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Related Concept Videos

Band Theory02:35

Band Theory

When two or more atoms come together to form a molecule, their atomic orbitals combine and molecular orbitals of distinct energies result. In a solid, there are a large number of atoms, and therefore a large number of atomic orbitals that may be combined into molecular orbitals. These groups of molecular orbitals are so closely placed together to form continuous regions of energies, known as the bands.
The energy difference between these bands is known as the band gap.
Conductor, Semiconductor,...
Energy Bands in Solids01:01

Energy Bands in Solids

Isolated atoms have discrete energy levels that are well described by the Bohr model. And, it quantifies the energy of an electron in a hydrogen atom as En. Higher quantum numbers 'n' yield less negative, closer electron energy levels.
 Band Formation:
When atoms are brought close together, as in a solid, these discrete energy levels begin to split due to the overlap of electron orbitals from adjacent atoms. This split occurs because of the Pauli exclusion principle, which states that no two...
Anionic Chain-Growth Polymerization: Mechanism01:04

Anionic Chain-Growth Polymerization: Mechanism

The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael acceptor.
Anionic Chain-Growth Polymerization: Overview01:20

Anionic Chain-Growth Polymerization: Overview

The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance,...
Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...
Semiconductors01:22

Semiconductors

There is variation in the electrical conductivity of materials - metals, semiconductors, and insulators that are showcased with the help of the energy band diagrams.
Metals such as copper (Cu), zinc (Zn), or lead (Pb) have low resistivity and feature conduction bands that are either not fully occupied or overlap with the valence band, making a bandgap non-existent. This allows electrons in the highest energy levels of the valence band to easily transition to the conduction band upon gaining...

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Updated: May 27, 2026

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations
13:56

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations

Published on: October 12, 2019

Atomistic band gap engineering in donor-acceptor polymers.

Gregory L Gibson1, Theresa M McCormick, Dwight S Seferos

  • 1Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.

Journal of the American Chemical Society
|December 2, 2011
PubMed
Summary
This summary is machine-generated.

We synthesized donor-acceptor copolymers, varying sulfur to selenium to tellurium atoms. This single atom substitution allows tuning of optical band gaps, offering new control over polymer properties beyond traditional theory.

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids

Published on: May 27, 2020

Area of Science:

  • Materials Science
  • Organic Electronics
  • Polymer Chemistry

Background:

  • Donor-acceptor (D-A) copolymers are crucial in organic electronics.
  • Tuning electronic properties of D-A copolymers is essential for device performance.
  • The impact of heavy atom substitution on D-A copolymer properties requires further investigation.

Purpose of the Study:

  • To synthesize and characterize a series of D-A copolymers with systematic sulfur (S), selenium (Se), and tellurium (Te) substitution.
  • To investigate the effect of single atom substitution (S, Se, Te) on the optical and electronic properties of D-A copolymers.
  • To understand the underlying mechanisms governing these property changes using computational and spectroscopic methods.

Main Methods:

  • Synthesis of S-, Se-, and Te-containing D-A copolymers.
  • Optical spectroscopy (UV-Vis absorption) to determine optical transitions and band gaps.
  • Solvatochromism studies to probe environmental effects on optical properties.
  • Density Functional Theory (DFT) and time-dependent DFT (TD-DFT) calculations for theoretical analysis.

Main Results:

  • Successful synthesis of S-, Se-, and Te-containing D-A copolymers, with Te-polymers requiring post-polymerization substitution.
  • Observation of a dual-band optical absorption profile in all polymers.
  • Significant red-shift in the low-energy optical transition and a decrease in its intensity with increasing atomic number (S to Se to Te).
  • Band gaps decreased from 1.59 eV (S) to 1.46 eV (Se) to 1.06 eV (Te).
  • High-energy band remained relatively constant in energy and intensity.
  • Observed trends deviate from standard D-A theory, suggesting additional factors influence optical properties.

Conclusions:

  • Single atom substitution (S, Se, Te) in D-A copolymers provides a novel strategy for band gap engineering.
  • The red-shift in low-energy absorption is attributed to decreased ionization potential, increased bond length, and reduced acceptor aromaticity.
  • The decrease in low-energy band intensity is linked to reduced electronegativity and charge separation capability of the acceptor unit.
  • Established D-A theory needs to be augmented with considerations of single atom substitution effects for comprehensive property control.