Exploring the Optoelectronic Properties of D-A and A-D-A 2,2′-bi[3,2-b]thienothiophene Derivatives

The synthesis of some novel donor-acceptor and acceptor-donor-acceptor systems containing a 2,2′-bi[3,2-b]thienothiophene donor block and various electron-accepting units is described alongside their photophysical properties studied using electrochemistry, optical spectroscopy and theoretical calculations. The obtained results show that the energy levels can be modulated by changing the strength of the acceptor unit. Among the three investigated end-groups, 1,1-dicyanomethylene-3-indanone exhibited the largest bathochromic shift and the lowest band gap suggesting the strongest electron-withdrawing character. Moreover, the emissive properties of the investigated systems vary greatly with the nature of the terminal group and are generally lower compared to their precursor aldehyde derivatives.


Introduction
Among the aromatic-fused 5,5 heterocyclic systems with one heteroatom in each ring, fused bithiophenes stand out due to their versatile properties and vast applications [1][2][3][4][5][6][7]. There are four possible arrangements of the heteroatoms (Chart 1) with stable aromatic structures I-III, while compounds with a core structure IV showing a S(IV) atom are not stable and could not be isolated.

Introduction
Among the aromatic-fused 5,5 heterocyclic systems with one heteroatom in each ring, fused bithiophenes stand out due to their versatile properties and vast applications [1][2][3][4][5][6][7]. There are four possible arrangements of the heteroatoms (Chart 1) with stable aromatic structures I-III, while compounds with a core structure IV showing a S(IV) atom are not stable and could not be isolated.  [7] as its derivatives revealed a wide array of applications. Thus, researchers have found it useful in medicinal chemistry as antibacterial agents [8,9], powerful inhibitors of platelet aggregation [10] or in glaucoma treatment [11], as materials with liquid crystal behavior [12,13] or non-linear optical properties [14,15] and in polymer chemistry as conducting (co)polymers [16][17][18].

Optical Properties
The physicochemical properties of the symmetrical and unsymmetrical systems based on [3,2-b]-TT 4-11 were investigated by UV-vis and fluorescence spectroscopy. The absorption properties have been studied in diluted CHCl3 (except for compound 8 that was solved in dichloromethane as it decomposed in chloroform) solutions (ca 10 −5 M), as well as in thin films spun-cast on glass from chloroform solutions for the target (A-)D-A systems 7-11. The corresponding data are gathered in Table 1. The UV-vis spectra (Figure

Optical Properties
The physicochemical properties of the symmetrical and unsymmetrical systems based on [3,2-b]-TT 4-11 were investigated by UV-vis and fluorescence spectroscopy. The absorption properties have been studied in diluted CHCl3 (except for compound 8 that was solved in dichloromethane as it decomposed in chloroform) solutions (ca 10 −5 M), as well as in thin films spun-cast on glass from chloroform solutions for the target (A-)D-A systems 7-11. The corresponding data are gathered in Table 1. The UV-vis spectra (Figure

