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

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)­zinc­(II) (<b>MCP</b>) and (5,15-bis­(4-carboxylphenyl)-15,20-ditolylporphyrinato)­zinc­(II) (<b>DCP</b>), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd₃(dppm)₃(CO)²⁺ (<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>, dppm = (Ph₂P)₂CH₂) via ionic CO₂^–···Pd₃²⁺ interactions. The photophysical properties in their triplet states were studied. The position of the T₁ state of <b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b> (∼8190 cm^–1) has been proposed using DFT computations and was corroborated by the presence of a T_<i>n</i> → 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 (³dye*···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b> → dye···³<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>*) operates through a Förster mechanism exclusively with <i>k</i>_ET values of ∼1 × 10⁵ s^–1 on the basis of transient absorption measurements at 298 K

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, <b>[Zn</b>_<b>2</b><b>]</b>, as a slipped dimer acceptor using a BODIPY unit, <b>[bod]</b>, as the donor, bridged by the linker C₆H₄CCC₆H₄. The rate of singlet energy transfer, <i>k</i>_ET(S₁), at 298 K (<i>k</i>_ET(S₁) = 4.5 × 10¹⁰ s^–1) extracted through the change in fluorescence lifetime, τ_F, of <b>[bod]</b> in the presence (27.1 ps) and the absence of <b>[Zn</b>_<b>2</b><b>]</b> (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 τ_F of <b>[bod]</b> increases to 45.3 ps, indicating that <i>k</i>_ET(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

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

9,18,27,36-Tetrakis­[<i>meso</i>-(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⁺ ([Pd₃²⁺]; dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt) with that of 5,10,15,20-tetrakis­[<i>meso</i>-(4-carboxyphenyl)]­porphyrinatozinc­(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd₃²⁺]_<i>x</i> (<i>x</i> = 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···[Pd₃²⁺]_<i>x</i> assemblies (<i>K</i>₁₄ = 36000 M^–1) are slightly more stable than those for TCPP···[Pd₃²⁺]_{<b><i>x</i></b>} (<i>K</i>₁₄ = 27000 M^–1). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd₃²⁺] assembly compared to that for TCPP···[Pd₃²⁺]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd₃²⁺] → 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···[Pd₃²⁺]) occurring in the various observed species (dye···[Pd₃²⁺]_<i>x</i>; <i>x</i> = 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

Push–Pull Porphyrin-Containing Polymers: Materials Exhibiting Ultrafast Near-IR Photophysics

Four push–pull polymers of structure (CC–<b>[Zn]</b>–CC–<b>A</b>)_<i>n</i> (<b>A</b> = isoindigo (<b>P1</b>), bis­(α-methylamino-1,4-benzene)­quinone (<b>P2</b>), 2-(<i>N</i>-methylamino-1,4-benzene)-<i>N</i>-1,4-benzene-maleimide (<b>P3</b>), and 2,2′-anthraquinone (<b>P4</b>); <b>[Zn]</b> = [bis­(<i>meso</i>-aryl)­porphyrin]­zinc­(II) = donor) and models <b>M1</b> and <b>M2</b> (<b>A</b>′–CC–<b>[Zn]</b>–CC–<b>A</b>′; <b>A</b>′ = respectively naphtoquinone and 2-anthraquinone) were prepared and characterized (¹H and ¹³C NMR, elemental analysis, GPC, TGA, cyclic voltammetry, steady state and ultrafast time-resolved UV–vis and emission spectroscopy) and studied by density functional theory (DFT) and time-dependent DFT (TDDFT) in order to address the nature of the low-lying singlet and triplet excited states. <b>P1</b> (fully conjugated polymer), <b>P2</b> (formally nonconjugated but exhibit strong electronic communication accross the chain) and <b>P4</b> (formally nonconjugated but local conjugation between the donor and acceptor) are near-IR emitters (λ_max > 750 nm). <b>M1</b> and <b>M2</b> are mono-CC–<b>[Zn]</b>–CC species, and <b>P3</b> exhibits a very modest CT contribution (as maleimide is a weak acceptor) and are not near-IR emitters. The nature of the S₁ and T₁ excited states are CT processes donor* → acceptor. In <b>P1</b>–<b>P4</b>, a dual fluorescence (7.7 < τ_F < 770 ps; except one value at 2.5 ns; <b>P3</b>) is depicted, which are assigned to fluorescences arising from the terminal and central units of the polymers identified from the comparison with <b>M1</b> and <b>M2</b>. The high and low energy fluorescences are respectively short (77 < τ_F < 166 ps) and long-lived (688 < τ_F < 765 ps) suggesting S₁ energy transfers with rates, k_ET, of 7.1 (<b>P1</b>), 12 (<b>P2</b>) and 4.5 (ns)⁻¹ (<b>P4</b>). The fs transient absorption spectra exhibit particularly very short triplet lifetimes (2.3 < τ_T1 < 87 ns) explaining the absence of phosphorescence. Also ultrafast lifetimes (85 < τ < 1290 fs) for species excited in the 0–0 peak of the Q-band (650 nm; i.e., ππ* porphyrin level) indicating its rather efficient nonradiative deactivation (S_<i>n</i> ∼ > S₁ and S_<i>n</i> ∼ > <i>T</i>_m). When cooling takes place or the solution concentration is increased, new red-shifted fluorescence bands appear, evidencing aggregate formation. Both fluorescence and transient absorption lifetimes of <b>P1</b>–<b>P4</b> become shorter and their band intensity lower. Finally, the position of the optically silent phosphorescence has been predicted to be in the 1300 (<b>P1</b>, <b>P2</b>) and 1000 nm (<b>P3</b>, <b>P4</b>) zones (DFT)

