Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following ...electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π‐conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (CH…O, OH…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance‐assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.
This article is categorized under:
Structure and Mechanism > Molecular Structures
Structure and Mechanism > Reaction Mechanisms and Catalysis
Hydrogen bonds are chemical interactions that can bind molecules and molecular fragments together to create elaborate structures and functions. In the past 30 years, many hydrogen bond design principles have emerged, and together they propel the field of supramolecular chemistry.
Here we report five blue‐phosphorescent platinum bis‐phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1–3) are of the ...general formula cis‐Pt(CNR)2(C≡CPh)2, in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis‐Pt(CNR)(ADC)(C≡CPh)2 (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λmax ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1–3 exhibit modest photoluminescence quantum yields (ΦPL), incorporation of one acyclic diaminocarbene ligand results in a three‐fold to 16‐fold increase in ΦPL while still maintaining an identical deep blue color profile.
The rising of the photoluminescence quantum yield: This work describes a new design for blue‐phosphorescent platinum acetylide complexes, using strong σ‐donor acyclic diaminocarbene (ADC) auxiliary ligands. Conversion of one isocyanide in the precursor complex to and ADC, by nucleophilic addition of a secondary amine, engenders up to a 16‐fold increase in photoluminescence quantum yield.
Cyclometalated iridium complexes have emerged as top-performing emitters in organic light-emitting diodes (OLEDs) and other optoelectronic devices. A persistent challenge has been the development of ...cyclometalated iridium complexes with deep blue luminescence that have the requisite color purity, efficiency, and stability to function in color displays. In this work we report a new class of cyclometalated iridium complexes with saturated blue luminescence. These complexes have the general structure Ir(C^C:
NHC
)
2
(C^C:
ADC
), where C^C:
NHC
is an N-heterocyclic carbene (NHC) derived cyclometalating ligand and C^C:
ADC
is a different type of cyclometalating ligand featuring an acyclic diaminocarbene (ADC). The complexes are prepared by a cascade reaction that involves nucleophilic addition of propylamine to an isocyanide precursor followed by base-assisted cyclometalation of the ADC intermediate. All three emit deep blue light with good quantum efficiencies (
Φ
PL
= 0.13-0.48) and color profiles very close to the ideal primary blue standards for color displays.
A new structural class of mixed-carbene cyclometalated iridium complexes with intense, high-purity blue luminescence are described.
A combined computational and experimental study reveals that
ortho
-,
meta
- and
para
-aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium ...exchange experiments show that the
ortho
-isomer undergoes a facile photoprotonation at a carbon atom
via
excited-state intramolecular proton transfer (ESIPT). The
meta
-isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction
via
proton-coupled electron transfer (PCET). The
para
-isomer undergoes a water-assisted ESPT reaction. All three reactions take place in the singlet excited-state, except for the photoredox process of the
meta
-isomer, which involves a triplet excited-state. Computations illustrate the important role of excited-state antiaromaticity relief in these photoreactions.
ortho
-,
meta
- and
para
-aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer, which are driven by excited-state antiaromaticity relief.
Dewar proposed the σ‐aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised ...doubts—“There is no need to involve ‘σ‐aromaticity’,”—other analyses, also indirect, resulted in wide‐ranging estimates of the σ‐aromatic stabilization energy. Moreover, the aromatic character of “in‐plane”, “double”, and cyclically delocalized σ‐electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A 2007, 111, 8163–8169) challenged the existence of an induced σ‐ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the σ‐aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc‐PVTZ level, the σ‐aromatic stabilization energy of cyclopropane is, at most, 3.5 kcal mol−1 relative to propane, and is close to zero when n‐butane is used as reference. Trisilacyclopropane also has very little σ‐aromatic stabilization, compared to Si3H8 (6.3 kcal mol−1) and Si4H10 (4.2 kcal mol−1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.
The existence of σ aromaticity in cyclopropane (see picture) has been challenged by recent magnetic analysis. Ab initio valence bond computations reveal directly that the σ‐aromatic stabilization energy of cyclopropane is at most 3.5 kcal mol−1 relative to propane. This small energy difference raises the question whether there is any need to invoke the concept of σ aromaticity for cyclopropane.
