Tunable J-type aggregation of silicon phthalocyanines in a surface-anchored metal-organic framework thin film.

Organic chromophores and semiconductors, like anthracene, pentacene, perylene, and porphyrin, are prone to aggregation, and their packing in the solid state is often hard to predict and difficult to control. As the condensed phase structures of these chromophores and semiconductors are of crucial importance for their optoelectronic functionality, strategies to control their assembly and provide new structural motifs are important. One such approach uses metal-organic frameworks (MOFs); the organic chromophore is converted into a linker and connected by metal ions or nodes. The spatial arrangement of the organic linkers can be well-defined in a MOF, and hence optoelectronic functions can be adjusted accordingly. We have used such a strategy to assemble a phthalocyanine chromophore and illustrated that the electronic inter-phthalocyanine coupling can be rationally tuned by introducing bulky side grounds to increase steric hindrance. We have designed new phthalocyanine linkers and using a layer-by-layer liquid-phase epitaxy strategy thin films of phthalocyanine-based MOFs have been fabricated and their photophysical properties explored. It was found that increasing the steric hindrance around the phthalocyanine reduced the effect of J-aggregation in the thin film structures.

Distance Matters: Effect of the Spacer Length on the Photophysical Properties of Multimodular Perylenediimide-Silicon Phthalocyanine-Fullerene Triads.

A multimodular donor-acceptor conjugate featuring silicon phthalocyanine (SiPc) as the electron donor, and two electron acceptors, namely tetrachloroperylenediimide (PDI) and C60 , placed at the opposite ends of the SiPc axial positions, was newly designed and synthesized, and the results were compared to the earlier reported PDI-SiPc-C60 triad. Minimal intramolecular interactions between the entities was observed. Absorption, fluorescence, computational and electrochemical studies were performed to evaluate the excitation energy, geometry and electronic structure, and energy levels of different photoevents. Steady-state absorption, fluorescence and excitation spectral studies revealed efficient singlet-singlet energy transfer from 1 PDI* to SiPc in the PDI-SiPc dyad and the PDI-SiPc-C60 triad. The measured rates for these photochemical events were found to be much higher than those reported earlier for the triad, due to closer proximity between the PDI and SiPc entities. The distance also affected the charge separation path in which involvement of PDI, and not C60 , in charge separation in the present triad was witnessed. The present investigation brings out the importance of donor-acceptor distances in channeling photochemical events in a multimodular system.

Guest-responsive polaritons in a porous framework: chromophoric sponges in optical QED cavities.

Introducing porous material into optical cavities is a critical step toward the utilization of quantum-electrodynamical (QED) effects for advanced technologies, e.g. in the context of sensing. We demonstrate that crystalline, porous metal-organic frameworks (MOFs) are well suited for the fabrication of optical cavities. In going beyond functionalities offered by other materials, they allow for the reversible loading and release of guest species into and out of optical resonators. For an all-metal mirror-based Fabry-Perot cavity we yield strong coupling (∼21% Rabi splitting). This value is remarkably large, considering that the high porosity of the framework reduces the density of optically active moieties relative to the corresponding bulk structure by ∼60%. Such a strong response of a porous chromophoric scaffold could only be realized by employing silicon-phthalocyanine (SiPc) dyes designed to undergo strong J-aggregation when assembled into a MOF. Integration of the SiPc MOF as active component into the optical microcavity was realized by employing a layer-by-layer method. The new functionality opens up the possibility to reversibly and continuously tune QED devices and to use them as optical sensors.

Correction: Guest-responsive polaritons in a porous framework: chromophoric sponges in optical QED cavities.

[This corrects the article DOI: 10.1039/D0SC02436H.].

Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates.

