External Heavy-Atom Effect via Orbital Interactions Revealed by Single-Crystal X-ray Diffraction

Article abstract of DOI:10.1021/acs.jpca.6b03867

Enhanced spin-orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as "base" or "acid" relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ? MO of the C-X bond (π/σ?) and n electron donation to a π? MO of the quinolinium moiety (n/π ?) are responsible for the EHE (RTP) in the solid state, respectively.

Full text of DOI:10.1021/acs.jpca.6b03867

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Article
External Heavy Atom Effect via Orbital Interactions
Revealed by Single-Crystal X-Ray Diffraction
Xingxing Sun, Baicheng Zhang, Xinyang Li, and Guoqing Zhang
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03867 • Publication Date (Web): 18 Jun 2016
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External Heavy Atom Effect via Orbital Interactions
Revealed by SingleꢀCrystal XꢀRay Diffraction
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Xingxing Sun,1,2 Baicheng Zhang,2 Xinyang Li2 and Guoqing Zhang2*
1CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, Hefei, 230026, China
2BioꢀX Interdisciplinary Sciences, Hefei National Laboratory for Physical Sciences at the
Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui,
230026, China
ABSTRACT: Enhanced spinꢀorbit coupling through external heavyꢀatom effect (EHE) has been
routinely used to induce roomꢀtemperature phosphorescence (RTP) for purely organic molecular
materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions
between the external heavy atom and the luminophore, is of essential importance in molecular
design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy
atoms serving to realize the EHEꢀrelated RTP. In this report, we conduct an investigation on how
heavyꢀatom perturbers and aromatic luminophores interact based on data obtained from
crystallography. We synthesized two classes of molecular systems including Nꢀhaloalkylꢀ
substituted carbazoles and quinolinium halides, where the luminescent molecules are considered
as “base” or “acid” relative to the heavyꢀatom perturbers, respectively. We propose that electron
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donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the CꢀX bond (π/σ*)
and n electron donation to a π* MO of the quinolinium moiety (n/ π*) are responsible for the
EHE (RTP) in the solid state, respectively.
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Introduction.
The external heavy atom effect (EHE) in the field of molecular spectroscopy has been
extensively studied since the early 1950s.1 The EHE is typically manifested as enhanced spinꢀ
orbit coupling, which increases the rates of both intersystem crossing (ISC) and phosphorescence
decay2 of the excited state luminophore. It is considered that the EHE is executed through the
orbital interactions between the heavyꢀatom perturber and the luminophore.3 Although the EHE
has been routinely used to boost molecular phosphorescence4,5,6,7,8, 9,10,11,12 for various
applications,13,14,15 very few reports are focused on the mechanistic perspective.16
Here, we synthesized two types of organic aromatic molecules, carbazole and quinolinium
derivatives (Chart 1), based on the relative energyꢀlevel of the HOMO (or basicity) / LUMO (or
acidity) and combine them with heavyꢀatom perturbers to generate roomꢀtemperature
phosphorescence (RTP)17,18 in the solid state. The orbital interactions can easily be revealed from
singleꢀcrystal Xꢀray diffraction data, in addition to other complementary evidence such as optical
and NMR spectroscopy. For the “basic” carbazole, we found that the π orbital interacts with the
antiꢀbonding orbital (σ*) with the alkyl halide (CꢀX); whereas for the “acidic” quinolinium, its
π* orbital interacts with filled p orbital from the counterion halide (Xꢀ) strongly to generate
almost purely roomꢀtemperature phosphorescence (RTP). As can be expected, stronger EHE is
observed for better orbital match in addition to the wellꢀknown factor of atomic number.
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Chart 1. Representative structures of carbazoleꢀbased “nꢀσ*”and quinoliniumꢀbased “nꢀπ*”
organic luminescent molecules.
Results and Discussion.
The halogenꢀsubstituted Nꢀalkyl carbazole derivatives (CZꢀnCꢀX, n = 2, 4, 6; X = Cl, Br, I)
were easily synthesized from carbazole and various endꢀsubstituted alkyl halides in the presence
of a phaseꢀtransfer catalyst, tetrabutyl ammonium bromide. The reaction was conducted at 50 °C
for 24 h and the crude product was purified by column chromatography (ethyl acetate/ petroleum
ether = 1/20, v/v), yielding a colorless solid (Supporting Information, SI). The quinolinium salt
(Qs) derivatives were synthesized via the SN2 substitution reaction between various quinolines
and corresponding alkyl halides in acetone at room temperature for 24 h, and then the product
was obtained by filtration and recrystallization. The structures of all compounds were analyzed
with singleꢀcrystal xꢀray diffraction (SCꢀXRD).
