Understanding the Electronic Interactions, Vertical Excitation Analysis, and the Photovoltaic Properties of 5-(2-ethylhexyl)-1,3-di(furan-2-yl)-4H-thieno[3,4-c]pyrrole-4,6-dione

Organic photovoltaic (OPV) are a promising new class of photovoltaic as they offer several advantageous features including large surface area to volume ratio, low cost, lightweight properties, and durability. The limitation of OPV that prevented their adoption for use in the past was their low power conversion efficiency (PCE) but that drawback has been solved by the development of the donor-acceptor-donor (D-A-D) system with high conversion efficiencies. Herein, 5-(2-ethylhexyl)-1,3-di (furan-2-yl)-4H-thieno [3,4-c]pyrrole-4,6(5H)-dione (FTPF), a donor-acceptor-donor monomer was investigated for its optoelectronic, excited state, and photovoltaic properties using a density functional theory (DFT) and time-dependent density function theory (TD-DFT) at the B3LYP/6-31+G(d,p) azolide groups of the acceptor unit, but n→π* charge transfer (CT) in DMF. The S 0 →S 5 in water and S 0 →S 4 are n→π* LE type excitations, while S 0 →S 5 in DMF conformed to a delocalized π→π* excitation extended over the D-A-D conjugated backbone. FTPF provided efficient electron injection in all studied solvent; showing that FTPF is a sure-bet for opto-electronic application.


Introduction
Photovoltaics (PVs) are used in solar cells to convert optical energy into electrical energy for various applications [1][2][3][4][5][6][7][8]. Typically, inorganic semiconductors like crystalline silicon are used as photovoltaics in solar cells but they are costly and suffer from efficiency limitations [1,[9][10][11][12]. Organic photovoltaics (OPVs) are a promising new class of photovoltaics as they offer several advantageous features including large surface area to volume ratio, low cost, lightweight properties and durability [13][14][15][16][17][18]. OPVs can even be made up to 1000 times thinner than crystalline silicon photovoltaics offering significant savings on materials needed [14,18]. The limitation of OPVs that prevented their adoption for use in the past was their low conversion efficiency (CE) but that drawback has been solved by the development of the donoracceptor-donor (D-A-D) system with their high conversion efficiencies [14,[19][20]. The D-A-D systems are classified by high conjugated bonding systems which allow strong electronic transitions. Dyes are other category of organic materials applied in photovoltaics, due to their high π-electron conjugations, leading to great electronic excitations and transitions [21][22][23]. These systems are characterized with low HOMO/LUMO energy gap, which gives insight into why these molecules act as semiconductors [24].
Studying highly conjugated systems in order to understand their properties and to improve their energy conversions and hence applicability is crucial in the optical and electronic industries [25][26][27][28]. Density functional theory (DFT) is a very reliable theory for predicting the density of electrons around a molecule thereby enabling easy simulation and prediction of that molecule's reactions and behaviour under certain conditions with high level of accuracy [29][30][31][32][33][34]. DFT and other computational chemistry methods are employed in independent studies of molecules systems or also as a validating study premise for experimentally studied chemical interactions [29,30,34].
In order understand the electronic interactions within the D-A-D system of FTPF and to further explore other informative optoelectronic and photovoltaic properties of their conjugated π-electron system, we employed the DFT method of investigation to perform theoretical calculations on FTPF ( Fig.   1.). The UV-Vis spectral determination is conducted in gas, hexane, THF and DMF while the other spectral properties (FT-IR and NMR) was analyzed in gas phase to obtain data necessary for characterization and structure determination. Potential energy distribution (PED) analysis was performed to achieve the FT-IR absorption parameters [38][39]. Other electronic property determinations like natural bonding orbital (NBO) analysis [40] and vertical excitation analysis of the first 5 excitations of FTPF are also pivotal in this study, so as to register the important intra-monomer, hence polymeric electronic transitions of our titled molecule [21][22][23]. The electronic excitations and oscillator strength parameters obtained from the UV-vis calculations were employed for the investigation of the photovoltaic properties.

