Photophysical and redox properties of new donor-acceptor-donor (DAD) compounds containing benzothiadiazole (A) and dimethyldihydroacridine (D) units: a combined experimental and theoretical study.

Four donor-acceptor-donor compounds consisting of 9,9-dimethyl-9,10-dihydroacridine donors differently linked to a benzothiadiazole acceptor were designed using DFT calculations and synthesized, namely 4,7-bis(4-(9,9-dimethyl-9,10-dihydroacridine)phenyl)benzo[c][1,2,5]thiadiazole (1), 4,7-bis(2,5-dimethyl-4-(9,9-dimethyl-9,10-dihydroacridine)phenyl)benzo[c][1,2,5]thiadiazole (2), 4,7-bis(3,5-di(9,9-dimethyl-9,10-dihydroacridine)phenyl)benzo[c][1,2,5]thiadiazole (3), and 4-(3,5-di(9,9-dimethyl-9,10-dihydroacridine)phenyl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (4). As predicted theoretically, all studied compounds were electrochemically active both in the reduction as well as in the oxidation modes. They underwent one electron quasi-reversible reduction. Oxidation of 1 and 2 involved a two electron process transforming them into dications and carrying out, in parallel, their dimerization. Oxidation of 3 and 4 resulted in their oligomerization (polymerization). The electrochemically determined ionisation potentials (IP) of 1-4 were similar, covering a narrow range of 5.28-5.33 eV and were consistent with DFT calculations. Larger differences were found for experimentally determined electron affinity (EA) values, being significantly lower for 2 (|EA| = 2.59 eV) as compared to 1, 3 and 4 whose |EA| values were higher by 0.15-0.25 eV, again consistent with DFT calculations. DFT calculations predict positive values of ΔE(S1-T1) for all compounds i.e. in the range of 0.18 eV to 0.43 eV for 1, 3 and 4 and a significantly lower value for 2 (0.06 eV), indicating a possible RISC process in this case. DFT calculations of ΔE(S1-T2) lead to negative and very small values for 2-4 implying a possible involvement of higher lying triplets in the generation of singlet excitons. The investigated derivatives exhibited fluorescence in the orange-red spectral range (550-770 nm) and were strongly dependent on the solvent polarity. The highest PLQY value of 37% was measured for 1 in toluene. The PLQY values significantly improved upon deoxygenation of the studied solutions. Solid state samples also exhibited higher PLQY values as compared to those determined for DCM solutions. These findings were rationalized by partial suppression of the vibrationally induced emission quenching in the solid state due to the intermolecular interaction confinement.

Expedient Synthesis of Oxaboracyclic Compounds Based on Naphthalene and Biphenyl Backbone and Phase-Dependent Luminescence of their Chelate Complexes.

The approach to a series of six- and seven-membered oxaboraheterocycles based on naphthalene or biphenyl backbones was developed. The key synthetic step involved Br/Li exchange in respective potassium (bromoaryl)trifluoroborates followed by quenching with selected electrophiles (CO2 , DMF, Me2 Si(H)Cl) and hydrolytic workup. Two ring-expanded benzoxaborole congeners were obtained by an additional reduction step with LiAlH4 or NaBH4 . The obtained boracyclic compounds were characterized in detail by NMR spectroscopy and single-crystal X-ray diffraction. Specifically, biphenyl-based systems show dynamic behaviour interpreted in terms of inversion of non-planar seven-membered boraheterocycles. The acidity of the obtained compounds varies very strongly (pKa ranges from 3.1-9.6) depending on their structure. Due to the enhanced boron Lewis acidity, selected compounds were used as a basis for luminescent complexes with 8-hydroxyquinoline. A strong phase-dependent variation of emission-band maximum (480-527 nm) and photoluminescence quantum yield (10-95 %) was observed, which was rationalized in terms of specific aggregation effects.

Copolymers Containing 1-Methyl-2-phenyl-imidazole Moieties as Permanent Dipole Generating Units: Synthesis, Spectroscopic, Electrochemical, and Photovoltaic Properties.

New donor-acceptor conjugated alternating or random copolymers containing 1-methyl-2-phenylbenzimidazole and benzothiadiazole (P1), diketopyrrolopyrrole (P4), or both acceptors (P2) are reported. The specific feature of these copolymers is the presence of a permanent dipole-bearing moiety (1-methyl-2-phenyl imidazole (MPI)) fused with the 1,4-phenylene ring of the polymer main chain. For comparative reasons, polymers of the same main chain but deprived of the MPI group were prepared, namely, P5 with diketopyrrolopyrrole and P3 with both acceptors. The presence of the permanent dipole results in an increase of the optical band gap from 1.51 eV in P3 to 1.57 eV in P2 and from 1.49 eV in P5 to 1.55 eV in P4. It also has a measurable effect on the ionization potential (IP) and electrochemical band gap (EgCV), leading to their decrease from 5.00 and 1.83 eV in P3 to 4.92 and 1.79 eV in P2 as well as from 5.09 and 1.87 eV in P5 to 4.94 and 1.81 eV in P4. Moreover, the presence of permanent dipole lowers the exciton binding energy (Eb) from 0.32 eV in P3 to 0.22 eV in P2 and from 0.38 eV in P5 to 0.26 eV in P4. These dipole-induced changes in the polymer properties should be beneficial for photovoltaic applications. Bulk heterojunction solar cells fabricated from these polymers (with PC71BM acceptor) show low series resistance (rs), indicating good electrical transport properties. The measured power conversion efficiency (PCE) of 0.54% is limited by the unfavorable morphology of the active layer.

