Visible light-induced homolytic cleavage of perfluoroalkyl iodides mediated by phosphines

Article abstract of DOI:10.3390/molecules25071606

In an effort to explain the experimentally observed variation of the photocatalytic activity of ^tBu₃P, ⁿBu₃P and (MeO)₃P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine– and phosphite–IC₄F₉ adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of ^tBu₃P-IC₄F₉ and (MeO)₃P-IC₄F₉ into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in ⁿBu₃P-IC₄F₉, in particular with regard to the P–I–C bending angle, reduce the Franck–Condon factors and thus the absorption probability compared to ^tBu₃P-IC₄F₉. Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO)₃P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C–I cleavage of perfluorobutane iodide involves S₁ ←S₀ absorption of the adduct followed by intersystem crossing to the photochemically active T₁ state.

Full text of DOI:10.3390/molecules25071606

molecules
Article
Visible Light-Induced Homolytic Cleavage of
Perfluoroalkyl Iodides Mediated by Phosphines
Mario Bracker 1, Lucas Helmecke 2, Martin Kleinschmidt 1,* , Constantin Czekelius 2,*
and Christel M. Marian 1,*
1
Institute of Theoretical and Computational Chemistry, Heinrich-Heine-University Düsseldorf,
40204 Düsseldorf, Germany; mario.bracker@hhu.de
Institute of Organic and Macromolecular Chemistry, Heinrich-Heine-University Düsseldorf,
2
40204 Düsseldorf, Germany; lucas.helmecke@hhu.de
*
Correspondence: Martin.Kleinschmidt@hhu.de (M.K.); Constantin.Czekelius@hhu.de (C.C.);
Christel.Marian@hhu.de (C.M.M.); Tel.: +49-211-8113212 (M.K.); +49-211-8115362 (C.C.);
+49-211-8113210 (C.M.M.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Paulo Jorge Costa and Fabrizio Santoro
Received: 14 February 2020; Accepted: 25 March 2020; Published: 1 April 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: In an effort to explain the experimentally observed variation of the photocatalytic activity of
tBu3P, nBu3P and (MeO)3P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have
explored the absorption characteristics of several phosphine– and phosphite–IC4F9 adducts by means of
relativistic density functional theory and multireference configuration interaction methods. Based on
the results of these computational and complementary experimental studies, we offer an explanation
for the broad tailing of the absorption of tBu3P-IC4F9 and (MeO)3P-IC4F9 into the visible-light region.
Larger coordinate displacements of the ground and excited singlet potential energy wells in nBu3P-IC4F9,
in particular with regard to the P–I–C bending angle, reduce the Franck–Condon factors and thus
the absorption probability compared to tBu3P-IC4F9. Spectroscopic and computational evaluation of
conformationally flexible and locked phosphites suggests that the reactivity of (MeO)3P may be the
result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed
mechanism for the phosphine-catalyzed homolytic C–I cleavage of perfluorobutane iodide involves
S1
←
S0 absorption of the adduct followed by intersystem crossing to the photochemically active T1 state.
Keywords: halogen bond; iodoperfluoroalkylation; radical; absorption spectra; adiabatic transition;
intersystem crossing; density functional thory; multireference configuration interaction; spin–orbit coupling
1. Introduction
Organic compounds incorporating fluorine substituents or a perfluoroalkyl group show unique
properties and are therefore very important synthetic targets in pharmaceutical research and industry [1–4].
The introduction of fluorine as the most electronegative element results in a different polarization profile
of the molecule associated with improved pharmacodynamic and -kinetic properties. It is therefore not
surprising that the development of a synthetic methodology for the efficient, safe, and environmentally
benign preparation of these compounds by transition metal catalysis or photocatalysis has found increasing
interest in the last decades [5–10].
The introduction of perfluoroalkyl groups or perfluoroalkanoates by addition of the corresponding
perfluoroalkyl iodide to unsaturated hydrocarbons such as alkenes or alkynes is synthetically highly
Molecules 2020, 25, 1606; doi:10.3390/molecules25071606
www.mdpi.com/journal/molecules

