Design and Synthesis of Fluorescent Carbon-Dot Polymer and Deciphering Its Electronic Structure

Abstract

Herein, we have reported one-pot synthesis of a fluorescent polymer-like material (pCDs) by exploiting ruthenium doped carbon dot (CDs) as building blocks. The unusual spectral profiles of pCDs with double humped periodic excitation dependent photoluminescence (EDPL) and the regular changes in their corresponding average lifetime indicate the formation of high energy donor states and low energy aggregated states due to the overlap of molecular orbitals throughout the chemically switchable π-network of CDs on polymerization. To probe the electronic distribution of pCDs, we have investigated the occurrence of photoinduced electron transfer with a model electron acceptor, menadione using transient absorption technique, corroborated with low magnetic field, followed by identification of the transient radical ions generated through electron transfer. The experimentally obtained B 1/2 value, a measure of the hyperfine interactions present in the system, indicates the presence of highly conjugated π-electron cloud in pCDs. The mechanism of formation of pCDs and the entire experimental findings have further been investigated through molecular modeling and computational model. The DFT calculations demonstrated the probable electronic transitions from surface moieties of pCDs to the tethered ligands. ASSOCIATED CONTENT Supporting Information Detailed synthetic protocol, additional spectroscopic data, TEM, details of MD-simulations and theoretical calculations are available in supporting information.

Redox Modifications of Carbon Dots Shape Their Optoelectronics

“…Several recent reports suggest that the photo-luminescent (PL) behavior can be tuned by altering the synthetic technique, temperature, size, starting precursor, doping, surface functionalization etc. [21][22][23][24][25][26] For example, while many CDs show excitation wavelength dependent PL (EDPL), a direct violation of the Kasha-Vavilov's rule [27][28] , there also exist reports of excitation independent PL behavior (EIPL) in CDs.…”

“…It is believed that these functional groups act like surface energy traps and restricts the radiative decay pathway by trapping the generated photo-excited electrons. Removing the surface energy traps by surface passivation of the dangling ends often leads to an enhancement of PL by increasing the radiative decay from the surface traps, and hence increasing the excited state lifetime and quantum yield 28,30 . Several modifications to the CDs surface have been reported in the literature to tune the PL intensity [19][20] .…”

“…Similarly, the exciton/charge carrier formation upon photoexcitation, carrier mobility, parent spin state (singlet or triplet) of formation and electronhole recombination as well as their luminescence efficiency remain elusive. While the properties of the electronically injected charge carriers in the singlet excited states have been studied with some clarity, the studies on excited triplet states and the probable recombination rates of triplet and singlet excitons are very limited 28,30 . A better understanding of the plausible singlet and triplet states along with their photo-dynamics, bandgap and intersystem crossing (ISC) can open up more choices for modification and design for higher luminescence efficiency and hence their uses in LEDs and optoelectronics [39][40][41][42] .…”

“…For more than a decade since the discovery of fluorescent carbon dots (CDs) by Scrivens et al, they have become one of the most promising fluorophores of the carbon family . The unique properties of CDs such as water solubility, stability, tunable photophysics, low toxicity, and easy and inexpensive preparation have garnered much attention among the scientific and industrial communities for potential applications such as sensing, − imaging, − catalysis, medicine, , optoelectronics, − energy conversion, and storage. − Although a variety of “top-down” and “bottom-up” approaches have been developed to synthesize the CDs, the origin of their luminescent behavior is still an open question. , Several recent reports suggest that the photoluminescent (PL) behavior can be tuned by altering the synthetic technique, temperature, size, starting precursor, doping, surface functionalization, and so forth. − For example, while many CDs show excitation dependent PL (EDPL), a direct violation of Kasha–Vavilov’s rule, , there also exist reports of excitation-independent PL (EIPL) behavior in CDs.…”

Solvent Effect on Structural Elucidation of Photoluminescent Graphitic Carbon Nanodots

“… 30 , 31 Another view of such an absorption is also attributed to different charge-transfer states between individual CNDs. 23 , 32 , 33 Figure 3 a shows the normalized emission spectra of CNDs in various solvents with excitation at 365 nm. Emission maxima reported in Table 1 are in close range from 443 to 455 nm for all solvents, except in nitromethane that was red-shifted to 480 nm.…”