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C60 dyads has been investigated. For this, two C60-SiPc-C60 dyads, 1 and 2, varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C60-SiPc-C60 1 where the SiPc and C60 are separated by a phenyl spacer, faster electron transfer was observed with kcs equal to 2.7×109 s-1 in benzonitrile. However, in the case of C60-SiPc-C60 2, where SiPc and C60 are separated by a biphenyl spacer, a slower electron transfer rate constant, kcs=9.1×108 s-1, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of 3SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3-20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc.+-C60.- charge separated states (1.57 eV) populated the low-lying 3SiPc* (1.26 eV) prior returning to the ground state.

Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates.

The front cover artwork is provided by the groups of Prof. Sastre-Santos, Prof. D'Souza, and Prof. Karr. The effect of donor-acceptor distance in governing the kinetics of electron transfer is demonstrated using different linkers between silicon phthalocyanines and C60. The cover shows the molecules playing soccer. Read the full text of the Article at 10.1002/cphc.202000578.

Effect of the Number of Anchoring and Electron-Donating Groups on the Efficiency of Free-Base- and Zn-Porphyrin-Sensitized Solar Cells.

A series of porphyrin compounds, free base (H2P) and their Zn (II) metallated analogues (ZnP), bearing one, two or three carboxylic acid groups, have been synthesized, characterized, and used as sensitizers in dye sensitized solar cells (DSSCs). The performance of these devices has been analyzed, showing higher efficiencies of those sensitized with ZnP compounds. These results have been explained, on one hand, taking into account the electronic character of the metal ion, which acts as mediator in the injection step, and, on the other, considering the number of anchoring groups, which determines both the stereoelectronic character of the dye and the way it binds to TiO2 surface.

Sequential, Ultrafast Energy Transfer and Electron Transfer in a Fused Zinc Phthalocyanine-free-base Porphyrin-C60 Supramolecular Triad.

A supramolecular triad composed of a fused zinc phthalocyanine-free-base porphyrin dyad (ZnPc-H2 P) coordinated to phenylimidazole functionalized C60 via metal-ligand axial coordination was assembled, as a photosynthetic antenna-reaction centre mimic. The process of self-assembly resulting into the formation of C60 Im:ZnPc-H2 P supramolecular triad was probed by proton NMR, UV-Visible and fluorescence experiments at ambient temperature. The geometry and electronic structures were deduced from DFT calculations performed at the B3LYP/6-31G(dp) level. Electrochemical studies revealed ZnPc to be a better electron donor compared to H2 P, and C60 to be the terminal electron acceptor. Fluorescence studies of the ZnPc-H2 P dyad revealed excitation energy transfer from 1 H2 P* to ZnPc within the fused dyad and was confirmed by femtosecond transient absorption studies. Similar to that reported earlier for the fused ZnPc-ZnP dyad, the energy transfer rate constant, kENT was in the order of 1012  s-1 in the ZnPc-H2 P dyad indicating an efficient process as a consequence of direct fusion of the two π-systems. In the presence of C60 Im bound to ZnPc, photoinduced electron transfer leading to H2 P-ZnPc.+ :ImC60.- charge separated state was observed either by selective excitation of ZnPc or H2 P. The latter excitation involved an energy transfer followed by electron transfer mechanism. Nanosecond transient absorption studies revealed that the lifetime of charge separated state persists for about 120 ns indicating charge stabilization in the triad.

Edge-on and face-on functionalized Pc on enriched semiconducting SWCNT hybrids.

