Prasun K. Mandal

Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission

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.

Solvation dynamics of Nile Red in a room temperature ionic liquid using streak camera

Time-resolved fluorescence behavior of Nile Red (NR) has been studied using a streak camera in a room temperature ionic liquid, [bmim]PF6. Wavelength dependent fluorescence decay and time dependent fluorescence Stokes shift have been observed. The time-correlated spectral shift function, C(t), indicator for solvation dynamics of NR in [bmim]PF6, is found to be biexponential with two well-separated time constants: 0.13 and 1.25 ns. The short and the long component of the dynamics have been explained in terms of the translational motion of the anion and a collective motion of the cation and anion, respectively. The effect of two different conformers of the cation of this room temperature ionic liquid, as revealed in the recent studies, on the long time constant is also discussed.

Fluorescence studies in environmentally benign solvents: solvation dynamics of Coumarin 102 in [BMIM][BF 4 ]

The steady-state and time-resolved fluorescence behavior of Coumarin 102 (C-102) has been investigated in a ionic liquid at room temperature, 1-buty1-3-methylimidazolium tetrafluoroborate, abbreviated here as [BMIM][BF4]. From the steady-state fluorescence maximum of C-102 in [BMIM][BF4], the polarity of this ionic liquid has been estimated to be 50.4 in the ET(30) scale, suggesting that [BMIM][BF4] is more polar than acetonitrile but less polar than methanol. In contrast to its time-resolved behavior in conventional polar solvents, C-102 exhibits wavelength-dependent fluorescence decay behavior in [BMIM][BF4] because of slow solvation dynamics in this viscous medium. The solvation dynamics has been found to be biphasic in nature consisting of picosecond and nanosecond components. In addition to these slow resolvable components, evidence is found for a very fast component of solvation, which occurs within the time resolution of the set-up (25 ps).

Dual Fluorescence in GFP Chromophore Analogues: Chemical Modulation of Charge Transfer and Proton Transfer Bands

Dual fluorescence of GFP chromophore analogues has been observed for the first time. OHIM (o-hydroxy imidazolidinone) shows only a charge transfer (CT) band, CHBDI (p-cyclicamino o-hydroxy benzimidazolidinone) shows a comparable intensity CT and PT (proton transfer) band, and MHBDI (p-methoxy o-hydroxy benzimidazolidinone) shows a higher intensity PT band. It could be shown that the differential optical behavior is not due to conformational variation in the solid or solution phase. Rather, control of the excited state electronic energy level and excited state acidity constant by functional group modification could be shown to be responsible for the differential optical behavior. Chemical modification-induced electronic control over the relative intensity of the charge transfer and proton transfer bands could thus be evidenced. Support from single-crystal X-ray structure, NMR, femtosecond to nanosecond fluorescence decay analysis, and TDDFT-based calculation provided important information and thus helped us understand the photophysics better.

Fluorescence response of mono- and tetraazacrown derivatives of 4-aminophthalimide with and without some transition and post transition metal ions

The fluorescence response of two fluorophore–spacer–receptor systems, 1 and 2, wherein a 4-aminophthalimido moiety is linked to a monoaza- and a tetraazacrown macrocycle via a dimethylene spacer, has been explored in the absence and in the presence of various transition and post transition metal ions. Unlike the parent fluorophore, these systems are weakly fluorescent because of photoinduced electron transfer (PET) between the terminal moieties of the molecules. Of the two systems, PET is found to be more efficient in 2, which contains the tetraazacrown moiety as the receptor. Both systems display complex fluorescence decay behaviour with the average fluorescence lifetime much shorter than that of the parent fluorophore. The systems exhibit fluorescence enhancement in the presence of the metal ions, some of which are notorious quenchers of fluorescence. While the fluorescence output of 1 is rather similar with different metal ions, 2 exhibits high fluorescence enhancement with metal ions such as Zn2+, Cd2+, Pb2+, Hg2+ and Mn2+ while producing negligible enhancement with the remaining metal ions studied. This specific signaling behaviour of 2 has been rationalized taking into consideration somewhat unusual binding ability of the tetraazacrown receptor moiety.

Interaction of alkali, alkaline earth and transition metal ions with a ketocyanine dye: a comparative electronic spectroscopic study.

Interaction of a dye which is structurally similar to a ketocyanine dye with metal ions (alkali, alkaline earth and transition metal) has been studied by monitoring the electronic absorption, steady state and time resolved fluorescence parameters of the dye. The dye (S(0) state) forms a 1:1 complex with cations as indicated by the appearance of a new band at a longer wavelength. Equilibrium constant and other thermodynamic parameters for complexation have been determined. The interaction between the dye and the cation is mostly electrostatic in nature. Spectroscopic results have been supplemented by DFT calculation. For very low concentration of cations, where complexation is insignificant, the absorption band of the dye undergoes a slight blue shift. Enhancement of fluorescence intensity has been observed in the same concentration range. Both phenomena have been explained in terms of formation of a weak association complex where one/more cation replace equivalent solvent molecules in the cybotatic region around the dye. The binding constant of the weak association complex involving cation and the dye (S(1) state) has been determined and has been found to depend on the charge-to-size ratio of the cations. Measurement of fluorescence lifetime of the dye indicates that the association complex is slowly decaying relative to solvated dye. At higher concentration of metal ions, however, fluorescence of the dye is quenched by the metal ions. A red shift of fluorescence maximum has also been observed in this concentration range.

Chemical tweaking of a non-fluorescent GFP chromophore to a highly fluorescent coumarinic fluorophore: application towards photo-uncaging and stem cell imaging

Three step chemical tweaking of nonfluorescent parent GFP chromophore has yielded a three hundred times as bright coumarinic fluorophore. A fluorogenic compound has been prepared which after photo-uncaging yields the same highly bright fluorophore that could be used for live stem cell imaging.

Ultrafast Dynamics of a Green Fluorescent Protein Chromophore Analogue: Competition between Excited-State Proton Transfer and Torsional Relaxation

The competition between excited-state proton transfer (ESPT) and torsion plays a central role in the photophysics of fluorescent proteins of the green fluorescent protein (GFP) family and their chromophores. Here, it was investigated in a single GFP chromophore analogue bearing o-hydroxy and p-diethylamino substituents, OHIM. The light-induced dynamics of OHIM was studied by femtosecond transient absorption spectroscopy, at different pH. We found that the photophysics of OHIM is determined by the electron-donating character of the diethylamino group: torsional relaxation dominates when the diethylamino group is neutral, whereas ultrafast ESPT followed by cis/trans isomerization and ground-state reprotonation are observed when the diethylamino group is protonated and therefore inactive as an electron donor.

Carbon Dot with pH Independent Near Unity PLQY in Aqueous Medium: Electrostatics Induced FRET at Sub-Micromolar Concentration

We report the synthesis and dynamical behavior of a carbon dot (CD) with near 100% photoluminescence quantum yield in water for a very large pH range (1-12). This CD exhibits a rotational correlational time of only ∼130 ps, signifying the whole CD is not exhibiting photoluminescence. Unlike most carbon-based nanoparticles (which act as a quencher of fluorescence), this CD could act as a donor, and the Förster model could account for the experimental observables for the resonance energy transfer (RET) experiment quite well. Based on two dynamical measurements, it could be shown that the fluorescing moiety is located inside the core of the CD. Importantly, for this CD, RET experiments could be performed with a very low concentration (500 nM) of the acceptor. This kind of electrostatics-driven RET at very low concentration is quite important in bioimaging. This ultrabright CD is nontoxic and useful for bioimaging in mesenchymal stem cells.