solid charge transfer (CT) complexes of fluorescein (Flu) with definite acceptors (tetrafluoro para benzoquinone (Fla); 7,7,8,8-tetracyanoquinodimethane (TCNQ); 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and iodine) were prepared. Several characteristic analytic procedures such as transmission (FT-IR), diffuse reflectance, UV-Vis absorption, and cyclic voltammetry were used to determine the structural and optical properties of the prepared fluorescein charge transfer complex. Kubelka Munk model (K-M) and the absorption spectral fitting (ASF) methods were used to obtain the optical transitions of the solid fluorescein charge transfer complex and fluorescein CT complexes in methanol solution. The cyclic voltammetry method has been presented as capable of obtaining valuable information on quasi-reversible redox systems in the fluorescein CT-complexes. The electrochemical gap of the fluorescein CT complex was successfully determined from the cyclic voltammetry technique. Theoretical calculations (density functional theory) were made to corroborate experimental results for the synthesized CT complexes.


INTRODUCTION
Modern organic electronic devices based on π-conjugated small molecules have attracted great concentration over recent years. These π-conjugated organic compounds offer numerous advantages, such as easy processability, inexpensiveness, lighter weight, and versatile molecular design. They are broadly applied as active materials for photovoltaic cells, bioelectronic devices, light-emitting diodes (LEDs), and field-effect transistors [1]. Significant studies have been devoted to explaining the transport mechanisms in organic semiconductor materials, carrier trapping phenomena, and charge injection processes to increase stability and electrical performance [2]. The organic charge-transfer complexes have created an excessive deal of consideration due to their successful physicochemical characteristics. The need for high-mobility organic semiconductor materials for high-performance electronic devices is speedily growing. Charge-transfer complexes play a significant role in smart sensors, optoelectronic devices, and stored chemical energy. The novel optoelectronic devices based on organic semiconductor materials need a good understanding of the charge-transport mechanism in organic semiconductors [3][4][5]. Charge-transfer complexes have attracted the consideration of scientific researchers due to their impressive applications in material science, engineering, pharmacology, medicine, and optoelectronic devices. The interaction between an electron-accepting molecule (A) with an electron-donating molecule (D) forms a typical charge-transfer complex ( + ⇄ [ . ] ⇄ [ . . ] ). The formation of the donor-acceptor complex provides the advantages of new color with a broad absorption in the UV-Vis-NIR region as well as a controllable optical band gap and energy levels [6][7][8][9][10][11][12][13][14]. Fluorescein organic donor molecule is a red powder in the solid state and belongs to the xanthene family. Under UV-blue excitation, it emits a strong yellow-green light in a liquid solution. It is used in a wider range of modern applications due to its excellent fluorescence quantum yield, biocompatibility, and high molar absorptivity [15][16][17][18]. Tetrafluoro para benzoquinone (Fla), 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), iodine are usually used as acceptors materials to form charge-transfer complexes. For possible applications in photonic and materials science area, the present work is devoted to preparing solid charge-transfer complexes of fluorescein with definite acceptors mentioned above, then their structural, optical, and cyclic voltammetry characteristics were explored. Density functional theory (DFT/TD-DFT) was adopted to obtain the optimized structure of the CT complexes -[(Flu)(Fla)], [(Flu)(TCNQ)], and [(Flu)(DDQ)] with B-3LYP/6-31G (basis set). Experimental data were also corroborated theoretically through DFT.

