Polymeric near infrared emitters with bay-annulated indigo moieties

Copolymers alternating the BAI with thiophene-based moieties showed absorption and fluorescence in the NIR and were found to prevent ACQ.


Introduction
Natural dyes and pigments as indigo (Scheme 1) and its natural (indigoids) or synthetic derivatives are valued for their bright color and high photochemical stability and have been used since antiquity. 1-5 A bay-annulated indigo derivative known as Cibalackrot (7,14-diphenyldiindolo[3,2,1-de; 3′,2′,1′ij][1,5]naphthyridine-6,13-dione), an industrial dye that has been known for over a century 6 , has a highly rigid chemical structure, showing strong luminescence 7 and is, as indigo, capable of singlet fission 8,9 and can be used as an organic laser dye 10 . The need for better performing and stable organic materials in organic electronics inspired the resurgence of historically long known molecules as indigo 11,12 and their subsequent chemical transformation into new families of -conjugated building blocks 12 , also for construction of new macromolecular semiconductors. 11 In this work, indigo was functionalized by reaction with 2-thienylacetyl chloride, as described previously 13,14 , to produce the bay-annulated indigo derivative BAI1 (Scheme 1). The electron-deficient BAI1 moiety is a suitable building block for the generation of push-pull type conjugated polymers. 15 In recent years copolymers comprising these building blocks have been studied for their suitability in OFETs and OPVs applications. 13 Moreover, these donor-acceptor copolymers are also attractive in view of a potential application in organic solar cells 16 and as NIR detectors. More specifically, near-infrared photodetection (NIR-OPD, @800-3000 nm) 17 is valuable for numerous scientific as well as industrial (inspection, sorting, safety/security) and recreational applications. Nowadays, near-infrared photodetectors are commonly based on inorganic semiconductors such as InGaAs, InGaSb 18 or HgCdTe 19 , although organic-based NIR detectors 18,[20][21][22][23][24] have gained increasing interest. To date, a very limited amount of well performing NIR-OPDs have been reported 24 . Recently, push-pull copolymers based on BAI indigo core were found to have a suitable for photodetection beyond 1000 nm 19 .
Here, we investigate the ground-and excited-state characteristics of copolymers comprising BAI1 units, linked through thiophene-based donor units (cyclopentadithiophene CPDT-or benzodithiophene BDT-based) to verify the potential of such copolymers as NIR solid state emitters. A rational design of the polymers indicates that the introduction of planarized and symmetrical thiophene donors (CPDT, 4H-cyclopenta[1,2b:5,4-b']dithiophene, and BDT, benzo[1,2-b:4,5-b']dithiophene) should enable interchain π-π stacking and maximized on-chain electronic interaction, thus promoting the bulk charge transport. Furthermore, the donor blocks, of the donoracceptor copolymers, have been structurally varied to evaluate their influence on its electronic properties. Additionally and since the BAI acceptor units does not carry solubilizing side chains, the donor blocks have been also used to induce a sufficient solubility.

Thin films preparation
Polymer thin films were obtained with a desktop precision spincoating system, model P6700 series from Speedline Technologies, as described elsewhere. 26,27 Solutions for spincoating were prepared by adding 2 mg of the polymers to 200 μL of chloroform solution, with stirring in the dark, at room temperature, overnight. Thin films from the samples were obtained by deposition of ca. 50 μL from a solution of the compounds into a circular sapphire substrate (10 mm diameter) followed by spin-coating (2500 rpm) in a nitrogen-saturated atmosphere (2 psi).

