@phdthesis{Selby2022,
  author    = {Selby, Joshua},
  title     = {Design and Chiroptical Properties of Chirally Substituted Indolenine Squaraine Mono-, Oligo-, and Polymers},
  doi       = {10.25972/OPUS-28206},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-282067},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {A series of monomeric chirally substituted indolenine squaraine monomers were successfully synthesized and utilized for the construction of various oligo- and polymers, in order to study their chiroptical properties in terms of exciton chirality. The quaternary carbon atom at the 3-position of the indolenine subunit, as well as the alkyl side chain attached to the indolenine nitrogen were selected as the most suitable site for chiral functionalization. For the C(3)-chiral derivatives, two synthetic routes depending on the desired substitution at the stereogenic center were established. The chiral side chains were prepared via Evans asymmetric alkylation where the resulting branching point at the 2 position constituted the chiral center. While the chiral substitution only had minor effects on the linear optical properties and geometric structure of the chromophore, all compounds exhibited a distinct and measurable CD signal that correlated with the distance of the chiral center to the central chromophore. Polymers bearing chiral side chains exhibited a solvent- and temperature-dependent helix-coil equilibrium, which was influenced by the type of side chain used. CD spectroscopy revealed the helical conformation to possess a preferred twist sense, and temperature-dependent measurements showed the degree of homohelicity to be nearly complete in certain cases. Furthermore, a CPL signal was able to be obtained for the helical conformer of one polymer. Various (co)oligo- and polymers comprising the C(3)-chiral monomers only displayed a solvent-independent J-type absorption behavior and thus did not form helical conformations in solution. CD spectroscopy revealed a solvent-dependent adoption of quasi-enantiomeric conformers, which was elucidated by quantum chemical TDDFT calculations.},
  subject      = {Squaraine},
  language  = {en}
}
@phdthesis{Shen2021,
  author    = {Shen, Chia-An},
  title     = {Dicyanomethylene Squaraines: Aggregation and G-Quadruplex Complexation},
  doi       = {10.25972/OPUS-24359},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-243599},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Squaraine dyes have attracted more attention in the past decade due to their strong and narrow absorption and fluorescence along with the easily functionalized molecular structure. One successful approach of core functionalization is to replace one oxygen of the squaric carbonyl group with a dicyanomethylene group, which shifts the absorption and emission into the near infrared (NIR) region and at the same time leads to a rigid, planar structure with C2v symmetry. However, such squaraines tend to aggregate cofacially in solution due to dispersion forces and dipole-dipole interactions, usually leading to H-type exciton coupling with undesired blue-shifted spectrum and quenched fluorescence. Therefore, the goal of my research was the design of dicyanomethylene-substituted squaraine dyes that self-assemble into extended aggregates in solution with J-type coupling, in order to retain or even enhance their outstanding optical properties. Toward this goal, bis(squaraine) dyes were envisioned with two squaraine units covalently linked to trigger a slip-stacked packing motif within the aggregates to enable J-type coupling. In my first project, bis(squaraine) dye BisSQ1 was synthesized, in which two dicyanomethylene squaraine chromophores are covalently linked. Concentration and temperature-dependent UV/Vis/NIR spectroscopy experiments reveal that BisSQ1 undergoes cooperative self-assembly resulting in J-type aggregates in a solvent mixture of toluene/1,1,2,2-tetrachloroethane (TCE) (98:2, v/v). The J type exciton coupling is evident from the significantly red shifted absorption maximum at 886 nm and the fluorescence peak at 904 nm. In conclusion, this was a first example to direct squaraine dye aggregation in solution to the more desired slip-stacked packing leading to J-type exciton coupling by simply connecting two dyes in a head-to-tail bis chromophore structure. Connecting two squaraine dyes with an additional phenylene spacer (BisSQ2) leads to two different polymorphs with very distinct absorption spectra upon cooling down a solution of BisSQ2 in a solvent mixture of toluene/TCE (98:2, v/v) with different rates. Accordingly, rapid cooling resulted in rigid helical nanorods with an absorption spectrum showing a panchromatic feature, while slow cooling led to a sheet-like structure with a significant bathochromic shift in the absorption spectrum. It was discovered that the conventional molecular exciton model failed to explain the panchromatic absorption features of the nanorods for the given packing arrangement, therefore more profound theoretical investigations based on the Essential States Model (ESM) were applied to unveil the importance of intermolecular charge transfer (ICT) to adequately describe the panchromatic absorption spectrum. Moreover, the red-shift observed in the spectrum for the sheet-like structure can be assigned to the interplay of Coulomb coupling and ICT-mediated coupling. Furthermore, the same bis-chromophore strategy was adopted for constructing an NIR-II emitter with a bathochromically-shifted