In identical manner as organic compounds have overpowered inorganic materials in the field of in-solution fluorescence, a development from strictly mineral inorganic luminescent materials towards organic luminophores could be observed in the past decades.[41] The growing ambition of developing organic materials with tuneable emission properties has led to numerous publications in this field of research.
While the dynamic processes and electronic effects which dominate luminescence properties in solution are comparatively well known,[12-13, 23c, 31]
the knowledge of mechanisms and parameters which influence solid state fluorescence of organic materials are for the most part diffuse. Assumptions regarding the influence of molecular interactions on fluorescence properties differ significantly depending on the
described systems and are in some cases even contradictory.[42] Most of the quenching processes which strongly influence the emission intensities of dissolved fluorescent compounds (as utilized in most sensor molecules) are dependent on dynamic rotation around bonds in solution to achieve the required orbital overlap for electron transfer to the excited fluorophore.[13] In the solid state, these mechanisms no longer apply due to the rigidness of the molecules. Although electron transfer is also possible in the solid state, it is exceedingly rare and demands highly specific structural requirements.[13] Hence, other parameters such as formation of excited dimers (excimers)[43] or exciplexes,[44] packing effects,[42a, 42c, 45] and intermolecular interactions[46] often predominantly influence fluorescence properties of organic compounds in the solid state.
While fluorescence of organic compounds in solution is a common phenomenon, the occurrence of solid state fluorescence of organic compounds is generally considered as rare.[42b, 42c, 47] First systematic investigations of the influence of packing on solid state fluorescence were performed by Langhals et al. on pigment dyes.[42a, 45a]
These led to several basic assumptions, in particular regarding the effect of π-π-interactions on emission intensities and quantum yields. Langhals and co-workers found that two modifications of one dye, which do not differ in their fluorescence properties in solution, differed significantly in the crystalline state in terms of emission intensity and position of the absorption maximum. They ascribed these deviations to differences in the interactions of the fluorophores. The modification which showed the shorter π-π distance was weaker in its emission intensity and also exhibited the bathochromic shift stated above. Hence, these phenomena were ascribed to an emission supressing effect of strong π-π interaction. The Diketopyrrolopyrrole pigment used for these experiments exhibits a fairly small π system (Scheme 1-5).
Scheme 1-5: Diketopyrrolopyrrole pigment used by Langhals et al. for the investigation of packing effects on solid state fluorescence properties.[42a, 45a]
Although this system is quite extraordinary compared to “simple” aromatic fluorophores like naphthalene of anthracene, these assumptions have become the basis of argumentation in the majority of subsequent publications. Hence, the conclusions drawn from research on these pigment dyes were afterwards applied to numerous other systems and fluorophores. Since then also contradictory effects of π-π overlap have been reported. For example Dreuw et al. reported on a naphthalene derivative which exhibits strong solid state fluorescence despite short π-π distances and large π-π overlap of fluorophores in 2005.[42b] Though contradictory results have been repeatedly reported, the majority of publications follow the thesis of solid state fluorescence quenching by π-π interaction.[48]
Of numerous utilized organic fluorophores, especially anthracene moieties have proven of value in countless fluorescent compounds. While the in-solution-fluorescence properties of many anthracene derivatives have been thoroughly described in literature (c.f. 1.1), a quite manageable number of publications address corresponding solid state fluorescence phenomena. From 2005 onward, especially the workgroup around Miyata and Tohnai has contributed several publications to this research topic. By synthesizing alkyl ammonium salts from 2,6-anthracenedisulfonic acid (2,6-ADS) and primary amines, they succeeded in altering the packing motifs of their fluorophores depending on the length of the ammonium alkyl-chain.[49] By these different packing motifs, the luminescence properties of their compounds in the solid state were also significantly affected. Two different packing forms were observed: a strongly fluorescent two dimensional motif and a one-dimensional motif which was weakly fluorescent. The differences in the observed emission intensities were ascribed to weaker distortion of the anthracene fluorophore, which was determined by IR spectroscopy. Their research in this field was further expanded by alteration of the alkyl amines, leading to corresponding ammonium salts with varying steric demand.[50]
Also aromatic amines were used, which led to strongly fluorescent solids.[51]
Differences in quantum yields and emission wavelengths of the obtained structures were again assigned to fluorophore distortion and intermolecular distance of the fluorescent molecules within the respective packing motif.