Optical Properties
The physicochemical properties of the symmetrical and unsymmetrical systems based on [3,2-b]-TT 4-11 were investigated by UV-vis and fluorescence spectroscopy. The absorption properties have been studied in diluted CHCl 3 (except for compound 8 that was solved in dichloromethane as it decomposed in chloroform) solutions (ca 10 −5 M), as well as in thin films spun-cast on glass from chloroform solutions for the target (A-)D-A systems 7-11. The corresponding data are gathered in Table 1. The UV-vis spectra ( Figure 1) of all synthesized compounds reveal similar absorption features, with broad, intense absorption bands (λ max ) in the visible region (435-600 nm) assigned to π-π* electron transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), as the TD-DFT calculations revealed (vide infra), and less intense bands in the UV region (250-420 nm) ascribed also to π-π* transitions between orbitals separated by higher energies. The impact of the electron-accepting ability of the end-capping groups (dicyanovinylene, rhodanine and indanone derivatives, respectively) in the D-A and A-D-A systems on the electronic properties is observed by inspecting the trend of λ max values (Table 1), as all the absorption maxima are red-shifted compared to that in parent [3,2-b]-TT compound 4 and aldehydes 5 and 6 as well, following the decrease of the optical band gap. Comparison the UV-vis spectra of the unsymmetrical D-A systems (7 and 8) and those of symmetrical A-D-A systems (10, 11) supports the aforementioned effect, with significant bathochromic shifts observed in the case of the latter.
in the D-A and A-D-A systems on the electronic properties is observed by inspecting the trend of λmax values (Table 1), as all the absorption maxima are red-shifted compared to that in parent [3,2-b]-TT compound 4 and aldehydes 5 and 6 as well, following the decrease of the optical band gap. Comparison the UV-vis spectra of the unsymmetrical D-A systems (7 and 8) and those of symmetrical A-D-A systems (10, 11) supports the aforementioned effect, with significant bathochromic shifts observed in the case of the latter. Moreover, as outlined in Table 1, the molar absorption coefficient (ε) of the low energy absorption bands of the A-D-A systems (79.8-98 × 10 3 M −1 cm −1 ) is higher than that of the unsymmetrical counterparts (62.5-61.7 × 10 3 M −1 cm −1 ), suggesting a better ability of light harvesting in the symmetrical systems. On the other hand, among the unsymmetrical systems, the indanone derivative 9 has the best light-harvesting ability (ε = 89.3 × 10 3 M −1 cm −1 ), higher than that of the A-D-A derivative 10 bearing dicyanovinylene as accepting units and very similar to compound 11 that has rhodanine moieties as terminal groups.
A comparison of the spectra in solution and neat film reveals two different behaviours. For compounds containing malonodinitrile and rhodanine as the acceptor unit (7, 8, 10, 11), a broadening and a red-shift of the band could be observed that is commonly observed in formation of J-aggregates, while in the case of derivative 9 bearing an indanone derivative as the accepting moiety, a blue-shift of the absorption maximum band could be seen that occurs in H-aggregates [49]. The estimated solid-state band gap of the materials taken from the red-edge onset of the thin films varies between 1.92-1.53 eV and follows the same trend with the optical gap in solution (2.08-1.71 eV).  Moreover, as outlined in Table 1, the molar absorption coefficient (ε) of the low energy absorption bands of the A-D-A systems (79.8-98 × 10 3 M −1 cm −1 ) is higher than that of the unsymmetrical counterparts (62.5-61.7 × 10 3 M −1 cm −1 ), suggesting a better ability of light harvesting in the symmetrical systems. On the other hand, among the unsymmetrical systems, the indanone derivative 9 has the best light-harvesting ability (ε = 89.3 × 10 3 M −1 cm −1 ), higher than that of the A-D-A derivative 10 bearing dicyanovinylene as accepting units and very similar to compound 11 that has rhodanine moieties as terminal groups.
A comparison of the spectra in solution and neat film reveals two different behaviours. For compounds containing malonodinitrile and rhodanine as the acceptor unit (7, 8, 10, 11), a broadening and a red-shift of the band could be observed that is commonly observed in formation of J-aggregates, while in the case of derivative 9 bearing an indanone derivative as the accepting moiety, a blue-shift of the absorption maximum band could be seen that occurs in H-aggregates [49]. The estimated solid-state band gap of the materials taken from the red-edge onset of the thin films varies between 1.92-1.53 eV and follows the same trend with the optical gap in solution (2.08-1.71 eV). The fluorescence emission properties and the fluorescence quantum yield (QY) of the compounds have been measured in diluted chloroform (dichloromethane for 8) solutions. Surprisingly, compound 9 showed no fluorescence. The fluorescence spectra are presented in Figure 2 and the emission characteristic data are summarized in Table 2.
Surprisingly, compound 9 showed no fluorescence. The fluorescence spectra are pre-sented in Figure 2 and the emission characteristic data are summarized in Table 2.   As shown in Figure 2, the photoexcitation of precursor aldehyde derivatives 5 and 6 at λ exc = 434 nm and λ exc = 450 nm, respectively, provides an emission maximum near 500 nm related to a fluorescence quantum yield of 77% and 18%, respectively, relative to fluoresceine as the standard reference.
Interestingly, compared to precursor derivatives, the D-A and A-D-A systems also exhibit emissive properties (see Table 2), with a pronounced red-shift of the emission maximum and a significantly decreased QY.
A comparison between the mono and disubstituted [3,2-b]TT derivatives highlights that A-D-A systems 10 and 11 show poor photoluminescence properties. It is also worth mentioning that both rhodanine-based D-A and A-D-A derivatives (8 and 11) display very low fluorescence QY (<1%) compared to those bearing dicyanovinylene as the electronwithdrawing group (10% in the case of derivative 7 and 6% in case of derivative 10).
Lifetime measurements for compounds 5-11 and estimation of the radiative (k r ) and non-radiative (k nr ) decay constants (Table 3) show that non-radiative processes are overwhelming in compounds 8-11.
where QY is the experimentally determined quantum yield and τ represents the average lifetime of the compound.