Molecules 2020, 25, 1606
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valuable since a plethora of such starting materials are easily accessible and commercially available.
In addition, subsequent displacement of the iodine substituent by nucleophilic substitution or metalation
allows for further functionalization of the fluorinated molecule. The iodoperfluoroalkylations of
double bonds can be initiated by homolytic C–I bond cleavage using radical initiators [11
irradiation [13,14]. In cases where such conditions are not advisable for delicate substrates, photocatalytic
methods [15 17] using visible light irradiation offers advantages. Herein, transition metal complexes
based on iridium or ruthenium [18 19], as well as copper [20 22], have been successfully employed for
atom transfer radical addition (ATRA) reactions.
In addition to metal complexes, organic photocatalysts have also been reported for iodoperfluoroalkylation
reactions of unsaturated hydrocarbons, either by using them as triplet sensitizers [23 24] or by activation via the
,12] or UV
–
,
–
,
corresponding electron donor–acceptor (EDA) complexes [25–27]. In the latter case, the formation of a halogen
bond between the electron-deficient perfluoroalkyl iodide and a suitable Lewis base results in homolytic
bond cleavage upon irradiation of visible light. Amines [28–33], phenols [34], ketones [35,36], as well as
phosphines [37–41] have been reported for this purpose in either stoichiometric or catalytic quantities.
In our studies addressing the Lewis-base-mediated iodoperfluoroalkylation of simple alkenes [40,42,43]
we found that catalytic amounts of phosphines and phosphites effectively catalyze this process with complete
regioselectivity upon irradiation with visible light (461 nm) (Scheme 1). Mechanistic investigations involving
19F-NMR analysis suggested the intermediate formation of an EDA complex F(CF2)n–I · · · PR3 by halogen
bond formation to the phosphorus atom [44,45]. In addition, the reaction was shown to proceed via free
perfluoroalkyl radicals. It is therefore assumed that visible light absorption of the EDA complex formation
leads to homolytic bond cleavage and subsequent radical chain reaction.
Scheme 1. Phosphine-catalyzed iodoperfluoroalkylation of simple alkenes.
Two observations called for a more detailed analysis, however. First, among the numerous
tested phosphorus compounds, tri(tert-butyl)phosphine (tBu3P) showed the fastest conversion, while
tri(n-butyl)phosphine (nBu3P) with comparable lone pair donor properties was much less efficient.
In contrast, electron-deficient trimethylphosphite ((MeO)3P) performed very well. Second, the UV-vis
spectra of EDA complexes formed upon mixing perfluorobutyl iodide (C4F9I) with the above-mentioned
phosphorus (III) compounds—without an alkene present—showed increased absorption but substantially
different peak tailing into the visible light region [46]. In addition, in all cases the overlap of the EDA
complex absorption with the emission spectrum of the blue LED employed in the reaction (λmax = 461 nm)
was very small (Figure 1). As a result, the different catalyst performances cannot be attributed exclusively
to the absorption profile or the donor properties, but additional factors such as the geometry-dependency
of the corresponding singlet–triplet transitions need to be taken into account. Therefore, we aimed at
an improved understanding of the photochemical, homolytic bond cleavage process by calculation of
the excited-state geometries of the perfluorobutyl iodide adducts [47–49] as well as a modeling of the
intersystem crossing (ISC) process.

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Figure 1. UV-vis spectra of C4F9I, tBu3P, nBu3P, (MeO)3P and mixtures of the phosphorus compounds
with C4F9I.
2. Results
2.1. Quantum Chemical Characterization of the Compounds in the Franck–Condon Region
2.1.1. Perfluorobutyl Iodide
Due to the presence of the heavy iodine atom, relativistic calculations have to be performed for
modeling the photophysics and photochemistry of the compounds. Spin selection rules are therefore
not strictly obeyed. Spin–orbit interaction is particularly pronounced between electronic states involving
differently oriented
represent the nearly degenerate highest occupied molecular orbital (HOMO) and HOMO-1 in the C4F9I
ground-state geometry. The C–I
bonding (HOMO-2) and σ∗ antibonding (lowest unoccupied molecular
p-type orbitals, located at the iodine center, such as the lone-pair nI orbitals that
σ
orbital, LUMO) involve iodine p atomic orbitals as well (Figure 2).
The vertical absorption spectrum of the non-coordinated perfluorobutyl iodide molecule is
characterized by weak (nI → σ∗) transitions in the middle-UV region and a strong (σ → σ∗) transition in
the far UV (Figure S1). With regard to the current experiments, only the weak first absorption band is of
interest. The spectral broadening and the enhancement of that band, which are observed when spin–orbit
coupling (SOC) is switched on (Figure 2a), can be traced back to intensity borrowing of the spin-forbidden
3(nIσ∗) transitions from the optically bright 1(σσ∗) transition at 167 nm. Comparison between theory and
experiment (Figures 2a and S2) reveals excellent agreement of the spectral shapes and absorption maxima
λmax (theory: 268 nm, experiment: 270 nm).

