Phosphorescence, optically detected magnetic resonance, and electron paramagnetic resonance studies of the lowest excited triplet state of the anti isomer of a 1,5-diazabicyclo[3.3.0]octadienedione [9,10-dioxa-anti-(methyl, hydrogen)bimane]

Abstract

We have studied the T1 state of the anti isomer of a 1,5-diazabicyclo[3.3.O]octadienedione by means of phosphorescence emission and excitation spectroscopy, optically detected magnetic resonance (ODMR), and EPR. From the excitation spectra the locations of the T2(3n7r*) and Sl(ln?r*) states were determined in crystals and frozen solutions. It was suggested that the molecule is planar in crystals and acetonitrile but is twisted in methyltetrahydrofuran (MTHF). Contrary to the cases of many aromatic carbonyls, the radiative decay rate at the 0-0 band is the largest for the y sublevel, which indicates the importance of the spin-orbit mixing with the lm* state in the radiative mechanism. The radiative activity of the x sublevel at the vibronic band is due to the 37r7r*-3n7r* and 1n7r*-17r7r* vibronic mixings. The radiationless decay takes place predominantly from the x and y sublevels as expected. In the durene mixed crystal there are two differently oriented molecules which give rise to slightly different zfs and phosphorescence spectra: D = 0.1004 cm-l and IEI = 0.0245 cm-l, and D = 0.1000 cm-' and IEl = 0.0235 cm-l, were obtained for these species. IntroductionThe recently synthesized new class of compounds, synand anti-l,5-diazabicyclo[3.3.0]octadienediones have many interesting photophysical properties.14 These molecules are called syn-and anti-bimanes, and brief formulas are written syn-(R1,R2)bimane and anti-(R1,R2)bimane. The molecular structures are shown in Figure 1.syn-Bimanes are strongly fluorescent and very weakly phosphorescent. This is presumably because the lowest excited singlet, S1('7r7r*), state is located far below the %7r* state and the intersystem crossing is not effective. On the other hand, anti-bimanes are strongly phosphorescent and very weakly fluorescent. A recent picosecond pulse study showed that intersystem crossing is very fast (>lo1' s-').~ It was suggested that this high rate is due to the proximity of the 3n7r* state to the Sl('a7r*) state. The lowest excited

Bivalent glycoconjugates based on 1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione (“bimane”) as a central scaffold

“…Bimanes are fluorescent molecules, the syn isomers displaying much stronger effects than the anti forms (see Chart 1) [15]. The latter show, on the other hand, low temperature phosphorescence [16,17]. Chart 1.…”