Impact of the chemical nature and position of spacers on controlling the optical properties of silicon quantum dots.

The unique properties of silicon quantum dots (SQDs), including intriguing optical properties, biocompatibility, and ease of surface modification have made them excellent candidates for a variety of optoelectronic and biomedical applications. Unfortunately, the low quantum efficiency (QE), unstable photoluminescence, and poor colloidal stability of SQDs have hindered their wide applicability. Herein, we report the synthesis of four assemblies of SQDs (1.6-1.8 nm average diameter) functionalized with fluorescein dye through isothiocyanate (-NCS) and carboxylate (COO-) spacers in the benzene ring of the fluorescein to produce the dyads Am-SQD-Fl, DiAm-SQD-Fl, urea-SQD-Fl, and SQD-Fl. The photophysical measurements showed that the spacer played a key role in directing and controlling the optical properties of SQDs dyads, with the isothiocyanate spacer leading to a significant improvement in the QE of the dyad systems up to 65% and extending their photostability for at least one year. The interactions between the SQDs and fluorescein in the dyads Am-SQD-Fl, DiAm-SQD-Fl, and SQD-Fl were found to mainly proceed through photoinduced electron transfer at different rates, while energy transfer was confirmed to be the predominant process in the dyad urea-SQD-Fl. To demonstrate the suitability of the functionalized SQDs for bioimaging applications, the water-soluble dyads were examined for fluorescence imaging of human bone cancerous U2OS cells.

Is π-Stacking Prone To Accelerate Singlet-Singlet Energy Transfers?

π-Stacking is the most common structural feature that dictates the optical and electronic properties of chromophores in the solid state. Herein, a unidirectional singlet-singlet energy-transfer dyad has been designed to test the effect of π-stacking of zinc(II) porphyrin, [Zn2], as a slipped dimer acceptor using a BODIPY unit, [bod], as the donor, bridged by the linker C6H4C≡CC6H4. The rate of singlet energy transfer, kET(S1), at 298 K ( kET(S1) = 4.5 × 1010 s-1) extracted through the change in fluorescence lifetime, τF, of [bod] in the presence (27.1 ps) and the absence of [Zn2] (4.61 ns) from Streak camera measurements, and the rise time of the acceptor signal in femtosecond transient absorption spectra (22.0 ps), is faster than most literature cases where no π-stacking effect exists (i.e., monoporphyrin units). At 77 K, the τF of [bod] increases to 45.3 ps, indicating that kET(S1) decreases by 2-fold (2.2 × 1010 s-1), a value similar to most values reported in the literature, thus suggesting that the higher value at 298 K is thermally promoted at a higher temperature.

What does it take to induce equilibrium in bidirectional energy transfers?

Two dyads built with a co-facial slipped bis(zinc(ii)porphyrin), a free base and a bridge, [Zn2]-bridge-[Fb] (bridge = C6H4C[triple bond, length as m-dash]C, 1 and C6H4C[triple bond, length as m-dash]CC6H4, 2), exhibit S1 energy equilibrium [Zn2]* ↔ [Fb]* at 298 K, an extremely rare situation, which depends on the degree of MO coupling between the units. At 77 K, 2 becomes bi-directional due to the two large C6H4-[Zn2] and C6H4-[Fb] dihedral angles.

Platinum Complexes of N,N',N″,N‴-Diboronazophenines.

Azophenine, (α-C6H5NH)2(C6H5-N═C6H2═N-C6H5), well known to be non-emissive, was rigidified by replacing two amine protons by two difluoroboranes (BF2+) and further functionalized at the para-positions of the phenyl groups by luminescent trans-ArC≡C-Pt(PR3)2-C≡C ([Pt]) arms [Ar = C6H4 (R = Et), hexa(n-hexyl)truxene) (Tru; R = Bu)]. Two effects are reported. First, the linking of these [Pt] arms with the central azophenine (C6H4-N═C6H2(NH)2═N-C6H4; Q) generates very low energy charge-transfer (CT) singlet and triplet excited states (3,1([Pt]-to-Q)*) with absorption bands extending all the way to 800 nm. Second, the rigidification of azophenine by the incorporation of BF2+ units renders the low-lying CT singlet state clearly emissive at 298 and 77 K in the near-IR region. DFT computations place the triplet emission in the 1200-1400 nm range, but no phosphorescence was detected. The photophysical properties are investigated, and circumstantial evidence for slow triplet energy transfers, 3Tru* → Q, is provided.

