Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex.

A comprehensive spectroscopic and structural investigation of [CoII (l-N4 tBu2 )(dbsq)][B(p-C6 H4 Cl)4 ] (1, l-N4 tBu2 =N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq1- =3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) CoII semiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) CoII semiquinonate state instead of the frequently observed valence tautomerism from ls CoIII catecholate to hs CoII semiquinonate. DFT calculations demonstrate that the (closed shell) CoIII catecholate suffers from a triplet instability leading to the ls CoII semiquinonate ground state. The thorough temperature-dependent spectroscopic study of the SCO enables a photophysical investigation. Thus, by selective photoexcitation of the ls fraction of 1 in solution at room temperature, ultrafast conversion to the hs state is observed using femtosecond electronic and IR-vibrational (infrared) transient absorption spectroscopy. The kinetics of the photocycle is described by a stretched exponential with τ=3.3-3.6 ps and ß=0.52-0.54, representing an upper limit for the hs-ls relaxation time. This is, to our knowledge, the fastest interconversion ever determined for a SCO complex, and is attributed to the special situation that in 1 a CoII complex is coordinated to a π-radical ligand allowing very efficient coupling between the ls and hs spin states.

ESIPT and photodissociation of 3-hydroxychromone in solution: photoinduced processes studied by static and time-resolved UV/Vis, fluorescence, and IR spectroscopy.

The spectral properties of fluorescence sensors such as 3-hydroxychromone (3-HC) and its derivatives are sensitive to interaction with the surrounding medium as well as to substitution. 3-HC is a prototype system for other derivatives because it is the basic unit of all flavonoides undergoing ESIPT and is not perturbed by a substituent. In this study, the elementary processes and intermediate states in the photocycle of 3-HC as well as its anion were identified and characterized by the use of static and femtosecond time-resolved spectroscopy in different solvents (methylcyclohexane, acetonitrile, ethanol, and water at different pH). Electronic absorption and fluorescence spectra and lifetimes of the intermediate states were obtained for the normal, tautomer and anionic excited state, while mid-IR vibrational spectra yielded structural information on ground and excited states of 3-HC. A high sensitivity on hydrogen-bonding perturbations was observed, leading to photoinduced anion formation in water, while in organic solvents, different processes are suggested, including slow picosecond ESIPT and contribution of the trans-structure excited state or a different stable solvation state with different direction of OH. The formation of the latter could be favored by the lack of a substituent increasing contact points for specific solute-solvent interactions at the hydroxyl group compared to substituted derivatives. The effect of substituents has to be considered for the design of future fluorescence sensors based on 3-HC.

Transient IR spectroscopy and ab initio calculations on ESIPT in 3-hydroxyflavone solvated in acetonitrile.

Femtosecond polarization resolved UV/Vis and mid-infrared spectroscopy was used to thoroughly identify and characterize the relevant elementary chemical and physical processes in the photocycle of 3-hydroxyflavone (3-HF) in solution. In one set of experiments with the polar aprotic solvent acetonitrile-d(3), for the first time excited state intramolecular proton transfer (ESIPT), vibrational cooling/relaxation and rotational diffusion could be separated, and furthermore mid IR vibrational spectra of 3-HF excited states in solution phase were obtained. UV/Vis transient absorption data yield the time constant τ(Rot) = 22 ps for rotational diffusion and the time constant τ(VR) = 8.5 ps for vibrational cooling/relaxation in the tautomer excited state (S(1)'). Biphasic ESIPT with τ < 120 fs and τ = 2.4 ps as well as slow ground state recovery with τ > 500 ps was found. The time resolved mid IR data yield a time constant of ≈3.4 ps for the slow ESIPT step as well as the vibrational frequencies of S(0,) S(1)' and, in particular those of the short lived excited state S(1). Via quantum chemical calculations, structural parameters of these states are obtained. Various models were used, namely for the isolated molecule, aggregates with solvent as well as a polarizable continuum, that allow us to correlate the two ESIPT components with two mechanisms. Results are compared to those from previously published gas-phase experiments and indicate that the observed slow ESIPT is mediated by solute-solvent interaction via a hydrogen bond with the hydroxyl group of 3-HF.

A luminescent Pt2Fe spin crossover complex.

A heterotrinuclear [Pt2Fe] spin crossover (SCO) complex was developed and synthesized employing a ditopic bridging bpp-alkynyl ligand L and alkynyl coordinated PtII terpy units: [FeII(L-PtII)2]2(BF4)2 (1). We identified two different types of crystals of 1 which differ in their molecular packing and the number of co-crystallized solvent molecules: 1H (1·3.5CH2Cl2 in P1[combining macron]) and 1L (1·10CH2Cl2 in C2/c); while 1L shows a reversible SCO with a transition temperature of 268 K, the analogous compound 1H does not show any SCO and remains blocked in the HS state. The temperature-dependent magnetic properties of 1H and 1L were complementarily studied by Mössbauer spectroscopy. It has been shown that 1L performs thermal spin crossover and that 1L can be excited to a LIESST state. The vibrational properties of 1 were investigated by experimental nuclear resonance vibrational spectroscopy. The experimentally determined partial density of vibrational states (pDOS) was compared to a DFT-based simulation of the pDOS. The vibrational modes of the different components were assigned and visualized. In addition, the photophysical properties of 1 and L-Pt were investigated in the solid state and in solution. The ultrafast transient absorption spectroscopy of 1 in solution was carried out to study the PL quenching channel via energy transfer from photoexcited PtII terpy units to the FeII-moiety.