spectrum. In chloroform, BisSQ3 exhibits an absorption maximum at 961 nm with a significant bathochromic shift (1020 cm-1) compared to the reference mono-squaraine SQ, indicating intramolecular J-type coupling via head-to-tail arrangement of two squaraine dyes. Moreover, BisSQ3 shows a fluorescence peak at 971 nm with a decent quantum yield of 0.33\%. In less polar toluene, BisSQ3 self-assembles into nanofibers with additional intermolecular J-type coupling, causing a pronounced bathochromic shift with absorption maximum at 1095 nm and a fluorescence peak at 1116 nm. Thus, connecting two quinoline-based squaraines in a head-to-tail fashion leads to not only intra-, but also intermolecular J-type exciton coupling, which serves as a promising strategy to shift the absorption and emission of organic fluorophores into the NIR-II window while retaining decent quantum yields. In conclusion, my research illustrates based on squaraine dyes how a simple modification of the molecular structure can significantly affect the aggregation behavior and further alter the optical properties of dye aggregates. Elongated supramolecular structures based on dicyanomethylene substituted squaraine dyes were successfully established by covalently linking two squaraine units to form a bis-chromophore structure. Then, a simple but efficient general approach was established to direct squaraine dye aggregation in solution to the more desired slip-stacked packing leading to J-type exciton coupling by directly connecting two squaraine dyes in a head-to-tail fashion without spacer units. Moreover, the additional spacer between the squaraine dyes in BisSQ2 allowed different molecular conformations, which leads to two different morphologies depending on the cooling rates for a hot solution. Hence, this is a promising strategy to realize supramolecular polymorphism. In general, it is expected that the concept of constructing J-aggregates by the bis-chromophore approach can be extended to entirely different classes of dyes since J-aggregates possess a variety of features such as spectral shifts into the NIR window, fluorescence enhancement, and light harvesting, which are commonly observed and utilized for numerous fundamental studies and applications. Moreover, the insights on short-range charge transfer coupling for squaraine dyes is considered of relevance for all materials based on alternating donor-acceptor π-systems. The panchromatic spectral feature is in particular crucial for acceptor-donor-acceptor (ADA) dyes, which are currently considered as very promising materials for the development of bulk heterojunction solar cells.},
  subject      = {Squaraine},
  language  = {en}
}
@phdthesis{Schreck2018,
  author    = {Schreck, Maximilian},
  title     = {Synthesis and Photophysics of Linear and Star-Shaped Oligomers of Squaraine Dyes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-174272},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {In this thesis, the synthesis and photophysics of a great variety of squaraine dyes are presented. This variety is based on four parent squaraines containing either indolenine or quinoline heterocycles. By a suitable choice of the donor and acceptor unit, the optical properties can already be adapted to the properties desired on the stage of the monomer. To promote a further derivatisation of these dyes, diverse functional groups are attached to the monomers using transition metal-catalysed C-C coupling reactions. However, this has to be preceded by the synthesis of bromine-functionalised derivatives as a direct halogenation of squaraine dyes is not feasible. Therefore, the halogen function is already introduced in precursor molecules giving rise to a molecular building block system containing bromine-, boronic ester-, and alkyne-functionalised monomer units, which pave the way to a plethora of squaraine oligomers and polymers. The indolenine homopolymer pSQB-1 as well as the corresponding small molecular weight oligomers dSQB-1 and tSQB were synthesized applying Ni-mediated Yamamoto and Pd-catalysed Suzuki coupling methodologies, respectively. The motivation for this project relied on the fundamental investigations by V{\"o}lker et al. on pSQB-V. A progressive red-shift of the lowest energy absorption maximum from the dimer to the polymer was observed in CHCl3 compared to the monomer. With increasing number of monomer units, the exciton coupling decreases from the dimer to the polymer. In addition, the shape of the absorption band manifold shows a strong dependence on the solvent, which was also observed by V{\"o}lker et al. J-type aggregate behavior is found in chlorinated solvents such as CHCl3 and DCM, whereas H-type aggregates are formed in acetone. Temperature-dependent absorption studies in PhCN reveals a reversible equilibrium of diverse polymer conformers, which manifests itself in a gradual change from H-aggregate behavior to a mixture with a more pronounced J-aggregate behavior upon raising the temperature. It isassumed that both characteristic aggregate bands correlate in borderline cases with two polymer structures which can be assigned to a zig-zag and a helical structure. As no experimental evidence for these structures could hitherto be provided by NMR, TD-DFT computations on oligomers (22-mers) can reproduce very closely the characteristic features of the spectra for the two conformational isomers. The subsequent chapters are motivated by the goal to influence the optical properties through a control of the superstructure