Computational details
The density functional theory (DFT) computational method is an appropriate theoretical approach to studying the optical and spectral properties of highly conjugated systems (e.g. natural dyes, D-A-D systems), owing to its swiftness and accuracy in dynamic electron correlation recovery [21,[41][42]. The ground state geometry optimization of the 5-(2-ethylhexyl)-1,3-di(furan-2-yl)-4H-thieno [3,4-c]pyrrole-4,6-dione (FTPF); a donor-acceptor-donor monomer was conducted at the DFT/B3LYP theoretical method using 6-31+G (d, p) basis set [43,44] in gas, hexane, tetrahydrofuran (THF) and dimethylformamide (DMF) solvents using Gaussian09W and GaussView 6.0.16 packages [45]. Excitations are classified as charge transfer (CT), Rydberg or local excitations (LE) depending on the spatial distribution, overlapping extent, centroid position, etc. which are deduced from the various indices considered in this work [23]. Calculations for the various indices in this excitation study were done using equations from Le Bahers et al. (2011) [48]. The extent of overlap between hole and electron in a molecular system is defined by Sr index. Sr can be obtained using equation 1.
Where r is vector component with respect to position, ρ hole and ρ ele are the hole and electron densities, respectively. The most significant position of either hole and electron distribution in the x,y or z coordinate is defined by the centroid to the coordinate under consideration. For example, the X coordinate of centroid of hole is written in equation 2.
Where x is the X component of vector, r with respect to position.
The spatial separation of the centroids of hole from electron (i.e. Chole from Cele) is defined by the D index which is deduced by invoking equations 3 to 6. This defines the net charge transfer length magnitude. The extend of charge transfer is obtained from the D values for the various coordinates. Considering x, y and z coordinates, D is given by equations 3, 4 and 5, respectively.
t index is the mean difference between H in CT direction from D index. The degree of separation of hole and electron in CT direction is defined by the t index. H index, which is the overall measure of spatial extension of hole and electron distribution. H index is deduced from the root mean square distribution of electron (|σele |) and hole (|σhole |), using equation 7.
The hole delocalization index (HDI) and electron delocalization index (EDI) are electron density dependent parameters, and are important in explaining the type of excitation occurring in a conjugated system. Equations 8 and 9 are used to determine HDI and EDI.

Spectroscopic and Geometrical Properties
We carried out UV-Vis spectroscopic calculations of FTPF using Gaussian09W in gas, DMF, hexane and THF solvents in order to investigate the effect of these solvents on maximum wavelength (λmax) of the electronic absorption transitions. The 1 H-NMR and 13 C-NMR calculations were performed to determine the chemical shifts values for hydrogen and carbon atoms in the FTPF monomer unit [49,50].
A combined UV-Vis spectrum of FTPF monomer in the various considered solvents was plotted using Multiwfn programme for clear visualization. Also, potential energy distribution (PED) assignments for the various IR vibrations were conducted on VEDA software [47], in order to understand the different interatomic vibrations of the studied D-A-D molecule.

UV-Visible analysis
In this study, we considered the effect of three solvents on the UV-Vis spectrum obtained in gas phase. A non-polar hexane and aprotic polar DMF and THF were added to solvate FTPF to achieve the maximum wavelenght of absorption. The combined computational UV-Vis spectrum of FTPF can be visualized in Fig. 2., for a clear understanding and correlation of the contributions of various solvents on the optoelectronic behaviour of FTPF.
Dual absorption peaks were recorded for FTPF in DMF and THF, which is characteristic of two different allowed transitions in a conjugated system [51,52]. The peak corresponding to a high energy absorption occurred at 261 nm in DMF and 263 nm in THF nm (π→π* transition), while the n→π* transition occurred at 373 nm in DMF and 371 in THF (lower energy transition, traceable to intramolecular charge transfer (ICT) between D and A units) [35].