Additionally, chiral amines were reacted with 2,6-anthracenedisulfonic acid.[52] By addition of host molecules, ternary intercalation structures were formed with varying host-dependent fluorescence properties. Though in this case quenching effects
induced by distortion or molecular contact were not addressed, the red-shifted solid state emission of one molecular arrangement was attributed to formation of an excimer complex. In a recent publication, the 2,6-ADS fluorophore was replaced by 1,8-ADS, which made more complex structures with larger cavities accessible.[53]
Figure 1-5: Host dependent shift of emission in the intercalation structure of 1,8-ADS and triphenylmethylamine (TPMA) by Hinoue et al.[53]
This again made the intercalation of various host molecules possible. Depending on the host, remarkable shifts of the emission maxima of up to 120 nm were observed (Figure 1-5), which were shown to correlate with the degree of π-π overlap and the π-π distances. Similar phenomena have also been reported in other publications.[50, 54]
Furthermore, although mainly addressing photodimerization of anthracene derivatives in the solid state, the workgroup around Kohmoto also reported on several anthracene derivatives with remarkable solid state fluorescence properties resulting from packing induced intermolecular interactions.[55] The strong emission and also notable red-shift of emission were reasoned to be caused by excimer emission. Finally, Fei et al. reported on a strongly fluorescent host/guest complex between
9,10-bis(diphenylthiophosphoryl)anthracene and toluene molecules. The strong fluorescence was ascribed to the formation of a T-shaped excimer between the guest molecules and the fluorophore. C-H…π bonding between host and guest was highlighted as one on the crucial factors for the formation of the fluorescent arrangement (for a detailed explanation of C-H…π bonding please see 3.2).[44b, 44c]
Although argumentation and the resulting assumptions are conclusive in themselves in all publications described above, some results clearly contradict one another, especially in terms of the effects of π-π-interaction on emission properties.
Other factors which have repeatedly been referred to in this context, such as quenching due to distortion of aromatic fluorophores, or the role of C-H…π bonding are not considered in all publications. The alignment of all these points of argumentation is clearly an issue within the scientific field of solid state fluorescence and the contradictions and uncertainties require clarification.
1.3 Scope
In the light of the inconsistencies among the various hypotheses on the effects of structural properties on solid state fluorescence of organic compounds stated in 1.2, the investigation of the interdependencies of structural alterations and solid state fluorescence was a key aspect of this thesis. The development of a system for quantification of structural properties as a basis of the comparison of fluorescent compounds was targeted. Furthermore, suitable compounds for this comparison were to be synthesized and crystallized for the acquisition of their crystal structures. By alignment of the structural features with the acquired solid state fluorescence data, the derivation of requirements for – and principles of – solid state fluorescence was aimed for. This research was to be founded on phosphanyl- and phosphorylanthracenes, which have been a major exploratory focus of the Stalke group in the past years.
Furthermore, synthesis of metal complexes from phosphanyl and phosphorylanthracenes and transition metal cations was of interest. The investigation of the coordination modes of these compounds towards varying cations and monitoring the influence of complex formation on fluorescence properties in the solid state and in solution were major goals of this thesis. The exploration of pathways for
the synthesis of functionalized phosphanylanthracenes for the development of chelating phosphanylanthracenes was also sighted.
Moreover, investigation of the photoinduced electron transfer (PET) mechanism was focused on. The synthesis of new receptor units and corresponding sensor molecules according to the receptor-spacer-fluorophore principle as well as monitoring their sensitivity towards different analytes was aimed for. By introduction of substituents to the fluorophore, the influence of secondary substituents on quenching mechanisms was to be explored. The possibility of uniting a sensor molecule with an emission altering second substituent was set as the ultimate goal in this context. Finally the feasibility of transferring the PET mechanism from amines to other quencher systems was to be verified.
Because phosphanylanthracenes, which are primarily addressed in the context of solid state fluorescence, all feature substituents which are directly bound to the fluorophore, and molecules which will be described with regard to the PET mechanism all bear spacers between the fluorophore and the main functional group, the structure of this thesis is will also be divided into two main parts. One chapter will be dedicated to molecules without spacers and one chapter will focus on molecules containing spacers in their structures.