EXPERIMENTAL
The fluorescein dye (Flu) was received from Aldrich Company, and used without further purification. DDQ, TCNQ, and Fla were obtained from Merck Company, and with prior use, they were purified by re-crystallization from CHCl3. The solid CT complexes were synthesized by mixing 1 mmol of the donor in a methanol mixture with 1 mmol of DDQ, TCNQ, and Fla (acceptors) in the same solvent. At room temperature, the mixture was stirred for  1 h. After reducing the solvent volume by the evaporation of 80%, the solid precipitate is obtained. The separated fluorescein CT complexes were filtered off, washed many times with chloroform (CHCl3), and then the obtained solid precipitate of the CT complexes was dried under a vacuum. The diffuse reflectance spectra of the fluorescein CT-complex were recorded on Ocean Spectrometer model USB4000-XR1-ES. A Cary series spectrophotometer (Agilent Technologies) was used to measure the absorbance spectra of the fluorescein CT-complex over the range of 200-800 nm. The cyclic voltammetry (CV) measurements were described in detail in our published work [19]. To obtain the theoretical data for the CT complexes [(Flu)(Fla)], [(Flu)(TCNQ)], and [(Flu)(DDQ)], Gaussian 09RevD.01 program [20] was used with Pople's basic set B3LYP/6-31G, and Gradient corrected correlation [21]. B3LYP/6-31G level of theory was applied to obtain the

RESULTS AND DISCUSSION
The infrared frequencies of the synthesized fluorescein charge transfer complexes have a change in the band intensities and the frequency which are attributed to the electronic configuration changes and the symmetry of the CT complex. The IR vibration bands at 719, 795, 894, 1166, 1548, 1668, and 2239 cm -1 of the free DDQ organic molecule can be attributed to ν(CCl), δ(C-C), ν(C=C), ν(C=O), and ν(CN), respectively. The IR spectrum of the free Fla acceptor has some characteristic bands at 992 cm -1 for the ν(C-F), 1322 cm -1 for the ν(C=C stretching), and (1745 and 1672) cm -1 for the ν(C=O stretching). The IR vibration bands at 719, 795, 894, 1166, 1548, 1668, and 2239 cm -1 of the free TCNQ organic molecule can be attributed to 850 cm -1 , (1043 and 1116), 1346,1531, (2091 and 2208), 2963, and 3044 cm -1 can be attributed to δr(CH), δ(CC), δ(CH), ν(C=C), ν(CN), νs(CH) and νas(CH) vibration motions. From the IR vibration bands of the Flu organic donor molecule and the corresponding acceptors, it can be recognized that the ν(CN), ν(C-F), ν(C=O), and ν(C-Cl) vibration motions are shifted to higher or lower frequencies values in case of complexes.
The diffused reflectance spectroscopy ( ) is a significant procedure to explore the qualitative knowledge about the optical properties of the investigated fluorescein CT-complex in powder form (inhomogeneous media). Figure 1 shows the diffuse reflectance spectra ( ) of fluorescein CT-complex. The Kubelka-Munk model (K-M) is used to convert the diffuse reflectance spectra of the fluorescein CT-complex into absorption spectra [23][24][25][26][27][28][29].
where ( ) is the Kubelka-Munk function and is the absorption coefficient of the fluorescein CT-complex. Figure 1 (A-E) shows ( ( )ℎ ) / versus ( ℎ ) for fluorescein CTcomplexes. The estimated values of the allowed indirect gap are given in Table 1.   Figure 2 shows the electronic absorption spectra of fluorescein CT complexes in methanol. The molar absorptivity (εmolar) of the fluorescein CT-complexes can be used to estimate the oscillator strengths (f) and the electric dipole strength (q 2 ) from the following relations [29][30][31][32][33]: where ̅ is the wavenumber and Δ is the absorption half-band width. The spectral behavior of the molar absorptivity for the fluorescein CT complexes is displayed in Figure 2(A-E). A Gaussian fitting with asymmetric least squares smoothing baseline was done to calculate f and q 2 . The Gaussian fitting analysis and the calculated values of (q 2 ) and (f) for fluorescein CT-complex in methanol (C = 5×10 -7 M/L) are shown in Table 2.  The absorption spectrum fitting (ASF) method can be applied to precisely determine the optical band gap energies of fluorescein CT-complex in the UV-Vis region [34][35][36][37][38][39][40][41][42]: where n is the power taking the value of 2 for the allowed indirect transition, A is optical absorbance, Z is a constant, c is the velocity of light, and d is the thickness of the sample (1 cm).    [43]. The first system's oxidation peak (Epa1) was found to be +0.13 V, while the cathodic peak (Epc1) was observed to be -0.06 V. The reversibility of the reaction was demonstrated by the difference between the anodic and cathodic peak potentials (ΔE), which was determined to be equal to + 0.19 V. The formal potential (E1/2) was taken as the average of Epc1 and Epa1 is +0.035 V. In the second system, Epa2 and Epc2 appeared at+0.62 V and +0.55 V respectively. The calculated ΔE was found + 0.07 V and E1/2 is + 0.62 V [43][44][45]. E(onset oxidation) and E(onset reduction) peak values have been calculated to be 1.28 V and 0.68 V, respectively. HOMO, LUMO, and band gap ( ) were calculated to be, -5.68, -3.72, and 1.96 eV, respectively.