Photophysical and electrochemical studies
Absorption and fluorescence spectra were recorded on Agilent Cary 5000 UV-Vis-NIR and Horiba-Jobin-Ivon SPEX Fluorog 3-22 spectrometers respectively. Absorption spectra of the transparent thin films were obtained in absorption mode using a clean sapphire substrate as the reference sample. The fluorescence spectra were corrected for the wavelength response of the system. For the steady state and time resolved emission experiments, the absorption at the excitation wavelength was kept below 0.1 values to avoid aggregation effects. Acquisition of the photoluminescence spectra of the polymers at NIR were performed with Hamamatsu R5509-42 photomultiplier, cooled to 193 K in a liquid nitrogen chamber (Products for Research model PC176TSCE-005), connected to the fluorimeter. The colour parameters were determined according to the CIE (Commission Internationale de l'Eclairage proceedings) 1931 scale diagram 28 . The x and y colour parameters were determined with the acquisition of the transmittance spectra using Shimadzu UV-2600 equipped with Colour Analysis software. Fluorescence quantum yields (φ F ) of the polymers in solution were determined with three replicates, by William's method 29 , using indocyanine green (IR-125 in ethanol, φ F = 0.132) 30 as standard. The φ F of the model monomers in solution were measured using the absolute method with a Hamamatsu Quantaurus QY absolute photoluminescence quantum yield spectrometer, model C11347 (integration sphere). The φ F of the polymers in thin films were measured by the absolute method with an integrating sphere module, as described elsewhere. 26,31,32 The following equation was used to determine the φ F of the BAI polymers thin films: where φ F (solid) is the fluorescence quantum yield of polymer thin film, is the integrated area under the ∫ ( ) emission of the polymer thin film (which excludes the integration of the Rayleigh peak), is the ∫ _ ( ) λ integrated area under the Rayleigh peak of a sample containing only a clean sapphire support, and is the ∫ _ ( ) λ integrated area under the Rayleigh peak in the emission spectra of polymer in thin film. Fluorescence decays were measured using a home-built picosecond time-correlated single photon counting (ps-TCSPC) apparatus described elsewhere 33 . Excitation was performed with the second harmonic, 372 nm or 402 nm (generated with a Spectra Physics GWU-23PS module) from a picosecond Spectra Physics mode-lock Tsunami laser (Ti:Sapphire) model 3950 (80 MHz repetition rate, tuning range 700-1000 nm), pumped by a Millennia Pro-10s, continuous wave, solid-state laser (532 nm). The fluorescence decays and the instrumental response function (IRF) were collected using 1024 channels in a time scale up to 24.4 ps/channel (using a Spectra Physics frequency divider, Pulse picker model 3980-2s, to reduce the fundamental laser repetition rate). Deconvolution of the fluorescence decay curves was performed using modulation function method, as implemented by G. Striker in the SAND program, and previously reported in the literature. 34 The time resolved ultrafast transient absorption measurements were collected in a broadband HELIOS spectrometer (350-1600 nm) from Ultrafast Systems as elsewhere described. 35 The transient absorption data was obtained with excitation at 420 Please do not adjust margins Please do not adjust margins nm and 770 nm and probed in the 350-800 nm and 800-1600 nm range. The measurements in solution were obtained in a 2 mm quartz cuvette, with optical density ≈ 0.1-0.3 at the pump excitation wavelength. To avoid photodegradation low laser pump energies were used (≤100 nJ) at the excitation wavelengths 420 nm and 770 nm and the solutions were stirred during the experiments or kept in movement using a motorized translating sample holder. The spectral chirp of the data was corrected using Surface Xplorer PRO program from Ultrafast Systems. Global analysis of the data (using a sequential model) was performed using Glotaran software. 36 Cyclic voltammetry experiments were carried out using an Autolab potentiostat/galvanostat PGSTAT204 running with NOVA 2.1 software and a three-electrode system in a one-compartment electrochemical cell of capacity 5 mL. A glassy carbon electrode (GCE) (d = 3 mm) was the working electrode, glassy carbon (GC) (d = 1.6 mm) wire the counter electrode and Ag/Ag + (0.01 M silver nitrate, AgNO 3 , in 0.1 M tetrabutylammonium hexafluorophosphate, NBu₄PF₆, in MeCN) as the reference electrode. Ferrocene/ferrocenium (Fc/FC + ) redox couple were used as the internal reference. The reference and polymers were dissolved in 0.1 M NBu₄PF₆ in CHCl 3 solution to acquire cyclic voltammograms with a 50 mV/s scan rate with a window potential between -1.6V to 1.0 V (vs. Ag/Ag + ).