Molecules 2020, 25, 1606
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(b) LUMO: σ∗
(c) HOMO: nI
(d) HOMO-1: nI
(e) HOMO-2: σ
(a)
Figure 2. Calculated first absorption band of C4F9I (red: no spin–orbit coupling, only singlet transitions,
black: including spin–orbit coupling, singlet as well as triplet transitions) and molecular orbitals involved
in the transitions. The spectral envelope was obtained by broadening the line spectrum by Gaussians with
standard deviation σ = 1500 cm−1. HOMO denotes the highest occupied molecular orbital, LUMO the
lowest unoccupied molecular orbital.
We observe a small red shift of the absorption maximum by 6 nm when basis sets of higher quality
are employed, i.e., triple zeta plus polarization functions on all atoms (cf. Figure S3).
2.1.2. Phosphines
The first absorption band of the isolated tBu3P, nBu3P, and (MeO)3P molecules peak at wavelengths
shorter than 200 nm (Figure S4). Only their tails can be seen in the experimental observation window.
They involve the respective lone-pair orbital on phosphorus,
of the trialkylphosphine. In dichloromethane (DCM) solution, phophine–DCM adducts can be formed,
with a marked impact on the photophysics (Figure 3a) [47 49]. The first excited singlet state of the adduct
n
P, and a pair of nearly degenerate σ∗ orbitals
–
is an optically bright charge transfer (CT) state originating from the excitation of an nP electron on the
electron-rich phosphine to a σ∗ orbital on DCM (Figure 3). This CT excitation is somewhat red-shifted
with regard to the local phosphine excitations (λmax
= = 192 nm
205 nm in tBu3P–DCM compared to λmax
in tBu3P) but much stronger (oscillator strength f = 0.353 in tBu3P–DCM compared to f = 0.132 in tBu3P).
The adduct complex is therefore expected to dominate the residual intensity of the tBu3P absorption in the
low-energy regime. This assumption is supported by the experimental observation of a lower absorbance
of tBu3P in non-coordinating solvents such as pentane in the wave length region
> 250 nm (cf. Figure S5).
2.1.3. Phosphine–Perfluorobutyl Iodide Adducts
tBu3P, nBu3P, and (MeO)3P form perfluorobutyl iodide adducts with nearly linear C–I–P coordination
in the electronic ground state. In line with the donor capabilities of these phosphines, the elongation of
the I–C bond is most pronounced in the tBu3P–IC4F9 adduct and smallest in (MeO)3P–IC4F9. (For details,
see Table 1.)

Molecules 2020, 25, 1606
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(b) LUMO: σ∗
(c) HOMO: nP
(a)
t
Figure 3. Calculated absorption spectra of Bu3P in DCM (black: continuum solvent model, red:
tBu3P-DCM adduct surrounded by a continuum solvent model of DCM) and frontier molecular orbitals of
the tBu3P-DCM monoadduct.
Table 1. Selected geometry parameters of perfluorobutyl iodide and its phosphine and phosphite adducts
in the electronic ground state (S0), the first excited triplet (T1), and singlet (S1) state.
◦
Compound
P–I Bond Length/pm
I–C Bond Length/pm
P–I–C Bond Angle/
S0
T1
S1
S0
T1
S1
S0
T1
S1
C4F9I
216
tBu3P-IC4F9
297 303
297 308
343
331a
320a
a
225 309
224 292
220 318
270
281a
294a
179 110
104
96a
96a
nBu3P-IC4F9
(MeO)3P-IC4F9 316 294
177
179
76
78
S1/S0 conical intersection.
In all phosphine adducts, the HOMO is predominantly composed of the lone-pair orbital on
phosphorus (
P), whereas the LUMO is a σ∗-type orbital. MO plots are shown for tBu3P-IC4F9 in Figure 4,
whereas those of the other adduct compounds may be found in Figures S6 and S7.
n
(a) HOMO: nP
(b) LUMO: σ∗
Figure 4. Frontier molecular orbitals of the tBu3P-IC4F9 adduct at the S0 geometry.
The direct T1
←
S0 transitions of all phosphine adducts have negligible oscillator strengths in the
Franck–Condon (FC) region. The strongly absorbing 1(nPσ∗) CT state forms the first excited singlet state
t
of Bu3P-IC4F9. The triplet excitations from the two lone-pair orbitals on the iodine center (
n
I) to the

Molecules 2020, 25, 1606
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σ
∗-orbital are close in energy but SOC between these states is not very pronounced. For this reason,
the first S1
←
S0 absorption maxima computed in the absence and presence of SOC are nearly identical
n
(cf. Figure S8). A qualitatively similar energy scheme is obtained in the case of Bu3P-IC4F9 whose
calculated absorption maximum is somewhat blue-shifted. In (MeO)3P-IC4F9, the much weaker 1(nIσ∗)
transitions are energetically favored over the 1(nPσ∗) CT transition, but the states are close in energy.
In line with the lower electron donor capabilities of (MeO)3P, the peak maximum is even more blue-shifted.
While the trends among the phosphine adducts are reproduced correctly, the computed absorption
bands of the CT transitions (Figure 5) are red-shifted with respect to the corresponding experimental data
(Figure 1) by about 20 nm. Note, however, that the transparency of the dichloromethane solvent quickly
degrades for wavelengths shorter than 240 nm. To check the influence of the solvent on the absorption
characteristics, the experiment was repeated for tBu3P-IC4F9 in pentane solution (transparency
to 220 nm). And indeed, the first absorption band peaks at 255 nm in that solvent before the cut-off is
reached (cf. Figures S2, S9 and S10).
≥
90% up
Figure 5. Calculated absorption spectra of the phosphine and phosphite adducts in DCM including
spin–orbit coupling (SOC).
Despite the good agreement between theory and experiment with regard to absorption maxima,
quantum chemical calculations, performed at the respective ground-state geometries of the adducts, are
not sufficient to explain the blue-light excitation of the compounds and their photochemistry. To this end,
adiabatic excitation energies and minimum geometries of the lowest excited singlet and triplet states have
to be known.
Similar to the basis set dependence of the C4F9I absorption, we observe a small red shift (4 nm) and
t
a slight increase of the intensity of the first absorption band of the Bu3P-IC4F9 adduct when using a
better atomic orbital basis set (cf. Figure S11). In view of the small changes and the markedly higher
computational cost, we refrain from carrying out the elaborate excited-state geometry optimizations using
the TZVP basis sets.

Molecules 2020, 25, 1606
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2.2. Relaxed Excited-State Geometries of the Phosphine–Perfluorobutyl Iodide Adducts
2.2.1. The First Excited Triplet State
The T1 states form very shallow basins in the I–C dissociative region of the respective potential energy
surfaces (PESs) (cf. heatmaps in Figure 6a–c).
(a) tBu3P-IC4F9
(b) nBu3P-IC4F9
(c) (MeO)3P-IC4F9
Figure 6. Heatmaps of the T1 potential energy surfaces of the phosphine and phosphite adducts as
functions of the C–I bond length and the C–I–P angle. All other internal coordinates were kept fixed to the
minimum geometry parameters of the respective T1 state. The energy scales on the right refer to DFT and
multireference configuration interaction (DFT/MRCI) energies relative to the ground-state minimum. Note
the different scales of the x-axes.
Here, the biradicalic triplet is the lowest-energy state, and I–C bond cleavage can proceed nearly
without any barrier. In addition to the markedly increased I–C bond length, we notice significant changes
of the P–I–C bond angle with respect to the diamagnetic species (cf. Table 1). The P–I–C bond angle varies
◦
among the three adducts from approximately 110 in tBu3P-IC4F9 for the phosphine with the largest steric
demand to less than 80◦ in nBu3P-IC4F9 and (MeO)3P-IC4F9 (Figure 7). The electronic structures of the
T1 states (cf. Figure S12 for the singly occupied molecular orbitals, SOMOs) are in qualitative agreement
with the picture of a perfluoroakyl radical attached to a negatively charged iodine and a phosphine radical
cation (F9C4• · · · I•⊕PR3).
Adiabatically, the T1 minima are located between 1.42 and 1.62 eV (Table 2) above the corresponding
S0 minima—too high in energy to be reached by thermal activation. Direct light activation is doomed to fail
as well because the singlet–triplet mixing is too low in this CT state for making direct T1
←
S0 absorption

Molecules 2020, 25, 1606
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feasible (vide infra). The most probable activation pathway of the triplet channel is photoexcitation of the
S1 state followed by ISC to the T1 PES.
(a) tBu3P-IC4F9
(b) nBu3P-IC4F9
(c) (MeO)3P-IC4F9
Figure 7. Nuclear arrangement of the phosphine–IC4F9 adducts at the optimized T1 minima.
Table 2. Vertical and adiabatic excitation energies (∆Evert and ∆Eadia/eV) and corresponding transition
wavelengths (
λ
and λadia/nm) including SOC at the DFT/multireference spin–orbit configuration
vert
interaction (MRSOCI) level of theory. Triplet energies have been averaged over the three states with largest
T1 contributions.
Compound
T1
S1
∆Evert
λ
∆Eadia λadia
∆Evert
λ
∆Eadia λadia
vert
vert
tBu3P-IC4F9
nBu3P-IC4F9
(MeO)3P-IC4F9
3.70
3.91
4.48
335
317
276
1.42
1.46
1.62
870
850
765
4.84
5.07
5.34
256
245
232
1.70
1.82a
1.87a
729
681a
663a
a
S1/S0 conical intersection.
2.2.2. The First Excited Singlet State
Unfortunately, minima on the S1 PESs of the phosphine-IC4F9 adducts could not be determined
using the same computational protocol as for the T1 states. Upon relaxation of the S1 geometry at the
time-dependent density functional theory (TDDFT)-Tamm–Dancoff approximation (TDA) level, the S1
and S0 states of all adducts undergo conical intersections where the calculation stops. Subsequent
multireference configuration interaction (MRCI) single-point calculations revealed substantial double
excitation contributions to the So and S1 wave functions at these geometries. We therefore preformed
minimum searches based on numerical MRCI gradients to locate the lowest points on the S1 PESs.
The nuclear arrangements at these minima are displayed in Figure 8 together with the course of the S0
and S1 potential energies along linearly interpolated paths connecting the singlet ground and excited
state minima.
t
While the S0 and S1 potentials of Bu3P-IC4F9 still exhibit a small energy gap of about 0.20 eV
n
at the S1 minimum, S0 and S1 are practically degenerate at the relaxed S1 geometries of Bu3P-IC4F9
and (MeO)3P-IC4F9 and underdo conical intersections. In any case, the S0 and S1 wave functions are
strongly mixed in the neighborhood, and non-adiabatic coupling is large. On the one hand, we therefore
expect non-radiative deactivation by internal conversion (IC) to the electronic ground state to be fast
in all phosphine adducts. On the other hand, the T1-dominated states lie energetically close as well
(
∆ES −T ≤ 0.20 eV) and their mutual spin–orbit couplings (Figure 9a) should suffice to transfer substantial
1
1
population from S1 to T1 by ISC. Whether the photochemically active triplet channels can be reached
depends on subtle differences between the competing IC and ISC dynamics.
The entries in Table 2 show that the origins of the S1
in principle, be reached by irradiation with the blue LED (λmax
←
S0 transitions of all phosphine adducts can,
461 nm) used in the experiment. Despite
=

Molecules 2020, 25, 1606
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the higher vertical excitation energy of (MeO)3P–IC4F9 in the FC region compared to tBu3P–IC4F9, we find
nearly equal adiabatic S1-S0 energies in all adduct complexes. Moreover, while the computed oscillator
strength of the S1
←
S0 absorption of the nBu3P-IC4F9 adduct is somewhat smaller than in the tBu3P–IC4F9
adduct (Figure 9b), it is substantially larger than the corresponding quantity in the (MeO)3P–IC4F9 adduct.
One may therefore wonder why the experimentally observed absorption intensity of the nBu3P-IC4F9
adduct is so much weaker than that of the two other compounds in the blue-light regime.
Figure 8. Energy profiles of the ground and first excited singlet states (solid lines) and the first excited
triplet states (dotted lines) of the phosphine–IC4F9 adducts along linearly interpolated paths connecting
the S0 minimum (left, 0%) and the optimized S1 geometry (right, 100%).
(a)
(b)
ˆ
ˆ
Figure 9.
(solid lines) and (
connecting the S0 minimum (0%) and the optimized S1 geometry (100%).
(
a
) Spin–orbit coupling matrix elements |hT1| Hso
|S0i| (dotted lines) and |hT1| Hso |S1i|
b
) oscillator strengths of the S1 S0 transitions along linearly interpolated paths
←

Molecules 2020, 25, 1606
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On a related note, the high efficiency of (MeO)3P as catalyst in the iodoperfluoroalkylation contrasts
with both the smaller maximal absorption and its weaker Lewis basicity. Moreover, it seems to be correlated
with the broad tailing of the absorption into the visible light region. The HOMO of the (MeO)3P–IC4F9
adduct shows a substantial participation of the oxygen lone pairs (Figure 10a). This suggests a strong
influence of the conformational flexibility of the alkoxide substituents upon the HOMO energy. This should
lead to absorption band broadening due to a large ensemble of different conformers. Since a complete
analysis of the conformational space and its corresponding excited states of F(CF2)n-I· · · P(OMe)3 was
out of reach, we aimed at the spectroscopic and computational evaluation of different, conformationally
locked phosphites, such as the “caged” phosphite 4-methyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane and
tri(tert-butyl)phosphite (tBuO)3P [50
–52]. In the case of the caged phosphite, the oxygen lone pairs point
in the same direction as the phosphorus lone pair. Accordingly, substantial participation was not found in
the HOMO but only in the HOMO-2 orbital (Figure 10b). In contrast, the highly rigid (tBuO)3P with the
oxygen lone pairs pointing backwards shows their strong participation in the HOMO of the adduct with
(I-P-O-C) dihedral angles of 62–64◦ (Figure 10c).
(a) (MeO)3P-IC4F9: HOMO, side and rear view
(b) C5H9O3P-IC4F9: HOMO-2, side and rear view
(c) (tBuO)3P-IC4F9: HOMO, side and rear view
Figure 10. Frontier molecular orbitals of the phosphite–IC4F9 adducts at the S0 geometry.
The corresponding calculated absorption spectra (Figure 11a) for the three phosphite adducts show a
substantial difference in excitation energies supporting this assumption further. These findings correlate
with the experimental, spectroscopic analysis of the corresponding perfluoroalkyl iodide–phosphite
complexes (Figure 11b). Complex formation in solution is less pronounced than in the case of phosphines.
Therefore, absorption in the region of 240–320 nm is dominated by free perfluoroalkyl iodide. The caged

