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An Experiment on Photochromism and Kinetics for the Undergraduate Laboratory

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R. Martín Negri

Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, 1428 Buenos Aires, Argentina

Hernán E. Prypsztejn

Escuela Técnica No. 1, Otto Krause, Ave. Paseo Colón 650, 1063 Buenos Aires, Argentina

J. Chem. Educ., 2001, 78 (5), p 645

DOI: 10.1021/ed078p645

Publication Date (Web): May 1, 2001

Abstract

An experiment on photochromism and photochemical kinetics for the undergraduate physical chemistry laboratory is presented. Solutions of the photochromic compound 1',3'-dihydro-1',3',3'-trimethyl-6-nitro- spiro(2H-1-benzopyran-2,2'-2H-indole) (referred to as 6-NO₂-BIPS) are irradiated with UV light, inducing a change of color by photoisomerization of the closed-ring isomer (N) to an open-ring isomer (the merocyanine isomer, MC). The thermal back-isomerization from MC to N is followed by recording the absorbance decay of MC in the visible region. Relaxation times at different temperatures were obtained and the activation energy and entropy calculated for the back isomerization. The value of ΔH° for the thermal equilibrium was calculated by measuring the changes of the UV absorbance of N and the visible absorbance of MC when temperature is varied. The system presents very strong solvent effects, which are illustrated by comparing the remarkable differences of the 6-NO₂-BIPS photochromism in ethanol and toluene.

The experiment has been regularly performed since the second semester of 1998 in a general physical chemistry course at the Faculty of Science, University of Buenos Aires. A simplified version is also performed by students of two technical high schools of Buenos Aires City.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Laboratory Instruction

Keywords (Pedagogy):

Hands-On Learning / Manipulatives

Keywords (Subject):

Dyes / Pigments

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