and thus of the intramolecular aggregate formation. On the one hand, bulky groups are implemented in the 3-position of the indolenine scaffold to provoke steric repulsion and thus favoring J-aggregate behavior at the expense of helical arrangements. The resulting homopolymer pDiPhSQB bearing two phenyl groups per indolenine exhibits J-type aggregate behavior with red-shifted absorption maxima in all considered solvents which is explained to be caused by the formation of elongated zig-zag structures. Furthermore, single-crystal X-ray analysis of monomer DiPhSQB-2-Br2 reveals a torsion of the indolenine moieties as a consequence of steric congestion. The twist of the molecular geometry and the resulting loss of planarity leads to a serious deterioration of the fluorescence properties, however a significant bathochromic shift of ca. 1 200 cm-1 of the lowest absorption band was observed compared to parent SQB, which is even larger than the shift for dSQB-1 (ca. 1 000 cm-1). On the other hand, a partial stiffening of the polymer backbone is attempted to create a bias for elongated polymer chains. In this respect, the synthetic approach is to replace every second biarylaxis with the rigid transoid benzodipyrrolenine unit. Despite a rather low average degree of polymerization < 10, exclusively red-shifted absorption maxima are observed in all solvents used. In order to complete the picture of intramolecular aggregates through the selective design of H-aggregates, a squaraine-squaraine copolymer was synthesised containing the classic cisoid indolenine as well as the cisoid quinoline building block. Taking advantage of the highly structure directing self-assembly character of the quinoline moiety, the copolymer pSQBC indeed showes a broad, blue-shifted main absorption band in comparison with the monomer unit dSQBC. The shape of the absorption band manifold solely exhibited a minor solvent and temperature dependence indicating a persistent H-aggregate behaviour. Hence, as a proof of concept, it is shown that the optical properties of the polymers (H- and J-aggregate) and the corresponding superstructure can be inherently controlled by an adequate design of monomer precursors. The last chapter of this work deals, in contrast to all other chapters, with intermolecular aggregates. It is shown that the two star-shaped hexasquarainyl benzenes hSQA-1 and hSQA-2 exhibit a strong propensity for self-organisation. Concentration- and temperature-dependent studies reveal a great driving force for self-assembly in acetone. While the larger hSQA-2 instantaneously forms stable aggregates, the aggregates of hSQA-1 shows a pronounced kinetic stability. Taking advantage of the kinetic persistency of these aggregates, the corresponding kinetic activation parameters for aggregation and deaggregation can be assessed. The absorption spectra of both hexasquarainyl benzenes in the aggregated state reveal some striking differences. While hSQA-1 features an intensive, very narrow and blue-shifted absorption band, two red-shifted bands are observed for hSQA-2, which are closely located at the monomer absorption. The very small bandwidth of hSQA-1 are interpreted to be caused by exchange narrowing and pointed towards highly ordered supramolecular aggregates. The concentration-dependent data of the two hexasquarainyl benzenes can be fitted to the dimer-model with excellent correlation coefficients, yielding binding constants in excess of 10^6 M-1, respectively. Such high binding constants are very surprising, considering the unfavourable bulky 3,3-dimethyl groups of the indolenine units which should rather prevent aggregation. Joint theoretical and NMR spectroscopic methods were applied to unravel the supramolecular aggregate structure of hSQA-1, which is shown to consist of two stacked hexasquarainyl benzenes resembling the picture of two stacked bowls.},
  subject      = {Squaraine},
  language  = {en}
}
@phdthesis{Auerhammer2018,
  author    = {Auerhammer, Nina A.},
  title     = {Energy Transfer and Excitonic Interactions in Conjugated Chromophore Arrangements of Bodipys and Pyrenes and Squaraines},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166721},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {In this work the energy transfer and excitonic coupling in different chromophore arrangements were investigated. A difference in the coupling strength was introduced by varring the connecting unit and the spacial orientation relative to each other. The synthesis of the 2,7-substituted pyrene compounds could be optimised and good yields of HAB 1 and HAB 2 and small amounts of HAB 2 could be achieved by cobalt-catalysed trimerisation or Diels Alder reaction in the end. Absorption and fluorescence spectra reveal strong intramolecular interactions between the pyrene molecules in the HAB 1. Excitation spectra recorded at the high and low energy fluorescence suggest the contribution of two components to the spectra. One being similar to the ground state aggregate and a second species similar to undisturbed pyrene. All these feature can be accounted to two different fluorescent states which are due to electronical decoupling in the excited state. Due to the strong intramolecular coupling already in the ground state of the molecule, no energy transfer could be studied, as the six pyrene units cannot be seen as separate spectroscopic entities between which energy could be transferred. In the second part of this thesis dye conjugates of different size and alignment were synthesised to study