Mechanism of the Intramolecular Charge Transfer State Formation in <i>all-trans</i>-β-Apo-8′-carotenal: Influence of Solvent Polarity and Polarizability

In this work we analyzed the infrared and visible transient absorption spectra of <i>all-trans</i>-β-apo-8′-carotenal in several solvents, differing in both polarity and polarizability at different excitation wavelengths. We correlate the solvent dependence of the kinetics and the band shape changes in the infrared with that of the excited state absorption bands in the visible, and we show that the information obtained in the two spectral regions is complementary. All the collected time-resolved data can be interpreted in the frame of a recently proposed relaxation scheme, according to which the major contributor to the intramolecular charge transfer (ICT) state is the bright 1B_u⁺ state, which, in polar solvents, is dynamically stabilized through molecular distortions and solvent relaxation. A careful investigation of the solvent effects on the visible and infrared excited state bands demonstrates that both solvent polarity and polarizability have to be considered in order to rationalize the excited state relaxation of <i>trans</i>-8′-apo-β-carotenal and clarify the role and the nature of the ICT state in this molecule. The experimental observations reported in this work can be interpreted by considering that at the Franck–Condon geometry the wave functions of the S₁ and S₂ excited states have a mixed ionic/covalent character. The degree of mixing depends on solvent polarity, but it can be dynamically modified by the effect of polarizability. Finally, the effect of different excitation wavelengths on the kinetics and spectral dynamics can be interpreted in terms of photoselection of a subpopulation of partially distorted molecules

Valence Tautomerism in Co–Dioxolene Complexes: Static and Time-Resolved Infrared Spectroscopy Study

In this work, we studied the valence tautomerism process on two different Co–dioxolene complexes by means of transient infrared spectroscopy (TRIR). The molecules investigated are <i>ls</i>-Co^III(Cat-N-BQ)­(Cat-N-SQ) (<b>DQ</b>_<b>2</b>) and [<i>ls</i>-Co^III(tpy)­(Cat-N-SQ)]­PF₆ (<b>tpy</b>), where Cat-NBQ = 2-(2-hydroxy-3,5-ditert-butylphenyl-imino)-4,6-ditert-butylcyclohexa-3,5-dienone, Cat-N-SQ is the dianionic radical analogue, and tpy = 2,2′-6-2″-terpyridine. DFT calculations of the harmonic frequencies for the two complexes allow us to pinpoint the normal modes to be used as markers of the semiquinonate and benzoquinonate isomers. The photoinduced one-electron charge transfer process from the radical semiquinonate ligand to the metal center leads to a <i>ls</i>-Co^II(<i>x</i>)­(Cat-N-BQ) electronic state (where <i>x</i> is the other ligand). Following this first step, an ultrafast ISC process (τ < 200 fs) takes places, yielding the benzoquinonate isomer (<i>hs</i>-Co^II(<i>x</i>)­(Cat-N-BQ)). In the experiments, we employed different excitation wavelengths on resonance with different absorption bands of the two samples. Excitation in the ligand-to-metal charge transfer (LMCT) band at ∼520 nm and in the semiquinonate band at ∼1000 nm induces the valence tautomerism (VT) in both samples. From the time evolution of the TRIR spectra, we determine the time constants of the vibrational cooling in the tautomeric state (7–14 ps) and the ground state recovery times (∼350 ps for <b>tpy</b> and ∼450 ps for <b>DQ</b>_<b>2</b>). In contrast, when the pump frequency is set at 712 nm, on resonance with the benzoquinonate absorption band of the second active ligand of the <b>DQ</b>_<b>2</b>, no electron transfer takes place: the TRIR spectra basically show only ground state bleaching bands and no marker band of the tautomeric conversion shows up

A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating molecular structure and ISC property, will be useful for devising new strategies to induce ISC in heavy atom-free organic chromophores

Combination of Transient 2D-IR Experiments and Ab Initio Computations Sheds Light on the Formation of the Charge-Transfer State in Photoexcited Carbonyl Carotenoids