Optical and electrochemical properties
The photophysical properties of the three BAI copolymers were investigated in solution (2-MeTHF and toluene) and in thin films and compared to their monomeric units. In Figure 1, the absorption and emission spectra of PBAIC-1 and PBAIC-2 in 2-MeTHF solution are compared with BAI1 and C 12 CPDT, PBAIBD-1 is compared to BAI1 and BDT. The change of the phenyl groups of Cibalackrot to the electron-richer thiophene groups in BAI1 lead to a bathochromic shift of the absorption maximum of ~40 nm: 547 nm for Cibalackrot to 588 nm for BAI1 ( Figure  SI4; Table SI2). Generally, incorporation of the BAI chromophores into the copolymers causes a significant red shift of the absorption and emission of these in comparison to BAI1. The shape of the absorption spectra is found similar for the three copolymers, with two broad absorption bands: one (I) in the 300-500 nm region and another (II), broader, at 550-1000 nm. The bathochromic shift of band I of the copolymers (PBAIC-1, PBAIC-2 and PBAIBD-2) that may be assigned to the donor units, in comparison to the absorption bands of the monomeric donor chromophores (BDT and CPDT), may indicate the conjugative interaction between the subunits. The interplay of first and second absorption band of the copolymers causes a green color of the copolymers, in contrast to the purple color characteristic of BAI1 (Figure 1). The red shift of the second (II) absorption band in comparison with BAI1 can be associated to an extended -conjugation and a strong donor-acceptor character of the copolymers. PBAIC-1 and PBAIC-2 (with the CPDT moiety as electron donor unit) in 2-MeTHF solution display emission spectra with bands in the 800-1200 nm region, with peak maxima at 997 and 982 nm, respectively, while PBAIBD-1 shows an emission spectrum with two peaks: 778 nm (visible region) and 944 nm (NIR), see Table 1. The electronic spectra of the three copolymers in toluene ( Figure SI5) are almost identical to the spectra in 2-MeTHF solutions (see Table  SI3). The introduction of the donor CPDT unit (from PBAIC-1), instead of BDT (PBAIBD-1), leads to a red-shift of the absorption either in solution, or in solid state. Nonetheless, solid state absorption bands are red-shifted relative to the solution spectra ( Figure SI5). Fluorescence quantum yields (φ F ) of the BAIcontaining copolymers obtained in 2-MeTHF, at 298 K and 77 K, and in thin films at 298 K (Table 1) are roughly similar to the solid state (thin films) values. The spatial structure of PBAIC-1, PBAIC-2 and PBAIBD-1 as donor-acceptor copolymers seems to preclude aggregation caused quenching (ACQ) effects. Moreover, the φ F values of the BAI copolymers at 77 K are about four times higher than in solution at 293 K (see Figure SI6 and Table SI4), as factor of interest for potential applications, e.g. as photoactive layer of NIR organic photodiodes. The PL emission area of the three polymers presented a linear course with the increase of the absorption at the excitation wavelength, in solution ( Figure SI7), ruling out aggregation effects of the polymer in 2-meTHF. The electrochemical properties and energy levels of BAI copolymers were determined in chloroform solution ( Table 2). The characteristic double reduction the amide groups, correlated to the BAI moiety, quasi-reversible, is prominent in cyclic voltammogram of PBAIBD-1. Moreover, the oxidation and reduction potentials are similar to those found in the literature to D-A type polymers based on Bay-annulated indigo and thiophene 13,16,19 . The energies of the HOMO and LUMO levels were obtained according to equations (2)   The energy of the HOMO, E HOMO , was calculated from the onset oxidation potential, , plus 4.8, which is the reference energy level of ferrocene below the vacuum level and the oxidation potential of ferrocene, E FOC , given by the potential of FOC/FOC + vs Ag/Ag + measured by cyclic voltammetry in solution ( Figure 2). The LUMO energy, E LUMO , was obtained from the optical energy gap E 0-0 , measured from the interception of the normalized UV-Vis absorption and fluorescence spectra, through equation (3). The HOMO-levels of the CPDT-based polymers are similar with values of -4.96 eV (PBAIC-1) and -4.95 eV (PBAIC-2), respectively, and correlate well with values determined from the onset of absorption (Table SI5). The LUMO-levels, about -3.5-3.9 eV, is closely correlated to the E LUMO = -3.6 eV, previously attributed to the model compound BAI1 40 . CPDTbased polymers also presented smaller band gaps than PBAIBD-1 (built with BDT as donor unit), thus, CPDT may be a a stronger electron donor unit than BDT and therefore gives higher HOMO energy levels ( Table 2). The band gap energies of BAI copolymers vary between 1.3-1.5 eV, when determined either by CV, or UV-spectroscopy (Table  2). Narrow band gap (Eg) conjugated polymers, 1.1 ≤ Eg ≤ 2.1 eV, have especially low lying LUMO energies making them excellent candidates for n-type OFET materials. 