The electrochemical behavior of the [(Flu)(I2)] complex under the previously specified circumstances is shown in Figure 4-B using cyclic voltammetry. Two quasi-reversible redox systems and one irreversible cathodic peak were depicted in this figure. In the first redox system, Epc1 occurred at +0.07 V, whereas Epa1 occurred at +0.37 V. ΔE was found to be +0.3 V, with +0.22 V E1/2. The Epa2 came at + 0.65 V in the second redox system, whereas Epc2 appeared at +0.43 V. ΔE and E1/2 were determined to be + 0.22 V and + 0.54 V, respectively.
The irreversible cathodic peak appeared at -0.37 V. The [(Flu)(I2)] complex also showed an irreversible cathodic peak at -0.47 V and an anodic peak at +0.9 V. E(onset oxidation) and E(onset reduction) peak values were determined to be +1.3 V and -0.66 V, respectively. The computed HOMO, LUMO, and band gap ( ) were -5.7 eV, -3.77 eV, and 1.96 eV, respectively. The electrochemical behavior of the [(Flu)(DDQ)] complex is depicted in Figure 4-C using the cyclic voltammetric method. Two quasi-reversible redox systems and two irreversible cathodic peaks were seen in the [(Flu)(DDQ)] complex. At -1.14 V, an irreversible cathodic peak developed, whereas an anodic peak appeared at +0.2. For the two quasi-reversible redox couples, Epa1 appeared at -0.32 V whereas Epc1 appeared at -0.42 V. The values for ΔE and E1/2 are +0.1 V and -0.37 V, respectively. The Epa2 emerged at +0.73 V in the second system, whereas the Epc2 arrived at +0.53 V. ΔE was determined to be +0.2 V, while E1/2 was found to be +0.625 V. The E(onset oxidation) and E(onset reduction) peaks are +1.3 V and -1.   ] complex has one anodic peak at -0.34 V and three quasi-reversible redox systems. The first quasi-reversible redox pairs Epa1 and Epc1 were seen at 0.0 V and -0.31 V, respectively. Calculated values for ΔE and E1/2 are +0.31 V and -0.155 V, respectively. In the second oxidation-reduction system, Epc2 showed up at +0.06 V while Epa2 showed up at +0.47 V. The ΔE was calculated and found +0.41 V, while the E1/2 was 0.265 V. The third quasi-reversible redox couple, Epa3, appeared at +0.92V, while Epc3 appeared at +0.57 V. The ΔE was found to be -0.35 V, with E1/2 obtaining at +0.745 V. The obtained values for the E(onset oxidation) and E(onset reduction) peaks are +1.35 and -0.75 V, respectively, while the calculated HOMO, LUMO, and band gap ( ) were -5.75, -3.65, and 2.1 eV, respectively. The electrochemical behavior of the [(Flu)(TCNQ)] complex under the previously specified circumstances is shown in Figure 4-E using cyclic voltammetry. Two irreversible peaks, one cathodic peak at -1.2 V and one anodic peak at -0.3 V, as well as two quasi-reversible redox systems, could be seen in this graph. Epa1 was detected at -0.63 V, while Epc1 was detected at -0.81 V. The calculated E was determined to be +0.18 V, with a formal potential of -0.72 V. Epa1 and Epc1 were found to be -0.63 V and -0.81 V, respectively. The computed ΔE was +0.18 V, with a formal potential of -0.72 V. In the second redox system, Epa2 occurred at + 0.25 V while Epc2 appeared at + 0.4 V. ΔE and E1/2 were calculated to be + 0.21 and + 0.145 V, respectively. The peaks for E(onset oxidation) and E(onset reduction) are, respectively, -0.83 and -1.7 V. The band gap ( ), HOMO, and LUMO calculated values were 5.23, -2.7, and 2.53 eV, respectively.    Table 3.  CT complex with minimum energy. Experimental data were also corroborated theoretical through DFT and found then in good agreement. The obtained data are vital for applied material science research fields such as organic photonic applications.