Photoluminescence is a fascinating phenomenon which has a huge impact on our daily life. Many important applications are based on this principle, such as imaging diagnostics, bioanalytic, photocatalysis, solar cells, or optoelectronic devices. The emerging demands for versatile photoluminescent materials have encouraged generations of scientists to develop different types of luminophores, ranging from molecular dyes to luminescent nanomaterials. Among them, luminescent transition metal complexes (TMCs), which consist of one (or more) metal center and several organic or inorganic ligands, are drawing increasing interest due to their unique photophysical and photochemical properties, such as large Stokes shift, long-lived triplet excited state, sharp emission band (f-block metal complexes), and multiple stale oxidation states (d-block metal complexes). These distinct optical properties are not only of great research interest, but also have led to commercial applications, such as imaging agents, optical sensors, light-harvesting materials, optical barcoding, and displays. The demands and desires of optoelectronic devices and higher requirements in bioanalytic make the development of new TMCs necessary, which needs to be examined in detail not only in terms of their chemical but much more importantly their photophysical properties. In this work, a series of new types of luminescent TMCs are involved, including Cr(III)-, Pt(II)-, and Pd(II) complexes. Based on their optical studies, a series of proof-of-concept applications were designed by introducing these metal complexes to different nanomatrix, such as polymeric nanoparticles and metal-organic frameworks (MOFs), resulting various luminescent nanosensors or energy-conversion materials. The major part of this work is based on the [Cr(ddpd)2]3+ complex (ddpd = N, N′‐dimethyl‐N, N´-dipyridine‐2‐ylpyridine‐2,6‐diamine) and his derivatives. Fundamental photophysical studies of these Cr(III) complexes showed that their photoluminescence properties can be significantly enhanced by ligand and solvent deuteration. Moreover, a choice of bulky counter anions can provide an enhancement in the photoluminescence properties as well as the oxygen sensitivity. In addition, based on the photophysical understanding of the [Cr(ddpd)2]3+ complex, a proof-of-concept study of photon upconversion in molecular chromium ytterbium salts was completed. Upon an excitation of the Yb3+ sensitizers at 976 nm, these solid-state salts produced upconverted luminescence of the Cr3+ activator at 780 nm at room temperature. Another proof-of-concept study based on the [Cr(ddpd)2]3+ complex was investigated by designing and developing multianalyte nanosensors for simultaneously measuring temperature (“T”), oxygen (“O”), and pH (“P”) in aqueous phase under one excitation wavelength. Apart from the [Cr(ddpd)2]3+ complexes, four novel Pt(II)- and Pd(II)-complexes bearing tetradentate ligands were also studied regarding their photophysical properties in solutions and in polystyrene nanoparticles (PS-NPs). In PS-NPs, the aggregation-induced Metal-Metal-to-Ligand Charge-Transfer (3MMLCT) state of the fluorinated Pt(II) complex is red-shifted compared to the monomeric emission and performs insensitive to oxygen, allowing the particles as self-referenced oxygen nanosensor in both the luminescence intensity and lifetime domains. Additionally, a triplet–triplet annihilation upconversion (TTA-UC) system was developed based on a crystalline MOF. A Pd(II) porphyrin complex acted as the sensitizer immobilized in the MOF walls, while a 9,10-diphenylanthracene annihilator was filled in the channels. Upon green light excitation at 532 nm, the resulting MOF crystalline showed an upconverted blue emission with delayed lifetime from 4 ns to 373 µs and a triplet–triplet energy transfer efficiency of 82%.