Enriched semiconducting single-walled carbon nanotubes (SWCNT (6,5) and SWCNT (7,6)) and HiPco nanotubes were covalently functionalized with either zinc phthalocyanine or silicon phthalocyanine as electron donors. The synthetic strategy resulted in edge-on and face-on geometries with respect to the phthalocyanine geometry, with both phthalocyanines held by an electronically conducting diphenylacetylene linker. The extent of functionalization in the MPc-SWCNT (M = Zn or Si) donor-acceptor nanohybrids was determined by systematic studies involving AFM, TGA, XPS, optical and Raman techniques. Intramolecular interactions in MPc-SWCNT nanohybrids were probed by studies involving optical absorbance, Raman, luminescence and electrochemical studies. Different degrees of interactions were observed depending on the type of MPc and mode of attachment. Substantial quenching of MPc fluorescence in these hybrids was observed from steady-state and three-dimensional fluorescence mapping, which suggests the occurrence of excited state events. Evidence for the occurrence of excited state charge transfer type interactions was subsequently secured from femtosecond transient absorption studies covering both the visible and near-infrared regions. Furthermore, electron-pooling experiments performed in the presence of a sacrificial electron donor and a second electron acceptor revealed accumulation of one-electron reduced product upon continuous irradiation of the nanohybrids. In such experiments, the ZnPc-SWCNT (6,5) nanohybrid outperformed other nanohybrids and this suggests that this is a superior donor-acceptor system for photocatalytic applications.

Supramolecular complex of a fused zinc phthalocyanine-zinc porphyrin dyad assembled by two imidazole-C60 units: ultrafast photoevents.

A new zinc phthalocyanine-zinc porphyrin dyad (ZnPc-ZnP) fused through a pyrazine ring has been synthesized as a receptor for imidazole-substituted C60 (C60Im) electron acceptor. Self-assembly via metal-ligand axial coordination and the pertinent association constants in solution were determined by 1H-NMR, UV-Vis and fluorescence titration experiments at room temperature. The designed host was able to bind up to two C60Im electron acceptor guest molecules to yield C60Im:ZnPc-ZnP:ImC60 donor-acceptor supramolecular complex. The spectral data showed that the two binding sites behave independently with binding constants similar in magnitude. Steady-state fluorescence studies were indicative of an efficient singlet-singlet energy transfer from zinc porphyrin to zinc phthalocyanine within the fused dyad. Accordingly, the transient absorption studies covering a wide timescale of femto-to-milli seconds revealed ultrafast energy transfer from 1ZnP* to ZnPc (kEnT ∼ 1012 s-1) in the fused dyad. Further, a photo induced electron transfer was observed in the supramolecularly assembled C60Im:ZnPc-ZnP:ImC60 donor-acceptor complex leading to charge separated states, which persisted for about 200 ns.

Exfoliation and supramolecular functionalization of graphene with an electron donor perylenediimide derivative.

The liquid exfoliation of graphite to few layered graphene sheets together with the non-covalent supramolecular functionalization of exfoliated graphene by the synthesized N,N'-di(2-ethylhexyl)-1-(N''''-methylpiperazin-N'''-yl)perylene-3,4,9,10-tetracarboxydiimide (Pip-PDI) is reported. The aromatic Pip-PDI has the ability to non-covalently interact with the exfoliated graphene sheets, stabilizing them and preventing their reassembly. On the other hand, the presence of the piperazine moiety on the bay position of the PDI core makes it an ideal electron donor, nicely coupled with the electron accepting exfoliated graphene, hence, forming a novel donor-acceptor nanoensemble, which was characterized by complementary spectroscopic and microscopy techniques. Theoretical calculations predicted the absence of a meaningful charge-separated state within the Pip-PDI/graphene ensemble, which was also proven by time-resolved fluorescence and transient absorption measurements.

Multichromophoric Perylenediimide-Silicon Phthalocyanine-C60 System as an Artificial Photosynthetic Analogue.

