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Organic optoelectronics as a field has grown immensely over the last few decades currently offering flexible devices at low production cost and changing the future of consumer electronics. Despite success in OLED display and photovoltaic applications, organic materials have yet not found major applications in lasers due to degradation under strong excitation conditions and low electrical conductivity. In this case, organic crystals offer best combination of optical and electrical properties for organic lasers. However, strong intermolecular interactions in densely packed molecular crystals lead to formation of excitons (a quasiparticle corresponding to strongly bound charge pair created by electrical or optical excitation) that induce high losses due to reabsorption and exciton annihilation in organic gain material. This work presents a thorough study of excitonic effects for lasing in organic crystals based on bifluorene derivatives. In an effort to avoid the negative effects and achieve low lasing threshold in crystals two strategies were devised aiming to control excitonic coupling via modification of molecular and crystal structure. First strategy was based on modification of molecular rigidity, where flexible fragments allowing for stronger molecular vibrations were found to diminish the effects of excitonic coupling and thus improve lasing performance. Another strategy was based on doping of crystal structure with lower concentration of highly emissive molecules. Here, strong excitonic coupling in crystals comprised of rigid bifluorene derivatives was found to mediate extremely efficient energy transfer to dopant, which allowed to eliminate reabsorption and exciton annihilation. Both strategies were proven to be successful for achieving one of the lowest amplified spontaneous emission threshold values recorded for organic crystals, and hence proved the potential of bifluorene crystals for laser applications.

In this thesis we investigate electronic energy transfer and relaxation in molecular aggregates. The main focus of this work is the development and application of the stochastic time-dependent variational approach to open quantum systems. First of all, we derive the equations of motion for an electronic system interacting with vibrational degrees of freedom. Then we analyze the dynamics and regimes of excitonic polaron formation in a model system. The accuracy of the approach is benchmarked, comparing it to a widely used theoretical method. The stochastic time-dependent variational approach is then applied to model the dynamics of optical excitations and the ultrafast time resolved fluorescence spectra in the Light Harvesting Complex 2. Finally, the effect of nonlinear coupling between the molecular system and the vibrational modes on the energy transfer and optical spectra is demonstrated.

In this thesis we investigate electronic energy transfer and relaxation in molecular aggregates. The main focus of this work is the development and application of the stochastic time-dependent variational approach to open quantum systems. First of all, we derive the equations of motion for an electronic system interacting with vibrational degrees of freedom. Then we analyze the dynamics and regimes of excitonic polaron formation in a model system. The accuracy of the approach is benchmarked, comparing it to a widely used theoretical method. The stochastic time-dependent variational approach is then applied to model the dynamics of optical excitations and the ultrafast time resolved fluorescence spectra in the Light Harvesting Complex 2. Finally, the effect of nonlinear coupling between the molecular system and the vibrational modes on the energy transfer and optical spectra is demonstrated.

In organic molecular assemblies, photo-induced energy and charge transfer is accompanied by significant intramolecular nuclear rearrangements. Therefore, optical excitations are described as quasiparticles excitons. Coupling to vibrational degrees of freedom is often a dominating factor in the description of the exciton dynamics, therefore the vibronic exciton theory has to be used. In this thesis, the significance of dephasing and decoherence of vibronic excitons in molecular systems is investigated and the signatures of electronic-vibrational interaction in the two-dimensional electronic spectra are analyzed with the focus on evolution of molecular excitation and quantum coherences. It is shown, that (i) quantum coherences of electronic, vibrational or mixed nature can be discriminated through the analysis of coherent oscillations in the two-dimensional spectrum, (ii) the effect of static energetic disorder on the coherences depends on the nature of the states involved in the quantum superposition, (iii) for vibronically-coupled systems, the lifetime of excitonic coherences are determined by the coupling to discrete modes of intramolecular vibrations and by proximity of the system parameters to an excitonic--vibrational resonance, (iv) the electronic and vibronic energy level structure of the six-porphyrin nanoring is determined by the non-trivial combined effect of energetic disorder, vibronic coupling and small geometric deformations of the nanoring.

In organic molecular assemblies, photo-induced energy and charge transfer is accompanied by significant intramolecular nuclear rearrangements. Therefore, optical excitations are described as quasiparticles excitons. Coupling to vibrational degrees of freedom is often a dominating factor in the description of the exciton dynamics, therefore the vibronic exciton theory has to be used. In this thesis, the significance of dephasing and decoherence of vibronic excitons in molecular systems is investigated and the signatures of electronic-vibrational interaction in the two-dimensional electronic spectra are analyzed with the focus on evolution of molecular excitation and quantum coherences. It is shown, that (i) quantum coherences of electronic, vibrational or mixed nature can be discriminated through the analysis of coherent oscillations in the two-dimensional spectrum, (ii) the effect of static energetic disorder on the coherences depends on the nature of the states involved in the quantum superposition, (iii) for vibronically-coupled systems, the lifetime of excitonic coherences are determined by the coupling to discrete modes of intramolecular vibrations and by proximity of the system parameters to an excitonic--vibrational resonance, (iv) the electronic and vibronic energy level structure of the six-porphyrin nanoring is determined by the non-trivial combined effect of energetic disorder, vibronic coupling and small geometric deformations of the nanoring.

