Paul-Ludovic Karsenti

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.

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.

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.

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.

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.

Very fast singlet and triplet energy transfers in a tri-chromophoric porphyrin dyad aided by the truxene platform

A trichromophoric dyad composed of an octa-β-alkyl-palladium(II)porphyrin (donor) and two tri-meso-aryl-zinc(II)porphyrins (acceptors) held by a truxene spacer exhibits very fast rates for triplet energy transfers at 77 (kET(T1) = 1.63 × 108 s-1) and 298 K (kET(T1) = 3.44 × 108 s-1), whereas the corresponding singlet energy transfer rates, kET(S1) = 3.9 × 1010 s-1 (77 K) and kET(S1) = 6.0 × 1010 s-1 (298 K), are also considered fast. The interpretation for these results is that the energy transfer processes proceed via a through bond Dexter mechanism (i.e. double electron exchange) supported by comparison with literature data and evidence for a moderate MO coupling between the donor and acceptor chromophores in the frontier MOs.

Spatial organization and optical properties of layer-by-layer assembled upconversion and gold nanoparticles in thin films

Multilayers of NaYF4:Yb,Tm@NaYF4 upconversion nanoparticles (UCNPs), up to 10 UCNP/polyelectrolyte bilayers, were prepared using the layer-by-layer (LBL) method. The assembled thin films of UCNPs were found to display good stability of upconversion luminescence (UCL) upon 980 nm near-infrared (NIR) excitation. Moreover, LBL assembled films comprising four UCNP/polyelectrolyte bilayers and four gold nanoparticles (AuNP)/polyelectrolyte bilayers were prepared while adjusting their deposition order to give rise to three different spatial organizations of the two types of nanoparticles. The UV-vis-NIR extinction spectra and UCL emission spectra revealed that the spatial organization determined by the sequence of deposition influences the interaction between UCNPs and AuNPs and thus their optical properties. The alternating deposition of UCNP and AuNP layers appears to be the best way to preserve their “individual” properties, as a result of reducing both the agglomeration of AuNPs and the scattering of the 980 nm excitation light that results in an apparent quenching effect on the UCL of UCNPs. By contrast, with the same numbers of UCNP and AuNP layers, the successive deposition of UCNPs followed by the successive addition of AuNP result in more agglomeration of AuNPs leading to a broad surface plasmon resonance (SPR) band in the NIR region and also a significant quenching of the UCL intensity of UCNPs. This study demonstrates an easy and effective way to prepare NIR-responsive and plasmonic hybrid thin films with the possibility to tune their optical properties through spatially controlled organization of the nanoparticles.

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.

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.

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.