Electrochemical Properties
To study the electrochemical properties of the synthesized compounds 7-11, cyclic voltammetry (CV) measurements were carried out in chloroform using Bu 4 NPF 6 as a supporting electrolyte. Cyclic voltammograms of A-D-A systems 10 and 11 ( Figure 3) reveal reversible oxidation and reduction waves that suggest the formation of the corresponding stable cation and anion radicals. Unsymmetrical D-A derivatives 7-9 are reversibly oxidized and undergo irreversible reduction processes, as shown in Figure 3. In the case of compound 7, the oxidation process is not fully reversible; in the reverse scan, a cathodic wave of weak intensity can be observed. This feature may be the result of the reduction product formed by the coupling of radical-cation [49].
As depicted in Table 1, the nature and number of the electron-acceptor group exerts a noticeable effect on both the anodic and cathodic peak potentials, which suggests that the HOMO and LUMO are delocalized over the entire molecule.
The electrochemical gap (E g ) estimated from the onset of the redox processes has the same trend as observed in the UV-vis spectroscopy experiments (see Table 1). Thereby, in the D-A series, the more electron-withdrawing group (EWG) from dicyanovinylene (7) to 1,1-dicyanomethylene-3-indanone (9) causes a decrease of the band gap by 0.62 eV. The same behaviour is observed in the A-D-A systems when the dicyanovinylene (10) is replaced with N-ethyl rhodanine (11).

Theoretical Investigations
To have a clearer idea of the impact of the structure on the optoelectronic properties of the obtained D-A and A-D-A molecules, we performed density functional theory (DFT) calculations at the B3LYP-D3/Def2-TZVP level of theory [51][52][53] using the Gaussian 09 package [54] (see ESI for details). First, the geometries of compounds 7-11 were optimized without any symmetry constraint and with the convergence criteria set to tight. Moreover, to mimic the experimental conditions as much as possible, calculations were carried out in chloroform using the PCM model, except for compound 8, which was solvated in dichloromethane. After the optimization step, all structures were found to be fully coplanar ( Figure S24 in Supplementary Materials), which is not so surprisingly since there are no steric repulsions between the building blocks of each compound (i.e., the two TT units and acceptor part(s)) and also due to their tendency to facilitate conjugation.  As depicted in Table 1, the nature and number of the electron-acceptor group exerts a noticeable effect on both the anodic and cathodic peak potentials, which suggests that the HOMO and LUMO are delocalized over the entire molecule.
The electrochemical gap (Eg) estimated from the onset of the redox processes has the same trend as observed in the UV-vis spectroscopy experiments (see Table 1). Thereby, in the D-A series, the more electron-withdrawing group (EWG) from dicyanovinylene (7) to 1,1-dicyanomethylene-3-indanone (9) causes a decrease of the band gap by 0.62 eV. The same behaviour is observed in the A-D-A systems when the dicyanovinylene (10) is replaced with N-ethyl rhodanine (11).