Increasing the lifetimes of charge separated states in porphyrin-fullerene polyads.

Two linear polyads were designed using zinc(ii)porphyrin, [ZnP], and N-methyl-2-phenyl-3,4-fullero-pyrrolidine (C60) where C60 is dangling either at the terminal position of [ZnP]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP]-C60 (1) or at the central position of [ZnP]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP(C60)]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP] (2) in order to test whether the fact of having one or two side electron donors influences the rate of electron transfer, ket. These polyads were studied using cyclic voltammograms, DFT computations, steady state and time-resolved fluorescence spectroscopy, and femtosecond transient absorption spectroscopy (fs-TAS). Photo-induced electron transfer confirmed by the detection of the charge separated state [ZnP˙+]/C60˙- from fs-TAS occurs with rates (ket) of 3-4 × 1010 s-1 whereas the charge recombinations (CRs) are found to produce the [ZnP] ground state via two pathways (central [ZnP˙+]/C60˙- (ps) and terminal central [ZnP˙+]/C60˙- (ns) producing [1ZnP] (ground state) and [3ZnP*]). The formation of the T1 species is more predominant for 2.

Ultrafast energy and electron transfers in structurally well addressable BODIPY-porphyrin-fullerene polyads.

Two electron transfer polyads built upon [C60]-[ZnP]-[BODIPY] (1) and [ZnP]-[ZnP](-[BODIPY])(-[C60]) (2), where [C60] = N-methyl-2-phenyl-3,4-fulleropyrrolidine, [BODIPY] = boron dipyrromethane, and [ZnP] = zinc(ii) porphyrin, were synthesized along with their corresponding energy transfer polyads [ZnP]-[BODIPY] (1a) and [ZnP]-[ZnP]-[BODIPY] (2a) as well as relevant models. These polyads were studied using cyclic voltammetry, DFT computations, steady state and time-resolved fluorescence spectroscopy, and fs transient absorption spectroscopy. The rates for energy transfer, kET, [BODIPY]* → [ZnP] are ∼2.8 × 1010 s-1 for both 1a and 2a, with an efficiency of 99%. Concurrently, the fast appearance of the [C60]-˙ anion for 1 and 2 indicates that the charge separation occurs on the 20-30 ps timescale with the rates of electron transfer, ket, [ZnP]*/[C60] → [ZnP]+˙/[C60]-˙ of ∼0.9 × 1010 to ∼3.8 × 1010 s-1. The latter value is among the fastest for these types of polyads. Conversely, the charge recombination operates on the ns timescale. These rates are comparable to or faster than those reported for other more flexible [C60]-[ZnP]-[BODIPY] polyads, which can be rationalized by the donor-acceptor separations.

Rendering cross-conjugated azophenine derivatives emissive to probe the silent photophysical properties of emeraldine.

An azophenine derivative was synthesized by coupling truxene and azophenine via the copper-free Sonagashira reaction using Pd2(dba)3 and As(PPh)3 as catalysts. The crystal structure of this heavy azophenine model (∼4000) was made and the identity of the structure was confirmed. By introducing truxene groups into this cross-conjugated structure, the deactivating rotations around the NH-C6H4 bonds were slowed down, which rendered this derivative near-IR emissive at 298 K. This species provided then the appropriate spectral and kinetic signatures for knowing where and what to look for in emeraldine, which was called non-emissive. Besides, two other compounds were also synthesized as models for this azophenine derivative for comparison and interpretation purposes.

Effect of Conjugation Length on Photoinduced Charge Transfer in π-Conjugated Oligomer-Acceptor Dyads.