In the dissertation, excitation energy transfer was studied in an excitonic dimer under various conditions of inter-chromophore resonance interaction and in different regimes of the system-bath coupling. We examine the manifestation of quantum coherence in the energy transfer in molecular dimer, within the hierarchy of the relaxation processes: vibrational relaxation, energy redistribution within single exciton manifold and relaxation to the ground state. In order to capture the coherence effects in the limit of weak resonance interaction a novel technique has been presented. We are able to reproduce the coherence effects in the simulation of static and dynamic spectroscopic experiments. The study of an excitonic heterodimer under various system-bath coupling conditions revealed both coherent and incoherent excitation evolutions. In the situation of different reorganization energies, in the case of strong excitonic mixing, the vibrationally relaxed excited state energy levels may become swapped with respect to the monomeric counterparts. The study of the relaxation of the excitonic dimer to the ground state revealed that the resonant coupling strength and the energy gap between the states control the rate of the process but not the character. We have explained the dissipation mechanism in the artificial carotenoid-pthalocyanine dyads, and the results may be helpful in determining the mechanism of energy dissipation during photosynthesis, known as the non-photochemical... [to full text] Šioje disertacijoje nagrinėjama sužadinimo energijos pernaša eksitoniniame dimere, esant įvairaus stiprio rezonansinei sąveikai tarp chromoforų skirtinguose sistemos ir termostato sąveikos režimuose. Nagrinėjamas kvantinio koherentiškumo vaidmuo energijos pernašos procesų hierarchijoje: virpesinėje relaksacijoje, sužadinimo pernašoje vieneksitoninėje juostoje bei relaksacijoje į pagrindinę būseną. Koherentiniams efektams silpnose rezonansinės sąveikos riboje aprašyti buvo išplėtotas naujas metodas. Juo remiantis modeliuojami koherentiškumo nulemti efektai statinės ir dinaminės spektroskopijos eksperimentuose. Tiriant eksitoninį dimerą įvariuose sistemos ir termostato sąveikos režimuose stebima tiek koherentinė, tiek nekoherentinė sužadinimo evoliucija. Nustatyta, jog esant skirtingoms chromoforų reorganizacijos energijoms ir stipriam eksitoniniam maišymuisi, eksitoninių būsenų potencinės energijos paviršių minimumai gali susikeisti vietomis lygininat su nesąveikaujančių chromoforų minimumais. Relaksacijos į pagrindinę būseną eksitoniniame dimere tyrimas atskleidė, jog rezonansinė sąveika ir būsenų energijų skirtumas daugiausia turi įtakos procesų spartai, bet ne pobūdžiui. Darbe paaiškintas disipacijos mechanizmas sintetinėse karotenoido-ftalocianino diadose; gauti rezultatai naudingi aiškinant apsauginį fotosintezės mechanizmą, vadinamą nefotocheminiu gesimu.

Disertacijos santraukoje reziumuojamas sužadinimo energijos pernašos eksitoniniame dimere tyrimas. Nagrinėjamas kvantinio koherentiškumo vaidmuo energijos pernašos procesų hierarchijoje: virpesinėje relaksacijoje, sužadinimo pernašoje vieneksitoninėje juostoje bei relaksacijoje į pagrindinę būseną. Glaustai aprašomas naujas metodas, išplėtotas koherentiniams efektams silpnos rezonansinės sąveikos riboje aprašyti. Pristatomas eksitoninio dimero tyrimas įvariuose sistemos ir termostato sąveikos režimuose, kuomet stebima tiek koherentinė, tiek nekoherentinė sužadinimo evoliucija. Nustatyta, jog esant skirtingoms chromoforų reorganizacijos energijoms ir stipriam eksitoniniam maišymuisi, eksitoninių būsenų potencinės energijos paviršių minimumai gali susikeisti vietomis lygininat su nesąveikaujančių chromoforų minimumais. Pristatomas relaksacijos į pagrindinę būseną eksitoniniame dimere tyrimas, kuris atskleidė, jog rezonansinė sąveika ir būsenų energijų skirtumas daugiausia turi įtakos procesų spartai, bet ne pobūdžiui. Darbe paaiškintas disipacijos mechanizmas sintetinėse karotenoido-ftalocianino diadose; gauti rezultatai naudingi aiškinant apsauginį fotosintezės mechanizmą, vadinamą nefotocheminiu gesimu. The summary of doctoral dissertation presents the investigation of excitation energy transfer, studied in an excitonic dimer under various conditions of inter-chromophore resonance interaction and in different regimes of the system-bath coupling. The manifestation of quantum coherence in the energy transfer in molecular dimer is considered within the hierarchy of the relaxation processes: vibrational relaxation, energy redistribution within single exciton manifold and relaxation to the ground state. A new method to capture the coherence effects in the limit of weak resonance interaction is presented. The study of an excitonic heterodimer under various system-bath coupling conditions, which revealed both coherent and incoherent excitation evolutions, is summarized. An outline of the study of the relaxation of the excitonic dimer to the ground state is given, which revealed that the resonance coupling strength and the energy gap between the states control the rate of the process but not the character. The dissipation mechanism in the artificial carotenoid-pthalocyanine dyads is explained, and the results may be helpful in determining the mechanism of energy dissipation during photosynthesis, known as the non-photochemical quenching.