Theoretical Investigations
To have a clearer idea of the impact of the structure on the optoelectronic properties of the obtained D-A and A-D-A molecules, we performed density functional theory (DFT) With the optimized geometries in hand, we further performed time-dependent density functional theory calculations (TD-DFT) at the same level of theory and in the appropriate solvent (see ESI for details about the method) in order to find out the origins of the experimentally observed absorption bands. The theoretical UV-vis spectra agree well with the experimental results, with their comparison depicted in Figure 4, along with the most relevant transitions in the 300-1000 nm range and with an oscillator strength higher than 0.15. According to the TD-DFT results, in all cases, the lowest-energy absorption (and the most intense band at the same time) observed in UV-vis spectra is due to transition from the ground state (S 0 ) to the first excited electronic state (S 1 ), i.e., S 0 →S 1 , which involves the electron density moving from the HOMO to the LUMO with a probability of~100%. Lowerintensity absorption bands also appear in the calculated spectra pertaining to transitions to higher excited electronic states. For example, the second absorption band (ca. 387 nm) in 7 can be associated with a combination of HOMO-1→LUMO and HOMO-2→LUMO transitions, while in 10, a HOMO-2→LUMO transition is mainly responsible for the band around 418 nm. One should note that the S 0 →S 2 transition is forbidden in all cases, except for compound 9, where it is characterized by a low oscillator strength.
relevant transitions in the 300-1000 nm range and with an oscillator strength higher than 0.15. According to the TD-DFT results, in all cases, the lowest-energy absorption (and the most intense band at the same time) observed in UV-vis spectra is due to transition from the ground state (S0) to the first excited electronic state (S1), i.e., S0→S1, which involves the electron density moving from the HOMO to the LUMO with a probability of ~100%. Lower-intensity absorption bands also appear in the calculated spectra pertaining to transitions to higher excited electronic states. For example, the second absorption band (ca. 387 nm) in 7 can be associated with a combination of HOMO-1→LUMO and HOMO-2→LUMO transitions, while in 10, a HOMO-2→LUMO transition is mainly responsible for the band around 418 nm. One should note that the S0→S2 transition is forbidden in all cases, except for compound 9, where it is characterized by a low oscillator strength. Concerning the low-energy absorption bands, their theoretical absorption maxima ranges from 546 nm to 645 nm within the D-A series and from 604 to 648 nm in the A-D-A type compounds, depending on the type and number of EWGs attached on the bi-TT core. Moreover, the theoretical bandgap (HOMO-LUMO) (Egap, Table 1) trend follows the experimental one and can be further explained by inspecting the energy levels of the Concerning the low-energy absorption bands, their theoretical absorption maxima ranges from 546 nm to 645 nm within the D-A series and from 604 to 648 nm in the A-D-A type compounds, depending on the type and number of EWGs attached on the bi-TT core. Moreover, the theoretical bandgap (HOMO-LUMO) (E gap , Table 1) trend follows the experimental one and can be further explained by inspecting the energy levels of the frontier molecular orbitals (FMOs) for the individual EWGs and for the bi-TT central unit, respectively (Table S1). More precisely, the InOCN has the lowest lying LUMO among all EWGs and contributes considerably to the lowering of the hybrid LUMO of molecule 9. Interestingly, the rhodanine acceptor moiety keeps the E gap of both the D-A (8) and A-D-A (11) systems below the E gap of DCV counterparts (i.e., 7 and 10), despite its inability to decrease LUMO eigenvalues as DCV does. The explanation is that the rhodanine part decreases the HOMO values with only 0.05 eV (in 8) and 0.1 eV (in 11) relative to parent dimer 5, which is less than DCV moiety, which considerably decreases the HOMO in 7 and 10 (with 0.2 eV and 0.5 eV, respectively) relative to the same parent [3,2-b]-TT central unit.
As HOMO and LUMO have the strongest impact on the optoelectronic properties of our studied compounds, it is useful to depict their spatial distribution in order to obtain more information about the nature of the electronic transitions. On this note, careful inspection of the wave functions shows a distribution of HOMO over the entire molecule in all (A-)D-A motifs, with a lower coefficient on the acceptor moiety in the case of the DCVand InOCN-based compounds. Similarly, LUMO is delocalized over both the donor and acceptor units within the A-D-A molecules, but with a considerably higher coefficient on the acceptor part in the case of the D-A counterparts ( Figure 5). This orbital distribution suggests that the HOMO→LUMO transition within the D-A series can be regarded as a combination of local excitation (LE) and internal charge transfer (ICT), while in the case of the A-D-A counterparts, only local transitions take place. To further prove these statements, we plotted the electron density difference between first excited state and ground state for each molecule (Figure 6). Upon excitation, the electron density moves from the yellow region to the blue one; thus, in the case of compounds 7-8, the S 0 →S 1 has a mixed character of ICT from uncapped TT moiety toward the EWG and LE mainly within the other half of the [3,2-b]-TT core and also within the EWG, especially in compound 8. On the other hand, in compound 10 and 11, only LE takes place. The absence of ICT in our A-D-A molecules is most likely due to a few cumulative causes: (i) the absence of a π-spacer between D and A that could facilitate the localization of the HOMO on the donor side and the LUMO on the acceptor moiety (i.e., charge separation); (ii) the presence of two acceptor moieties, which pull the electron density in opposite direction and do not allow its polarization toward one EWG; and (iii) the imbalance between the donation ability of the [3,2-b]-TT platform and accepting ability of EWGs.

Figure 6.
Electron difference density plots between first excited state and ground state for compounds 7-11. Density depletion is depicted in yellow and density accumulation is plotted in blue (isosurface value = 0.0001).

