Carbon dots (CDs) usually emit a strong blue light and excitation wavelength dependent long wavelength lights. This significantly limits their applications because one has to use a series of different excitation light sources to get different colors and the long wavelength emissions are usually very weak. We found that one type of CDs synthesized from p-phenylenediamine could emit various long wavelength lights (green to red) independent of the excitation wavelength when dispersed in different solvents. The photoluminescence quantum yields of the same CDs were 10–35% in different solvents for different color emissions. Based on this solvent-color effect, we further mixed the same CDs with different polymers to form solid CD films for various color emissions, and these film emissions were also excitation wavelength independent. Multicolor LEDs were demonstrated with the same CDs in solution and solid film states for color displays.

Excitation wavelength independent visible color emission of carbon dots

https://www.rsc.org/suppdata/cc/c1/c1cc10400d/c1cc10400d.pdf

A colorimetric and fluorescent chemosensor for the detection of an explosive—2,4,6-trinitrophenol (TNP)

Organic solid-state luminescence switching (SLS) materials with the ability to reversibly switch the luminescence by altering the mode of molecular packing without changing the chemical structures of their component molecules have attracted considerable interest in recent years. In this work, we design and synthesize a new class of 2,7-diphenylfluorenone derivatives (compounds 1–6) that exhibit prominent aggregation-induced emission (AIE) properties with high solid-state fluorescence quantum yields (29–65%). Among them, 2,7-bis(4-methoxyphenyl)-9H-fluoren-9-one (2) and 2,7-bis(4-ethylphenyl)-9H-fluoren-9-one (6) display reversible stimuli-responsive solid-state luminescence switching. Compound 2 transforms between red and yellow crystals (the emission wavelength switches between 601 and 551 nm) under the stimuli of temperature, pressure, or solvent vapor. Similarly, compound 6 exhibits SLS behavior, with luminescence switching between orange (571 nm) and yellow (557 nm). Eight X-ray single-crystal structu...

Fluorenone Organic Crystals: Two-Color Luminescence Switching and Reversible Phase Transformations between π–π Stacking-Directed Packing and Hydrogen Bond-Directed Packing

Pentacenequinone derivative 3 forms luminescent supramolecular aggregates both in bulk as well as in solution phase. In bulk phase at high temperature, long-range stacking of columns leads to formation of stable and ordered columnar mesophase. Further, derivative 3 works as sensitive chemosensor for picric acid (PA) and gel-coated paper strips detect PA at nanomolar level and provide a simple, portable, and low-cost method for detection of PA in aqueous solution, vapor phase, and in contact mode.

Self-assembled pentacenequinone derivative for trace detection of picric acid.

Excimer-induced high-efficiency fluorescence due to pairwise anthracene stacking in a crystal with long lifetime

Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrile-water mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene-modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15-20 vol % of water, and 2) aggregate formation in anthracene-modified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre-arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face-to-face, angular, and T-shaped geometry. The formation of different types of anthracene excimers was confirmed by steady-state and time-resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground-state aggregation. Most excitingly, increased pi conjugation in the molecular framework of anthracene-substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature.

Aggregation-controlled excimer emission from anthracene-containing polyamidoamine dendrimers.

Significant aggregation is observed in pyrene-modified zero- and first-generation polyamidoamine (PAMAM) dendrimers above their critical aggregation concentration (CAC, >10(-6) M). The pyrene units are attached to the dendrimer skeleton through imine bonds, which play a pivotal role in enhancing the aggregation propensity of the PAMAM dendrimers. Scanning electron microscopy studies suggest that pyrene-modified PAMAM dendrimers aggregate into doughnut-shaped assemblies. As a result of aggregation, the pyrene chromophores are pre-arranged in a face-to-face geometry in the ground state, and readily generate pyrene "static excimer" on photoexcitation. The static pyrene excimer emits with an unprecedented quantum yield of 0.62 ± 0.01 in dichloromethane, and also exhibits remarkable positive solvatochromism from 498 to 638 nm, which leads to the highest bathochromic shift for pyrene excimer emission in solution reported so far. Lippert-Mataga analysis of the system suggests that general and specific solvent effects play a crucial role in the positive solvatochromism exhibited by the system. Luminescence quenching studies on both monomer and aggregate systems were carried out in the presence of various metal ions, and the results imply that pyrene-modified PAMAM dendrimer can be utilized for selective detection of Hg ions in the presence of a wide variety of transition, alkali, and alkaline earth metal ions. This report presents the first dendrimer-based chromophoric system exhibiting positive solvatochromism over a range of 140 nm, and shows that pyrene-modified PAMAM dendrimers can be effectively utilized to generate wavelength-tunable emitting systems displaying bluish green, greenish yellow, and orange-red colors at room temperatures.

Tunable emission of static excimer in a pyrene-modified polyamidoamine dendrimer aggregate through positive solvatochromism.

Peripheral functionalization of PAMAM dendrimers with naphthalene units leads to significant ground state aggregation in the system above the critical aggregation concentrations (CAC). Upon photo-excitation of the ground state aggregates, static type excimer formation of naphthalene moiety is observed. Significant red-shifted emission from naphthalene excimers is achieved through generating the static type excimers in polar solvents such as methanol and acetonitrile-water mixtures. Control experiments suggest that the presence of dendritic scaffold in the system play a pivotal role in generating intense static excimer emission in naphthalene modified PAMAM dendrimers, in solution phase at room temperature.

/pdf/utilizing-dendritic-scaffold-for-feasible-formation-of-569cp30pib.pdf

P. K. Lekha

Papers

Aggregation-controlled excimer emission from anthracene-containing polyamidoamine dendrimers.

Tunable emission of static excimer in a pyrene-modified polyamidoamine dendrimer aggregate through positive solvatochromism.

Utilizing dendritic scaffold for feasible formation of naphthalene excimer