Effective and fast algorithms for calculating rate constants for internal conversion (IC) and intersystem crossing (ISC) in the Franck-Condon and Herzberg-Teller approximations have been developed ...and implemented. The methods have been employed for calculating IC and ISC rate constants for the pyrromethene-567 dye (
PM567
), hetero8circulene (
4B
) and free-base porphyrin (
H
2
P
). The fluorescence quantum yields obtained by comparing calculated rate constants for the radiative and non-radiative processes are in good agreement with experimental data.
Effective and fast algorithms for calculating rate constants for internal conversion (IC) and intersystem crossing (ISC) in the Franck-Condon and Herzberg-Teller approximations have been developed and implemented.
Delayed fluorescence from molecules with an inverted singlet-triplet gap (DFIST) is the consequence of the unusual reverse order of the lowest excited singlet (S
1
) and triplet (T
1
) states of ...thermally activated delayed fluorescence (TADF) emitters. Heptazine (1,3,4,6,7,9,9
b
-heptaazaphenalene) derivatives have an inverted singlet-triplet gap thanks to the combination of multiple resonance (MR) effects and a significant double excitation character. Here, we study computationally the effect of gold(
i
) metalation and coordination on the optical properties of heptazine (molecule
4
) and the phosphine-functionalized 2,5,8-tris(dimethylphosphino)heptazine derivatives (molecules
1-3
).
Ab initio
calculations at the approximate second-order coupled cluster (CC2) and extended multiconfigurational quasi degenerate perturbation theory at the second order (XMC-QDPT2) levels show that molecules
1-4
have an inverted singlet-triplet gap due to the alternating spatial localization of the electron and hole of the exciton in the heptazine core. A non-vanishing one-electron spin-orbit coupling operator matrix element between T
1
and
and a fast S
1
← T
1
intersystem crossing rate constant (
k
ISC
) calculated at the XMC-QDPT2(12,12) level of theory for molecule
4
suggest that this new family of complexes may be the first organometallic DFIST emitters reported.
Substitution with gold(
i
)-containing moieties results in non-vanishing oscillator strengths and spin-orbit coupling leading to fast intersystem crossing in light-emitting heptazine derivates with an inverted singlet-triplet gap.
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3.
When are Antiaromatic Molecules Paramagnetic? Valiev, Rashid R; Baryshnikov, Glib V; Nasibullin, Rinat T ...
Journal of physical chemistry. C,
09/2020, Volume:
124, Issue:
38
Journal Article
Peer reviewed
Open access
Magnetizabilities and magnetically induced current densities have been calculated and analyzed for a series of antiaromatic cyclo4kcarbons (k = 2–11), isonphlorins (n = 4–8), expanded ...porphyrinoids, and meso–meso, β–β, β–β triple-linked porphyrin and isophlorin arrays. The cyclo4kcarbons with k = 2–6 are predicted to be closed-shell paramagnetic molecules due to the very strong paratropic ring current combined with its large radius. Larger cyclo4kcarbons with k = 6–11 are diamagnetic because they sustain a paratropic ring current whose strength is weaker than −20 nA T–1, which seems to be the lower threshold value for closed-shell paramagnetism. This holds not only for cyclo4kcarbons but also for other organic molecules like expanded porphyrinoids and oligomers of porphyrinoids. The present study shows that meso–meso, β–β, β–β triple-linked linear porphyrin and isophlorin arrays have a domainlike distribution of alternating diatropic and paratropic ring currents. The strength of their local paratropic ring currents is weaker than −20 nA T–1 in each domain. Therefore, linear porphyrin and isophlorin arrays become more diamagnetic with increasing length of the ribbon. For the same reason, square-shaped meso–meso, β–β, β–β triple-linked free-base porphyrin and isophlorin tetramers as well as the Zn(II) complex of the porphyrin tetramer are diamagnetic. We show that closed-shell molecules with large positive magnetizabilities can be designed by following the principle that a strong paratropic current ring combined with a large ring-current radius leads to closed-shell paramagnetism.