A series of π-conjugated oligomer-acceptor dyads were synthesized that feature oligo(phenylene ethynylene) (OPE) conjugated backbones end-capped with a naphthalene diimide (NDI) acceptor. The OPE segments vary in length from 4 to 8 phenylene ethynene units (PEn-NDI, where n = 4, 6 and 8). Fluorescence and transient absorption spectroscopy reveals that intramolecular OPE → NDI charge transfer dominates the deactivation of excited states of the PEn-NDI oligomers. Both charge separation (CS) and charge recombination (CR) are strongly exothermic (ΔG0CS ∼ -1.1 and ΔG0CR ∼ -2.0 eV), and the driving forces do not vary much across the series because the oxidation and reduction potentials and singlet energies of the OPEs do not vary much with their length. Bimolecular photoinduced charge transfer between model OPEs that do not contain the NDI acceptors with methyl viologen was studied, and the results reveal that the absorption of the cation radical state (OPE+•) remains approximately constant (λ ∼ 575 nm) regardless of oligomer length. This finding suggests that the cation radical (polaron) of the OPE is relatively localized, effectively occupying a confined segment of n ≤ 4 repeat units in the longer oligomers. Photoinduced intramolecular electron transfer dynamics in the PEn-NDI series was investigated by UV-visible femtosecond transient absorption spectroscopy with visible and mid-infrared probes. Charge separation occurs on the 1-10 ps time scale with the rates decreasing slightly with increased oligomer length (ßCS ∼ 0.15 Å-1). The rate for charge-recombination decreases in the sequence PE4-NDI > PE6-NDI ∼ PE8-NDI. The discontinuous distance dependence in the rate for charge recombination may be related to the spatial localization of the positive polaron state in the longer oligomers.

pH-Induced Surface Modification of Atomically Precise Silver Nanoclusters: An Approach for Tunable Optical and Electronic Properties.

Noble metal nanoclusters (NCs) play a pivotal role in bridging the gap between molecules and quantum dots. Fundamental understanding of the evolution of the structural, optical, and electronic properties of these materials in various environments is of paramount importance for many applications. Using state-of-the-art spectroscopy, we provide the first decisive experimental evidence that the structural, electronic, and optical properties of Ag44(MNBA)30 NCs can now be tailored by controlling the chemical environment. Infrared and photoelectron spectroscopies clearly indicate that there is a dimerization between two adjacent ligands capping the NCs that takes place upon lowering the pH from 13 to 7.

Real-time observation of ultrafast electron injection at graphene-Zn porphyrin interfaces.

We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via electrostatic and π-π stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP˙(+) and GC˙(-), as indicated by a spectral feature at 650-750 nm, which is attributed to a ZnTMPyP radical cation resulting from the ET process.

To what extent can charge localization influence electron injection efficiency at graphene-porphyrin interfaces?

Controlling the electron transfer process at donor-acceptor interfaces is a research direction that has not yet seen much progress. Here, with careful control of the charge localization on the porphyrin macrocycle using ß-cyclodextrin as an external cage, we are able to improve the electron injection efficiency from cationic porphyrin to graphene carboxylate by 120%. The detailed reaction mechanism is also discussed.

Quantum confinement-tunable ultrafast charge transfer at the PbS quantum dot and phenyl-C61-butyric acid methyl ester interface.

Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion.

Dendron to central core S1-S1 and S2-S(n) (n>1) energy transfers in artificial special pairs containing dendrimers with limited numbers of conformations.

Two dendrimers consisting of a cofacial free-base bisporphyrin held by a biphenylene spacer and functionalized with 4-benzeneoxomethane (5-(4-benzene)tri-10,15,20-(4-n-octylbenzene)zinc(II)porphyrin) using either five or six of the six available meso-positions, have been synthesized and characterized as models for the antenna effect in Photosystems I and II. The presence of the short linkers, -CH2O-, and long C8H17 soluble side chains substantially reduces the number of conformers (foldamers) compared with classic dendrimers built with longer flexible chains. This simplification assists in their spectroscopic and photophysical analysis, notably with respect to fluorescence resonance energy transfer (FRET). Both steady-state and time-resolved spectroscopic measurements indicate that the cofacial free bases and the flanking zinc(II)-porphyrin antennas act as energy acceptor and donor, respectively, following excitation in either the Q or Soret bands of the dendrimers. The rate constants for singlet electronic energy transfer (k(EET)) extracted from the S1 and S2 fluorescence lifetimes of the donor in the presence and absence of the acceptor are ≤ (0.1-0.3)×10(9) and ∼2×10(9)  s(-1) for S1→S1 (range from a bi-exponential decay model) and about 1.5×10(12)  s(-1) for S2→S(n) (n>1). Comparisons of these experimental data with those calculated from Förster theory using orientation factors and donor-acceptor distances extracted from computer modeling suggest that a highly restricted number of the many foldamers facilitate energy transfer. These foldamers have the lowest energy by molecular modeling and consist of one or at most two of the flanking zinc porphyrin antennas folded so they lie near the central artificial special pair core with the remaining antennas located almost parallel to and far from it.

