Letter pubs.acs.org/JPCL
Origin of Excitation Dependent Fluorescence in Carbon Nanodots Arjun Sharma,† Trilochan Gadly,‡ Alka Gupta,§ Anand Ballal,§ Sunil Kumar Ghosh,‡ and Manoj Kumbhakar*,† †
Radiation & Photochemistry Division, ‡Bio-organic Division, and §Molecular Biology Division, Bhabha Atomic Research Center, Mumbai 400085, India S Supporting Information *
ABSTRACT: The fascinating aspect of excitation dependent fluorescence in carbon nanodots has led to several hypotheses, starting from particle size distribution to the presence of different emissive states and even to sluggish solvent relaxation around nanodot. In this contribution we provide definitive evidence for the involvement of discrete multiple electronic states for the excitation dependent emission in carbon nanodots. The presence of different types of aggregates even at very dilute solutions used in ensemble fluorescence spectroscopy, where fluorescence intensity shows linear dependence with absorbance, is the origin of these multiple electronic states. Inhomogeneous broadening due to slow solvent relaxation leading to excitation dependent spectral shift has negligible influence in conventional solvents.
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due to incomplete solvation or red-edge effect in very high viscosity solvents, polymers, organized assemblies, etc., as reported with regular fluorophore.19 Red edge effect is observed only in the case of slow relaxation from distributed excited states, and this effect is not resolved from author’s experiments.20−22 Moreover, ground state heterogeneity, as reported in other cases, is also expected to result in excitation dependent spectral shift and needs to be duly considered.20,21 Therefore, despite several promises, slower solvent relaxation leading to multicolor fluorescence in CNDs fails to explain the observed discrete energy levels at different excitation wavelengths (Figures 2 and 3 in ref 16). Second, reorientation of solute, i.e., anisotropy decay time, is independent of excited state energy relaxation by solvation. Additionally, if the identity of CNDs remains the same over the entire emission spectrum, it is impossible to explain the observed significant variation of anisotropy decay time with wavelength. This is because slower solvation only influences the measured initial anisotropy value, but not the reorientation time.23 Moreover, incomplete solvation up to around 20 ns after photoexcitation of CND in regular polar media is astonishingly slow in comparison to 100 nm) for CNDs is way beyond the generally observed shift of ∼10 nm © 2016 American Chemical Society
Received: August 9, 2016 Accepted: September 2, 2016 Published: September 2, 2016 3695
DOI: 10.1021/acs.jpclett.6b01791 J. Phys. Chem. Lett. 2016, 7, 3695−3702
Letter
The Journal of Physical Chemistry Letters The present work has been undertaken to address this intense debate on the origin of large excitation-dependent fluorescence spectral shift. Our results highlight significant fundamental insight into the photoluminescence of CNDs from different steady-state and time-resolved ensemble spectroscopic investigations and has substantiated various discrete proposals suggested for the exotic observation of huge excitation dependent spectral shift, without violating the classical Kasha−Vavilov rule.23,27,28 CNDs investigated were synthesized from citric acid and urea using simple heating mantle under nitrogen (CND1) and air atmosphere (CND2). For amino-rich CNDs, citric acid and diaminopropane under nitrogen atmosphere were used as precursor (CND3). The highly pure polar fractions of these CNDs are used for the present investigation. All the spectroscopic measurements were performed with very dilute solutions (550 nm) shows the shortest fluorescence lifetime, while the intense blue emissive states (at
