Azobenzene is a prototypical photochromic molecule existing in two isomeric forms, which has numerous photochemical applications that rely on a precise knowledge of the molar absorption coefficients (ε). Careful analysis revealed that the previously reported absorption spectra of the "pure" isomers were in fact mutually contaminated by small amounts of the other isomer. Therefore, the absorption spectra of both trans- and cis-azobenzene in methanol were re-determined at temperatures of 5-45 °C. The thermodynamically more stable trans-azobenzene was prepared by warming the solution in the dark. To obtain the spectrum of cis-azobenzene three methods were used, which gave consistent results within the limits of error. The method based on the subtraction of derivative spectra coupled with a global analysis of the spectra recorded during thermal cis-trans isomerization is shown to give slightly more reliable results than the method using isomeric ratios determined by H-NMR. The described methods are readily generalizable to other azobenzene derivatives and to other photochromic systems. The practical implication of the re-determined ε values is demonstrated by a very high precision of spectrophotometric species analysis in azobenzene isomeric mixtures. The new ε values imply that the previously reported quantum yields must be revised.

A new photoremovable protecting group, (6-hydroxy-3-oxo-3H-xanthen-9-yl)methyl (1), with a molar absorption coefficient ε of ∼4 × 10(4) m(-1) cm(-1) at ∼520 nm for the release of carboxylates or phosphates is reported. Three derivatives of 1 (diethyl phosphate, acetate, and bromide) were isolated as complexes with DDQ and shown to release the ligands with quantum yields ≤2.4% in aqueous solution.

The quantum yields of azobenzene photoisomerization in methanol solution were redetermined using newly obtained molar absorption coefficients of its cis- and trans-isomers. The results differ substantially from those published previously, especially in the range of the nπ* absorption band. Besides actinometry, these findings are relevant for applications of azobenzene derivatives in optical switching.

Three monosubstituted 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives were synthesized and their spectroscopic properties were characterized and compared to those of iodine and chlorine-atoms containing analogues as well as an unsubstituted BODIPY derivative. The fluorescence quantum yields were found to decrease, whereas the intersystem crossing quantum yields (ΦISC), determined by transient spectroscopy, increased in the order of the H → Cl → Se/I → Te substitution. The maximum ΦISC, found for the 3-phenyltellanyl derivative, was 59%. The results are interpreted in terms of the internal heavy-atom effect of the substituents.

Abstrak: Antioksidan merupakan senyawa yang menghambat atau menunda reaksi oksidasi molekul dengan cara menghambat proses inisiasi atau propagasi reaksi oksidasi berantai . Struktur kimiawi antioksidan, sumber radikal bebas, dan sifat fisiko-kimia sediaan sampel yang berbeda dapat memberikan hasil uji aktivitas antioksidan yang beragam. Oleh karena itu, diperlukan suatu metode analisa aktivitas antioksidan yang selektif untuk suatu jenis sampel tertentu. Studi perbandingan metode uji aktivitas antioksidan DPPH, FRAP, dan FIC telah dilakukan terhadap asam askorbat (AA), asam galat (AG), dan kuersetin. Ketiga metode uji antioksidan dibedakan berdasarkan pada jenis mekanisme reaksinya, sedangkan sampel standar antioksidan dipilih berdasarkan struktur polihidroksi atau polifenol yang umumnya mewakili dasar struktur antioksidan bahan alam. Metode uji aktivitas antioksidan terhadap radikal DPPH ditemukan paling efektif dan efisien diantara ketiga metode uji yang digunakan dengan nilai IC 50 berturut-turut 1,27; 2,44; dan 2,77 mg/L untuk AG, kuersetin dan AA. Adapun metode FIC terbukti paling tidak efektif dan efisien karena sensitivitasnya yang sangat rendah dan daya kelatnya lebih kecil dari 20%. Korelasi antara metode uji untuk semua standar antioksidan terbukti sangat tinggi (R>0,98), khususnya antara FRAP dan DPPH. Hal ini mengindikasikan adanya keterkaitan sangat kuat antara daya hambat radikal bebas dengan potensial reduksi senyawa polihidroksi (polifenol) terhadap ion besi. Secara umum kedua metode ini sangat dimungkinkan bisa saling menggantikan satu sama lain.Abstract: Antioxidants are compounds that prevent or postpone oxidation reaction of a molecule by inhibiting initiation or propagation process of the chain oxidation reaction. The chemical structure of antioxidants, free radical sources, and physico-chemical properties of different sample preparations can provide various antioxidant activity test results. Therefore, selectivity of an antioxidant activity analysis method for a particular sample type is compulsary. The comparative study of antioxidant activity assays (DPPH, FRAP, and FIC) against ascorbic acid (AA), gallic acid (GA), and quercetin was accomplished. The three antioxidant activity assays were distinguished based on their typical reaction mechanism, while the samples of antioxidant standard were selected on the polyhydroxy or polyphenol based-structure, which are commonly available as the basic skeleton of the natural antioxidants compounds. The antioxidant assay against a radical of DPPH was found to be the most effective and efficient among the three assays with IC 50 1. 27, 2.44, and 2.77 mg/L for GA, quercetin and AA, respectively. In contrast, FIC method was proven to be the least one due to its poor sensitivity and low chelating capability (<20%). The correlation between the antioxidant assay methods was found to be really high (R>0,98), particularly between the FRAP and DPPH assay. This result indicated that the radical inhibition has strong correlation with the reduction power of t...