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SPECTROSCOPIC PROPERTIES OF THE LOWEST‐LYING EXCITED STATES OF 2‐AMINOPYRIMIDINE, CYTOSINE, URACIL AND THEIR DERIVATIVES *
Author(s) -
Kleinwachter Vladimir,
Drobnik Jaroslav,
Augenstein Leroy
Publication year - 1966
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1966.tb09847.x
Subject(s) - intersystem crossing , chemistry , phosphorescence , photochemistry , singlet state , triplet state , excited state , cndo/2 , fluorescence , atomic physics , molecule , physics , organic chemistry , quantum mechanics
— The absorption and emission spectra of pyrimidine derivatives show that sub‐stituting an amino or hydroxyl group at the 2nd and 4th positions of the pyrimidine ring (to give the derivatives found in biological systems) creates excitation patterns quite different from those for pyrimidine itself: the lowest singlet and triplet states are of (?,π*) and not ( n,π* ). The relative intensities of fluorescence and phosphorescence depend upon at least two factors: (a) the extent of intersystem crossing which depends mostly on the relative energies of the lowest (π,π*) singlet and lowest ( n,π* ) triplet states, where the energy of the latter is influenced strongly by the environment; and (b) the extent of mixing of (π,π*) and ( n,π* ) triplet states, which depends initially upon the closeness of the energies of the two states. The measured and inferred energies for these various states are summarized. In most cases the intensity of luminescence is lower than that found in purines, presumably much of the excitation energy is dissipated in photochemical changes. Also the lifetime of phosphorescence is shorter than in purines. Substitution at the 5th carbon increases the luminescence yield. When a bromine atom is substituted on the 5th carbon of uracil no effect on intersystem crossing is found—presumably the electron density of the π* orbital is minimal in the vicinity of the bromine. Polarization measurements show that the two lowest π‐π* transition moments in 2‐aminopyrimidine are oriented almost perpendicular to each other in the molecular plane whereas phosphorescence is polarized out of plane.