Design of triads for probing the direct through space energy transfers in closely spaced assemblies.

Using a selective stepwise Suzuki cross-coupling reaction, two trimers built on three different chromophores were prepared. These trimers exhibit a D(^)A1-A2 structure where the donor D (octa-ß-alkyl zinc(II)porphyrin either as diethylhexamethyl, 10a, or tetraethyltetramethyl, 10b, derivatives) through space transfers the S1 energy to two different acceptors, di(4-ethylbenzene) zinc(II)porphyrin (A1; acceptor 1) placed cofacial with D, and the corresponding free base (A2; acceptor 2), which is meso-meso-linked with A1. This structure design allows for the possibility of comparing two series of assemblies, 9a,b (D(^)A1) with 10a,b (D(^)Â1-A2), for the evaluation of the S1 energy transfer for the global process D*→A2 in the trimers. From the comparison of the decays of the fluorescence of D, the rates for through space energy transfer, kET for 10a,b (kET ≈ 6.4 × 10(9) (10a), 5.9 × 10(9) s(-1) (10b)), and those for the corresponding cofacial D(^)A1 systems, 9a,b, (kET ≈ 5.0 × 10(9) (9a), 4.7 × 10(9) s(-1) (9b)), provide an estimate for kET for the direct through space D*→A2 process (i.e., kET(D(^)A1-A2) - kET(D(^)A1) = kET(D*→A2) ∼ 1 × 10(9) s(-1)). This channel of relaxation represents ∼15% of kET for D*→A1.

The pt-organometallic version of perigraniline: going blue.

Four conjugated push-pull organometallic polymers ([Pt]-AQ)n ([Pt] = trans-bis(phenylacetylene)bis(tributylphosphine)platinum(II); AQ = 2-bromo-, 2,6-dibromo-, 2,6-diamino-, and unsubstituted anthraquinone diimine) were prepared and characterized by UV-vis spectroscopy and electrochemistry. A low-energy charge transfer, CT, band ([Pt]*→AQ; confirmed by density functional theory calculations), was found in the 445-500 nm window rather than the expected red-shifted range above 630 nm. X-ray structures of four model compounds reveal that steric hindrance induces large dihedral angles between the C6 H4 and NCC2 planes, rendering π-orbital overlap difficult between the [Pt] and AQ units. The position of the CT band is mainly driven the reduction potential of the anthraquinone diimine unit.

Reduced and oxidized forms of the Pt-organometallic version of polyaniline.

This work represents an effort to synthesize all four forms of polyaniline (PANI) in its organometallic versions. Polymers containing substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in the backbone exhibiting the general structure (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C-PtL(2))(n) along with the corresponding model compounds (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C)(PtL(2)Cl)(2) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C)(PtL(2)Cl)(2) (L = PBu(3); X = H, F, Cl) were synthesized. The polymers and corresponding model compounds were characterized (including (1)H and (31)P NMR, IR, mass spectra, elemental analysis, and X-ray structure determinations) and investigated for their redox properties in the absence and in the presence of acid. Their optical properties, including ns transient spectroscopy were also investigated. These properties were interpreted through density functional theory (DFT) and time-dependent DFT (TDDFT) computations. These materials are found to be oligomers (GPC) with thermal stability (TGA) reaching 350 °C. The greatest stabilities were found in the cases with X = F. Using a data bank of 8 X-ray structures of diimine derivatives, a relationship between the C═N bond distance and the dihedral angle between the benzoquinone ring and the flanking phenyl planes is noted. As the size of the substituent X on the benzoquinone center increases, the degree of conjugation decreases as demonstrated by the C═N bond length. The largest dihedral angles are noted for X = Cl. These polymers exhibit drastic chemical differences when X is varied (X = H, F, Cl). The completely reduced polymer (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) (i.e., X = H) was not chemically accessible whereas in the cases of X = F, Cl, these materials were obtained and represent the first examples of fully reduced organometallic versions of PANI (i.e., leucoemaraldine). For the (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) polymers, the completely oxidized form for X = H was isolated (pernigraniline), but for X = F and Cl, only the largely reduced mixed-valence form (i.e., emaraldine) was obtained via chemical routes. In acidic solutions, the chemically accessible polymer for X = H, (C≡CC(6)H(4)-N═C(6)H(4)═N-C(6)H(4)C≡C-PtL(2))(n), exhibits two chemically reversible waves indicating that the reduced form (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) can be generated. The absorption spectra of the highly colored diimine-containing species exhibit a broad charge transfer band (assigned based on DFT calculations (B3LYP); C(6)H(4)C≡C-PtL(2)-C≡CC(6)H(4) → N═C(6)X(4)═N) in the 450-800 nm window red shifting according X = H → Cl → F, consistent with their relative inductive effect. The largest absorptivity is measured for X = H because this polymer is fully oxidized whereas for the cases where X = F and Cl, these polymers exists in the mixed valence form. The ns transient absorption spectra of two polymers (X = F; reduced and mixed-valence polymers) were measured. The triplet excited state in the mixed-valence polymer is dominated by the reduced diamine residue and the T(1)-T(n) absorption of the diimine is entirely quenched.