Ultrafast Electron Transfers in Organometallic Supramolecular Assemblies Built with a NIR-Fluorescent Tetrabenzoporphyrine Dye and the Unsaturated Cluster Pd₃(dppm)₃(CO)²⁺

The sodium 9,18,27,36-tetra-(4-carboxyphenyl­ethynyl)­tetrabenzo­porphyrinatozinc­(II) (<b>TCPEBP</b>) and sodium 5,10,15,20-tetra-(4-carboxy­phenyl­ethynyl)­porphyrinatozinc­(II) (<b>TCPEP</b>, for comparison purposes) salts were prepared to investigate the ionic driven host–guest assemblies made with the unsaturated redox-active cluster Pd₃(dppm)₃(CO)²⁺ (<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>, dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt). Nonemissive dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} assemblies (<i>x</i> = 1–4) are formed in methanol with <i>K</i>_1<i>x</i> (binding constants) values of 83 200 (<b>TCPEBP</b>) and 70 400 M^–1 (<b>TCPEP</b>; average values extracted from graphical methods (Benesi–Hildebrand, Scott, and Scatchard), matching those obtained from fluorescence quenching experiments (static model)). These values are consistent with the more electron rich <b>TCPEBP</b> dye. This conclusion is corroborated by electrochemical data, which indicate a lower oxidation potential of the <b>TCPEBP</b> dye (+0.46 V) vs <b>TCPEP</b> (+0.70 V vs SCE) and by shorter calculated average Pd···O distances (DFT (B3LYP): 3.259 vs 3.438 Å, respectively). Using the position of the 0–0 component of the Q-bands and the electrochemical data, the excited-state driving forces for dye*···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} <b> → </b> dye^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} are estimated for <b>TCPEBP</b> (+1.22 V vs SCE) and <b>TCPEP</b> (1.08 V vs SCE). The time scale for this process occurs within the laser pulse (fwhm <75–110 fs) during the measurements of the femtosecond transient absorption spectra. Conversely, the back electron transfers (dye^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} <b> → </b> dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>}) occur well within 1 ps (respectively 650 and 170 fs for <b>TCPEBP</b> and <b>TCPEP</b>). Arguments are provided that the reorganization energy governs this difference

