Synthesis, Excited-State Dynamics, and Reactivity of a Directly-Linked Pyromellitimide−(Porphinato)zinc(II) Complex

N-[5-(10,20-Diphenylporphinato)zinc(II)]−N‘-(octyl)pyromellitic diimide (PZn−PI), a meso-pyromellitimide-substituted (porphinato)zinc(II) compound, has been fabricated from the reaction of (5-amino-10,20-diphenylporphinato)zinc(II) with pyromellitic dianhydride in the presence of octylamine. Interrogation of the photoinduced charge separation (CS) and thermal charge recombination (CR) electron-transfer (ET) dynamics for PZn−PI in CH2Cl2 via pump−probe transient absorption spectroscopic methods shows that τCS and τCR are 770 and 5200 fs, respectively. These ET dynamics differ from those elucidated previously for closely related 5-quinonyl-substituted (porphinato)metal compounds, and derive from the fact that the low-lying excited states for PZn−PI are porphyrin-localized, possessing little charge-transfer character. The synthesis of N-{5-[15-(2-(triisopropylsilyl)ethynyl)-10,20-diphenylporphinato]zinc(II)}−N‘-(octyl)pyromellitic diimide demonstrates that PZn−PI can be halogenated at its 15-meso-position and used subsequently as a substrate in metal-catalyzed cross-coupling reactions; the reactivity of PZn−PI is unusual with respect to many directly linked donor−acceptor compounds in that it is stable to these oxidizing and reducing reaction conditions

Dual-Frequency 2D IR on Interaction of Weak and Strong IR Modes

Dual-frequency 2D IR heterodyne photon-echo spectroscopy of C⋮N and CO stretching vibrational modes in 2-cyanocoumarin is reported. We have shown that the interaction among these modes provides convenient and useful structural constraints for molecules. Implementation of two pulse sequences, 4, 4, and 6 μm and 6, 6, and 4 μm, allowed the clear determination of contributions caused by vibrational relaxation. Positive correlation between C⋮N and CO frequency distributions was observed in 2-cyanocoumarin. Because C⋮N modes are highly localized and have frequencies in a spectral region with minimal water absorption, the C⋮N/CO interactions have a strong potential for use as structural reporters in proteins. In addition to CN/CO peaks, the cross-peaks responsible for the C⋮N/CC interaction are also observed in the 2D IR spectra, where CC is a coumarin ring stretching mode. We have demonstrated that 2D IR spectroscopy can utilize interactions of strong IR modes with weak local modes as structural reporters

Ultrafast Singlet Excited-State Polarization in Electronically Asymmetric Ethyne-Bridged Bis[(porphinato)zinc(II)] Complexes

The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5‘-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5‘,-15‘-ethynyl-10‘,20‘-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S1→Sn absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S1→S0 oscillator strength on a 2−20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin−porphyrin conjugation, which varies as a function of the pigment−pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (τ ≈ 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S1→S0 transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S1-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation

Synthesis, Electronic Structure, and Electron Transfer Dynamics of (Aryl)ethynyl-Bridged Donor−Acceptor Systems

The ET dynamics of a series of donor−spacer−acceptor (D−Sp−A) systems featuring (porphinato)zinc(II), (aryl)ethynyl bridge, and arene diimide units were investigated by pump−probe transient absorption spectroscopy. Analysis of these data within the context of the Marcus−Levich−Jortner equation suggests that the π-conjugated (aryl)ethynyl bridge plays an active role in the charge recombination (CR) reactions of these species by augmenting the extent of (porphinato)zinc(II) cation radical electronic delocalization; this increase in cation radical size decreases the reorganization energy associated with the CR reaction and thereby attenuates the extent to which the magnitudes of the CR rate constants are solvent dependent. The symmetries of porphyrin-localized HOMO and HOMO-1, the energy gap between these two orbitals, and D−A distance appear to play key roles in determining whether the (aryl)ethynyl bridge simply mediates electronic superexchange or functions as an integral component of the D and A units

Structure Dependent Energy Transport: Relaxation-Assisted 2DIR Measurements and Theoretical Studies

Vibrational energy relaxation and transport in a molecule that is far from thermal equilibrium can affect its chemical reactivity. Understanding the energy transport dynamics in such molecules is also important for measuring molecular structural constraints via relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy. In this paper we investigated vibrational relaxation and energy transport in the ortho, meta, and para isomers of acetylbenzonitrile (AcPhCN) originated from excitation of the CN stretching mode. The amplitude of the cross-peak among the CN and CO stretching modes served as an indicator for the energy transport from the CN group toward the CO group. A surprisingly large difference is observed in both the lifetimes of the CN mode and in the energy transport rates for the three isomers. The anharmonic DFT calculations and energy transport modeling performed to understand the origin of the differences and to identify the main cross-peak contributors in these isomers described well the majority of the experimental results including mode excited-state lifetimes and the energy transport dynamics. The strong dependence of the energy transport on molecular structure found in this work could be useful for recognizing different isomers of various compounds via RA 2DIR spectroscopy

Highly Conjugated (Polypyridyl)metal−(Porphinato)zinc(II) Compounds: Long-Lived, High Oscillator Strength, Excited-State Absorbers Having Exceptional Spectral Coverage of the Near-Infrared

