Miroslav Menšík

Roles of octabutoxy substitution and J-aggregation in stabilization of the excited state in nickel phthalocyanine.

Nickel phthalocyanine (NiPc) complexes are known to show a rapid nonradiative deactivation of the photoexcited state through the internal conversion. This could be exploited in practical applications, such as photoprotection and photodynamic therapy. The butoxy substitution of NiPc plays an important role for drug delivery but also greatly influences its photophysics. We prepared novel peripherally substituted 2,3,9,10,16,17,23,24-octabutoxy nickel(II) phthalocyanine and characterized the deactivation pathway of its photoexcited state in solution by femtosecond transient absorption spectroscopy and quantum chemical calculations. We bring experimental evidence for the kinetic model, in which the photoexcitation evolves in two independent branches. In the first branch, assigned to the monomer, it undergoes ultrafast intersystem crossing to a triplet state, which subsequently decays to the ground state through a pathway involving lower-lying triplet states, with a ground-state recovery lifetime of 814 ps. It is about three-times longer than the lifetime published for unsubstituted NiPc. In the second branch, the photoexcitation decayed to a triplet state with an orders of magnitude longer lifetime, with the quantum yield of about 4%. This state showed spectral features of J-aggregates. These findings are important for the applications that rely on singlet oxygen formation or fast nonradiative deactivation of the excited state.

Time-Resolved Transient Optical Absorption Study of Bis(terpyridyl)oligothiophenes and Their Metallo-Supramolecular Polymers with Zn(II) Ion Couplers

α,ω-Bis(terpyridyl)oligothiophenes spontaneously assemble with Zn(II) ions giving conjugated constitutional dynamic polymers (dynamers) of the metallo-supramolecular class, which potentially might be utilized in optoelectronics. Their photophysical properties, which are of great importance in this field of application, are strongly influenced by the dynamic morphology. It was assessed in this study by using ultrafast pump-probe optical absorption spectroscopy. We identified and characterized relaxation processes running in photoexcited molecules of these oligomers and dynamers and show impacts of disturbed coplanarity of adjacent rings (twisting the thiophene-thiophene and thiophene-terpyridyl bonds by attached hexyl side groups) and Zn(II) ion couplers on these processes. Major effects are seen in the time constants of rotational relaxation, intersystem crossing, and de-excitation lifetimes. The photoexcited states formed on different repeating units within the same dynamer chain do not interact with each other even at very high excitation density. The method is presented that allows determining the equilibrium fraction of unbound oligothiophene species in a dynamer solution, from which otherwise hardly accessible values of the average degree of polymerization of constitutionally dynamic chains in solution can be estimated.

Singlet fission in thin films of metallo-supramolecular polymers with ditopic thiophene-bridged terpyridine ligands

The singlet fission (SF) phenomenon is currently investigated for its potential to overcome the Shockley–Queisser energy conversion efficiency limit of single-junction photovoltaic (PV) cells. One of the hurdles of using SF for PV cells is the limited choice among the presently available SF materials. We compare the photophysical behavior of thin solid films of two novel compounds: a ditopic ligand bis(terpyridine-4′-yl)terthiophene (T) and a metallo-supramolecular polymer (MSP), prepared by the coordination of T to Zn2+ ions (PT). The transient absorption kinetics in PT after photoexcitation into its second electronic excited state was consistent with an ultrafast (SF) process with a time constant of 160 fs. The lifetime of such a formed triplet state reached only 1 ns, due to mutual bimolecular annihilation, which also hindered the determination of the yield of the triplet state formation. In contrast, the metal-free thin films of T showed no signs of SF, but rather profound exciton relaxation through an excimer formation. Powder WAXS diffractograms pointed out a lower degree of structural order in the PT than that in the T – which may be the probable reason for differences in energy relaxation pathways of these two materials. To the best of our knowledge, singlet fission has not yet been reported in the MSP class of materials.

Optical line width in semiconductor quantum dots

The line-width of the optical transitions in quantum dots is studied theoretically on the basis of the electron coupling to the longitudinal optical phonons in polar semiconductors. With using the self-consistent Born approximation to the electronic self-energy, we are able to reproduce one of the main experimental results obtained on CdSe and CuBr quantum dots, namely the linear dependence of the width of the optical line on the inverse of the quantum dot diameter. In addition to it, the theory allows to expect certain resonance features on the linear dependence of line width. We remind that perhaps extensions of the present line-width theory might be suitable for to describe adequately the behavior of the line width in CdSe and InAs quantum dots.