Sequential photoinduced energy transfer followed by electron transfer and the formation of charge-separated states, which are primary events of natural photosynthesis, have been demonstrated in a newly synthesized multichromophoric covalently linked triad, PDI-SiPc-C60 . The triad comprises a perylenediimide (PDI), which primarily fulfils antenna and electron-acceptor functionalities, silicon phthalocyanine (SiPc) as an electron donor, and fulleropyrrolidine (C60 ) as a second electron acceptor. The multi-step convergent synthetic procedure developed here produced good yields of the triad and control dyads, PDI-SiPc and SiPc-C60 . The structures and geometries of the newly synthesized donor-acceptor systems have been established from spectral, computational, and electrochemical studies with reference to appropriate control compounds. Ultrafast energy transfer from 1 PDI* to SiPc in the case of PDI-SiPc and PDI-SiPc-C60 was witnessed. An energy-level diagram established from spectral and electrochemical data suggested the formation of two types of charge-separated states, that is, PDI-SiPc.+ -C60.- and PDI.- -SiPc.+ -C60 from the 1 SiPc* in the triad, with generation of the latter being energetically more favorable. However, photochemical studies involving femtosecond transient spectroscopy revealed the formation of PDI-SiPc.+ -C60.- as a major charge-separated product. This observation may be rationalized in terms of the closer spatial proximity to SiPc of C60 compared to PDI in the triad. The charge-separated state persisted for a few nanoseconds prior to populating the 3 SiPc* state during charge recombination.

Axially Substituted Silicon Phthalocyanine as Electron Donor in a Dyad and Triad with Azafullerene as Electron Acceptor for Photoinduced Charge Separation.

The synthesis of a donor-acceptor silicon phthalocyanine (SiPc)-azafullerene (C59 N) dyad 1 and of the first acceptor-donor-acceptor C59 N-SiPc-C59 N dumbbell triad 2 was accomplished. The two C59 N-based materials were comprehensively characterized with the aid of NMR spectroscopy, MALDI-MS as well as DFT calculations and their redox and photophysical properties were evaluated with CV and steady-state and time-resolved absorption and photoluminescence spectroscopy measurements. Notably, femtosecond transient absorption spectroscopy assays revealed that both dyad 1 and triad 2 undergo, after selective photoexcitation of the SiPc moiety, photoinduced electron transfer from the singlet excited state of the SiPc moiety to the azafullerene counterpart to produce the charge-separated state, with lifetimes of 660 ps, in the case of dyad 1, and 810 ps, in the case of triad 2. The current results are expected to have significant implications en route to the design of advanced C59 N-based donor-acceptor systems targeting energy conversion applications.

Design and Sensing Properties of a Self-Assembled Supramolecular Oligomer.

Supramolecular polymers are a class of macromolecules stabilized by weak non-covalent interactions. These self-assembled aggregates typically undergo stimuli-induced reversible assembly and disassembly. They thus hold great promise as so-called functional materials. In this work, we present the design, synthesis, and responsive behavior of a short supramolecular oligomeric system based on two hetero-complementary subunits. These "monomers" consist of a tetrathiafulvalene-functionalized calix[4]pyrrole (TTF-C[4]P) and a glycol diester-linked bis-2,5,7-trinitrodicyanomethylenefluorene-4-carboxylate (TNDCF), respectively. We show that when mixed in organic solvents, such as CHCl3 , CH2 ClCH2 Cl, and methylcyclohexane, supramolecular aggregation takes place to produce short oligomers stabilized by hydrogen bonding and donor-acceptor charge-transfer (CT) interactions. The self-associated materials were characterized by 1 H NMR and UV/Vis/NIR absorption spectroscopy, as well as by concentration- and temperature-dependent absorption spectroscopy and dynamic light scattering (DLS) analyses of both the monomeric and oligomerized species. The self-associated system produced from TTF-C[4]P and TNDCF exhibits a concentration-dependent aggregation behavior typical of supramolecular polymers. Further support for the proposed self-assembly came from theoretical calculations. The fluorescence emitting properties of TNDCF are quenched under conditions that promote the formation of supramolecular aggregates containing TTF-C[4]P and TNDCF. This quenching effect has been utilized as a probe for the detection of substrates in the form of anions (i.e., chloride) and nitroaromatic explosives (i.e., 1,3,5-trinitrobenzene). Specifically, the addition of these substrates to mixtures of TTF-C[4]P and TNDCF produced a fluorescence "turn-on" response.