Construction of (CuX)2n cluster-containing (X = Br, I; n = 1, 2) coordination polymers assembled by dithioethers ArS(CH2)(m)SAr (Ar = Ph, p-Tol; m = 3, 5): effect of the spacer length, aryl group, and metal-to-ligand ratio on the dimensionality, cluster nuclearity, and the luminescence properties of the metal-organic frameworks.

Reaction of CuI with bis(phenylthio)propane in a 1:1 ratio yields the two-dimensional coordination polymer [{Cu(µ(2)-I)(2)Cu}{µ-PhS(CH(2))(3)SPh}(2)](n) (1). The 2D-sheet structure of 1 is built up by dimeric Cu(2)I(2) units, which are connected via four bridging 1,3-bis(phenylthio)propane ligands. In contrast, treatment of 2 equiv of CuI with 1,3-bis(phenylthio)propane in MeCN solution affords in a self-assembly reaction the strongly luminescent metal-organic 2D-coordination polymer [Cu(4)I(4){µ-PhS(CH(2))(3)Ph}(2)](n) (2), in which cubane-like Cu(4)(µ(3)-I)(4) cluster units are linked by the dithioether ligands. The crystallographically characterized one-dimensional (1D) compound [{Cu(µ(2)-Br)(2)Cu}{µ-PhS(CH(2))(3)SPh}(2)](n) (3) is obtained using CuBr. The outcome of the reaction of PhS(CH(2))(5)SPh with CuI also depends of the metal-to-ligand ratio employed. Mixing CuI and the dithioether in a 2:1 ratio results in formation of [Cu(4)I(4){µ-PhS(CH(2))(5)Ph}(2)](n) (4) in which cubane-like Cu(4)(µ(3)-I)(4) clusters are linked by the bridging dithioether ligand giving rise to a 1D necklace structure. A ribbon-like 1D-polymer with composition [{Cu(µ(2)-I)(2)Cu}{µ-PhS(CH(2))(5)SPh}(2)](n) (5), incorporating rhomboid Cu(2)I(2) units, is produced upon treatment of CuI with 1,5-bis(phenylthio)pentane in a 1:1 ratio. Reaction of CuBr with PhS(CH(2))(5)SPh produces the isomorphous 1D-compound [{Cu(µ(2)-Br)(2)Cu}{µ-PhS(CH(2))(5)SPh}(2)](n) (6). Strongly luminescent [Cu(4)I(4){µ-p-TolS(CH(2))(5)STol-p}(2)](n) (7) is obtained after mixing 1,5-bis(p-tolylthio)pentane with CuI in a 1:2 ratio, and the 2D-polymer [{Cu(µ(2)-I)(2)Cu}(2){µ-p-TolS(CH(2))(5)STol-p}(2)](n) (8) results from reaction in a 1:1 metal-to-ligand ratio. Under the same reaction conditions, 1D-polymeric [{Cu(µ(2)-Br)(2)Cu}{µ-p-TolS(CH(2))(5)STol-p}(2)](n) (9) is formed using CuBr. This study reveals that the structure of the self-assembly process between CuX and ArS(CH(2))(m)SAr ligands is hard to predict. The solid-state luminescence spectra at 298 and 77 K of 2 and 4 exhibit very strong emissions around 535 and 560 nm, respectively, whereas those for 1 and 5 display weaker ones at about 450 nm. The emission lifetimes are longer for the longer wavelength emissions (>1.0 µs arising from the cubane species) and shorter for the shorter wavelength ones (<1.4 µs arising from the rhomboid units). The Br-containing species are found to be weakly fluorescent.