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Aromaticity of Even-Number Cyclo[n]carbons (n = 6–100) Baryshnikov, Glib V; Valiev, Rashid R; Nasibullin, Rinat T ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
12/2020, Volume:
124, Issue:
51
Journal Article
Peer reviewed
Open access
The recently synthesized cyclo18carbon molecule has been characterized in a number of studies by calculating electronic, spectroscopic, and mechanical properties. However, cyclo18carbon is only one ...member of the class of cycloncarbonsstandalone carbon allotrope representatives. Many of the larger members of this class of molecules have not been thoroughly investigated. In this work, we calculate the magnetically induced current density of cycloncarbons in order to elucidate how electron delocalization and aromatic properties change with the size of the molecular ring (n), where n is an even number between 6 and 100. We find that the Hückel rules for aromaticity (4k + 2) and antiaromaticity (4k) become degenerate for large C n rings (n > 50), which can be understood as a transition from a delocalized electronic structure to a nonaromatic structure with localized current density fluxes in the triple bonds. Actually, the calculations suggest that cycloncarbons with n > 50 are nonaromatic cyclic polyalkynes. The influence of the amount of nonlocal exchange and the asymptotic behavior of the exchange–correlation potential of the employed density functionals on the strength of the magnetically induced ring current and the aromatic character of the large cycloncarbons is also discussed.
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Magnetically induced ring currents are calculated from the magnetic shielding tensor by employing the Ampère-Maxwell law. The feasibility of the method is demonstrated by integrating the
zz
component ...of the shielding tensor along the symmetry axis of highly symmetric ring-shaped aromatic, antiaromatic and nonaromatic molecules. The calculated ring-current strengths agree perfectly with the ones obtained by integrating the current-density flux passing through a plane cutting half the molecular ring. The method can be used in combination with all electronic structure codes capable of calculating nuclear magnetic resonance (NMR) shielding tensors in general points in space. We also show that nucleus independent chemical shifts (NICS) along the symmetry axis are related to the spatial derivative of the strength of the global ring-current along the
z
axis.
We present a method based on Ampère-Maxwell's integral law for calculating strengths of magnetically induced ring currents.
Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue ...photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated
levels of theory also show that 1 has a large inverted singlet-triplet (IST) gap, a non-vanishing spin-orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.
A theoretical model is proposed that allows the estimation of the quantum yield of phosphorescence of dye molecules in the vicinity of plasmonic nanoparticles. For this purpose, the rate constants of ...the radiative and nonradiative intramolecular transitions for rhodamine 123 (
Rh123
) and brominated rhodamine (
Rh123-2Br
) dyes have been calculated. The plasmon effect of Ag nanoparticles on various types of luminescence processes has been studied both theoretically and experimentally. We show that in the presence of a plasmonic nanoparticle, the efficiency of the immediate and delayed fluorescence increases significantly. The phosphorescence rate of the rhodamine dyes also increases near plasmonic nanoparticles. The long-lived luminescence
i.e.
, delayed fluorescence and phosphorescence is more enhanced for
Rh123-2Br
than for
Rh123
. The largest phosphorescence quantum yield is obtained when the dye molecule is at a distance of 4-6 nm from the nanoparticle surface. Our results can be used in the design of plasmon-enhancing nanostructures for light-emitting media, organic light-emitting diodes, photovoltaic devices, and catalysts for activation of molecular oxygen.
A theoretical model is proposed that allows the estimation of the quantum yield of phosphorescence of dye molecules in the vicinity of plasmonic nanoparticles.
New perhalophenyl three-coordinated gold(
i
) complexes using the chelate ligand 1,2-bis(diphenylphosphino)benzene (dppBz) and AuR(tht) (R = C
6
F
5
(
1
),
o
-C
6
BrF
4
(
2
),
p
-C
6
BrF
4
(
3
),
o
...-C
6
F
4
I (
4
),
p
-C
6
F
4
I (
5
); tht = tetrahydrothiophene) have been prepared. The crystal structures of compounds
1
and
2
consist of distorted three-coordinated Au(
i
) complexes displaying different Au-P distances at the same gold atom. The complexes show intense photoluminescent emission in the solid state at room temperature (RT) and at 77 K. The study of the dependence of the emission lifetime with temperature suggests the existence of thermally activated delayed fluorescence (TADF) processes at RT. We have computed the rate constants for intersystem crossing and reverse intersystem crossing of the photophysical processes through first-principle calculations, supporting the experimental observations with very good agreement.
Four new perhalophenylgold(
i
)-diphosphino complexes have been described as TADF emitters. The position and the electronegativity of the halogen atoms in the aromatic ring allow tuning of the photophysical properties.
Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (
1
) are studied from combined experimental and computational viewpoints. The intense blue ...photoemission of
1
in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated
ab initio
levels of theory also show that
1
has a large inverted singlettriplet (IST) gap, a non-vanishing spinorbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of
1
. IST gaps are also obtained for the oxygen-containing (
2
) and selenium-containing (
3
) analogues of
1
at the ADC(2) level of theory, but not for the tellurium one (
4
). Calculations of the magnetically induced current density demonstrate that the heptazine core of
1
is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.
The calculated energy inversion of the first excited states of a heptazine phenylthiolate molecule is consistent with the intense blue fluorescence in the solid state and in toluene solution.
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