41 Moreover, these BAI copolymers also display optical absorption in the near-IR region of the electromagnetic spectrum, ideal for OPV devices. 21 Also, because these polymers contain both donoracceptor moieties on their single main backbones, they are promising candidates for replacing polymer blends used as active layers in manufacturing all polymeric solar cells, and thus, overcome drawbacks such as miscibility between the donor and acceptor polymer and gradual phase separation of polymer blend films. [42][43][44]

Time-resolved spectroscopy studies
Femtosecond transient absorption (fs-TA) was further used to characterize the excited state dynamics of the copolymers and two model monomers (BAI1 and BDT) in solution and in thin films (Figures SI8-SI11 and Table SI6). From Figure 3, it is seen that the fs-TA spectra of the copolymers are composed of a broad positive excited state absorption (ESA) in the 880-1600 nm range, and negative TA bands between 350 nm and 955 nm matching with those in Figures 1 attributed to ground-state absorption (GSA).
In solution after about 5 ps (PBAIC2) and 10 ps (PBAIBD1) the initially formed ESA band blue-shifts by 20 nm and thereon the band maxima remain unchanged. Since there are no additional bands at longer decay times (that could be attributed, e.g., to triplet excited state formation in the polymers) the positive TA bands are attributed to the S 1 →S n ESA. This behavior contrasts with that of the monomeric BAI1 chromophore where in addition to the S 1 →S n ESA (in the 350-480 nm and 665-800 nm), GSA, and stimulated emission in the (510-630 nm) bands, a triplet-triplet absorption band is also observed with triplet lifetime of  T = 54 s (presented in the ns-TA absorption spectra, Figure SI9). For PBAIBD1 a characteristic ESA band is seen in the 460-560 range, as also found for the monomeric BDT unit. However, contrary to monomeric BDT, in PBAIBD1 this band disappears after 106 ps (see Figure SI10-SI11 and Figure 2). For monomeric BDT the S 1 →S n ESA band was found to be strongly overlapped with the T 1 →T n ESA band, as shown by comparison with the triplet absorption spectra obtained by ns-TA, ( T = 99 s), Figure SI10. The data support (i) the pronounced conjugation between BAI1 and BDT moieties in the PBAIBD1 copolymer, which promotes electron-delocalization, as shown by the significant red-shift of the absorption band when compared with the monomeric BAI1 (Figure 1), and (ii) the absence of triplet state formation in the polymers, due to the fast deactivation of the excited state. The mechanism behind this will be discussed below. Global analysis of representative kinetic traces of the copolymers shows that transient decays are well fitted with the sum of three exponentials. The best-fit results and preexponential values are presented in Table 3 (also in table SI6 and Figure Figure SI12). BDT transient decays are well fitted with a double exponential decay law, with transient lifetimes of 313 ps (also in agreement with  F = 560 ps obtained by TCSPC, Figure SI13) and 99 s, that was fixed in the analysis to the triplet lifetime obtained by ns-TA ( Figure SI14 in SI). The shortest decay time component ( 1 ) which, at shorter emission wavelengths is associated to a negative preexponential for the PBAIC-1 copolymer (Table 3), should be considered to be associated to solvent dynamics (in THF with characteristic times ranging from 0.43 ps to 0.94 ps) 45 , or attributed to vibrational relaxation due to the depopulation of a hot vibrational state generated through the 770 nm excitation (since excitation was not performed in the lowest energy 0-0 electronic transition). 38 Yet, this is not the case with PBAIC-2 and PBAIBD-1, where a longer component is observed with 2.4 ps and 8.9 ps (Table 3), respectively, in addition to a longer  3 component in the order of hundreds of ps. These  1 and  2 decay components are associated to excitation energy migration within different segments of the polymer chain (higher to lower energy conjugated segments) ending up in the  3 decay component which mirrors the decay of the more relaxed segment to the ground state.
