Photophysical studies of cryptand based fluorophore systems in different environments

Abstract

The three secondary amino nitrogens of a heteroditopic cryptand have been derivatized with naphthyl groups and different spacer units to have a (fluorophore-spacer)₃-receptor configuration. The length and the nature of the spacer units have been varied in the systems to study the effect on the photophysical behavior and the photoinduced electron transfer (PET) process of these systems. The systems L₁ and L₂ (where the spacer is (CH₂)_n, with n=1,2, respectively) show intramolecular CT transition on the red side of the localized π -π* absorption band of naphthalene units in aprotic solvents. All the systems exhibit varying degree of PET depending upon the nature of the spacer and the environment. The efficiency of PET is found to be lower as the length of the spacer [in the case of (CH₂)_n, where n=1,2 (L₁ and L₂)] is increased suggesting that the interaction in these cases is predominantly through bond. Apart from emission from the locally excited (LE) state, these systems show emission from the intramolecular exciplex and/or intramolecular excimer states depending on the environment. The ligand where a C=0 group separates the fluorophore and the cryptand unit (L3) does not exhibit PET. However, the system exhibits excimer emission in the presence of H+. The use of ¤ CH₂CO¤ group as spacer where ¤ CH₂¤ group is attached directly to the naphthalene fluorophore (L₄) prevents efficient PET indicating a different orientation of naphthalene with respect to cryptand unit in comparison to L₂ (∅ _F for free L₄ is comparable to that of free naphthalene). This system exhibits a very weak exciplex emission in tetrahydrofuran (THF) compared to that observed in L₁ and L₂. The system where the fluorophore unit is built within the cryptand moiety (L₅; where secondary nitrogens are present in the cryptand moiety) does not show either intramolecular exciplex or excimer emission.