Self-assembled Dyads Research Articles

Self-Assembled Donor-Acceptor Dyad Molecules Stabilize the Heterojunction of Inverted Perovskite Solar Cells and Modules.

The heterointerface between a semiconducting metal oxide and a perovskite critically impacts on the overall performance of perovskite solar cells (PVSCs). Herein, we report a feasible yet effective strategy to suppress the interfacial reaction between nickel oxide and the perovskite via chemical passivation with self-assembled dyad molecules, which leads to the simultaneous improvement of the power conversion efficiencies (PCEs) and operational lifetimes of inverted PVSCs. As a result, inverted PVSCs consisting of simple methylammonium iodide perovskites have achieved an excellent PCE of 20.94% and decent photostability with 93% of the initial value after 600 h of 1 sun equivalent illumination. Moreover, this strategy can be readily translated into slot-die fabrication of perovskite modules, achieving a high PCE of 14.90% with an area of 19.16 cm2 (no shade in the interconnecting area) and a geometrical fill factor of 93%. Overall, this work provides an effective strategy to stabilize the vulnerable heterointerface in PVSCs.

Spectral investigations of meso-tetraalkylporphyrin-fullerene host–guest complexes

Meso-tetraalkylporphyrins and their Zn(II) complexes were synthesized and characterized by various spectroscopic techniques. Single crystal X-ray structure of meso-tetrapropylporphyrin (3) revealed the orientation of alkyl chains and planar conformation of porphyrin macrocycle. Spectroscopic, photophysical and electrochemical redox properties of self-assembled donor–acceptor dyads formed by meso-tetraalkylporphyrins and fullerene C 60 were investigated. These studies revealed 1:1 supramolecular dyad formation between the electron donor porphyrins and the electron acceptor, fullerene entities. The determined association constants (K) follow the order: H2TMeP (1) > H2TEtP (2) > H2TPrP (3) > H2THexP > H2TPP . The effect of alkyl chain length on porphyrin-fullerene complexation was investigated. The redox behavior of self-assembled dyads was investigated in PhCN containing 0.1 M TBAPF6 as supporting electrolyte. The oxidation potentials of dyads are positively shifted (20–100 mV) as compared to corresponding meso-tetraalkylporphyrins indicating the supramolecular interactions between the constituents in the ground state. The geometric and electronic structure of 1: C 60 was probed by DFT calculations which suggested the possibility of charge transfer from meso-tetraalkylporphyrin to fullerene C 60.

FRET Sensitization of Tungsten–Alkylidyne Complexes by Zinc Porphyrins in Self-Assembled Dyads

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.

Photoinduced Electron Transfer in Zinc Naphthalocyanine-Naphthalenediimide Supramolecular Dyads

Photoinduced electron transfer was studied in self-assembled donor-acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI-py:ZnNc (1) and NDI(CH(2))(2)-py:ZnNc (2) self-assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10(4) and 2.2×10(4) M(-1), respectively, from the steady-state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine-appended NDI to ZnNc in toluene. The energy-level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc ((1)ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge-separation of 1 and 2 were found to be 2.2×10(10) and 4.4×10(9) s(-1), respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge-separated states were found to be 8.50×10(8) s(-1) (1.2 ns) for 1 and 1.90×10(8) s(-1) (5.3 ns) for 2. These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self-assembled NDI-py:ZnNc and NDI(CH(2))(2)-py:ZnNc dyads are useful as photosynthetic models.

Light harvesting zinc naphthalocyanine–perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

Photoinduced electron-transfer dynamics of self-assembled donor-acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with tert-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (1) and ZnNc:PDIim (2) in toluene were found to be 2.40 × 10(4) and 1.10 × 10(5) M(-1), respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an ab initio B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc˙(+):PDI˙(-). The electrochemical results suggest the exothermic charge-separation process via the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation via the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI˙(-)) and the ZnNc radical cation (ZnNc˙(+)) in the visible and NIR regions. The rate constants of charge-separation (k(CS)) processes of the self-assembled dyads 1 and 2 were determined to be 4.05 × 10(10) and 1.20 × 10(9) s(-1), respectively. The rate constant of charge recombination (k(CR)) and the lifetime of charge-separated states (τ(CS)) of dyad 1 were determined to be 2.34 × 10(8) s(-1) and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10(7) s(-1)) and a longer lifetime of the charge separated state (45 ns) were observed in dyad 2 in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine-perylenediimide dyads as good photosynthetic models.