The steadyꢀstate photoluminescence emission spectra of Nꢀalkyl carbazoles are shown in
Figure 1a and Table 1, where all compounds are emissive in the solid state with the exception of
Nꢀsubstituted iodoethyl carbazole (CZꢀ2CꢀI). The higher energy band from 360 nm to 460 nm
with distinct carbazole vibronic features is ascribed to fluorescence (τ = 0.56 ns ꢀ 16.1 ns); the
lower energy counterpart from 525 to 625 nm is contributed by RTP (τ = 32 ms ꢀ 700 ms).
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Compared to carbazole emission in dilute solution,19 the relative intensity of the vibronic peaks
varies significantly presumably due to coupled vibrational transitions in the aggregation state of
the luminophores.20 As expected, the EHE is the weakest for Clꢀsubstitution and strongest for the
iodoꢀsubstituted carbazole, based on the relative intensity between the fluorescence and RTP
bands, which results in distinct color change from blue to orange to the naked eye under a UV
lamp (Figure 1b). The Cl derivatives, CZꢀ2CꢀCl and CZꢀ4CꢀCl, exhibit very weak
phosphorescence so that the RTP peak could only be observed in the amplified emission spectra
from 530 nm to 600 nm (inset of Figure 1a). It is also expected that the shorter chain derivatives
exhibit stronger EHE as well due to higher heavyꢀatom concentrations: e.g., for the Br
derivatives, CZꢀ6CꢀBr, CZꢀ4CꢀBr and CZꢀ2CꢀBr: when the alkyl chain length is decreased, the
RTP/fluorescence ratio gradually increases.
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Other than these wellꢀknown general features of EHE, some interesting subtle spectroscopic
features can also be observed from Figure 1a. In terms of the energy of fluorescence and RTP,
the shorter chain derivatives exhibit more blueꢀshifted emissions (e.g., 371 nm and 373 nm in the
fluorescence region and 544 nm and 555 nm in the RTP region for CZꢀ2CꢀBr and CZꢀ4CꢀBr,
respectively). Representative fluorescence and RTP lifetime decays are shown in Figure 1c and
1d. In the fluorescence region, the lifetime is 16.1 ns, 5.97 ns and 0.56 ns for CZꢀ4CꢀCl, CZꢀ4Cꢀ
Br and CZꢀ4CꢀI. In the RTP region, the measured lifetimes are 100 ms, 85 ms and 32 ms,
respectively. The trend is obvious for both fluorescence and RTP: both exhibit drastically
decreased lifetime values (e.g., fluorescence: from 11.86 ns to 0.56 ns; RTP: from 700 ms to 32
ms) for heavierꢀelement substitution although the quantum yields do not decease as much (i.e.,
10.8% to 3.6%, Table 1), which is ascribed to more pronounced EHE from the spinꢀorbit
coupling. All fluorescence decays can be fitted with three exponents, which is possibly due to the
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formation of various aggregates in the ground state and/or in the solid state. Shorterꢀlived
components are more prominent for Brꢀ and Iꢀcontaining derivatives as a result of the heavyꢀ
atom quenching effect. For the RTP decays, however, the decay is much closer to singleꢀ
exponential since triplet dimers or aggregates are less likely to form.
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Figure 1. a) Normalized steadyꢀstate photoluminescence emission spectra of various Nꢀalkyl
carbazole derivatives. Inset, amplified emission spectra of CZꢀ2CꢀCl and CZꢀ4CꢀCl from 530
nm to 600 nm. b). Photos of CZꢀ4CꢀCl (left), CZꢀ4CꢀBr (middle) and CZꢀ4CꢀI (right) crystals
emission under 365ꢀnm UV light. c) and d)Timeꢀresolved fluorescence and roomꢀtemperature
phosphorescence emission of CZꢀ4CꢀCl, CZꢀ4CꢀBr and CZꢀ4CꢀI crystals.
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Table 1: Luminescent characterization of carbazole derivatives in solid state.