Luminescent Organometallic Complexes Built upon the Nonemissive Azophenine

Azophenine, C₆H₂(NPh)₂(NHPh)₂, is renowned to be nonemissive in solution or in the solid state at 298 and 77 K. It was rendered luminescent in solution at room temperature without using any cyclization strategy of the N^∧N end by anchoring two or four <i>trans</i>-RCCPt­(PBu₃)₂(CC) units (R = hexa-<i>n</i>-hexyltruxene (<b>Tru</b>)) on the azophenine. Complexes of the general formulas C₆H₂(NC₆H₄CCSiMe₃)₂(NH<b>PtTru</b>)₂ (<b>DiPtTruQ</b>) and C₆H₂(N<b>PtTru</b>)₂(NH<b>PtTru</b>)₂(<b>TertPtTruQ</b>), where <b>Pt</b> = <i>trans</i>-C₆H₄CCPt­(PBu₃)₂CC, exhibit fluorescence (420 nm) and phosphorescence (512 nm) bands arising from upper localized ππ*/C₆H₄CC to <b>Tru</b>CC charge transfer singlet and triplet excited states in 2MeTHF at 298 and 77 K. This latter assignment is based on DFT computations (B3LYP). Moreover, <b>DiPtTru</b> and <b>TertPtTru</b> exhibit low-energy absorption bands with maxima in the 470–485 nm range extending all the way to 600–650 nm. These spectral features are associated with charge transfer (CT) excited states: namely, <b>TruPt</b> → <b>Q</b> (<b>Q</b> = C₆H₂N₂(NH)₂). No emission band (fluorescence or phosphorescence) associated with these CT states has been detected at 298 K, but weak fluorescence bands (λ_max ∼750 nm) decaying on the picosecond time scale have been observed in both cases. Biexponential decays were also often noted and likely reflect the presence of the possible conformers associated with the two possible dihedral angles made by the C₆H₄ plane and the central C₆H₂N₂(NH)₂ core. No evidence for electron transfer between the <b>TruPt</b> arms and <b>Q</b> was observed

Platinum Complexes of <i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>‴‑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 <i>para</i>-positions of the phenyl groups by luminescent <i>trans</i>-ArCC–Pt­(PR₃)₂-CC (<b>[Pt]</b>) arms [Ar = C₆H₄ (R = Et), hexa­(<i>n</i>-hexyl)­truxene) (<b>Tru</b>; R = Bu)]. Two effects are reported. First, the linking of these <b>[Pt]</b> arms with the central azophenine (C₆H₄–NC₆H₂(NH)₂N–C₆H₄; <b>Q</b>) generates very low energy charge-transfer (CT) singlet and triplet excited states (^3,1(<b>[Pt]</b>-to-<b>Q</b>)*) 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, ³<b>Tru</b>* → <b>Q</b>, is provided

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

Ultrafast Heat Transfer at the Nanoscale: Controlling Heat Anisotropy

Thermoplasmonics has benefited from increasing attention in recent years by exploiting the photothermal effects within plasmonic nanoparticles to generate nanoscale heat sources. Recently, it has been demonstrated that exciting gold nanoparticles with ultrashort light pulses could be used to achieve high-speed light management and nanoscale heat-sensitive chemical reaction control. In this work, we study non-uniform thermal energy transient distribution inside cross-shaped nanostructures with femtosecond transient spectroscopy coupled to a thermo-optical numerical model, free of fitting parameters. We show experimentally and numerically that the polarization of the excitation light can control the heat distribution in the nanostructures. We also demonstrate the necessity of considering nonthermal electron ballistic displacement in fast transient heat dynamics models

Ultrafast Heat Transfer at the Nanoscale: Controlling Heat Anisotropy

Thermoplasmonics has benefited from increasing attention in recent years by exploiting the photothermal effects within plasmonic nanoparticles to generate nanoscale heat sources. Recently, it has been demonstrated that exciting gold nanoparticles with ultrashort light pulses could be used to achieve high-speed light management and nanoscale heat-sensitive chemical reaction control. In this work, we study non-uniform thermal energy transient distribution inside cross-shaped nanostructures with femtosecond transient spectroscopy coupled to a thermo-optical numerical model, free of fitting parameters. We show experimentally and numerically that the polarization of the excitation light can control the heat distribution in the nanostructures. We also demonstrate the necessity of considering nonthermal electron ballistic displacement in fast transient heat dynamics models