Core/alloy-shell/shell quantum dots (CASS QDs) have been shown to exhibit excitation energy dependent PL efficiency. The magnitude of the normalized transient population has been shown to increase more than 5-fold with decreasing the excitation energy. For high energy excitation cooling of the exciton (predominantly electron) to band edge is much slower (rise time of ∼526 fs) in comparison to low energy excitation (rise time of <∼100 fs). Time constant associated with the excitation energy dependent dynamics of the hot electron trapping is ∼1 ps. Time constant related to excitation energy independent hot hole dynamics is ∼35 ps. Truncation time obtained from single particle investigation has been shown to increase four folds (20 to 80 s) and the magnitude of additional exponential time constant responsible for hole trapping has been shown to increase nearly two folds with decrease in excitation energy. Thus, by employing ensemble level ultrafast dynamics and single particle PL blinking dynamics, it could be shown that the extent of hot electron trapping decreases, and the extent of hot hole trapping increases, as the excitation energy is lowered. Thus, the extent of the nonradiative Auger process decreases, thereby leading to enhanced PL efficiency for lower energy excitation.

The molecular origin of the photoluminescence of carbon dots (CDs) is not known. This restricts the design of CDs with desired optical properties. We have synthesized CDs starting from carbohydrates by employing a simple synthesis method. We were able to demonstrate that the CDs are composed of aggregated hydroxymethylfurfural (HMF) derivatives. The optical properties of these CDs are quite unique. These CDs exhibit an excitation-independent PL emission maximum in the orange-red region (λ ∼ 590 nm). These CDs also exhibit excitation as well as monitoring wavelength-independent single exponential PL decay. These observations indicate that only one type of chromophore (HMF derivative) is present within the CDs. Several HMF derivatives are aggregated within the CDs; therefore, the aggregated structure cause a large Stokes shift (∼150 nm). By several control experiments, we showed that the same aggregated chromophore unit (HMF derivative), and not the individual fluorophores, is the fluorescing unit. The emission maximum and the single exponential PL lifetime are independent of the polarity of the medium. The existence of a low-lying trap state could be reduced quite significantly. A model has been proposed to explain the interesting steady state and dynamical photoluminescence behaviour of the CDs. As the molecular origin of their photoluminescence is known, CDs with desired optical properties can be designed.

The molecular origin of the photoluminescence (PL) of carbon dots (CDs) is not fully understood. In this article it is shown that CDs are composed of aggregated 2-pyridone derivatives employing π–π stacking and H-bonding, etc. The PL quantum yield of this CD is quite high in aqueous medium (∼75%). Unlike literature reports the PL emission maximum of this CD is excitation wavelength independent, and PL decay follows a single-exponential decay equation. These CDs have a long PL lifetime (from ∼10 to 15 ns), so that solvation is complete before emission. The extent of trap states could be reduced quite significantly. A high PLQY and long and single-exponential PL lifetime and it’s polarity dependence would make this CD a suitable probe for FRET and FLIM. It could be shown that unlike literature reports this CD as a single particle does not blink. Unlike literature reports where CDs are bleached within a few seconds these CDs at the single-particle level are alive for about a few minutes. All these aggregation-induced much improved optical properties will make this CD a suitable optical emitter at the ensemble as well as single-particle level toward bioimaging. As the molecular origin is now known several optical properties can now be tuned.

Contrary to the belief of the chemistry community, intense charge transfer is observed between meta-oriented donor–acceptor moieties in ultrasmall single-benzenic fluorophores. Red emission in any solvent has so far been reported with molecular fluorophores having the lowest molecular weight (MW) of 252.5 Da; however, we have achieved the same with a meta-fluorophore having an MW of only 203.1 Da. Red emission in a nonpolar solvent has so far been reported with a fluorophore having a minimum MW of 464.5 Da, but we have achieved the same with a meta-fluorophore having an MW of only 255.3 Da. Intense Stokes (260 nm) and solvatochromic (160 nm) shifts, high magnitudes of fluorescence quantum yield (>0.4), and excited-state lifetime (>16 ns) have been obtained in these single-benzenic meta-fluorophores, and these values are comparatively much higher in comparison to corresponding o-/p-fluorophores. These extraordinary meta-fluorophores have been employed to measure subcellular nanopolarity in live stem cells toward cost-effective white fluorescent ink and white light-emitting diodes (LEDs) in solid state.