Res. Chem. Intermed.
, Vol. 27, No. 1,2, pp. 5–20 (2001)
Irradiation-dependent equilibrium between open
and closed form of UV absorbers of the
, ANJA P. FLUEGGE
, JUERGEN KECK
, HORST E. A. KRAMER
and DAVID LEPPARD
Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany
Ciba Specialty Chemicals Inc., Rosental R 1059, CH-4002 Basle, Switzerland
Received 24 March 2000; accepted 28 April 2000
Abstract—2-(2-Hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) bearing one or more intramolec-
ular hydrogen bonds (IMHBs) show phosphorescence in polar solvents at 77 K which increases in
intensity with UV-irradiation time (¸
333 nm) until an equilibrium value is reached (phospho-
rescence evolution). Phosphorescence emission is produced from open conformers of the molecules
with intermolecular rather than intramolecular hydrogen bonds, which are formed in polar solvents
under the in uence of UV radiation. All IMHBs of an individual molecule must be broken to en-
able triplet state population. Reformation of the closed form is observed for HPTs after dark peri-
ods resulting in a lower initial phosphorescence intensity upon renewed irradiation (relaxation). The
methoxy derivatives (MPTs), where the OH groups are replaced by methoxy groups, can be employed
as spectroscopicmodels for the open form; they emit phosphorescenceindependent of irradiation time.
For the equilibrium between open and closed form of M-OH-P (2-(2-hydroxy-4-methoxyphenyl)-4,6-
diphenyl-1,3,5-triazine) under constant irradiation (¸
333 nm, 100 W-HBO lamp) a proportion of
5% of open form was evaluated.
This article is dedicated to Professor Haruo Shizuka in recognition of his outstand-
ing contributions in the eld of photochemistry.
UV absorbers with intramolecular hydrogen bonds (IMHBs), such as the well-
known 2-(2-hydroxyphenyl)benzotriazoles (HBzTs) or the 2-(2-hydroxyphenyl)-
4,6-diaryl-1,3,5-triazines (HPTs), are very common to protect polymers against
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