Manifestation of Donor-Acceptor Properties of N-Doped Polymer Carbon Dots During Hydrogen Bonds Formation in Different Solvents

Abstract

The effective use of polymer carbon dots (PCD) in various fields of science and technology requires a more detailed understanding of the mechanisms of their photoluminescence formation and change as a result of their interaction with the environment. In this study, PCD synthesized via a hydrothermal method from citric acid and ethylenediamine are studied in various solvents using FTIR spectroscopy, optical absorption spectroscopy, and photoluminescence spectroscopy. As a result of the analysis of the obtained dependencies of such PCD spectral characteristics as the photoluminescence FWHM, the photoluminescence quantum yield, the photoluminescence lifetime on the acidity and basicity of the solvent, a hypothesis was formulated on the formation mechanism of hydrogen bonds between the PCD surface groups and the molecules of the environment, and conclusions were made about the donor-acceptor nature of the synthesized PCD.

Keywords: absorption spectroscopy; acidity; basicity; donor–acceptor properties; hydrogen bonds; photoluminescence; photoluminescence lifetime; photoluminescence quantum yield; polymer carbon dots.

Figure 1

FTIR absorption spectra of PCD powders with varying nitrogen content evaporated from water.

Figure 2

PL and optical absorption spectra of all PCD in water (a,c) and isopropanol (b,d), and PL and optical absorption spectra of PCD with EDA:CA = 0.1 (e,g) and EDA:CA = 20 (f,h) in all the studied solvents.

Figure 2

PL and optical absorption spectra of all PCD in water (a,c) and isopropanol (b,d), and PL and optical absorption spectra of PCD with EDA:CA = 0.1 (e,g) and EDA:CA = 20 (f,h) in all the studied solvents.

Figure 3

Dependence of the PL Stokes shift on the orientational polarizability of the solvent for all samples. The values of Δf are presented in Table 1.

Figure 4

Dependencies of the PCD photoluminescence FWHM and PLQY on the acidity (a,c) and basicity (b,d) of the solvent. The errors in the calculated PLQY and measured PL FWHM are 3% and 2% of the corresponding values of these parameters for all PCD solutions.

Figure 5

PCD photoluminescence decay kinetics in different solvents.

Figure 6

Dependencies of the PCD PL lifetimes on the acidity and basicity of the solvents (errors in the calculated PCD PL lifetimes are 0.3% of the calculated values for all solutions).

Figure 7

Dependencies of the F1 and F2 percentage contributions into the total PCD PL intensity on the acidity and basicity of the solvent (errors in the calculated contributions are 0.5% of the contributions themselves for all solutions).

Figure 8

FTIR absorption spectra of PCD (class PCD1) treated with different solvents. On the right is an enlarged fragment of the FTIR spectra in the 1650–1800 cm⁻¹ region, normalized to the maximum of the band for visual clarity.

Figure 9

Dependencies of the polymer carbon dots PL FWHM and PLQY on the C=O band peak position in the FTIR absorption spectra for PCD1 solutions (sample with a precursor ratio of 0.1:1).