Infrared Vibrational Studies
Every molecule responds to IR light through signatory interatomic vibrations whose frequencies depend on the nature of bonds (different functional groups) and the modes of vibration is determined by the number of atoms in the molecule and spatial arrangement of the molecule. For a non-linear system like FTPF, the number of vibrations is given by equation 10, where N is the total number of atoms in the molecule [51,52]. The potential energy distribution (PED) assignments for the various frequencies of vibrational were generated for FTPF at B3LYP/6-31+G(d,p) levels.
Number of vibrational modes = 3N + 6 (10) Results for the PED assignments (in %) for IR vibrational frequencies are reported on Table 1.

C-C vibrations
The C-C single bond stretching vibrations of FTPF occurred within the range 1124-1390 cm -1 , with all having relative low PED, a basic property of saturated C-C IR stretches of alkyl groups. Unsaturated C=C stretching vibrations occurred at higher frequencies when compared to the saturated vibrations, which agrees to the theory that stronger bond exhibits vibration at higher frequencies [53].

C-O vibrations
Naturally, C=O stretching vibrations occurs around 1600-1800 cm -1 , the carbonyl stretching vibrations for FTPF occurred at the following frequencies; 1788.

C-N vibrations
The N-H stretching vibrations are absent showing that there is no protonated nitrogen atom in FTPF molecule. Also, N-C stretching vibrations that occurred in FTPF are not significant, due to their ignorable PED. Notable bending and torsional vibrations involving HCN and CNCC, occurred at 1329.54 cm -1 (PED; 11) and 165.51 cm -1 (PED; 11), respectively. C-S bond vibrations gave ignorable vibrations; hence they are exempted from our discussion.

Nuclear Magnetic Resonance (NMR) Spectroscopy
The chemical shift for the different protons and carbon atoms in FTPF corresponding to their level of interaction with external magnetic field or photons of radiofrequency were determine and the NMR spectrum plotted ( Table 2,

H-NMR;
The protons H29 to H33 exhibited the expected chemical shift within the range 0-2ppm, which varies depending on whether each proton is a methyl, methylene or methine in nature, and also the spatial stereo-chemical orientation which may project the proton towards or away from an electronegative group.
These factors determine the de-shielding values of different atoms considering their proximity to heteroatoms. Protons from H41-H45 showed chemical shifts within the range 2.5ppm to 4.5ppm due to the de-shielding effect of the electronegative Nitrogen. H44 being above the plane and closest to the heteroatom is more de-shielded as evident in its chemical shift of 4.02ppm, while H45 with a chemical shift lower as it is below the plane and hence relatively protected from the de-shielding effect of the electronegative heteroatom Nitrogen. H47 to H51 are all vinylic/aromatic protons of the furan ring, this is evident in their chemical shift values (ranging from 6.5ppm to 8.9ppm). They experience further deshielded properties when they are localized around the highly electronegative oxygen atoms [50].

C NMR;
The carbon atoms C2, C3, C6 and C7, as expected for a conjugated π acceptor system were observed downfield with chemical shift within the range of 100-130 ppm. C6 and C7 in a single bond each with the sulphur atom were also observed downfield at 124.54 ppm and 125.34 ppm, respectively.
Carbonyl carbons (C1 and C4) of the pyrroledione acceptor system were observed to suffer further de-

Bond Length, Bond Angle, and Dihedral Angles
The bond lengths and angles of all interatomic bonds and the dihedral angles in FTPF were calculated using the VEDA software. Results from the geometrical property analysis are reported in Tables S1, S2  The dihedral angles, which shows the angle between two intersecting planes is the clockwise angle suspended between two sets of three connected atoms [59]. The calculated dihedral angles of FTPF calculated at CAM-B3LYP/6-31+G(d,p) levels are reported in Table S3., of the supporting information.