Transient dynamical studies of ruthenium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2‘;6‘,2‘ ‘-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn), osmium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2‘;6‘,2‘ ‘-terpyridine)2+ bis-hexafluorophosphate (Os-PZn), ruthenium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-15-(4‘-nitrophenyl)ethynyl-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2‘;6‘,2‘ ‘-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn-A), osmium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-15-(4‘-nitrophenyl)ethynyl-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2‘;6‘,2‘ ‘-terpyridine)2+ bis-hexafluorophosphate (Os-PZn-A), and ruthenium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-ruthenium(II)-15-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2‘;6‘,2‘ ‘-terpyridine)4+ tetrakis-hexafluorophosphate (Ru-PZn-Ru), and ruthenium(II) [5-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-osmium(II)-15-(4‘-ethynyl-(2,2‘;6‘,2‘ ‘-terpyridinyl))-10,20-bis(2‘,6‘-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2‘;6‘,2‘ ‘-terpyridine) tetrakis-hexafluorophosphate (Ru-PZn-Os) show that these highly conjugated supermolecular chromophores feature electronically excited states that absorb over broad NIR spectral windows with considerable oscillator strength and manifest lifetimes (1−50 μs) that are extraordinarily long relative to those of classic low band-gap organic materials. The excited-state absorptive domains of these strongly coupled multipigment ensembles can be extensively modulated. For sequential one-photon absorptive processes, these compounds evince large σe, σe/σg, and σe − σg values. As the combination of all these properties within single chromophoric entities have heretofore lacked precedent within the NIR, these and closely related structures may find particular utility in a variety of technologically important optical-limiting applications

Effect of Mutation on the Dissociation and Recombination Dynamics of CO in Transcriptional Regulator CooA: A Picosecond Infrared Transient Absorption Study^†

The CO ligation process in a mutant (H77G) of CooA, the CO-sensing transcriptional regulator in Rhodospirillum rubrum, is studied with femtosecond time-resolved transient absorption spectroscopy in the mid-infrared region. Following photolyzing excitation, a transient bleach in the vibrational region of bound CO due to the CO photodissociation is detected. In contrast to the spectra of the wild-type (WT) CooA, the transient bleach spectra of H77G CooA show a bimodal shape with peaks shifting to the lower frequency during spectral evolution. The CO recombination dynamics show single-exponential behavior, and the CO escaping yield is higher than that of the WT CooA. A reorientation process of CO relative to the heme plane during recombination is revealed by anisotropy measurements. These phenomena indicate changes in the protein response to the CO ligation and suggest an alteration to the CO environment caused by the mutation. On the basis of these results, the role of His77 in the CO-dependent activation of CooA and a possible activation mechanism involving collaborative movement of the heme and the amino residues at both sides of the heme plane are discussed

Interrogating Conformationally Dependent Electron-Transfer Dynamics via Ultrafast Visible Pump/IR Probe Spectroscopy

We demonstrate for the first time the utility of time-resolved visible pump/mid-infrared (IR) probe spectroscopy to interrogate directly, and provide unique infomation regarding, conformationally dependent photoinduced ET dynamics and the subsequent structural evolution of the resulting charge-separated (CS) state. Exemplary polarized visible pump/IR probe experiments involving N-[5-(10,20-diphenylporphinato)zinc(II)]-N‘(octyl)pyromellitic diimide (PZn-PI) and [5-[4‘-(N-(N‘-octyl)pyromellitic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn()PI) show that it is possible to assess the mean PZn-to-PI interplanar torsional angle of electronically excited structural conformers that undergo ET within the sub-ps time domain for both of these donor−spacer−acceptor (D-Sp-A) systems. Further transient specroscopic experiments carried out on PZn()PI determine how this angle evolves with time. Because vibrational transition moments are often known and typically localized, this work underscores that polarized visible pump/IR probe spectroscopy defines a valuable tool to interrogate structrure in both electronically excited and CS states; this fact, coupled with the ultrafast time resolution and high senseitivity offered by this technique, make it ideally suited to probe a range of mechanistic issues relevant to charge-transfer reactions

Ultrafast Excited-State Dynamics of Nanoscale Near-Infrared Emissive Polymersomes

Formed through cooperative self-assembly of amphiphilic diblock copolymers and electronically conjugated porphyrinic near-infrared (NIR) fluorophores (NIRFs), NIR-emissive polymersomes (50 nm to 50 μm diameter polymer vesicles) define a family of organic-based, soft-matter structures that are ideally suited for deep-tissue optical imaging and sensitive diagnostic applications. Here, we describe magic angle and polarized pump−probe spectroscopic experiments that: (i) probe polymersome structure and NIRF organization and (ii) connect emitter structural properties and NIRF loading with vesicle emissive output at the nanoscale. Within polymersome membrane environments, long polymer chains constrain ethyne-bridged oligo(porphinato)zinc(II) based supermolecular fluorophore (PZnn) conformeric populations and disperse these PZnn species within the hydrophobic bilayer. Ultrafast excited-state transient absorption and anisotropy dynamical studies of NIR-emissive polymersomes, in which the PZnn fluorophore loading per nanoscale vesicle is varied between 0.1−10 mol %, enable the exploration of concentration-dependent mechanisms for nonradiative excited-state decay. These experiments correlate fluorophore structure with its gross spatial arrangement within specific nanodomains of these nanoparticles and reveal how compartmentalization of fluorophores within reduced effective dispersion volumes impacts bulk photophysical properties. As these factors play key roles in determining the energy transfer dynamics between dispersed fluorophores, this work underscores that strategies that modulate fluorophore and polymer structure to optimize dispersion volume in bilayered nanoscale vesicular environments will further enhance the emissive properties of these sensitive nanoscale probes

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule–plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions