Strategy for the synthesis of new 3- or 4-substituted-1,8-naphthalimides with –NC– and –C–C– bonds as well as differences in the optical properties between them

Abstract

The research focused on developing an optimal synthesis pathway for 3- and 4-amino/iodo derivatives of 1,8-naphthalimides, which are important reagents for obtaining new 1,8-naphthalimides derivatives with beneficial optical properties. During this work, the reactivity of individual compounds was discussed, which is primarily influenced by the position of substitution in the naphthalimide ring. In this respect, it was shown that amine derivatives substituted at the 4-position are characterized by lower basicity, which may explain their lower reactivity, for example in condensation reactions. The amine derivatives were used in condensation reactions with aldehydes to obtain derivatives containing an imine bond (–NC–), while iodo derivatives were used in Suzuki coupling reactions to obtain derivatives with a carbon–carbon bond (–C–C–). For the obtained compounds, the basic optical properties (UV-Vis, PL, quantum yield, emission lifetime, aggregation, etc.) were studied, comparing both the bond type (–C–C– vs. –NC–) and the site of substitution (3- vs. 4-posision). The properties of the obtained derivatives were analyzed and discussed and supported by appropriate theoretical calculations (DFT). Compounds with a –C–C– bond exhibited more intense emission than derivatives with a –NC– bond, which results from the photoinduced electron transfer (PET) from nitrogen atom to the naphthalimide ring in the imines studied. A bathochromic shift in emission was observed increasing solvent polarity for pyrene and naphthalene derivatives. Spectroscopic studies (¹H NMR, absorption and emission) of derivatives with the –NC– bond were also carried out in various solvents: weakly polar (dichloromethane) and polar (acetone, dimethyl sulfoxide) with the addition of trifluoroacetic acid in order to analyze the PET inhibition as well as hydrolysis processes, which are responsible for the increase in emission observed for the investigated imines. UV-vis and ¹H NMR studies of the imines in the presence of trifluoroacetic acid showed that the imine derivatives at the 4-position are more susceptible to hydrolysis than those substituted at the 3-position. Moreover, the analyzsed compounds with a –C–C– bond exhibited aggregation-induced emission (AIE) or aggregation-induced blue-shifted emission (AIBSE), whereas compounds containing an imine bond (–NC–) showed aggregation-caused quenching (ACQ).