Relaxation dynamics in three polypyridyl iron(II)-based complexes probed by nanosecond and sub-picosecond transient absorption spectroscopy

On the intersystem crossing rate in a Platinum(II) donor-bridge-acceptor triad

The rates of ultrafast intersystem crossing in acceptor-bridge-donor molecules centered on Pt(II) acetylides are investigated. Specifically, a Pt(II) trans-acetylide triad NAP--Pt--Ph-CH₂-PTZ [1], with acceptor 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and donor phenothiazine (PTZ), is examined in detail. We have previously shown that optical excitation in [1] leads to a manifold of singlet charge-transfer states, S*, which evolve via a triplet charge-transfer manifold into a triplet state ³NAP centered on the acceptor ligand and partly to a charge-separated state ³CSS (NAP^--Pt-PTZ⁺). A complex cascade of electron transfer processes was observed, but intersystem crossing (ISC) rates were not explicitly resolved due to lack of spin selectivity of most ultrafast spectroscopies. Here we revisit the question of ISC with a combination and complementary analysis of (i) transient absorption, (ii) ultrafast broadband fluorescence upconversion, FLUP, which is only sensitive to emissive states, and (iii) femtosecond stimulated Raman spectroscopy, FSR. Raman resonance conditions allow us to observe S* and ³NAP exclusively by FSR, through vibrations which are pertinent only to these two states. This combination of methods enabled us to extract the intersystem crossing rates that were not previously accessible. Multiple timescales (1.6 ps to ∼20 ps) are associated with the rise of triplet species, which can now be assigned conclusively to multiple ISC pathways from a manifold of hot charge-transfer singlet states. The analysis is consistent with previous transient infrared spectroscopy data. A similar rate of ISC, up to 20 ps, is observed in the trans-acetylide NAP--Pt--Ph [2] which maintains two acetylide groups across the platinum center but lacks a donor unit, whilst removal of one acetylide group in mono-acetylide NAP--Pt-Cl [3] leads to >10-fold deceleration of the intersystem crossing process. Our work provides insight on the intersystem crossing dynamics of the organo-metallic complexes, and identifies a general method based on complementary ultrafast spectroscopies to disentangle complex spin, electronic and vibrational processes following photoexcitation.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: III. [Fe(tpen)]Cl2 in acetonitrile

In order to achieve an in-depth understanding of the role played by the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(ii) complexes, an accurate description of the solvated complexes in the two spin states is required. To this end, we are applying state-of-the-art ab initio molecular dynamics (AIMD) simulations to the study of the structural and vibrational properties of iron(ii) polypyridyl complexes. Two aqueous LS complexes were investigated in this framework, namely, [Fe(bpy)3]2+ (bpy = 2,2'-bipyridine) [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830; Lawson Daku, Phys. Chem. Chem. Phys., 2018, 20, 6236] and [Fe(tpy)2]2+ (tpy = 2,2':6',2''-ter-pyridine) [Lawson Daku, Phys. Chem. Chem. Phys., 2019, 21, 650]. For aqueous [Fe(bpy)3]2+, combining the results of forefront wide-angle X-ray scattering experiments with those of the AIMD simulations allowed the visualization of the interlaced coordination and solvation spheres of the photoinduced HS state [Khakhulin et al., Phys. Chem. Chem. Phys., 2019, 21, 9277]. In this paper, we report the extension of our AIMD studies to the spin-crossover complex [Fe(tpen)]2+ (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) in acetonitrile (ACN). The determined LS and HS solution structures of the complex are in excellent agreement with the experimental results obtained by high-resolution transient X-ray absorption spectroscopy [Zhang et al., ACS Omega, 2019, 4, 6375]. The first solvation shell of [Fe(tpen)]2+ consists of ACN molecules located in the grooves defined by the chelating coordination motif of the tpen ligand. Upon the LS → HS change of states, the solvation number of the complex is found to increase from ≈9.2 to ≈11.9 and an inner solvation shell is formed. This inner solvation shell originates from the occupancy by about one ACN molecule of the internal cavity which results from the arrangement of the 4 pyridine rings of the tpen ligand, and which becomes accessible to the solvent molecules in the HS state only thanks to the structural changes undergone by the complex. The presence of this inner solvation shell for the solvated HS complex probably plays a key role in the spin-state dependent reactivity of [Fe(tpen)]2+ in liquid solutions.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: II. aqueous [Fe(tpy)2]Cl2

LS and HS Fe–O radial distribution functions and running coordination numbers for aqueous [Fe(tpy)₂]Cl₂: in both spin states, the first hydration shell of [Fe(tpy)₂]²⁺ consists in a chain of ∼15 hydrogen-bonded water molecules wrapped around the ligands.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: aqueous [Fe(bpy)3]Cl2, a case study

LS and HS IR spectra of aqueous [Fe(bpy)₃]²⁺ and corresponding HS–LS difference IR spectrum as obtained from state-of-the-art ab initio molecular dynamics simulations applied to the determination of the structural and vibrational properties of the solvated complex.

Enhanced energy transport in genetically engineered excitonic networks

One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.

Ultrafast resonance energy transfer in the umbelliferone–alizarin bichromophore

Fast and efficient intramolecular energy transfer takes place in the umbelliferone–alizarin bichromophore; the process is well described by the Förster mechanism.