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τFl1
τFl2 τFl3
τRTP1 τRTP2 τRTP3
(ms)/ (ms)/ (ms)/
λFl
λRTP
ΦFl(%)e ΦRTP(%)e
b
d
τFl0
τRTP
(ns)/ (ns)/ (ns)/
(%) (%) (%)
(nm)a
(nm)c
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(%) (%)
(%)
406,
429
6.34/ 13.2/ 12.5/
10.65 2.27 87.08
700/
100
CZꢀ
2CꢀCl
11.86 542
16.10 551
700
100
230
85
10.7
9.1
5.0
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0.1
<0.01
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410,
438
20.2/ 15.8/ 19.1/
5.42 92.78 1.80
100/
100
CZꢀ
4CꢀCl
CZꢀ
2Cꢀ
Br
371,
406
1.87/ 4.03/ 3.95/
15.53 3.81 80.66
544,
3.63
180/ 310/
592 64.19 35.81
CZꢀ
4Cꢀ
Br
3.07/ 7.78/ 17.8/
64.30 23.49 12.21
555,
604
85/
100
373
372
372
5.97
9.15
0.56
0.8
CZꢀ
6Cꢀ
Br
1.92/ 6.09/ 14.5/
5.52 55.30 39.18
551,
604
410/
100
410
32
0.5
1.13/ 0.12/ 5.40/
12.63 81.37 6.00
556,
24/ 51 /
604 35.80 42.03 22.17
7.6 /
CZꢀ
4CꢀI
2.4
a. Fluorescence emission peak excited at excitation maxima.
b. Apparent preꢀexponential weightꢀaveraged lifetime at emission maximum.
c. RTP emission maximum excited at excitation maxima.
d. Apparent preꢀexponential weightꢀaveraged lifetime at emission maximum.
e. Absolute photoluminescence quantum yields under air at 298 K.
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Figure 2. a) Normalized steadyꢀstate photoluminescence emission spectra of various
quinolinium salts with PF6 and halides as the counterꢀions: nonꢀsubstituted (QPF6 and QBr), 6ꢀ
methoxylsubsituted (MeOQPF6 and MeOQBr) and 6ꢀtrifluoromethylꢀsubstituted (CF3QBr and
CF3QI) quinolinium salts. b). Photos of QPF6 (left), CF3QBr (middle) and CF3QI (right) solid
emission under 365 nm UV light. c) RTP decay profiles for CF3QBr and CF3QI.
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Table 2: Luminescent characterization of Qs in the solid state.
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QPF6
QBr
MeOQPF6
MeOQBr
CF3QBr
533
CF3QI
λem (nm)a
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412
551
411
547
615
0.056 ns/
2.40
84 ꢁs/
38.04
4.17 ns/
58.87
173 ꢁs/
26.61
54 ꢁs/ 29.14
15 ꢁs/ 58.52
1.44 ꢁs/
100
τ1 / (%)
2.94 ns/
13.87
467 ꢁs/
54.10
1.20 ns/
11.12
433 ꢁs/
61.72
τ2 / (%)
τ3 / (%)
τb
12.9 ns/
83.73
14 ꢁs/ 7.86
11.3 ns/
30.01
1505 ꢁs/
11.6
450 ꢁs/
12.34
11.2 ns
286 ꢁs
5.96 ns
490 ꢁs
80.4 ꢁs
1.44 ꢁs
a. Photoluminescence emission peak excited at excitation maxima.
b. Apparent preꢀexponential weightꢀaveraged lifetime at emission maximum in the solid state.
We then performed the same experiments for all the quinolinium salts (Qs, Figure 2 and Table
2), where most Qs solids are emissive except for 1ꢀethylquinolinꢀ1ꢀium iodide (QI) and 1ꢀethylꢀ
6ꢀmethoxyquinolinꢀ1ꢀium iodide (MeOQI). As shown in Figure 2a, we found that the counterꢀ
ion plays a significant role on the energy of the emission spectrum for the Qs, where the most
blueꢀshifted emissions belong to the crystals with PF6 counterꢀions such as QPF6 (λem = 412 nm)
and MeOQPF6 (λem = 411 nm). On the other hand, Qs with halides counterꢀions have much redꢀ
shifted emission peaks with the trifluoroꢀQs iodide (CF3QI) exhibiting the most redꢀshifted
emission (λem = 615 nm). The photoluminescence lifetimes of these Qs crystals were also
measured. It was found that Qs with the PF6 counterꢀion have emission lifetimes in the ns range,
e.g., QPF6 (τ = 11.2 ns) and MeOQPF6 (τ = 5.96 ns), which is typical of fluorescence; while
those of Qs with the halide counterꢀions are much longerꢀlived (ꢀsꢀms), which are ascribed to
RTP. The results can be interpreted by a combination of chargeꢀtransfer (CT) transition and EHE.