the interaction of the transition-dipole moments. Therefore a systematic investigation of Sonogashira conditions was performed in order to obtain good yields of the desired compounds and keep dehalogenation at a minimum level. Nevertheless only the symmetrical triads could be purified as the asymmeric triads and pentades proved to decompose during purification. The pyrene containing triads Py2B and Py2SQB show small interactions already in the ground state represented by red shifts of the spectra and a broadening of the bands. Nevertheless, these interactions are in the weak coupling regime and energy transfer between the constituents is possible. On the contrary in the TA spectra it is obvious that always the whole triad, at least to some extend is excited. To question if the excitation of the high energy state is deactivated by energy transfer or rather IC in a superchromophore could not be distinguished in the course of this work. At present additional time-dependent calculations of the dynamics are in progress to get a deeper understanding of the photophysical processes taking place in the triads. The dye conjugates B2SQB-3 and (SQB)2B-4 can be assigned to the strong interaction range and hence are describable by exciton theory. The transition-dipole moments proved to be more than additive and increase for both compounds from absorption to fluorescence. This can be explained by an enhancement of the coupling in the relaxed excited state compared to the absorption into the Franck-Condon state due to a more steep potential energy surface in the excited state and hence smaller fluctuations. In the last part of this thesis the influence of disrupting electronical communication by implementing a rigid non-conjugated bridge in a bichromophoric trans-squaraine system was tested. While the flexible linked squaraines show complex spectra due to different conformers the SQA2Anth compound is rigified and no rotation is possible. This change in flexibility is represented in the steady-state spectra where just one main absorption and fluorescence band is present due to a single allowed excitonic state. The system proves to own an excited state that is completely delocalised over the whole molecule.},
  subject      = {Chromophor},
  language  = {en}
}
@phdthesis{Ceymann2016,
  author    = {Ceymann, Harald},
  title     = {Synthesis and Optical Spectroscopic Properties of Squaraine Superchromophores},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136850},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {In this work the successful synthesis, the linear and nonlinear spectroscopic properties as well as the electrochemical behaviour of some linear and star-shaped squaraine superchromophores that are based on indolenine derivatives were presented. The attempt to synthesise similar chromophores which contained only benzothiazole squaraines failed unfortunately. However, one trimer that contained mixed benzothiazole indolenine squaraines could be synthesised and investigated as well. The linear spectroscopic properties, like red-shift and broadening of the absorption, of all superchromophores could be explained by exciton coupling theory. The heterochromophores (SQA)2(SQB)-N, (SQA)(SQB)2-N and (SQA)(SQB)-NH displayed additional to the typical squaraine fluorescence from the lowest excited state some properties that could be assigned to localised states. While the chromophores with N-core showed very small emission quantum yields, the chromophores with the other cores and the linear oligomers display an enhancement compared to the monomers. Transient absorption spectroscopy experiments of the star-shaped superchromophores showed, that their formally degenerated S1 states are split due to a deviation of the ideal C3 symmetry. This is also the reason for the observation of an absorption band for the highest exciton state, which is derived from the S1-state of the monomers, as its transition-dipole moment would be zero in the symmetrical case. The linear oligomers and the star-shaped superchromophores with a benzene or triarylamine core showed at least additive, sometimes even weak cooperative, behaviour in the two-photon absorption experiments. Additional to higher two-photon absorption cross sections the chromophores showed a pronounced broadening of the nonlinear absorption, due to symmetry breaking and a higher density of states. Unfortunately it was not possible to solve the problem of the equilibrium of the cisoid and the transoid structure of donor substituted azulene squaraines, due to either instability of the squaraines or steric hindrance.},
  subject      = {Squaraine},
  language  = {en}
}
@phdthesis{Gsaenger2013,
  author    = {Gs{\"a}nger, Marcel},
  title     = {Organic Thin-Film Transistors Based on Dipolar Squaraine Dyes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-80588},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {In summary, it can be stated that the herein studied set of acceptor-substituted squaraine dyes can be seen as potent candidates for OTFTs. Furthermore, their transistor performance can be easily tuned to obtain hole mobilities up to 0.45 cm2/Vs from solution and 1.3 cm2/Vs from sublimation by choosing adequate deposition techniques. In the end, a probable structural model derived from studies of the thin-film morphology by methods such as optical spectroscopy, AFM and X-ray even facilitated the clarification of the observed charge transport behavior.},
  subject      = {Organische Chemie},
  language  = {en}
}