The excited state dynamics of carbonyl carotenoids is very complex because of the coupling of single- and doubly excited states and the possible involvement of intramolecular charge-transfer (ICT) states. In this contribution we employ ultrafast infrared spectroscopy and theoretical computations to investigate the relaxation dynamics of <i>trans</i>-8′-apo-β-carotenal occurring on the picosecond time scale, after excitation in the S₂ state. In a (slightly) polar solvent like chloroform, one-dimensional (T1D-IR) and two-dimensional (T2D-IR) transient infrared spectroscopy reveal spectral components with characteristic frequencies and lifetimes that are not observed in nonpolar solvents (cyclohexane). Combining experimental evidence with an analysis of CASPT2//CASSCF ground and excited state minima and energy profiles, complemented with TDDFT calculations in gas phase and in solvent, we propose a photochemical decay mechanism for this system where only the bright single-excited 1B_u⁺ and the dark double-excited 2A_g^– states are involved. Specifically, the initially populated 1B_u⁺ relaxes toward 2A_g^– in 200 fs. In a nonpolar solvent 2A_g^– decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1B_u⁺ state which then quickly relaxes to the GS (18 ps in chloroform). The 1B_u⁺ state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2A_g^– → 1B_u⁺ population transfer is evidenced by a cross peak on the T2D-IR map revealing that the motions along the same stretching of the conjugated chain on the 2A_g^– and 1B_u⁺ states are coupled

Combined Experimental and Theoretical Study of Efficient and Ultrafast Energy Transfer in a Molecular Dyad

We have characterized the dynamics and the efficiency of electronic energy transfer (EET) in a newly synthesized molecular dyad, composed of a styryl-pyridinium donor and a BODIPY acceptor. The kinetics of the process has been studied with femtosecond transient absorption spectroscopy in different solvents. In all the analyzed media EET is quantitative and very fast, as we find that almost 70% of the overall excitation energy is transferred from the donor to the acceptor on a subpicosecond time scale. The experimental measurements have been supported by a theoretical analysis; the electronic couplings between the donor and acceptor moieties have been calculated at the (TD)­DFT level and complemented by a conformational analysis of the full dyad. The computed energy transfer times are in good agreement with the experimental values; this allowed us to verify the correctness of the Förster equation, demonstrating that, although EET in the examined system occurs on an ultrafast time scale, the approximations introduced in the case of the weak coupling regime remain valid

Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet–Triplet Annihilation Upconversion

The triplet excited state properties of two BODIPY phenothiazine dyads (<b>BDP-1</b> and <b>BDP-2</b>) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). <b>BDP-1</b> has a longer linker between the phenothiazine and the BODIPY chromophore than <b>BDP-2</b>. Moreover, the two chromophores in <b>BDP-2</b> assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of <b>BDP-1</b> (34–40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (<i>k</i>_CS = 2.2 × 10¹¹ s^–1 and 2 × 10¹² s^–1 for <b>BDP-1</b> and <b>BDP-2</b>, respectively; <i>k</i>_CR = 4.5 × 10¹⁰ and 1.5 × 10¹¹ s^–1 for <b>BDP-1</b> and <b>BDP-2</b>, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends on both the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4 and 97.5% and lifetimes are 13 and 116 μs for <b>BDP-1</b> and <b>BDP-2</b>, respectively. The spin–orbit charge transfer (SO-CT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet–triplet annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%

Role of Local Structure and Dynamics of Small Ligand Migration in Proteins: A Study of a Mutated Truncated Hemoprotein from <i>Thermobifida fusca</i> by Time Resolved MIR Spectroscopy