Frontier Molecular Orbital (FMO) Analysis
FMO analysis is an important approach to the study of potential materials for electronic applications, it does not only expose the energies of molecular orbitals, but also explain the distributions of these orbitals around the molecular system [22,23]. For an appreciable knowledge on the electronic excitation properties of FTPF, the HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1 and LUMO+2 diagrams were generated at CAM-B3LYP/6-31+G(d,p) levels to give pictorial explanation of the energy band gap (Eg). The electron transition properties are more informative and explicit when more molecular orbitals are considered [22,23]. The FMO maps are visualized in Fig. 3 alongside the energies of the studied orbitals. HOMO is distributed mainly around the carbon and oxygen atoms of the donor and acceptor units of FTPF monomer, but no significant contribution was recorded from N and S atoms. From the lowest unoccupied molecular orbital (LUMO) isosurface, one can infer that LUMO spreads all over the donor and acceptor backbone including the S atom in the acceptor unit, but N. Visuals from the plots of LUMO+1, LUMO+2 and HOMO-1 show that they are also distributed around the donor and acceptor groups, but HOMO-2 is virtually distributed all over the FTPF monomer. Negative energy terms was obtained for both HOMO and LUMO (-5.7509 eV and -2.3274 eV, respectively) using DFT method at CAM-B3LYP/6-31+G(d,p) levels employed; an indication that all calculation converged appropriately at this level [21,52,[60][61][62]. The calculated HOMO-LUMO energy gap is 3.4256 eV, a relatively small energy gap which shows that FTPF polymer is a good candidate for application as opto-electrical component [22,23] in organic electronic devices like OPV cells. In extension, HOMO-1/LUMO+1 energy gap (4.7130 eV) calculated is also adequate when considering other possible transition energies if the molecule is incident with photons of appropriate energies. The computed HOMO and LUMO energies, and Eg of FTPF in gas, hexane, DMF and THF phases are reported in Table 3., to afford a comparative discussion. Significant increase in Eg is noticed as we introduced organic solvents, which implies that hexane, DMF and THF impedes excitation, and hence the optoelectronic properties depreciate in these solvents. The first transition in FTPF occurs with more ease than subsequent excitations in gas phase, as can be deduced from the difference in Eg at ground state (3.42 eV) and excited state (5.73 eV). Eg difference between ground and excited state are not significant in the organic solvated systems.

Density of State (DOS) Analysis
The Partial DOS, Total DOS and Overlap Partial DOS plots of the DAD system in gas, hexane, DMF and THF are presented in Fig. 4 to afford a comparative study of its behaviour in various solvents.  which is an indication that DOS, TDOS and OPDOS of our studied monomer are independent of solvent.

Scanning Tunneling Microscopy (STM)
To further study this FTPF system, we conducted simulations for the scanning tunneling microscopy (STM) which helps study the structural pattern in a chemical system. STM is directly proportional to electronic distribution in a system and its maps give full information about the flow of tunneling current (I) around atoms in a molecule [63,64]. STM plots for FTPF was done in gas, hexane, DMF and THF, in order to investigate the effect of solvent on the tunneling current around the D-A-D molecule. regions. It is worthy to note that change in solvents had no effect on the STM map of FTPF, as can be seen from the maps for gas, hexane, DMF and THF, in Fig. 5.

Molecular Electrostatic Potential (MEP) Analysis
The electrostatic potential plot comprises a mixture of colours that ranges from blue, red to green.
The zones with negative potentials, i.e. high electron densities are transcribed as red with negative values for isosurfaces; while the electron deficient zones are blueish (positive isosurface values are recorded for such surfaces) [63,65].  In the simulated ESP map, the yellowish-red region shows the transition from a low electron density region to a high electron density region (red) [63]. Such distributions are visible around the non-polar ethylhexyl group on the A unit. Reactivity, polar properties, electronic transitions and its accompanied properties are not expected around these regions [63,65].