Does a nitrogen matter? Synthesis and photoinduced electron transfer of perylenediimide donors covalently linked to C59N and C60 acceptors.

The first perylenediimide (PDI) covalently linked to an azafullerene (C59N) is described. PDI-C59N and PDI-C60 dyads where PDI acts as an electron-donor moiety have been synthesized by connection of the balls to the PDI 1-bay position. Photoexcitation of the PDI unit in both systems results in formation of the charge-separated state by photoinduced electron transfer from the singlet excited state of the PDI moiety to the C59N or to the C60 moiety. The charge-separated state has a lifetime of 400 ps in the case of PDI-C59N and 120 ps for the PDI-C60 dyad in benzonitrile at 298 K. This result has significant implications for the design of organic solar cells based on covalent donor-acceptor systems using C59N as an electron acceptor, indicating that longer-lived charge-separated states can be attained using C59N systems instead of C60 systems.

Synthesis and photophysics of silicon phthalocyanine-perylenebisimide triads connected through rigid and flexible bridges.

Three new bisperylenebisimide-silicon phthalocyanine triads [(PBI)(2)-SiPcs 1, 2, and 3] connected with either rigid or flexible bridges were synthesized and characterized. A new synthetic approach to connect SiPc and PBI moieties through click chemistry produced triad 3 with an 80% yield. In (PBI)(2)-SiPc 1, PBI and SiPc are orthogonal and were connected with a rigid connector; triads 2 and 3 bear flexible aliphatic bridges, resulting in a tilted (2) or nearly parallel arrangement (3) of PBI and SiPc. Photoinduced intramolecular processes in these (PBI)(2)-SiPcs were studied and the results are compared with those of the reference compounds SiPc-ref and PBI-ref. The occurrence of electron-transfer processes between the SiPc and PBI units was confirmed by time-resolved emission and transient absorption techniques. Charge-separated (CS) states with lifetimes of 0.91, 1.3 and 2.0 ns for triads 1, 2, and 3, respectively, were detected using femtosecond laser flash photolysis. Upon the addition of Mg(ClO(4))(2), an increase in the lifetime of the CS states to 59, 110 and 200 µs was observed for triads (PBI)(2)-SiPcs 1, 2, and 3, respectively. The energy of the CS state (SiPc(·+)-PDI(·-)/Mg(2+)) is lower than the energy of both silicon phthalocyanine ((3)SiPc*-PDI) and perylenebisimide (SiPc-(3)PDI*) triplet excited states, which decelerates the metal ion-decoupled electron-transfer process for charge recombination to the ground state, thus increasing the lifetime of the CS state. The photophysics of the three triads demonstrate the importance of the rigidity of the spacer and the orientation between donor and acceptor units.

Multistep electron transfer systems based on silicon phthalocyanine, [60]fullerene and trinitrofluorenone.

The synthesis and photodynamics in the absence and in the presence of Mg(2+) ions of a novel TNF-C(60)-SiPc-C(60)-TNF pentad are reported. The redox gradient approach allows to obtain a long-lived CS state of 160 ns and 200 micros in the absence and in the presence of Mg(2+) ions, respectively.

Synthesis and photophysical studies of a new nonaggregated C60-silicon phthalocyanine-C60 triad.

A C60-SiPc-C60 triad showing no aggregation is synthesized and characterized. Photoexcitation of the triad results in formation of the charge-separated state by photoinduced electron transfer from the singlet excited state of the SiPc moiety to the C60 moiety. The charge-separated state has a lifetime of 5 ns in benzonitrile at 298 K.

Fullerene acting as an electron donor in a donor-acceptor dyad to attain the long-lived charge-separated state by complexation with scandium ion.

A long-lived charge-separated (CS) state of fullerene-trinitrofluorenone linked dyad in which fullerene acts as an electron donor is formed by photoinduced electron transfer from C60 to TNF in the presence of Sc(OTf)3; the CS lifetime is determined as 23 ms in PhCN at 298 K.