The nature of the bonding and the aromaticity of the heavy Group 14 homologues of cyclopropenylium cations E3H3+ and E2H2E′H+ (E, E′=C–Pb) have been investigated systematically at the BP86/TZ2P DFT ...level by using several methods. Aromatic stabilization energies (ASE) were evaluated from the values obtained from energy decomposition analysis (EDA) of charged acyclic reference molecules. The EDA‐ASE results compare well with the extra cyclic resonance energy (ECRE) values given by the block localized wavefunction (BLW) method. Although all compounds investigated are Hückel 4n+2 π electron species, their ASEs indicate that the inclusion of Group 14 elements heavier than carbon reduces the aromaticity; the parent C3H3+ ion and Si2H2CH+ are the most aromatic, and Pb3H3+ is the least so. The higher energies for the cyclopropenium analogues reported in 1995 employed an isodesmic scheme, and are reinterpreted by using the BLW method. The decrease in the strength of both the π cyclic conjugation and the aromaticity in the order C≫Si>Ge>Sn>Pb agrees reasonably well with the trends given by the refined nucleus‐independent chemical shift NICS(0)πzz index.
Sniffing out aromaticity: The π aromaticity of homologues of the cyclopropenylium cations E3H3+ and E2H2E′H+ (E, E′=C–Pb) was evaluated. All the compounds follow the 4n+2 Hückel aromaticity rule, but their energetic stabilization varies and the aromaticity and degree of cyclic π‐electron conjugation decreases in the order C≫Si>Ge>Sn>Pb. Orbital plots reveal that the occupied π MO responsible for aromaticity is increasingly localized on going from carbon to lead.
Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are 4n + 2 π-aromatic in the ground state, become 4n + 2 ...π-antiaromatic in the first ¹ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S₁ (¹ππ*) state, but not in the “aromatic” S₂ (¹ππ*) state. Stokes’ shifts of structurally related compounds e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.
Photoacids like substituted naphthalenes (X = OH, NH
3
+
, COOH) are aromatic in the S
0
state and antiaromatic in the S
1
state. Nucleus independent chemical shifts analyses reveal that ...deprotonation relieves antiaromaticity in the excited conjugate base, and that the degree of "antiaromaticity relief" explains why some photoacids are stronger than others.
Photoacids like substituted naphthalenes (X = OH, NH
3
+
, COOH) are aromatic in the S
0
state and antiaromatic in the S
1
state. Antiaromaticity relief explains why some are more photoacidic than others.
Density functional theory computations suggest that formally non-aromatic organic dyes, like diketopyrrolopyrrole, naphthodipyrrolidone, indigo, and isoindigo, show increased 4
n
π-antiaromatic ...character and decreased LUMO orbital energies upon hydrogen bonding, making them suitable molecular candidates for applications in n-type organic field effect transistors.
Hydrogen bonding increases antiaromaticity and lowers the LUMO energy levels of non-aromatic π-conjugated cores.
Is it possible to achieve molecules with starlike structures by replacing the H atoms in (CH)nq aromatic hydrocarbons with aluminum atoms in bridging positions? Although D4h C4Al42− and D2 C6Al6 are ...not good prospects for experimental realization, a very extensive computational survey of fifty C5Al5− isomers identified the starlike D5h global minimum with five planar tetracoordinate carbon atoms to be a promising candidate for detection by photoelectron detachment spectroscopy. BOMD (Born–Oppenheimer molecular dynamics) simulations and high‐level theoretical computations verified this conclusion. The combination of favorable electronic and geometric structural features (including aromaticity and optimum C–Al–C bridge bonding) stabilizes the C5Al5− star preferentially.
Star chemistry! A molecular star (C5Al5−) has been identified computationally (see figure). The energetics at high level and extensive explorations at the DFT level on its potential energy surface indicate that the starlike structure could be a global minimum, which makes it promising for experimental realization.
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