Atomic Dipole Corrected Hirshfield (ADCH) Atomic Charge Properties
We further studied the distribution of charges around FTPF monomer by conducting ADCH population analysis, so as to provide appropriate and corrected Hirshfield charge distribution than the one initially employed by Tanaka & Chujo [68]. ADCH approach is a more improved method in the determination of the activity sites within a molecule as a dependent property of electron density distribution obtainable from DFT studies [52]. The ADCH and Hirshfield results for all atomic elements in FTPF monomer in gas, hexane, DMF and THF solvent systems, calculated with CAM-B3LYP method at 6-31+G(d,p) basis level are presented on Table 4.  Considering the effect of solvent on the ADCH charge distribution, from results in Table 4, one can see that no significant change was observed for change in solvent from gas, hexane, DMF to THF, and all inferences obey the electrophilic and nucleophilic activities on the various atoms of FTPF monomer. It is important to note that DMF gave the highest shift from results obtained in no solvent (gas). Also, it is visible that the magnitude of ADCH values for both positive and negative charges of different atoms are higher than the Hirshfield charges, which are more adequate in determination and explanation of charge moment distributions [52].

Vertical Excitation Studies using Hole-Electron Analysis
In order to understand the types and nature of the first five excitations (S0 → S1, S0 → S2, S0 → S3, S0 → S4 and S0 → S5) [23], the hole-electron excitation properties of FTPF was studied in gas and DMF.

Hole-Electron excitations
For this study, quantitative excitation determinant indices (Sr, D, H, t, hole delocalization and electron delocalization) were generated using Multiwfn programme [46]. For molecular orbitals, the net configuration coefficients equal 1. This implies that ʃρ hole (r)dr and ʃρ ele (r)dr are both equal to 1. All hole and electron distribution should satisfy this situation [48]. Computationally, all the considered indices were determined by employing equations 1-9 [48]. One can interpret if an excitation is a localized excitation (LE), charge transfer (CT) or a Rydberg type depending on the values of various indices studied. Also, maps of hole and electron, centroids of hole and electron, Sr and charge density difference (CCD) shows the positions of excitation around the molecule under consideration [22,23]. Results from the hole-electron excitation studies of FTPF monomer in gas and DMF are reported in Table 5., while the plotted excitation isosurfaces are provided in Fig. 7.   Fig. 7. Isosurface maps of Hole and Electron, Centroids of Hole and Electron, Sr and CDD of FTPF generated with TD-SCF CAM-B3LYP method at 6-31+G(d,p) basis set.
Sr is a function used to measure the overlap between hole and electron densities, it quantifies the spatial presence of hole and electron [48]. Sr is an important parameter for defining the excitation types from hole-electron interaction, in conjunction with other indices. The D index defines the total magnitude of CT length, it is equal to the distance between the hole and electron centroids (Chole and Cele) [48]. H index is an overall estimation of the average extent of spatial extension of hole and electron distribution in all coordinates. The assignment of the breath of distribution of hole and electron during an excitation process is defined by H index [48]. The measure of hole and electron separation degree in charge transfer dimension is connoted in t index value. For t<0, hole and electron separation is not large due to charge transfer, while when t>0, one can say there exist a substantial separation between hole and electron. The magnitude of t index can be transformed to the extent of separation between hole and electron for the particular excitation. The spatial distribution of hole and electron at excited state are defined by the HDI and EDI respectively. The values of HDI and EDI is a signal for whether the excitation is highly localized (high HDI and EDI) or delocalized (low HDI and EDI) [48].    Fig. 7., where the blue (hole) and green (electron) lobes are spread around both donors and acceptor, this is as a result of π→π* transitions along the conjugated furanyl-pyrroledione-furanyl ring backbone. EDI (8.08 and 8.14 in gas and DMF) validates a local excitation (LE) of n→π* characterized to originate from heteroatoms (carbonyl oxygen) of the pyrroledione A unit. It is visible from Fig. 7., that during excitation, the hole is domiciled on the carbonyl oxygen, while electron extends locally around the A unit.

Second Excitation (S0→S2); From D index values in
Sr isosurfaces shows the overlap of hole and electron on the acceptor group, which confirms the localization of S0→S4 excitation.