Design and photophysical properties of zinc(II) porphyrin-containing dendrons linked to a central artificial special pair.

The click chemistry synthesis and photophysical properties, notably photo-induced energy and electron transfers between the central core and the peripheral chromophores of a series of artificial special pair-dendron systems (dendron = G1, G2, G3; Gx = zinc(II) tetra-meso-arylporphyrin-containing polyimides) built upon a central core of dimethylxanthenebis(metal(II) porphyrin) (metal = zinc, copper), are reported. The dendrons act as singlet and triplet energy acceptors or donors, depending on the dendrimeric systems. The presence of the paramagnetic d(9) copper(II) in the dendrimers promotes singlet-triplet energy transfer from the zinc(II) tetra-meso-arylporphyrin to the bis(copper(II) porphyrin) unit and slow triplet-triplet energy transfer from the central bis(copper(II) porphyrin) fragment to the peripheral zinc(II) tetra-meso-arylporphyrin. If bis(zinc(II) porphyrin) is the central core, evidence for chain folding is observed; this is unambiguously demonstrated by the presence of triplet-triplet energy transfer in the heterobimetallic systems, a process that can only occur at short distances.

Reactivity of CuI and CuBr toward Et2S: a reinvestigation on the self-assembly of luminescent copper(I) coordination polymers.

CuI reacts with SEt(2) in hexane to afford the known strongly luminescent 1D coordination polymer [(Et(2)S)(3){Cu(4)(mu(3)-I)(4)}](n) (1). Its X-ray structure has been redetermined at 115, 235, and 275 K in order to address the behavior of the cluster-centered emission and is built upon Cu(4)(mu(3)-I)(4) cubane-like clusters as secondary building units (SBUs), which are interconnected via bridging SEt(2) ligands. However, we could not reproduce the preparation of a coordination polymer with composition [(Et(2)S)(3){Cu(4)(mu(3)-Br)(4)}](n) as reported in Inorg. Chem. 1975, 14, 1667. In contrast, the autoassembly reaction of SEt(2) with CuBr results in the formation of a novel 1D coordination polymer of composition [(Cu(3)Br(3))(SEt(2))(3)](n) (2). The crystal structure of 2 has been solved at 115, 173, 195, and 235 K. The framework of the luminescent compound 2 consists of a corrugated array with alternating Cu(mu(2)-Br)(2)Cu rhomboids, which are connected through two bridging SEt(2) ligands to a tetranuclear open-cubane Cu(4)Br(4) SBU, ligated on two external Cu atoms with one terminal SEt(2). The solid-state luminescence spectra of 1 and 2 exhibit intense halide-to-metal charge-transfer emissions centered at 565 and 550 nm, respectively, at 298 K. A correlation was also noted between the change in the full width at half-maximum of the emission band between 298 and 77 K and the relative flexibility of the bridging ligand. The emission properties of these materials are also rationalized by means of density functional theory (DFT) and time-dependent DFT calculations performed on 1.

Energy transfers in monomers, dimers, and trimers of zinc(II) and palladium(II) porphyrins bridged by rigid Pt-containing conjugated organometallic spacers.

A series of linear monomers (spacer-M(P)), dimers (M(P)-spacer-M'(P)), and trimers (M(P)-spacer-M'(P)-spacer-M(P)) of spacer/metalloporphyrin systems (M' = Zn, M = Zn, Pd, P = porphyrin, and spacer = trans-C(6)H(4)C[triple bond]CPtL(2)C[triple bond]CC(6)H(4)- (L = PEt(3))) including mixed metalloporphyrin compounds, were synthesized and characterized. The S(1) and T(1) energy transfers Pd(P)*-->Zn(P) occur with rates of approximately 2 x 10(9) s(-1), S(1), and 0.15 x 10(3) (slow component) and 4.3 x 10(3) s(-1) (fast component), T(1). On the basis of a literature comparison with a related dyad, the Pt atom in the conjugated chain slows down the transfers. The excitation in the absorption band of the trans-C(6)H(4)C[triple bond]CPtL(2)C[triple bond]CC(6)H(4)- spacer in the 300-360 nm range also leads to T(1) energy transfer (spacer* --> M(P); M = Zn, Pd) with rates of 10(4) s(-1).