Adam Langlois

Slow and Fast Singlet Energy Transfers in BODIPY-gallium(III)corrole Dyads Linked by Flexible Chains

Red (no styryl), green (monostyryl), and blue (distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene) corrole dyads have been prepared in high yields using click chemistry, and their photophysical properties are reported. An original and efficient control of the direction of the singlet energy transfers is reported, going either from BODIPY to the gallium-corrole units or from gallium-corroles to BODIPY, depending upon the nature of the substitution on BODIPY. In one case (green), both directions are possible. The mechanism for the energy transfers is interpreted by means of through-space Förster resonance energy transfer (FRET).

Phosphorescent Cu(I) complexes based on bis(pyrazol-1-yl-methyl)-pyridine derivatives for organic light-emitting diodes†

Mononuclear Cu(I) complexes based on bis(pyrazol-1-yl-methyl)-pyridine derivatives and ancillary triphenylphosphine have been prepared and characterized by 1H NMR, mass spectroscopy and single-crystal X-ray analysis. The thermogravimetric analysis shows that the complexes exhibit high thermal stability. The electronic absorption spectra display two features in the regions of 230–260 and 290–350 nm attributable to mixed ligand-to-ligand (LLCT) and metal-to-ligand-charge-transfer (MLCT) excited states, which is supported by the results of density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on these Cu(I) complexes. These complexes are strongly emissive in the solid state at ambient temperature. Intense blue or green emission in the poly(methyl methacrylate) film is observed in the region of 475–518 nm for these complexes with the emission lifetimes in the microsecond time scale (12–20 μs), indicating that the emission may be phosphorescence emission. Increasing the steric hindrance of the substituents on the pyrazole unit results in a blue-shift of the emission bands and enhanced emission quantum efficiency in PMMA films. The two most emissive complexes have been used for the fabrication of phosphorescent organic light-emitting diodes (POLEDs).

Platinum(II) cyclometallates featuring broad emission bands and their applications in color-tunable OLEDs and high color-rendering WOLEDs

Two phosphorescent platinum(II) cyclometallated complexes with phenoxy groups (1 and 2) have been developed. The modified organic ligands derived by combining the phenoxy moiety and 2-phenylpyridine conferred them with a more flexible structure, leading to superior intermolecular interaction properties of the resulting Pt(II) metallophosphors. Because of the excimer formation induced by Pt(II) complexes 1 and 2, the emission color can be tuned over a wide range from cyan to orange by simply increasing the concentration of the Pt(II) metallophosphors. Inspired by their broad emission band, color tunability and outstanding electroluminescence (EL) performance, these two Pt(II) phosphors complemented with blue fluorescent emitter 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) were employed in manufacturing high color-rendering white organic light-emitting devices (WOLEDs). In such simple two-emitter systems, 1-based WOLEDs exhibited reasonable EL performance with an external quantum efficiency (ηext) of 11.7%, luminance efficiency (ηL) of 29.1 cd A−1, power efficiency (ηp) of 16.9 lm W−1 and color rendering index (CRI) of 77, whereas 2-based WOLEDs demonstrated an ηext of 10%, ηL of 21.7 cd A−1, ηp of 10.7 lm W−1 and CRI of 88.

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.

Maple™-assisted calculations of the J-integral: a key parameter for the understanding of excited state energy transfer in porphyrins and other chromophores

The spectral overlap between the emission of a donor molecule and the absorption of an acceptor molecule, quantifiable using the J-integral calculation, is a parameter of extreme importance when studying the excited state energy transfer by either the Forster or Dexter mechanism. Despite its extreme importance in both mechanisms, it is often misinterpreted, approximated or incorrectly calculated. The calculation of the J-integral is not trivial especially when one wishes to carry out the calculation on measured spectroscopic data. A detailed description for the correct calculation of the J-integral is herein reported and presents a Maple™ assisted template that is capable of performing this calculation in the two different energy scales (nm and cm -1 ) to yield the value of the J-integral in given units. Specific examples using porphyrin-containing compounds are provided. This Maple™ program is flexible and can be easily adapted to the needs of a researcher. A call for the standardization of the calculation of the J-integral for the facile comparison with other overlap integrals found in the literature is made.

Origin of the temperature dependence of the rate of singlet energy transfer in a three-component truxene-bridged dyads

We report a truxene-based dyad built upon one donor (tri-meso-phenylzinc(II)porphyrin) and two acceptors (octa-b-alkylporphyrin free base) in which the donor exhibits free rotation around a Ctruxene-Cmeso single bond at 298 K in fluid solution but not at 77 K in a glass matrix, whereas the acceptors have very limited motion as they are blocked by b-methyl groups. This case is interesting because all the structural and spectroscopic parameters affecting the rate for singlet energy transfer according to a Forster Resonance Energy Transfer are only weakly temperature dependent, leaving only the Dexter mechanism explaining the larger variation in rate of energy transfers with the temperature hence providing a circumstantial evidence for a dual mechanism (Foster and Dexter) in truxene-based dyads (or polyads) in the S1 excited states.

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.

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Forster theory, evidence for two pathways for S1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, kET(S1)1 and kET(S1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched components ( 10 (ns)-1), are observed, hence, demonstrating the bidirectional paths with no back energy transfer. A 298 K, only two components are detected (0.44 ([Zn2]) and 2.64 ns (FB)) and the resulting reduced τFs indicates back energy transfer, therefore cycling and equilibrium. Their global rates are 0.31 and 1.8 (ns)-1 for kET(S1)1 and kET(S1)-1 at 298 K. This large temperature dependence on kET(S1) is fully consistent with the participation of thermal activation. Finally, DFT calculations (B3LYP) were used to illustrate a clear correlation between the relative kET(S1)s and the amplitude of the MO couplings between the artificial special pair and the antenna.