Fifth Excitation (S0→S5)
; D index of the fifth excitation is relatively high; 1.69 Å in gas, but extremely reduced in DMF (0.07 Å), this is where the effect of change in solvent is highest. The t index is also bipolar in gas and DMF, excitation is proposed to be local or Rydberg in gas (t=0.02), but CT in DMF (t=-1.81), another major shift. From Sr values, one can propose an n→π* transition for S0→S5 in gas, and π→π* in DMF. Also, observing the hole and electron isosurface on Fig. 7., where the hole and electron of S0→S5 excitation is localized on the acceptor unit in gas, but extends all over both acceptor and two donor units in DMF. HDI and EDI values (12.18 and 6.10, respectively in gas) supports a localized system, but this pair reduced for DMF, hence a delocalized transition is proposed.
The effect of DMF solvent is seen for S0→S3 and S0→S5, which a greater twist of the type of excitation transition in S0→S5 (where an n→π* changed to π→π* due to the change from gas to DMF solvent). The various studied excitations are summarized in Table 6., for easy correlation and visualization, their behaviour in the two systems are presented. Table 6. Summary of the difference kinds and locations of Excitation of FTPF in gas and DMF from Hole-Electron Excitation studies with TD-SCF CAM-B3LYP method at 6-31+G(d,p) basis set.

Excitations Excitation Type
Delocalized π→π* Rydberg excitation originating from C=C π bonds of D-A-D system. Same.

S0→S3
A localized n→π* transition around the carbonyl on the azolide moiety of A unit. An n→π* CT originating from the lone pairs of carbonyl oxygen.

S0→S4
An n→π* LE originating from the carbonyl oxygen atoms of the A unit. Same.

S0→S5
An n→π* LE on the A unit (equivalent to

Heat Maps Analysis
In order to further expose the major character of our studied D-A-D monomer, the contributions of the different fragments to hole and electron were plotted in the form of heat maps [23]. From heat maps, we can understand the source fragment of the excited electrons and their location after excitation, this gives more meaning to the results from hole and electron indices discussed in section 3.7.3. in order to plot the heat map of 5-(2-ethylhexyl)-1,3-di (furan-2-yl)-4H-thieno [3,4-c]pyrrole-4,6(5H)-dione, the molecule was split into three (3) fragments; Frag 1 is the two furanyl donors units, frag 2 is the pyrroledione acceptor unit and frag 3 is the ethylhexyl group. The plotted heat maps for FTPF monomer in gas and DMF is presented in Fig. 8.  Fig. 8., the colour distribution on heat map for S0→S1, S0→S3 and S0→S4 in gas and DMF are respectively equivalent, which suggest a possible equivalent domicile for hole and electron.