Phonon induced exciton decay

Abstract The time dependence of the exciton decay induced by the single vibrational mode was calculated. Neither exponential nor polynomial time decay was found. Three consecutive time intervals appeared. The competing character of the exciton-vibrational and vibration-bath coupling processes is discussed.

Analysis of collision-controlled time dependence of diffusion coefficient of polaron pairs from transient absorption spectra of conducting polymers

We derived relations for time-dependent diffusion coefficient of photoexcited species determined from transient absorption spectra for various types of collision processes controlling decay of photoexcited species of conducting polymers. The method was applied for time-resolved experimental data of polaron pairs decay in 10-nm-thick thin films of regioregular poly(3-hexyl-thiophene).Graphical abstract

Discrete dissociation model of photogenerated inter-chain charge transfer states in external electric field

A new theoretical model of photogeneration yield of free charge carriers on the electric field in π-conjugated polymers was introduced. It generalizes the model of Arkhipov et al. (Chem Phys Lett 372:886–892, 2003a) where the dissociation of the inter-chain charge transfer states is controlled by a thermally activated transition from the lowest quantum state over a potential barrier. Contrary to the previous model based on the effective mass approximation, the new model determines the potential and transfer integrals between HOMO orbitals, using quantum chemical methods. The model successfully explained experimental dependences of photogenerated charge yield in poly{1-[4-(trimethylsilyl)phenyl]-2-phenylacetylene} exhibiting a strong sensitivity on electric field strength over a very large interval. This indicates the model applicability for estimation of fill factor in solar cells. The model was found stable toward various distributions of charge transfer states (on-chain delocalization, charge transfer to side groups, helix structure, etc.) and tunneling effects behind potential barrier.Graphical abstract

Electric field dependence of charge mobility in linear conjugated polymers

A combined quantum mechanical and semi-classical approach was used for the description of the charge carrier transport, taking into account realistic microscopic lamellar structure present in many common conjugated polymers. Unlike previous theoretical models that consider a polymer as a 3D point lattice of single-state sites or model the polymer chain as a molecule with single charge state, our molecular-scale model takes into account the density of states of conjugated polymer chain segments determined using a quantum–mechanical tight-binding model. Because of the relatively fast charge carrier delocalization on the conjugated polymer chain segments, the short-distance on-chain motion is separated from the slower inter-chain hopping. Inter-chain hopping rates, described by means of the Marcus theory, are calculated self-consistently with the chain segment occupation. The present model describes the electric field dependence of the hole mobility in conjugated polymers and includes the influence of the transverse electric field, which is important for simulation of the gate-voltage dependences of the charge carrier transport in organic field effect transistors.Graphical Abstract

Charge-carrier/exciton transfer between two quasi-zero-dimensional nanostructures

Quantum transport theory is applied to obtain formula for the irreversible transfer of electronic excited state (exciton), or a charge carrier, between two quasi-zero-dimensional nanostructures. The electron-phonon interaction is included in a non-perturbative way. The formula is expected to be suitable for analysis of experimental data on exciton or charge transfer. We emphasize the use of the transfer formula in the problem of the electrical conduction of polymers. Menšík.

Quantum dots with indirect band gap: power-law photoluminescence decay

In this work the experimental effect of a slow decay of the photoluminescence is studied theoretically in the case of quantum dots with an indirect energy band gap. The slow decay of the photoluminescence is considered as decay in time of the luminescence intensity, following the excitation of the quantum dot sample electronic system by a short optical pulse. In the presented theoretical treatment the process is studied as a single dot property. The inter-valley deformation potential interaction of the excited conduction band electrons with lattice vibrations is considered in the self-consistent Born approximation to the electronic self-energy. The theory is built on the non-equilibrium electronic quantum transport theory. The time dependence of the photoluminescence decay is estimated upon using a simple effective mass model. The numerical calculation of the considered model shows the power-law time characteristics of the photoluminescence decay in the long-time limit of the decay. We demonstrate that th...