Molecules 2020, 25, 1606
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phosphite shows very small broadening in absorption and was also completely inactive as catalysts;
(tBuO)3P showed increased absorption and substantial tailing into the visible light region. It should be
noted, however, that tailing was even more pronounced for (MeO)3P and a stronger absorption above
440 nm correlates with higher activity as catalyst.
(a)
(b)
Figure 11.
(a) Calculated and (b) measured absorption spectra of the three phosphite–IC4F9 adducts.
In addition, experimental absorption spectra of the pure phosphites in DCM solution are shown.
3. Discussion
To understand the different photochemical behaviors of the adducts, we note that the probability
WrFaCd(f − i) of a radiative transition from an initial electronic state i to a final electronic state f,
ꢀ
ꢀ ꢀ
2
ꢀ
ꢀ
ꢀ
4e2
3c3h¯
2
ꢀ
ꢀ ꢀ
WrFaCd(f − i) =
(Ef,b − Ei,a)3 hΨ |µˆ|Ψ i hv |v i
,
(1)
ꢀ
ꢀ ꢀ
4 ∑∑
f
i
f,b i,a
a
b
ꢀ
ꢀ
ꢀ
ꢀ
does not depend solely on the electric transition dipole moment hΨ |µˆ|Ψ i and the energy difference
ꢀ
ꢀ
f
i
ꢀ
ꢀ
ꢀ
ꢀ
(
Ef − Ei) of the two states. Through the Franck–Condon factors hv |v i 2, i.e., the squared overlaps of
ꢀ
ꢀ
f,b i,a
the vibrational wave functions, it depends on the coordinate displacements of the two potential minima as
well. In Equation (1),
e
denotes the charge of the electron,
c
the velocity of light, and h¯ the Planck constant
h divided by 2π.
Inspection of the molecular structures in Figure 8 reveals that the S1 minimum of the tBu3P-IC4F9
adduct lies geometrically somewhat closer to the corresponding ground-state equilibrium than the
nBu3P-IC4F9 and (MeO)3P-IC4F9 adducts. The closer geometrical distance between the S0 and S1 minima
is also reflected in the slightly shallower slope of the ground-state energy profile in Figure 8. In particular,
the small overlap of the ground- and excited-state vibrational wave functions in the C–I–P angle bending
coordinate of nBu3P-IC4F9 hampers the transition from the nearly linear ground-state equilibrium structure
t
to the bent structure in the S1 potential well. This overlap is not large in Bu3P-IC4F9 either, but it
appears sufficient to explain the stronger residual absorption of the S1 state in the blue wavelength region
(cf. Figure 1). We note further that the oscillator strength of the S1
the three complexes (Table 2).
←
S0 transition is the largest one among
In all three cases, the S1 minimum lies geometrically in the exit channel of the T1 state toward C–I
dissociation. The mutual SOC of the S1 and T1 states in this region implies that S1 T1 ISC is possible in
competition with S1
S0 internal conversion back to the S0 minimum. Based on these data, we propose