Excitation transfer in metal-ligand coordinated free-base porphyrin-magnesium phthalocyanine and free-base porphyrin-magnesium naphthalocyanine dyads

Singlet excitation transfer in self-assembled dyads formed via axial coordination of imidazole appended free-base tetraphenylporphyrin, H2PIm , to either a magnesium phthalocyanine, MgPc , or a magnesium naphthalocyanine, MgNc is investigated in non-coordinating solvents using spectroscopic (optical and mass), electrochemical, and time-resolved transient absorption techniques. The newly assembled dyads are fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral data are found to be in the range of 103–104 M-1 for the 1:1 dyads suggesting fairly stable complex formation. However, the anticipated 2:1 complex for Mg coordination is not observed under the present solution conditions. Electrochemical and computational studies suggested that photoinduced electron transfer to be a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. However, selective excitation of H2PIm entity in these dyads resulted in rapid excitation transfer and the position of the imidazole linkage on the H2P entity seem to direct the overall efficiency of excited energy transfer. Kinetics of energy transfer is monitored by transient absorption measurements using pump-probe technique and is compared with the earlier reported H2PIm:ZnPc and H2PIm:ZnNc dyads. The time constants are in the order of 0.2–18 ps depending upon the type and relative orientation of the donor and acceptor entities of the dyad indicating ultrafast excitation transfer, and agree fairly well with theoretically estimated values assuming Förster energy transfer mechanism.

Electron transfer reaction of light harvesting zinc naphthalocyanine–subphthalocyanine self-assembled dyad: spectroscopic, electrochemical, computational, and photochemical studies

Electron transfer reaction of a self-assembled donor-acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and subphthalocyanine appended with pyridine coordinating ligand, SubPc(py), was investigated in the present study. The SubPc(Py) : ZnNc self-assembled dyad absorbs the light in a wide section of the UV/Vis/NIR spectra. The formation constant of SubPc(py) : ZnNc in o-dichlorobenzene was found to be 1.2 × 10(5) M(-1) from the steady-state absorption and emission measurements, suggesting stable complex formation. The geometric and electronic calculations by using ab initio B3LYP/6-311G methods showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the subphthalocyanine entity, suggesting that the charge-separated state of the supramolecular complex is (SubPc(py))˙(-) : ZnNc˙(+). The electrochemical results suggest the exothermic charge-separation process via the singlet states of both SubPc(py) and ZnNc entities. Upon coordination the pyridine appended subphthalocyanine to ZnNc; the main quenching pathway involved charge separation via the singlet excited states of ZnNc and SubPc(py). A clear evidence of the intramolecular electron transfer from the singlet state of ZnNc to SubPc(py) was monitored by femtosecond laser photolysis in o-dichlorobenzene by observing the characteristic absorption band of the ZnNc radical cation in the NIR region at 960 nm. The rate of charge-separation process was found to be 1.3 × 10(10) s(-1), indicating fast and efficient charge separation. The rate of charge recombination and the lifetime of the charge-separated state were found to be 1.0 × 10(9) s(-1) and 1 ns, respectively. The absorption in a wide section of the solar spectrum and high charge-separation/charge-recombination ratio suggests the usefulness of self-assembled SubPc(Py) : ZnNc for being a photosynthetic model.

Photodynamics of excitation energy transfer in self-assembled dyads. Evidence for back transfer

Three self-assembled photonic dyads comprising a zinc porphyrin donor and a free base acceptor have been studied by time-resolved fluorescence spectroscopy. The driving force of the assembly is the site selective binding of an imidazole connected to a free base porphyrin. Three spacers have been incorporated between the imidazole connector and the free base porphyrin, providing three different distances separating the donor and the acceptor. The high efficiencies and the rates of energy transfer in the set of dyads is consistent with the Forster energy transfer mechanism. Evidence for Forster back transfer has been obtained, and its efficiency and rate have been quantitatively evaluated for the first time.

Spectroscopic, Electrochemical, and Photochemical Studies of Self-Assembled via Axial Coordination Zinc Porphyrin−Fulleropyrrolidine Dyads

Spectroscopic, redox, and photochemical behavior of self-assembled donor−acceptor dyads formed by axial coordination of zinc tetraphenylporphyrin, (TPP)Zn, and fulleropyrrolidine bearing either pyridine or imidazole coordinating ligands were investigated. The UV−vis, 1H NMR, and ESI-mass spectral studies, as well as computational studies, revealed supramolecular 1:1 dyad formation between the electron donor [(TPP)Zn] and the electron acceptor, fulleropyrrolidine entities. The determined formation constant K values followed the order o-pyridyl ≪ m-pyridyl ≃ p-pyridyl ≪ N-phenyl imidazole entities of the fulleropyrrolidine. The evaluated thermodynamic parameters revealed stable complexation with complex dissociation enthalpies ranging between 26 and 32 kJ mol-1. The 1H NMR studies revealed axial coordination of the pyridine or imidazole ligands to the central zinc of (TPP)Zn, while the ESI-Mass spectral studies performed in CH2Cl2 matrix revealed the expected molecular ion peak of the self-assembled dyads. ...