Figure 6.
Electron difference density plots between first excited state and ground state for compounds 7-11. Density depletion is depicted in yellow and density accumulation is plotted in blue (isosurface value = 0.0001).

Materials and Methods
3.1. General Data 1 H NMR (400 or 600 MHz) and 13 C NMR (100 or 150 MHz) spectra were recorded in CDCl 3 or CD 2 Cl 2 at room temperature using the solvent line as a reference.
Thin layer chromatography (TLC) was conducted on silica gel 60 F 254 TLC plates. All plates were visualized by UV irradiation at 254 and 365 nm.
Solvents were dried and distilled under argon using standard procedures. Chemicals were purchased from TCI Chemicals, and Alfa Aesar and Sigma Aldrich and were used without further purification.
Melting points were determined with a Kleinfeld apparatus and are uncorrected. HRMS were recorded using an LTQ XL OBITRAP mass spectrometer equipped with ESI/APCI sources. UV-vis optical data in solution and in films were recorded with an UV-vis 1900 Shimadzu spectrometer. Fluorescence spectra were recorded on a JASCO FP-8300 spectrofluorometer using glass cuvettes (1 cm). Solutions for UV-vis, CV, HRMS and fluorescence measurements were prepared in HPLC grade solvents (dichloromethane, chloroform, acetonitrile).
Films were obtained from chloroform solutions using a classical spin coater. Fluorescence quantum yields were calculated using fluoresceine (in 0.1 N NaOH, QY = 0.95) or rhodamine B (in ethanol, QY = 0.7) as standards.
Fluorescence lifetime measurements were performed on a MicroTime200 time-resolved confocal fluorescence microscope system (PicoQuant, Berlin, Germany), described in detail elsewhere [55]. The excitation was provided by picosecond diode laser heads operating at selected wavelengths (405 nm for 5 and 6; 520 nm for 7, 8, 10, 11; and 640 nm for 9) and at a 40 MHz repetition rate. The signal was collected from the compounds in a CHCl 3 solution with a UPlanSApo 60 ×/NA = 1.2 water immersion objective and filtered thereafter with a 50 µm pinhole and adapted long-pass emission filters.
Cyclic voltammetry (CV) measurements were carried out with a Biologic SP-150 potentiostat using a three-electrode cell equipped with a platinum electrode, a calomel reference electrode (SCE) and a platinum wire counter electrode. All experiments were performed in 0.10 M Bu 4 NPF/CHCl 3 at 100 mV/s, Pt electrode references, SCE.

Procedure for the Synthesis of 4
Under stirring and in an argon atmosphere, n-butyllithium (1.5 M in hexane) (1.83 mL, 2.75 mmol) was added dropwise over 10 min at −78 • C to a solution of 3,6-bis(hexyloxy)thieno[3,2b]thiophene (850 mg, 2.5 mmol) in dry THF (65 mL) and the stirring was continued for another 2 h at the same temperature. Then, CuCl 2 (370 mg, 2.75 mmol) was added in one portion and the reaction mixture was gradually warmed up to room temperature and then stirred overnight. After adding water (30 mL) and triethylamine (10 mL), the mixture was extracted with dichloromethane (3 × 30 mL) and the combined organic layers were dried over magnesium sulfate. After filtration, the solvent was removed under reduced pressure. The crude product was purified with silica gel gradient column chromatography (elution system started with pentane/CH 2 Cl 2 = 2/0.2 and ended with pentane/CH 2 Cl 2 = 2/0.4) to afford a yellow solid (410 mg, 0.60 mmol, 48%).

Conclusions
In conclusion, a series of symmetrical and unsymmetrical conjugated systems based on a 2,2 -bi[3,2-b]thienothiophene donor block and various electron-withdrawing groups as acceptors was synthesized and characterized. An in-depth study of the terminal electronacceptor unit's impact on the optical and electrochemical properties was carried out. It was shown that the introduction of electron-withdrawing groups in parent 2,2 -bi[3,2b]thienothiophene led to a red shift of the main absorption band due to a decrease in the HOMO-LUMO band gap. All investigated systems display emission maxima in the green region, with the A-D-A compounds exhibiting poorer photoluminescence properties compared to their D-A counterparts. Finally, theoretical studies shed light on the observed optoelectronic behaviours by revealing the type and nature of the transitions that can take place and also by showing how various electron-accepting groups tune the optical bandgaps.