Molecules 2020, 25, 1606
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the following mechanism for the blue-light-initiated phosphine-catalyzed homolytic C–I cleavage of
perfluorobutane iodide:
1(R3P + IC4F9)
→
1(R3P − IC4F9) bluꢀe LED 1(R3P − IC4F9)∗
ISC
•
3(R3P⊕• − I · · · C4F9)∗ → R3PIe• + C4F9.
(2)
t
n
While the different behavior of the phosphines Bu3P and Bu3P can be correlated primarily by
their absorption profile, the surprisingly high reactivity of (some) phosphites may be the result of oxygen
lone-pair participation and concomitant broadening of absorption. The strong tailing in the case of (MeO)3P
and the associated relatively high absorption above 440 nm may be explained by its high conformational
flexibility. Due to rotation about the P–O bond, energetically disfavored conformers are accessible in small
fractions that show optimal orbital alignment for excitation. In the case of the conformationally locked
(tBuO)3P, the oxygen lone pairs are already in a suitable position but the associated energy barriers are
higher and the optimal geometry for excitation may be out of reach.
4. Materials and Methods
4.1. Computational Methods
Closed-shell ground-state geometries were optimized with resricted Kohn–Sham (KS) density
functional theory (DFT) using Turbomole [53
–
55] in conjunction with the B3-LYP functional [56
,57],
Grimme D3-BJ dispersion corrections [58 59] and the continuum solvent model COSMO [60
,
,
61] for
mimicking a DCM solvent environment. Unless stated otherwise, the def2-SVP atomic orbital basis
sets [62] from the Turbomole library were utilized for all atoms, save for iodine, which was represented by
a relativistic small-core effective core potential and the corresponding def2-TZVPD basis [63]. To check the
sensitivity of the results with respect to the choice of atomic orbital basis set, additional test calculations on
the absorption properties of C4F9I and its tBu3P adduct were performed employing the larger def2-TZVP
basis sets for H, C, F, and P [64,65], while keeping the iodine basis set unmodified. Triplet geometry
optimizations were performed with the corresponding time-dependent DFT (TDDFT) [66] module using
the Tamm–Dancoff approximation [67] (TDA) to TDDFT. In (MeO)3P-IC4F9, where TDDFT-TDA did not
converge, we employed unrestricted KS DFT instead for the geometry optimization of the triplet state.
Numerical second derivatives for vibrational analyses were computed with SNF [68].
Minimum searches at the TDDFT level were not successful for the excited singlet states of the adducts.
In these cases, we employed a computational protocol based on numerical DFT/MRCI gradients [69].
The combined DFT and multireference configuration interaction (DFT/MRCI) [70,71] approach was used
in conjunction with the semiempirical R2018 Hamiltonian [72] and the standard configuration selection
threshold of 1.0 Eh to determine spin–orbit free excitation energies and oscillator strengths. The KS orbitals
and orbital energies for the DFT/MRCI calculations were optimized utilizing the BH-LYP [57
functional, empirical dispersion corrections [58 59], and an implicit DCM solvent environment [60
Auxiliary basis sets for the resolution-of-the-identity approximation of the two-electron integrals [70
,
73] density
61].
74
,
,
,
]
were taken from the Turbomole library [55], i.e., we chose the def2-TZVPD auxiliary basis set [75] on
iodine and the def2-SVP sets [64] on all other atoms.
Interstate spin–orbit coupling matrix elements (SOCMEs) for determining the probability of ISC were
computed by the SPOCK program [76
an effective one-center mean-field approximation to the Breit–Pauli SOC operator [79
–
78] using a spin–orbit effective core potential for iodine [63] and
80] for the lighter
,
elements. For efficiency reasons, vertical excitation energies and oscillator strengths of multiplicity-mixed
wave functions were determined by quasi-degenerate perturbation theory (QDPT) in the basis of

