Non-traditional intrinsic luminescence: inexplicable blue fluorescence observed for dendrimers, macromolecules and small molecular structures lacking traditional/conventional luminophores

Abstract

Abstract This article reviews activities related to a new photoluminescence phenomenon involving inexplicable blue fluorescence emissions first observed in simple aliphatic amines and poly(amidoamine) (PAMAM) dendrimers over two decades ago. Although this luminogenic phenomenon follows certain traditional luminescence paradigm (TFP) principles, it exhibits features that represent a distinct paradigm shift from traditional systems by not adhering to several critical TFP criteria. Firstly, this new photoluminescence does not require excitation of delocalized electrons found in traditional aromatic or extended π-systems. Secondly, the active luminophores do not function as single, independent molecular entities responding to traditional concentration dependent emission quenching criteria, but instead these emissions are enhanced by collective molecular associations. Surprisingly, this new “photoluminescence effect” is observed in a wide range of common organic materials. Quite remarkably, it occurs in the absence of traditional/conventional luminophores and involves the aggregation/clustering and/or physico-chemical confinement of normally non-emissive, electron rich, hetero-atomic, functionalized moieties. As such, this new fluorescence phenomenon is referred to as non-traditional intrinsic luminescence (NTIL). The article begins with a historical overview of traditional luminescence paradigm (TFP). This sets the stage for an in-depth overview of all NTIL emissive molecular/macromolecular structures, architectures, assemblies, moieties and elemental compositions documented from first observations to the present. It is notable that these NTIL activities were reported independently within four parallel investigational pathways, all of which have collectively converged into a framed consensus concerning a rational mechanism. This consensus presents compelling evidence that this NTIL emission phenomenon results from the molecular immobilization/rigidification or physico-chemical confinement of collections/ multiples of certain common non-emissive, electron rich moieties. These electron rich moieties are referred to as hetero-atomic sub-luminophores (HASLs). They include commonly non-emissive hetero-atomic entities such as: amines, imines, amides, nitriles, ureas, ethers, hydroxyls, esters, carboxylic acids, pyrrolidones, oxazolines, imidazolines, etc. that become emissive when forced into highly associated confined assemblies. Molecular immobilization and confinement features associated with these non-aromatic emissive HASLsx assemblies are in some aspects reminiscent of certain aromatic aggregation induced emission (AIE)-type materials pioneered by Tang et al. That withstanding, these NTIL active materials exhibit many unprecedented and differentiated features not found in traditional luminescent substrates. This review will examine these unique NTIL emissive materials and properties followed by a discussion of potential life science labeling applications and conclude with a unifying emission mechanism currently proposed for this new emerging area of luminescent materials.