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For QPF6 and MeOQPF6, there is no heavy atom perturbation nor significant CT involved;
mostly the fluorescence emission characteristic πꢀπ* arising from the quinolinium ring can be
observed. For the QsX crystals, a combination of CT and EHE can result in the prominent
emergence of triplet emissions. For instance, in the photoluminescence decay of CF3QBr, a fast
component in the ꢀs domain and a slower one in the ms range are both present (Figure 2c and
Table 2). Whereas for the iodide counterpart, a singleꢀexponent decay with a fitted apparent
lifetime of 1.44 ꢀs was recorded. Since the bromide anion is a weaker electron donor compared
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to iodide, it is then possible that 3CT is slightly above the Qs 3πꢀπ* state, both ꢀs and ms
lifetimes corresponding to 3CT and 3πꢀπ* could be resolved. However, the CF3QI 3CT is well
below the Qs 3πꢀπ* state and thus a single exponent decay from 3CT is observed. The results are
consistent with a previous report by Kosower et al,21 where the presence of multiple chargeꢀ
transfer bands for pyridinium iodide is observed.
Figure 3. Molecular packing of CZꢀ2CꢀCl, CZꢀ2CꢀBr, CZꢀ2CꢀI, CZꢀ4CꢀCl, CZꢀ4CꢀBr, CZꢀ
4CꢀI and CZꢀ6CꢀBr crystals (red dotted lines denote the weak interactions).
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To further understand the structureꢀphotoluminescence properties relationship in the
crystalline state, singleꢀcrystal Xꢀray Diffraction (SCꢀXRD) measurements were performed for
all the carbazole and Qs compounds. The molecular packings of carbazole derivatives are
presented in Figure 3. The nitrogen atom on the carbazone ring seems to be in close proximity
with the CꢀX moiety, which is likely due to the πꢀσ* interaction. Specifically, from the aspect of
the linker length, for the ethylene linker (CZꢀ2CꢀCl, CZꢀ2CꢀBr, CZꢀ2CꢀI), the CꢀX “tail” all
folds back onto the carbazole ring, which is a highꢀenergy conformation because of the steric
effect. It is possible that the stabilized energy from the intramolecular πꢀσ* interaction over
compensates the energy rise from sterics. For the longerꢀlinker derivatives (CZꢀ4CꢀCl, CZꢀ4Cꢀ
Br, CZꢀ4CꢀI, CZꢀ6CꢀBr), intermolecular contact via this πꢀσ* interaction is preferred since
both orbital stabilization and sterics can both be achieved. In addition, for CZꢀ2CꢀBr, CZꢀ4Cꢀ
Br and CZꢀ6CꢀBr, the distances between the nitrogen atom and bromine atom are 3.346 Å,
3.750 Å and 4.483 Å, respectively, indicating gradually weaker πꢀσ* interactions. The results are
consistent with the spectroscopic data, where the RTP ratio decreases from CZꢀ2CꢀBr to CZꢀ
4CꢀBr to CZꢀ6CꢀBr. On the other hand, for different halogen derivatives with the same linker
length (4C), the distance between the halogen and nitrogen atom increases (3.694 Å, 3,750 Å and
3.950 Å for CZꢀ4CꢀCl, CZꢀ4CꢀBr and CZꢀ4CꢀI). Considering the increasing atom radius of Cl
(1.0 Å), Br (1.15 Å) and I (1.40 Å),22 the distance of ArC···HCH (another weak interaction
denoted by the red dotted line) actually decreases from 2.859 Å to 2.830 Å to 2.791 Å from CZꢀ
4CꢀCl to CZꢀ4CꢀBr to CZꢀ4CꢀI, which could be related to the increasingly stronger πꢀσ*
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interaction, given that the antiꢀbonding σ* orbital primarily resides at the back of the carbon
atom (the opposite side of the halogen atom).