9,10-Anthraquinones Disubstituted with Linear Alkoxy Groups: Spectroscopy, Electrochemistry, and Peculiarities of Their 2D and 3D Supramolecular Organizations.

Spectroscopic, electrochemical, and structural properties of 2,6-dialkoxy-9,10-anthraquinones (Anth-OCn, n = 4, 6, 8, 10, and 12) of increasing alkoxy substituents length were investigated. UV-vis spectroscopy showed a substitution-induced bathochromic shift of the least energetic band from 325 nm in the case of unsubstituted anthraquinone to ca. 350 nm for the studied derivatives. Similarly as unsubstituted anthraquinone, the studied compound showed two reversible one electron reductions to a radical anion and spinless anions, respectively. The first reduction was affected by electron-donating properties of the substituents, its potential being shifted to ca. -1.5 V (vs Fc/Fc+), i.e., by 80 to 95 mV as compared to the case of unsubstituted anthraquinone. This corresponded to a decrease of |EA| from 3.27 to 3.19-3.17 eV. The experimental spectroscopic and electrochemical data were in full agreement with the DFT calculations. The introduction of the alkoxy substituent improved solution processibility of the studied compounds and facilitated the formation of their ordered supramolecular 2D aggregation on HOPG as well as single crystal growth from solutions. Comparative structural investigations carried out on single crystals and monolayers deposited on HOPG revealed two, mutually related, effects of the substituent length on the resulting supramolecular organization. The first one concerns both the 2D organization in the monolayers and 3D molecular arrangement in crystals: increasing substituent length evolution of the structure occurs from herringbone-type to lamellar. The second effect, observed in monolayers of the derivatives with longer substituents, concerns gradual evolution of their lamellar structures with increasing substituent length. This evolution is induced by the structure of the graphite substrate and involves increasing correlation of the molecules orientation (anthraquinone cores as well as alkoxy substituents) with the symmetry of the graphite substrate. As a result, their 2D and 3D structures become dissimilar.

Nanosecond-Lived Excimer Observation in a Crystal of a Rhodium(I) Complex via Time-Resolved X-ray Laue Diffraction.

The rare observation of transient Rh···Rh excimer formation in a single crystal is reported. The estimated excited-state lifetime at 100 K is 2 ns, which makes it the shortest-lived small-molecule species caught experimentally using the laser-pump/X-ray-probe time-resolved Laue method. Upon excitation with 390 nm laser light, the intermolecular Rh···Rh distance decreases from 3.379(4) to 3.19(1) Å, and the metal-metal contact gains more bonding character. On the basis of the experimental results and theoretical modeling, the structural changes determined with 100 ps time resolution reflect principally the S0 → S1 electronic transition.

D-A-D Compounds Combining Dithienopyrrole Donors and Acceptors of Increasing Electron-Withdrawing Capability: Synthesis, Spectroscopy, Electropolymerization, and Electrochromism.

Five D-π-A-π-D compounds consisting of the same donor unit (dithieno[3,2-b:2',3'-d]pyrrole, DTP), the same π-linker (2,5-thienylene), and different acceptors of increasing electron-withdrawing ability (1,3,4-thiadiazole (TD), benzo[c][1,2,5]thiadiazole (BTD), 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 1,2,4,5-tetrazine (TZ), and benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)) were synthesized. DTP-TD, DTP-BTD, and DTP-DPP turned out to be interesting luminophores emitting either yellow (DTP-TD) or near-infrared (DTP-BTD and DTP-DPP) radiation in dichloromethane solutions. The emission bands were increasingly bathochromically shifted with increasing solvent polarity. Electrochemically determined electron affinities (|EA|s) were found to be strongly dependent on the nature of the acceptor changing from 2.86 to 3.84 eV for DTP-TD and DTP-NDI, respectively, while the ionization potential (IP) values varied only weakly. Experimental findings were strongly supported by theoretical calculations, which correctly predicted the observed solvent dependence of the emission spectra. Similarly, the calculated IP and EA values were in excellent agreement with the experiment. DTP-TD, DTP-BTD, DTP-TZ, and DTP-NDI could be electropolymerized to yield polymers of very narrow electrochemical band gap and characterized by redox states differing in color coordinates and lightness. Poly(DTP-NDI) and poly(DTP-TD) showed promising electrochromic behavior, not only providing a rich color palette in the visible but also exhibiting near-infrared (NIR) electrochromism.