The Excited-State N-H Tautomerization Rate in Free-Base Corroles.

Corrole is a tetrapyrrolic dye with a structure that resembles porphyrin, apart from a single missing carbon. The absence of this carbon results in the re-arrangement of the double bonds within the macrocycle, and the presence of three pyrrolic protons in the central cavity in its free-base form. These protons lead to the existence of two distinct tautomeric structures that exist in a dynamic equilibrium. Although the ground-state energies of the tautomers are similar, the excited states show a significant difference in energy which unbalances the equilibrium between the tautomers and results in rapid excited-state tautomerization, favouring one tautomeric species over the other. Although the excited-state tautomerization process has been known for a long time, very few studies have been performed on it, leaving many key aspects of the process poorly understood. Herein we show how ultrafast photoluminescence can be used to experimentally determine the rates of excited-state tautomerization and activation energies of three free-base corrole derivatives thus allowing us to completely describe the excited-state dynamics of the unusual excited state of free-base corrole and opening the door to the development of new materials that can exploit its unique characteristics.

Strong Host-Guest Dependence on the Emissive Properties of MOF-5 and [Zn2(BTTB)(DMF)2•(H2O)3]n.

3D-[Zn4O(1,4-BDC)3•x(solvent)]n (MOF-5; BDC = 1,4-benzodicarboxylate) and 3D-[Zn2(BTTB)(DMF)2•(H2O)3]n (MOF-D; BTTB = 4,4',4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoate) have been investigated by means of steady-state UV-visible and fluorescence and time-resolved emission spectroscopy, as a function of solvent and power of the excitation irradiation. The low-temperature X-ray structures (173 K) were permitted to locate solvent molecules (here H2O) in the lattice. They were found distributed in the middle in the voids with no evidence of specific interactions (H-bond, coulombic, and dipole-dipole) with the framework. The fluorescence decays of the ligands (ππ* excited state), τF, for the host-guest composites MOF-5@solvent and MOF-D@solvent (solvent = air, MeCN, EtCN, MeOH, EtOH, and DMF) were found bi-exponential (short τF1 (ps), and long τF2 (ns)) with one important feature: upon cooling from 298 to 77 K, MOF-5's τF1 decreases and τF2 increases, while the opposite trend is generally observed in MOF-D. The low values for τF1 (ps) in MOF-5 are associated with the augmented probability of solvent-ligand collisions leading to nonradiative deactivation, which upon cooling to 77 K increases further as the scaffolding contracts. The augmentation in τF2 is readily associated with the increased rigidity of the ligands that are not submitted to this effect (at the surface of the MOF and as pendent groups). For the low emitter MOF-D, the reversed situation is noted but not as clearly due to the uncertainties in the data. Upon increasing the excitation flux, the fluorescence intensity increases linearly with the laser power indicating the absence of singlet-singlet annihilation, inferring the absence of efficient exciton migration. This observation is explained by the small absorptivity coefficients, which leads to a small J spectral overlap between absorption and fluorescence according to the Forster and Dexter theories, and consequently, a small rate for energy migration. This conclusion drastically changes the perception of the photocatalytic mechanism of MOF-5 and other MOFs exhibiting similar absorption features (i.e., no antenna effect).

A Fused Poly(truncated rhombic dodecahedron)-Containing 3D Coordination Polymer: A Multifunctional Material with Exceptional Properties.

The design of new and inexpensive metal-containing functional materials is of great interest. Herein is reported a unique thermochromic near-IR emitting coordination polymer, 3D-[Cu8I8(L1)2]n, CP2, which is formed when ArS(CH2)4SAr (L1, Ar = 4-C6H4OMe) reacts with 2 equiv of CuI in EtCN. In MeCN, CP1 ([Cu4I4(L1)(MeCN)2]n, consisting of an alternating [-Cu4I4-L1-Cu4I4-L1-]n chain where the Cu4I4 cubane units bear two metal-bound MeCN molecules, is formed. Heat-driven elimination of these MeCN's in solid CP1 also leads to CP2 through a predisposed organization of the Cu4I4 units prone to fusion after MeCN eliminations (i.e., a rare case of template effect). The CP2 structure exhibits parallel 1D-(Cu8I8)n chains, (z-axis; designated 1D-[CuI]n) as secondary building units (SBU) held together by parallel thioether ligands (x,y-axes), forming a nonporous 3D network. The structure of this 1D-[CuI]n SBU is unprecedented and consists of a series of fused and twisted open Cu4I4 cubanes forming a fused poly(truncated rhombic dodecahedron). Unexpectedly, the compact 3D CP2 exhibits a solid-to-solid phase transition at 100 °C and a hysteresis of ∼20 °C. CP1 emits intensively (298 K: λemi = 564 nm; Φe = 0.35), whereas CP2 presents a strongly red-shifted weaker emission (298 K: λemi ∼ 740 nm, Φe < 0.0001). Moreover, CP2, which is stable over long periods of time, exhibits thermochromism where the emission intensity of the near-IR band decreases significantly at the benefit of a ligand-centered phosphorescence at 415 nm. Altogether, these properties listed above make CP2 exceptional. The low-energy singlet and triplet excited states have been assigned to ligand/metal-to-ligand charge transfer based on DFT and TD-DFT computations.