Photovoltaic Properties
The open circuit voltage ( ) is defined as the energy difference between the redox potential of the electrolyte's redox couple ( − 3 − ⁄ ) and the quasi-Fermi level of the semiconductor's conduction band . It is mathematically represented as: Where, is the conduction band edge of 2 , q is the unit charge, T is the absolute temperature, k is the Boltzmann constant, is the number of electrons in the conduction band, is the density of accessible states in the conduction band and is the redox potential of the electrolyte. ∆CB is the shift of CB when the dyes are adsorbed [69,70]. It is mathematically represented as: Where is the dipole moment of the individual dye molecule perpendicular to the surface of 2 , and is the dye surface concentration, 0 and are the vacuum permittivity and dielectric permittivity, respectively. The value of can also be approximately obtained from the difference between and . It is used for this purpose because the studied monomer system is singly not in the adsorbed state on 2 [70].
can be mathematically expressed as: LHE ( ) is the light harvesting efficiency at maximum wavelength, is the electron injection efficiency, and is the charge collection efficiency. To obtain a high , LHE and should be as high as possible. The LHE can be mathematically expressed as: Where f is the oscillator strength of the dye corresponding to , is related to the thermodynamic driving force ∆ of electron injection from the excited states of dye to conductive band 2 ∆ (The free energy difference for electron injection) is mathematically represented as: Where * is the redox potential of the oxidized dye at excited state. is the redox potential of the oxidized dye at ground state and ∆ is the lowest vertical excitation energy. 2 is the energy of the conductive band edge of 2 [69][70][71][72].
∆ (The driving force for dye regeneration) is mathematically represented as: A value of ∆ greater than 0.2 eV for an oxidized dye could be the efficient electron injection.
To determine the value of Jsc and the overall potential conversion efficiency (µ), the calculated values of open circuit voltage (Voc), the oscillator strength f, Light harvesting efficiency (LHE), λmax, the force energy difference for electron injection (△G ing ), the driving forces of regeneration(△G reg ) were calculated in Gas, Hexane, DMF, and THF phases as presented in the Table 7., it is worthy to note that a conjugated system with little energy band gap is beneficial to a red-shifted absorption spectrum and give rise to more electrons corresponding to an increase in nc and thus, increase the efficiency of Voc. In this study, it was observed that in all the solvent, △G inj is greater than 0.2 eV and therefore, FTPF monomer in the four phases provide efficient electron injection. However, △G inj values for DMF and THF are larger than in other phases, this implies that both solvents ensure provision of highest electron injection in FTPF monomer system. It is also observed that the △G reg is greater than 0.4 in all the phases and at all the excitation hence; an implication that DMF and THF have more effect on △G reg compared to the other studied solvents. Results in Table 7., shows that among the studied phases, the highest f value comes from DMF and THF in the vertical excitation which could be accounted on the basis of solvent polarity.
Also its was observed that the LHE varies in different phases and show greater stability in the first and second excitation state for both phases. The highest values of Voc were obtained in Hexane and DMF.

Natural Bonding Orbital (NBO) Analysis
The major stabilization energies of FTPF monomer was analyzed using natural bonding orbital analysis.

CONCLUSION
We studied the spectra and excitation properties of FTPF monomer in gas, hexane, DMF and THF phases, using DFT and TD-DFT approach. A dual absorption was recorded for FTPF in DMF and THF, corresponding to n→π* and π→π* transitions, this correlates to the experimental results. Although, a single (π→π*) absorption occurred in gas and hexane phase. We conclude that FTPF experience more excitations in polar (DMF and THF) than non-polar (gas and hexane) solvents, a major red shift was also observed as we move from gas/hexane to polar DMF/THF. From FMO, we discovered that the calculated energy gap of FTPF is very low, and is close to the values obtained experimentally by Çakal et al, a good opto-electronic property required for organic cells. Little or no solvent effect was recorded in the DOS and STM results. Isosurface values on ESP plots showed that decrease in the distribution of electron densities around FTPF monomer follows the trend O>N>S. ADCH analysis exposed the best sites for electrophilic substitution reactions are the carbonyl oxygen atoms of the acceptor fragment and the terminal carbon atoms of the ethylhexyl substituents.
We analyzed the hole-electron excitations for the first 5 excitations, for good understanding about the intramolecular electronic transitions in FTPF monomer. S0→S1 is a delocalized π→π* Rydberg excitations originating from the C=C π bonds within the D-A-D system. A LE was observed for S0→S2, while S0→S3 in water occurred as an n→π* from the carbonyl and azolide groups of the acceptor unit.
S0→S3 was observed to be n→π* charge transfer originating from the carbonyl oxygen atoms in DMF.
S0→S5 in water and S0→S4 are n→π* LE type excitations, while S0→S5 in DMF conformed to a delocalized π→π* excitation extended over the D-A-D conjugated backbone. The obtained photovoltaic properties suggest that FTPF provides efficient electron injection in all solvents, although best properties are observed in DMF and THF phases. NBO analysis results showed that there exists high contribution from several intra-and inter-fragment electron delocalizations that contribute immensely to the stabilization energy of FTPF monomer. In summary, FTPF is an adequate D-A-D monomer for an optoelectrically active polymer applicable in organic photovoltaics.