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To further back the πꢀσ* interaction hypothesis, we studied the UVꢀVis, fluorescence and 1Hꢀ
NMR spectroscopy of all the compounds in dilute solutions. For the UVꢀVis absorption spectra,
Figure 4 and Table 3, the three carbazole compounds with the ethylene linker, irrespective of the
halogen type, exhibit the most blueꢀshifted absorption (λabs = 326 nm) at the lowest transition,
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followed by the counterparts with the butylene linker (λabs = 330 nm) and then the hexylene
derivative (λabs = 332 nm). The fluorescence emission spectra exhibit the same trend with
decreasing emission energy from C2 to C4 to C6ꢀlinker derivatives. A possible explanation is
that the nearby nitrogen n orbital and CꢀX σ* mixing can lead to decreased potential energy for
the lone pair participating the conjugation and thus the πꢀπ* transition is blueꢀshifted. However,
the orbitals mixing between the p electrons on the halogen and the carbazole π* may also result
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in increased energy for π*. To further elucidate the interaction, HꢀNMR spectra were
investigated in CDCl3 solution as well. The results in Figure 4c clearly indicate that the electron
density is significantly depleted around the nitrogen atom as the proton closest to nitrogen atom
(He) exhibits the biggest downfield shift from 4.32 ppm to 4.37 ppm to 4.69 ppm for C6, C4 and
C2ꢀlinker derivatives. This nonꢀlinear decrease is unlikely to be from the induction effect from
Br alone. Meanwhile the protons on the aromatic ring, Ha, Hb, Hc and Hd, also all show
downfield chemical shift from C6 to C2 derivatives, indicating the electron density on the
conjugated system (carbazole ring) decrease. Based on all the results above, we ascribe the
orbital interaction more likely to be (N)nꢀ(CꢀX)σ* rather than (X)nꢀ(CZ)π*. Finally, we
performed theoretical calculations for the lowest singlet transition states for both CZꢀ2CꢀBr and
CZꢀ4CꢀBr (Figure 4d). Calculations were performed using wB97XD/ccꢀpVDZ with the Xꢀray
CIF coordinates and a polarizable continuous medium (Tomasi) model solvent
dichloromethane.23,24 From the calculations, it is clear that the 2C derivative has significant σ*
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participation but no such interaction is noted for the 4C counterpart. The calculations also
indicate that the π orbital energy is indeed lower for the 2C derivative while the π* is raised
presumably due to the (CZ)πꢀ(CꢀX)σ* mixing.
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Figure 4. a) and b). UV absorption spectra and steadyꢀstate emission spectra of various
carbazole derivatives in dilute CH2Cl2 solution. c) 1HꢀNMR spectra of CZꢀ2CꢀBr, CZꢀ4CꢀBr
and CZꢀ6CꢀBr. d) Calculated main singlet transition state for CZꢀ2CꢀBr and CZꢀ4CꢀBr (orbital
energy is given in Hartree).
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Table 3: Luminescence characterization in dilute CH2Cl2 solution.
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τFL (ns) c
6.14
λ
abs (nm) a
λ
FL (nm) b
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326, 339
330, 344
326, 339
330, 344
332, 345
328, 341
331, 345
343, 360
348, 365
343, 360
349, 365
350, 367
346
CZꢀ2CꢀCl
CZꢀ4CꢀCl
CZꢀ2CꢀBr
CZꢀ4CꢀBr
CZꢀ6CꢀBr
CZꢀ2CꢀI
5.79
4.96
6.02
9.32
6.13
348, 364
5.09
CZꢀ4CꢀI
a. Lowest singlet transition.25
b. Photoluminescence emission peak excited at excitation maxima.
c. Preꢀexponent weightꢀaveraged fluorescence lifetime.
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Figure 5. Crystal packing of MeOQBr, MeOQI, MeOQPF6, QBr, QI, QPF6, CF3QBr and
CF3QI .
The SCꢀXRD structures of the Qs derivatives are presented in Figure 5. From the crystal
structures, two of the Qs include a water molecule in the crystals and the rest contain only the
quinolinium ring and the halide counter ion. As can be seen, the “acidic” quinoline ring is in
close proximity with filled p orbital from the counterion halide (Xꢀ), which is likely due to the
much stronger (Xꢀ)nꢀ(quinoline)π* interaction, compared to that of πꢀσ* for the carbazole
derivatives based on calculations for some representative compounds (Table 4). Comparing the
energies of the π* orbital for MeOQBr and MeOQPF6, the former shows a notable increase (ꢀ
1.012 eV) vs. the latter (ꢀ1.143 eV), which is ascribed to the better mixing with bromide lone
pairs. The calculated energy difference between the bromide lone pair and quinolinium π* is
0.2418 hartrees (h); the difference between the conjugationꢀparticipating nitrogen lone pair with
CꢀBr σ* is 0.3611 h. From the calculated results, for MeOQBr, the lowest singlet transition state
could be a CT one if the orbital overlap is reasonable in the solid state; for MeOQPF6, however,
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even in the solid state, the lowest singlet transition remains to be πꢀπ*. The calculation results
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are consistent with spectroscopic data in Figure 2.