Unusual triplet-triplet annihilation in a 3D copper(i) chloride coordination polymer.

A new coordination polymer (CP) defined as [Cu2Cl2(EtS(CH2)4SEt)4]n (CP2) was prepared by reacting EtS(CH2)4SEt with CuCl in acetonitrile in a 1 : 2 stoichiometric ratio. The X-ray structure reveals formation of non-porous 3D material composed of parallel 2D-[Cu2Cl2S2]n layers of Cl-bridged Cu2(µ-Cl)2 rhomboids assembled by EtS(CH2)4SEt ligands. A weak triplet emission (Φe < 0.0001) is observed in the 400-500 nm range with τe of 0.93 (298 K) and 3.5 ns (77 K) as major components. CP2 is the only 2nd example of emissive thioether/CuCl-containing material and combined DFT/TDDFT computations suggest the presence of lowest energy M/XLCT excited states. Upon increasing the photon flux (i.e. laser power), a triplet-triplet annihilation (TTA) is induced with quenching time constants of 72 ps (kQ = 1.3 × 1010 s-1) and 1.0 ns (kQ = 7.1 × 108 s-1) at 298 and 77 K, respectively, proceeding through an excitation energy migration operating via a Dexter process. Two distinct (Io)1/2 (Io = laser power) dependences of the emission intensity are depicted, indicating saturation as the observed emission increases with the excitation flux. These findings differ from that previously reported isomorphous CP [Cu2Br2(µ-EtS(CH2)4SEt)4]n (CP1), which exhibits no TTA behaviour at 77 K, and only one (laser power)2 dependence at 298 K. The ∼18-fold increase in kQ upon warming CP2 from 77 to 298 K indicates a temperature-aided TTA process. The significant difference between the presence (slower, CP2) and absence (CP1) of TTA at 77 K is explained by the larger unit cell contraction of the former upon cooling. This is noticeable by the larger change in inter-rhomboid CuCu separation for CP2.

Azophenine as Central Core for Efficient Light Harvesting Devices.

The notoriously non-luminescent uncycled azophenine (Q) was harnessed with Bodipy and zinc(II)porphyrin antennas to probe its fluorescence properties, its ability to act as a singlet excited state energy acceptor and to mediate the transfer. Two near-IR emissions are depicted from time-resolved fluorescence spectroscopy, which are most likely due to the presence of tautomers of very similar calculated total energies (350 cm-1 ; DFT; B3LYP). The rates for energy transfer, kET (S1 ), for 1 Bodipy*→Q are in the order of 1010 -1011  s-1 and are surprisingly fast when considering the low absorptivity properties of the lowest energy charge transfer excited state of azophenine. The rational is provided by the calculated frontier molecular orbitals (MOs) which show atomic contributions in the C6 H4 C≡CC6 H4 arms, thus favoring the double electron exchange mechanism. In the mixed-antenna Bodipy-porphyrin star molecule, the rate for 1 Bodipy*→porphyrin has also been evaluated (≈16×1010  s-1 ) and is among the fastest rates reported for Bodipy-zinc(II)porphyrin pairs. This astonishing result is again explained from the atomic contributions of the C6 H4 C≡CC6 H4 and C≡CC6 H4 arms thus favouring the Dexter process. Here, for the first time, this process is found to be sensitively temperature-dependent. The azophenine turns out to be excellent for electronic communication.

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.

Excited State N-H Tautomer Selectivity in the Singlet Energy Transfer of a Zinc(II)-Porphyrin-Truxene-Corrole Assembly.

An original corrole-containing polyad for S1 energy transfer, in which one zinc(II)-porphyrin donor is linked to two free-base corrole acceptors by a truxene linker, is reported. This polyad exhibits a rapid zinc(II)-porphyrin*→free-base corrole transfer (4.83×1010  s-1 ; 298 K), even faster than the tautomerization in the excited state processes taking advantage of the good electronic communication provided by the truxene bridge. Importantly, the energy transfer process shows approximately 3-fold selectivity for one corrole N-H tautomer over the other even at low temperature (77 K). This selectivity is due to the difference in the J-integral being effective in both the Förster and Dexter mechanisms. The data are rationalized by DFT computations.

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.

Metal Dependence on the Bidirectionality and Reversibility of the Singlet Energy Transfer in Artificial Special Pair-Containing Dyads.

