Tautomeric forms of PPI dendrimers functionalized with 4-(4′-ethoxybenzoyloxy)salicylaldehyde chromophores

Abstract

Bonding of the promesogenic unit derived from 4-(4′-ethoxybenzoyloxy)salicylaldehyde to the amino terminated PPI dendrimer chains results in formation of the salicylidenimine chromophore groups. Absorption and fluorescence investigations of the dendrimer solutions supported by the quantum chemistry calculations revealed that the chromophore groups may exist in enol and keto tautomeric forms with relative concentrations depending on the dendrimer generation and solvent. The dendrimer fluorescence is attributed to nπ^∗ states of keto tautomers which may also be formed from excited enol tautomers.

Introduction

During recent decades, light sensitive organic composites have received significant attention due to their importance in basic research and plentiful technological application perspectives [1], [2], [3]. In order to improve photochromic behavior and expand application areas, many photochromic materials have been chemically incorporated in different matrices. Polymers, liquid crystals or dendrimers have been widely used for these purposes [4], [5]. As a result, recent studies have shown that dendrimers due to their attractive structural features are perfect candidates for implementation of these achievements [6].

Dendrimers are uniform, three-dimensional spherical macromolecules composed of the central core, internal branching units and terminal groups on periphery [7]. Well-defined molecular architecture of dendrimers is responsible for versatile structural modifications within the entire volume [8], [9], [10], [11]. Numerous chromophore groups have been used for dendrimer functionalization in order to create materials with controlled physico-chemical properties [12], [13]. Consequently dendrimers functionalized with the light sensitive molecules are potential candidates for the creation of artificial light harvesting systems, nonlinear optical, optoelectronic or photovoltaic devices [14], [15], [16]. Light-induced orientation of photochromic molecules is an extremely important feature in the development of photochromic materials [17], [18], [19]. Various light-sensitive materials are also attractive for medical applications due to their light controllable host–guest properties [12], [20], [21].

Aromatic molecules containing Schiff base groups are especially important light sensitive materials in photochemistry. Their photochromic properties [22], [23] are directly related to the presence of intramolecular hydrogen bonds and proton transfer reactions. These features suggest application of Schiff base compounds in fabrication of molecular memories, optical switches, etc. From an academic point of view they are also seen as perfect materials for the study of the proton transfer and keto-enol tautomerism.

Photochromic behavior of Schiff base compounds is usually accompanied by the photoinduced structural changes of molecules such as keto-enol tautomerism and trans–cis isomerization. In aromatic Schiff bases, a hydroxyl group is responsible for the H-donating properties, while a nitrogen atom participates as an acceptor. Schiff base compounds have a dominant enol tautomeric form in the ground state, while a hydrogen atom rapidly transfers and binds to the nitrogen atom upon its photoexcitation. This process is considered as the excited state intramolecular proton transfer (ESIPT). The created excited cis-keto tautomers undergo relaxation to the ground state, and finally back proton transfer takes place reproducing the original enol tautomeric form. The ESIPT process was found to take place on a femtosecond time scale, while the back proton transfer occurs during several picoseconds [22], [24]. There are many different types of Schiff base containing compounds revealing light induced reversible tautomerization. Salicylideneanilines are among the most widely investigated [25].

Here we present investigations of the poly(propylene-imine) (PPI) dendrimers functionalized with 4-(4′-ethoxybenzoyloxy)salicylaldehyde moieties (PPI-SA). Functional moieties attached to core chains form salicylidenimine chromophore groups, which determine the near UV absorption and fluorescence properties of the dendrimers. We present absorption and fluorescence investigations of the PPI-SA dendrimers in solutions combined with the quantum chemistry calculations of their electronic and conformational structure. Our investigations show that the terminating groups may exist in several tautomeric forms with relative concentrations depending on the dendrimer generation. Keto-enol tautomeric changes take place in the excited state, determining fluorescence properties of the investigated dendrimers.