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Table 4. Calculated orbital energies for Qs and carbazole derivatives. a
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MeOQBr
MeOQPF6
CZꢀ2CꢀBr
quinolinium π*: ꢀ0.0374 h
bromide lone pair: ꢀ0.2792 h
bromide lone pair: ꢀ0.2823 h
quinolium π: ꢀ0.3207 h
quinolinium π*: ꢀ0.0420 h
quinolinium π: ꢀ0.3238 h
quinolinium π: ꢀ0.3564 h
fluorine lone pair: ꢀ0.4092h
bCꢀX σ*: +0.0681 h
CZ π*: +0.0269 h
cCZ π: ꢀ0.2930 h
dCZ π: ꢀ0.3381 h
ebromide lone pair: ꢀ0.3645 h
a. Calculations were performed using wB97XD/ccꢀpVDZ with the Xꢀray CIF coordinates and a
polarizable continuous medium (Tomasi) model solvent dichloromethane. Energy units are
hartrees.
b. Mostly σ* with slight CZ π* mixing.
c. Mostly CZ π with N lone pair mixing.
d. Mostly CZ π with slight bromide lone pair mixing.
e. Mostly bromide lone pair with slight CZ π mixing.
Finally, we investigated the photoluminescence spectra of these carbazole and quinolinium
salt derivatives in solution at 77 K. The emission spectra of dilute carbazole derivative solutions
(10ꢀ5 M) in methyltetrahydrofuran (mTHF) are shown in Figure 6a: in addition to the
fluorescence band (~340ꢀ400 nm), the phosphorescence band (~410ꢀ500 nm with secondsꢀlong
“afterglow”) could also be observed for each derivative. However, the relative intensity of the
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phosphorescence band is the highest for CZꢀ2CꢀBr due to the strongest orbital mixing, which is
consistent with the NMR spectra (Figure 4c). For the quinolinium derivatives (Figure 6b),
however, no phosphorescence band is present from the spectra other than the highꢀenergy
fluorescence band from 300ꢀ350 nm (note that the hump in the spectra is probably from
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ꢀ
aggregates due to the poor solubility of the PF6 derivatives). The lack of phosphorescence for
the quinolinium derivatives can be explained by the separation of the Qs π* orbital and the
counterion n orbital as a result of solvation.
Figure 6. Steadyꢀstate emission spectra of various carbazole (a, λex = 310 nm) and quinolinium
salt (b, λex = 280 nm) derivatives in dilute mTHF solutions (10ꢀ5 M) at 77 K.
Conclusion
In conclusion, we synthesized two classes of organic aromatic molecules, carbazole and
quinolinium derivatives with heavyꢀatom perturbers to investigate external heavyꢀatom induced
RTP in the solid state. In the carbazole case, the heavyꢀatom perturber serves as an electron
acceptor, or “acid”, due to the presence of a low lying σ* (CꢀX) orbital. The crystal structure
suggests that both intramolecular and intermolecular mixings of the σ* orbital with the carbazole
π orbital are possible. In the quinolinium salts case, the heavyꢀatom perturber is an anion
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(electron donor) and is likely to interact with the lowꢀlying π* orbital of the quinolinium ring.
We thus conclude that regardless of electron acceptor or donor, the external heavyꢀatom effect
can be realized as long as direct and matching orbital interactions are present.
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Supporting Information. Materials, methods, syntheses of compounds, 1HꢀNMR spectroscopy,
supplementary figures and crystallographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Prof. Guoqing Zhang, gzhang@ustc.edu.cn, Tel: +86 (551) 6360 6607
ACKNOWLEDGMENT
The authors thank the support from Fundamental Research Funds for the Central Universities
(WK2340000068 and CX2060200022) and the National Natural Science Foundation of China
(21222405). We also thank Prof. Carl O. Trindle (University of Virginia) for the helpful
theoretical discussions.
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