The demetalation of a precursor dyad, 3, built upon a zinc(II)-containing artificial special pair and free-base antenna, leads to a new dyad, 4, for singlet energy transfer composed of cofacial free-base porphyrins (acceptor), [Fb]2 bridged by a 1,4-C6H4 group to a free-base antenna (donor), [Fb]. This dyad exhibits the general structure [M]2-C6H4-[Fb], where [M]2 = [Fb]2, and completes a series reported earlier, where [M]2 = [Mg]2 (2) and [Zn]2 (3). The latter dyads exhibit a bidirectional energy-transfer process at 298 K for 2 and at 77 K for 3. Interestingly, a very scarce case of cycling process is observed for the zinc-containing dyad at 298 K. The newly reported compound 4 exhibits a quasi unidirectional process [Fb]*→[Fb]2 (major, kET = 2 × 1011 s-1 at 298 K), where the remaining is [Fb]2*→[Fb] (minor, kET = 8 × 109 s-1 at 298 K), thus completing all possibilities. The results are analyzed in terms of molecular orbital couplings (density functional theory computations), Förster resonance energy transfer parameters, and temperature dependence of the decay traces. This study brings major insights about artificial special pair-containing dyads and clearly contributes to a better understanding of the communication between the two main components of our models and those already described in the literature.

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.

Dense ionization and subsequent non-homogeneous radical-mediated chemistry of femtosecond laser-induced low density plasma in aqueous solutions: synthesis of colloidal gold.

The "cold" low density plasma channels generated by the filamentation of powerful femtosecond laser pulses in aqueous solutions constitute a source of dense ionization. Here, we probed the radiation-assisted chemistry of water triggered by laser ionization via the radical-mediated synthesis of nanoparticles in gold chloride aqueous solutions. We showed that the formation of colloidal gold originates from the reduction of trivalent ionic gold initially present in solution by the reactive radicals (e.g. hydrated electrons) produced upon the photolysis of water. We analyzed both the reaction kinetics of the laser-induced hydrated electrons and the growth kinetics of the gold nanoparticles. Introduction of radical scavengers into the solutions and different initial concentrations of gold chloride provided information about the radical-mediated chemistry. The dense ionization results in the second order cross-recombination of the photolysis primary byproducts. Competition with recombination imposes the non-homogeneous interaction of reactive radicals with solute present in irradiated aqueous solutions. Such a laser-induced non-homogeneous chemistry suggests similarities with the radiation chemistry of water exposed to conventional densely ionizing radiation (high dose rate, high linear energy transfer).

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.

Metal Linkage Effects on Ultrafast Energy Transfer.

We report the preparation of several new porphyrin homodimers bridged by a platinum(II) ion in which very intense electronic communication through the coordination link occurs. Moreover, the synthesis of a new porphyrin dyad and its photophysical properties are reported. This dyad exhibits the fastest singlet energy transfer ever reported for synthetic systems between a zinc(II) porphyrin and a porphyrin free base. This extremely fast transfer (∼100 femtoseconds) is in the same range as the fastest one measured in natural systems. This feature is due to the platinum(II) linker, which allows for strong MO couplings between the two porphyrin units as experimentally supported by electrochemistry and corroborated by DFT computations.

Triplet Energy Transfers in Well-Defined Host-Guest Porphyrin-Carboxylate/Cluster Assemblies.

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)], dppm = (Ph2P)2CH2) via ionic CO2(-)···Pd3(2+) interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd3(2+)] (∼8190 cm(-1)) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680-700 nm arising from a T1-T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1-T1 energy transfer ((3)dye*···[Pd3(2+)] → dye···(3)[Pd3(2+)]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 10(5) s(-1) on the basis of transient absorption measurements at 298 K.

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster.

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.

Ultrafast Singlet Energy Transfer in Porphyrin Dyads.

A weakly fluorescent Pt-bridged dyad composed of zinc(II) porphyrin (Zn; donor) and free base (Fb; acceptor) has been designed and exhibits an ultrafast singlet energy transfer between porphyrins. The use of larger atoms within the central linker significantly increases the MO coupling between the two chromophores and inherently the electronic communication.

The 3D [(Cu2Br2){µ-EtS(CH2)4SEt}]n material: a rare example of a coordination polymer exhibiting triplet-triplet annihilation.

EtS(CH2)4SEt, L1, forms with CuI a luminescent 2D polymer [Cu4I4{µ-L1}2]n (CP1), which exhibits no triplet excitation energy migration, but with CuBr, it forms a 3D material (CP2), [(Cu2Br2){µ-L1}]n consisting of parallel (Cu2Br2S2)n layers bridged by L1's. CP2 shows T1-T1 annihilation at 298 K but not at 77 K.

Unexpected drastic decrease in the excited-state electronic communication between porphyrin chromophores covalently linked by a palladium(II) bridge.

A dyad built up of a zinc(II) porphyrin and the corresponding free base, [Zn-Fb], fused to N-heterocyclic carbene (NHCs) ligands, respectively acting as singlet energy donor and acceptor, and a bridging trans-PdI2 unit, along with the corresponding [Zn-Zn] and [Fb-Fb] dimers were prepared and investigated by absorption and emission spectroscopy and density functional computations. Despite favorable structural and spectroscopic parameters, unexpectedly slow singlet energy transfer rates are measured in comparison with the predicted values by the Förster theory and those observed for other structurally related dyads. This observation is rationalized by the lack of large molecular orbital (MO) overlaps between the frontier MOs of the donor and acceptor, thus preventing a double electron exchange through the trans-PdI2 bridge, and by an electronic shielding induced by the presence of this same linker preventing the two chromophores to fully interact via their transition dipoles.