Carbon monoxide recombination dynamics in a mutant of the truncated hemoglobin from <i>Thermobida fusca</i> (3F-<i>Tf</i>-trHb) has been analyzed by means of ultrafast Visible-pump/MidIR-probe spectroscopy and compared with that of the wild-type protein. In 3F-<i>Tf</i>-trHb, three topologically relevant amino acids, responsible for the ligand stabilization through the formation of a H-bond network (TyrB10 TyrCD1 and TrpG8), have been replaced by Phe residues. X-ray diffraction data show that Phe residues in positions B10 and G8 maintain the same rotameric arrangements as Tyr and Trp in the wild-type protein, while Phe in position CD1 displays significant rotameric heterogeneity. Photodissociation of the ligand has been induced by exciting the sample with 550 nm pump pulses and the CO rebinding has been monitored in two mid-IR regions respectively corresponding to the ν­(CO) stretching vibration of the iron-bound CO (1880–1980 cm^–1) and of the dissociated free CO (2050–2200 cm^–1). In both the mutant and wild-type protein, a significant amount of geminate CO rebinding is observed on a subnanosecond time scale. Despite the absence of the distal pocket hydrogen-bonding network, the kinetics of geminate rebinding in 3F<i>-Tf</i>-trHb is very similar to the wild-type, showing how the reactivity of dissociated CO toward the heme is primarily regulated by the effective volume and flexibility of the distal pocket and by caging effects exerted on the free CO on the analyzed time scale

Efficient Photoinduced Charge Separation in a BODIPY–C₆₀ Dyad

A donor–acceptor dyad composed of a BF₂-chelated dipyrromethene (BODIPY) and a C₆₀ fullerene has been newly synthesized and characterized. The two moieties are linked by direct addition of an azido substituted BODIPY on the C₆₀, producing an imino–fullerene–BODIPY adduct. The photoinduced charge transfer process in this system was studied by ultrafast transient absorption spectroscopy. Electron transfer toward the fullerene was found to occur selectively exciting both the BODIPY chromophore at 475 nm and the C₆₀ unit at 266 nm on a time scale of a few picoseconds, but the dynamics of charge separation was different in the two cases. Eletrochemical studies provided information on the redox potentials of the involved species and spectroelectrochemical measurements allowed to unambiguously assign the absorption band of the oxidized BODIPY moiety, which helped in the interpretation of the transient absorption spectra. The experimental studies were complemented by a theoretical analysis based on DFT computations of the excited state energies of the two components and their electronic couplings, which allowed identification of the charge transfer mechanism and rationalization of the different kinetic behavior observed by changing the excitation conditions

Tailoring Photoisomerization Pathways in Donor–Acceptor Stenhouse Adducts: The Role of the Hydroxy Group

Donor–acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activatable negative photochromes. They are closely related to (mero)­cyanine dyes with the sole difference being a hydroxy group in the polyene chain. The presence or absence of the hydroxy group has far-reaching consequences for the photochemistry of the compound: cyanine dyes are widely used as fluorescent probes, whereas DASAs hold great promise for visible light-triggered photoswitching. Here we analyze the photophysical properties of a DASA lacking the hydroxy group. Ultrafast time-resolved pump–probe spectroscopy in both the visible and IR region show the occurrence of <i>E–Z</i> photoisomerization on a 20 ps time scale, similar to the photochemical behavior of DASAs, but on a slower time scale. In contrast to the parent DASA compounds, where the initial photoisomerization is constrained to a single position (next to the hydroxy group), ¹H NMR <i>in situ</i>-irradiation studies at 213 K reveal that for nonhydroxy DASAs <i>E–Z</i> photoisomerization can take place at two different bonds, yielding two distinct isomers. These observations are supported by TD-DFT calculations, showing that in the excited state the hydroxy group (pre)­selects the neighboring C₂–C₃ bond for isomerization. The TD-DFT analysis also explains the larger solvatochromic shift observed for the parent DASAs as compared to the nonhydroxy analogue, in terms of the dipole moment changes evoked upon excitation. Furthermore, computations provide helpful insights into the photoswitching energetics, indicating that without the hydroxy group the 4π-electrocyclization step is energetically forbidden. Our results establish the central role of the hydroxy group for DASA photoswitching and suggest that its introduction allows for tailoring photoisomerization pathways, presumably both through (steric) fixation via a hydrogen bond with the adjacent carbonyl group of the acceptor moiety, as well as through electronic effects on the polyene backbone. These insights are essential for the rational design of novel, improved DASA photoswitches and for a better understanding of the properties of both DASAs and cyanine dyes