R. Schmidt's research while affiliated with University Hospital Frankfurt and other places

Abstract— Photophysical properties of two chlorin type molecules (CHLI) and (CHLII) were investigated in different solvents. Quantum yields of fluorescence φF of S, → T, intersystem crossing φT, and of singlet oxygen (1Δg) formation φΔ, as well as the Stern-Volmer constants for the quenching of the S, states by oxygen and the bimolecular rate constants of quenching of 1Δg by the chlorins were measured. The values of φT and φΛ can be given as 0.57 and 0.58 for CHLI and 0.69 and 0.58 for CHLII. The values of the fluorescence quantum yields, the strong absorption of the chlorins in the red (Λ > 630 nm) and the high values of the quantum yields for 1Δg formation recommend the chlorin derivatives as potential markers and photosensitizers for tumor therapy.

Abstract The self-sensitized photooxidation of meso-diphenylhelianthrene to its endoperoxide was kincticallv examined. It is recommendable as a convenient, reliable, and wavelength independent actinometer for the visible wavelength range. The calibration values, the actinometric procedure and the evaluation method are given in detail.

Abstract— The photocycloreversion quantum yields Q of the endoperoxides of anthradichromene, benzodixanthene and dimethylhomoocoerdianthrone have been investigated in the 248 ≤γ≤ 405 nm wavelength range. Two types of wavelength dependencies can be distinguished phenomenologically. For the excitation of the Sn(n≥ 3) states of dimethylhomöocoerdianthrone endoperoxide γ-independent Q-values have been observed throughout 365 ≥γ≥ 265 nm. The endoperoxides of anthradichromene and benzodixanthene in contrast exhibit a quite different and unusual dependence of Q on γ. For both compounds Q first increases with photon energy until a fairly sharp maximum in Q is reached at 302 nm. With further increasing energy Q falls off and approaches a wavelength independent plateau between 280 and 248 nm. The S1-states of the endoperoxides investigated are inactive for photocycloreversion, which is generally found in endoperoxide photochemistry. The wavelength dependencies observed reveal that the photocycloreversion of endoperoxides deviates remarkably from the commonly accepted generalization that photochemical reactions in solution phase should only occur from the lowest excited singlet or triplet states.

The highly reversible thermally stable photochromic system consisting of heterocoerdianthrone-endoperoxide (HCDPO) and its parent compounds HCD and 02 is proposed as a new reusable liquid chemical actinometer in the UV region 248 ≤λ≤ 334 nm. The foremost advantageous features of this new system are: (1) high reproducibility and accuracy, (2) high sensitivity, (3) almost wavelength independent quantum yields, (4) no loss in accuracy even after 100 repeated actinometric cycles and (5) very easy handling and straightforward calculability of the radiation quantum flux.

Abstract— –Photophysical properties of [26] porphyrin (26 P) were investigated in chloroform. The quantum yields of fluorescence, of S1→ T1 intersystem crossing and singlet oxygen formation were measured. The purity, stability, the strong absorption in the red (δmax= 783 nm; εmax= 28 000 M1 cm-1) and the ability of singlet oxygen formation recommend 26 P as potential photosensitizer for tumor therapy.

Photooxygenation of aromatic compounds is often accompanied by color changes. Because of the resulting ease of detection, such reactions found very early particular interest. It was observed that most endoperoxides are also photochemically rather unstable compounds, forming rearranged products. By designing new endoperoxides undergoing little rearrangement, the cycloreversion reaction became the major pathway, which made endoperoxides more attractive as photochromic systems. Few of them have been synthesized so far but they were investigated in detail. Among them, highly reversible systems of outstanding properties were found. In the near future, the interest will focus on a broader variation of color range and a further improvement of reversibility. Both goals may be achieved by introducing electron-donating substituents and by testing new bridging groups -X- (-X-=-S- is currently under investigation). With respect to possible applications, the sometimes-moderate solubility will be improved by suitable variation of molecular structures. Especially, the unique system permitting a switch between photostability and photoreversibility will be refined by optimizing the sensitizer and improving the solubility. Thus, the topic is currently an active research area with a great potential for applications and further progress is expected.

Photoacoustic calorimetry was used to investigate the deactivation of electronically excited dimers of rhodamine B and eosin Y in water. Temperature dependence experiments were used to evaluate a possible photodissociative deactivation path, which should be accompanied by chemical volume changes. The quantitative evaluation demonstrates that photodissociation plays no role. Radiationless deactivation occurs for both dimers with unit quantum yield. Since the entire absorbed photon energy is released very rapidly as heat, deactivation occurs by internal conversion but not by intersystem crossing.

The photophysical properties of acenaphthylene (ACN), fluoranthene, rubicene and decacyclene were investigated in toluene by electronic spectroscopy and photoacoustic calorimetry. The photophysical parameters of the related molecules pyracyclene (PYR), periflanthene (PF) and 1,16-benzoperiflanthene, taken from the literature, were included in the discussion. A Siebrand plot was used to analyse the energy dependence of the rate constant kic of internal conversion. With the exception of ACN and PF and possibly PYR, the molecules follow the energy gap law, demonstrating that internal conversion should be promoted by CH modes. For ACN and PF, deviations to much larger values of kic were observed. This effect could be caused by a strong displacement of the So and S1 potential energy curves, which is related to the large differences in the CC bond lengths in the two states.

Efficiencies EFTc of triplet-sensitized cycloversion (T-cycloversion) of several aromatic endoperoxides (APOs) have been determined. From the dependence of EFTc on the triplet state energy of the sensitizer the energies ET(ππ*) of reactive triplet states of APO are estimated ranging from ET(ππ*)≲ 170 kJ mol−1 for the endoperoxide of mesodiphyenylhelianthrene (MDHPO) to ET(ππ*)≈250 kJ mol−1 for the endoperoxide of rubrene. Quantaum yields QTc of T-cycloversion could be derived from [APO]- dependent measurements of EFTc Values of QTc=0.07±0.01 for MDHPO and QTc=0.042±0.008 for the endoperoxide of benzodixanthene resulted. T-cycloversion of the endoperoxide of homoeocoerdianthrone (HOCDPO) was found to be an adiabatic and spin-allowed reactions. S1-excited HOCD and O2(3Σ−g are formed in equimolar amounts. Thus T-cycloversion of APOs parallels in an amazing way singlet state cycloreversion.

The quantum yield of singlet oxygen O2(1Δg) formation by the ketone N-methylacridone (NMA) varies strongly with solvent: 0.05 (methanol) ⩽QNMAΔ ⩽0.29 (tetrachloromethane). The analysis of the solvent and O2-concentration-dependent data on NMA fluorescence demonstrates that O2(1Δg) is only formed in quenching of the ketone's triplet state by O2 with an efficiency of SΔ=0.29 being independent of solvent polarity. Thus the solvent effect on the quantum yield of intersystem crossing determines the solvent dependence of QNMAΔ. The small value of SΔ could lead to the expectation that the T1 state of NMA is an nπ* state. However, NMA definitely has a T1(ππ*) state. Therefore a strict classification of values of SΔ corresponding to T1(nπ*) ketones (SΔ ≈ 0.3) and T1(ππ*) ketones (0.7 ≲ SΔ≲1) does not exist. Results of photochemical 16O/18O isotope exchange experiments exclude the participation of a trioxetane in the diabatic deactivation route to the ground state ketone and O2(3Σ−g).