Molecules 2020, 25, 1606
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DFT/MRCI wave functions. Adiabatic excitation energies including SOC were obtained by means of
multireference spin–orbit configuration interaction (MRSOCI) [81] calculations.
4.2. Experimental Procedures
All preparations involving air- and moisture-sensitive compounds were carried out inside a glove box
(Vacuum Atmospheres model OMNI-LAB) under N2 atmosphere (Air Liquide ALPHAGAZTM 5.0). Glassware
was dried for 2 h at 120 ◦C and cooled down in vacuo.
Nonafluoro-1-iodobutane was purchased from TCI and was filtered through a column packed
with aluminum oxide 90 basic 0.063–0.200 mm (activity stage I) and an activated molecular sieve
(4 Å) under N2 atmosphere. The clear and colorless liquid was stored in amber glass vials under
N2 atmosphere. Tri-tert-butylphosphine was purchased from Sigma Aldrich. 4-Methyl-2,6,7-trioxa-1-
phosphabicyclo[2.2.2]octane [82] and tri(tert-butyl)phosphite [50,83] were prepared according to the literature.
Pentane and dichloromethane were dried with the solvent purification system MP-SPS 800 from M.
Braun and degassed with freeze-pump-thaw. UV-vis measurements were performed on a Perkin Elmer
Lamda 2 UV-vis spectrometer in Hellma cuvettes (10 × 10 mm, Suprasil quartz glass).
5. Conclusions
The results of the present computational chemistry study support the mechanism of a
phosphine-assisted light-induced homolytic C–I bond cleavage, proposed earlier by some of us [40
on the basis of experimental observations. In addition, they provide an explanation for the fact that
tBu3P is a much better photocatalyst in the blue-light regime than nBu3P. While the origin of the S1
S0
]
←
absorption band should be energetically accessible to blue-light radiation in both butylphosphine–IC4F9
adducts, the larger coordinate displacements of the ground and excited singlet potential energy wells in
nBu3P-IC4F9 reduce the overlaps of the vibrational wave functions and thus the absorption probability
compared to tBu3P-IC4F9. Due to the presence of iodine and its involvement in the electronic transitions,
spin–orbit coupling is strong enough to enable intersystem crossing and to facilitate the population of the
biradicalic triplet state, which is the photochemically active state.
t
The fact that both Bu3P and (MeO)3P are active catalysts for the iodoperfluoroalkylation can be
rationalized by two different factors. In the case of the bulky phosphine as strong Lewis base donor,
the overall increase in absorption results in a tailing into the visible light region sufficient for radical
generation. In the case of the electron-deficient, conformationally flexible phosphite, the overall increase
of absorption is only small, but efficient oxygen lone-pair participation in the HOMO, found in a small
fraction of conformers, may be the reason for the observed broad tailing into the longer wavelength region
and concomitant catalytic activity.
Supplementary Materials: The following are available online. Description of general experimental procedures,
synthesis of phosphites, reactions, NMR spectra, UV-vis measurements and further computational details, Figure S1:
Calculated absorption spectra of C4F9I in CH2Cl2 with and without SOC, Figure S2: Experimental UV-vis spectra
t
t
of C4F9I, Bu3P, and Bu3P + C4F9I in CH2Cl2, Figure S3: Atomic orbital basis set dependence of the calculated
absorption spectrum of C4F9I (190–400 nm) in CH2Cl2, Figure S4: Computed absorption spectra of the phosphines
(tBu3P, nBu3P) and the phosphite (MeO)3P in CH2Cl2 with SOC, Figure S5: Comparison of the experimental UV-vis
spectra of tBu3P in pentane and in CH2Cl2, Figure S6: Frontier molecular orbitals of the nBu3P-IC4F9 adduct, Figure
S7: Frontier molecular orbitals of the (MeO)3P-IC4F9 adduct, Figure S8: Calculated absorption spectra of tBu3P–C4F9I
t
t
in CH2Cl2 with and without SOC, Figure S9: Experimental UV-vis spectra of C4F9I, Bu3P, and Bu3P + C4F9I in
pentane, Figure S10: Comparison of the experimental UV-vis spectra of tBu3P + C4F9I in pentane and in CH2Cl2,
Figure S11: Atomic orbital basis set dependence of the calculated DFT/MRCI singlet absorption spectrum of the
tBu3P–C4F9I adduct complex, Figure S12: Singly occupied MOs (SOMOs) of the phosphine and phosphite adducts in
the relaxed T1 state, Figures S13–S18: Solvent influence on the measured absorption spectra, Figures S19–S23: Impact

Molecules 2020, 25, 1606
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of spin–orbit coupling on the calculated spectra, Figures S24–S39: Minimum nuclear arrangements with selected
geometry parameters
Author Contributions: Conceptualization, M.K., C.C., and C.M.M.; theoretical investigation, M.B.; experimental
investigation, L.H.; writing—original draft preparation, C.C. and C.M.M.; visualization, M.B. and L.H.; supervision,
M.K., C.C., and C.M.M.; funding acquisition, M.K., C.C., and C.M.M. All authors have read and agreed to the
published version of the manuscript.
Funding: This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—
396890929/GRK 2482.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study;
in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish
the results.
Abbreviations
The following abbreviations are used in this manuscript:
ATRA
CT
atom transfer radical addition
charge transfer
DCM
DFT
EDA
HOMO
IC
dichloromethane
density functional theory
electron donor–acceptor
highest occupied molecular orbital
internal conversion
ISC
intersystem crossing
KS
Kohn–Sham
LED
LUMO
MRCI
light-emitting diode
lowest unoccupied molecular orbital
multi-reference configuration interaction
MRSOCI multi-reference spin–orbit configuration interaction
PES
potential energy surface
QDPT
SOC
quasi-degenerate perturbation theory
spin–orbit coupling
SOCME
SOMO
TDDFT
TDA
spin–orbit coupling matrix element
singly occupied molecular orbital
time-dependent density functional theory
Tamm–Dancoff approximation
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