The photochemistry of 6-phenanthridinecarbonitrile II. The physical and photophysical properties

✍ Scribed by Bruno M. Vittimberga; Donald Sears

• Publisher: Journal of Heterocyclic Chemistry
• Year: 2001
• Tongue: English
• Weight: 117 KB
• Volume: 38
• Category: Article
• ISSN: 0022-152X
• DOI: 10.1002/jhet.5570380143

✦ Synopsis

Abstract

The physical and photophysical properties of 6‐phenanthridinecarbonitrile (1) have been examined. We previously reported that when 1 is irradiated in aqueous 2‐propanol, three products are formed [3]. These include dimethyl‐6‐phenanthridinylcarbinol (2), phenanthridine (3) and 6,6′‐biphenanthridine (4). Phenanthridinyl radical is formed in neutral media by hydrogen atom abstraction from an alcohol molecule by an excited state of 1 in a monophotonic process. The presents of acid effectively quenches all photochemical behavior. These products may all be explained assuming in‐cage and out‐of‐cage reactions. The free spin value, g~e~, was determined at 125 K and found to be 2.0043 which is close to the theoretical value for that of a free electron. The total emission spectrum of 1 at 77 K shows a fluorescence maximum at 378 nm and a much weaker phosphorescence maximum at 502 nm which represented less than 3% of the total emission. When benzophenone is added to the reaction mixture, the triplet state of 1 is populated, but photosensitized product formation does not occur. The result supports a singlet reactive state. When cis/trans‐piperylene is added to the reaction mixtures, it quenches the fluorescence of 1. The fluorescence quantum yield (Φ~f~) was found to be 0.227 in neat 2‐propanol. The addition of water causes an increase in Φ~f~ and a decrease in the Pk~a~ of the medium. The excited state lifetime (τ) was determined in neat 2‐propanol, using oxygen quenching, and found to be 3.4 ns. This number increased with increasing water concentration. The photoreactive state of 1 appears to be its π,π* singlet state making its behavior more like that of the corresponding hydrocarbon parent.