where  0 stands for the donor lifetime in the absence of acceptor and  1,2 for the decay time of the donor in the polymer (with acceptor units). Due to the chemical structure of the corresponding repeat units (Scheme 1), the monomeric C 12 CPDT or BDT are taken as model donor units of the copolymers. Therefore for PBAIC-1,2  0 = 166 ps 25 (related to C 12 CPDT) and for PBAIBD-1  0 = 560 ps (related to BDT, Figure  S11) were used in eq. 4. The two decay components ( 1 and  2 ) describe contributions of the energy donors in the presence of energy acceptors. 49 This three exponential fit of the transient decays basically mirrors the fact that energy transfer (ET) processes leads to nonexponential decays and that the two shortest decay times, and respective pre-exponential factors, characterize, as a whole, the ET process. Indeed, from the preexponential factors in solution (Table 3), except for the longer component (associated to a relaxed exciton segment decaying to the ground state) and the shorter component of PAIBC1 (associate to solvent dynamics) the ET process can be fully characterized by the two mentioned components. Considering the simplest case of Coulombic (Förster) energy transfer, in which the energy donor units are distributed in a medium with a uniform concentration of acceptors, C A , the fluorescence decay function of the donor, I D (t), in the presence of acceptors can be obtained by eq. 5, 49, 50 with R 0 the Förster radius and  0 the lifetime of the Donor in the absence of acceptor. Although energy transfer leads to nonexponential decays, the I D (t) for PBAIC-2 and PBAIBD-1 can be fitted with a sum of two exponentials terms (Figure 4), which corresponded to two shortest decay times in these copolymers (Table 3). For PBAIC2,  2 is dominant particularly at longer wavelengths (representing ~91 % of the total decay) and should be therefore seen as the governing the ET process (i.e., the ET process involving more distant D-A pairs); for PBAIBD1 similar contributions were found for the two decay times, 50 %, indicating that short and distant D-A pairs contribute equally to the on-chain hopping decay mechanism. Additionally, the rate constants for ET process varies from 1.8310 10 to 4.1110 11 s -1 ( Table 3), characteristic of conjugated organic polymers. 38,49

Conclusions
In this work, three new BAI-based donor-acceptor copolymers were synthesized, and their photoexcitation decay mechanisms investigated in solution and thin films. Even though the φ F of these copolymers varies from 1 up to 2% in solution, the film values are basically identical to solution, thus showing that ACQ is not taking place. While in solution, the BAI-containing copolymers presented broad long wavelength absorption bands, with maxima varying between 688-775 nm, in thin films, the red-shifted emission bands reaches 900-1300 nm, indicating that these copolymers are a promising class of low-bandgap, circa 1.4-1.5 eV, copolymers with solid state absorption and emission widely shifted into the NIR region. The decays in the PBAIBD-1 and PBAIC-2 copolymers were further associated to an excitation energy transfer hopping that could be rationalized with a Förster-type mechanism with differing donor-acceptor contributions.
Our work reaches not only the need of the development of new NIR-absorber polymers to featured applications e.g. in sensing and photovoltaics, but also reinforces the use bay-annulated indigo as building block for the generation of push-pull type conjugated polymers

Conflicts of interest
There are no conflicts to declare.