# Dithieno[3,2-*b;2',3'-f]phosphepinium-Based Near-Infrared Fluorophores: *p*<sub>x</sub>-π* Conjugation Inherent to Seven-Membered Phosphacycles

Keita Andoh,[a] Masahito Murai,*[a] Pierre-Antoine Bouit,[b] Muriel Hissler,[b] Shigehiro Yamaguchi*[a,c]

Supporting information for this article is given via a link at the end of the document.

**Abstract:** Positively charged phosphorus-containing heterocycles are characteristic core skeletons for functional molecules. While various phosphonium-containing five- or six-membered-ring compounds have been reported, the seven-membered-ring phosphepinium have not been fully studied yet. In this study, dithieno[3,2-*b*;2',3'-f]phosphepinium ions containing electron-donating aminophenyl groups were synthesized. An X-ray crystallographic analysis of the resulting donor-acceptor-donor dyes revealed a bent conformation of the central seven-membered ring. These compounds exhibit fluorescence in the near-infrared region with a bathochromic shift of ca. 70 nm compared to a phosphepine oxide congener and a large Stokes shift. High fluorescence quantum yields were obtained even in polar solvents due to the suppression of the nonradiative decay process. A theoretical study revealed that the phosphepinium skeleton is highly electron-accepting owing to the orbital interaction between a *p*<sub>x</sub> orbital of the phosphonium moiety and a π* orbital of the 1,3,5-hexatriene moiety. Due to the lower-lying *p*<sub>x</sub> orbital in the phosphonium moiety compared to that of the phosphine oxide and the bent conformation of the seven-membered ring, the phosphepinium ring permits effective *p*<sub>x</sub>-π* conjugation. A large structural relaxation with a contribution of a quinoidal resonance structure is suggested in the excited state, which should be responsible for the bright emission with a large Stokes shift.

## Introduction

Cationic organic fluorophores are attractive research targets with a wide range of applications including bioimaging dyes, chemosensors, and electrochromic devices.[1] Compared to positively charged carbon-, nitrogen-, and oxygen-containing π-conjugated fluorophores, phosphorus-centered cationic dyes have received less attention, even though organophosphonium salts are well-established as useful reagents and catalysts in organic synthesis, including the Wittig reaction.[2] Incorporating a charged λ⁴σ⁴-phosphorus atom, i.e., a phosphonium, into a π-skeleton imparts strong electron-accepting character and can help modulate the optoelectronic properties of the resulting compound.[3]

To maximize the effect of the phosphonium-bridge, an appropriate π-conjugated skeleton must be selected in order to maximize the orbital interactions with the phosphorus center. In this context, positively charged five-membered-ring phosphole derivatives, i.e., phospholiums,[4,5] are the most widely used

motifs. A variety of phospholium-based dyes have already been synthesized, mainly focusing on their fluorescence properties,[4a-1] which has led to applications in photochromic materials,[4g] liquid crystals,[4h] and batteries.[4i] In the phospholium skeleton, the phosphonium center is combined with a 1,3-butadiene. An extrapolation of this concept would be the construction of a

<figure><img src="image_1.png" /><figcaption>**Figure 1.** (a) Representative examples of phosphepine-based π-conjugated systems. (b) Orbital interactions in the LUMOs of seven- and five-membered-ring phosphacycles. (c) Molecular design and chemical structures of dithienophosphepinium-based NIR dyes **1** and **2** (ArF<sub>4</sub> = 3,5-ditrifluoromethylphenyl) and the phosphepine oxide congeners **3** and **4**.</figcaption></figure>

seven-membered-ring phosphepine consisting of a 1,3,5-hexatriene bridged by a phosphonium.[6] However, despite the

<footnote>[a] K. Andoh, Prof. Dr. M. Murai, and Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science, and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: masahito.murai@chem.nagoya-u.ac.jp, yamaguchi@chem.nagoya-u.ac.jp</footnote>

<footnote>[b] Prof. Dr. P.-A. Bouit, and Prof. Dr. M. Hissler
Université Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes (France)</footnote>

<footnote>[c] Prof. Dr. S. Yamaguchi
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8601 (Japan)</footnote>
fascinating nonplanar structure of seven-membered-ring
phosphepinium, studies employing this skeleton remain scarce,
compared to studies on the corresponding five-[4] and six-
membered-ring[7] phosphonium-bridged π-conjugated
homologues. A theoretical study[8e] has suggested that in order
to employ the phosphepine as a stable core skeleton for
functional molecules, it is crucial to introduce arene rings in a
fused fashion.[8-10] This has been demonstrated for benzene,[8]
indole,[9] and thiophene-fused phosphepine oxides,[10] and their
structures and photophysical properties have been reported
(Figure 1a). Stabilization can also be achieved by incorporating
nitrogen atoms into the seven-membered-ring skeleton to
construct diazaphosphepine derivatives.[11] In contrast to these
phophepine oxide derivatives, there are only three precedents of
stable phosphepinium-based molecules. In a pioneering study,
Winter synthesized tribenzo[b,d,f]phosphepinium,[6b] and then
Tsurusaki, Kamikawa, and coworkers reported the synthesis of a
dibenzo[b,f]phosphepinium by a gold-catalyzed 7-endo-dig
cyclization of o-ethynylarylphosphonium.[6c] Recently, Ren and
coworkers have reported the photophysical properties of indole-
fused phosphepinium derivatives, whose longest absorption and
emission wavelengths are blue-shifted relative to those of the
corresponding phosphepine oxide derivatives due to the
nonplanar π-conjugated skeletons by the non-covalent
interactions with the triflate (TfO⁻) as a counter anion.[6d]

Herein, we disclose a unique pₓ⁻π* conjugation in the seven-
membered-ring phosphepine structure, wherein a pₓ orbital of
the phosphorus center effectively overlaps with a π* orbital of the
1,3,5-hexatriene moiety (Figure 1b). This orbital interaction is
analogy to the well-established concept of the σ*⁻π* conjugation
that leads to the stabilization of the LUMO in five-membered-ring
heterole skeletons.[3b,12] The σ*⁻π* conjugation involves the
overlap of a σ*(p₂) orbital of the heavy-element moiety (i.e., a
phosphorus moiety in this case) and a π* orbital of the butadiene
moiety (Figure 1b). Meanwhile, an effective stabilization of the
LUMO by the pₓ⁻π* conjugation can be achieved in the
phosphepine structure, as the out-of-phase orbitals at the
terminal carbon atoms of the triene moiety match the symmetry
of the pₓ orbital at the phosphorus center. The positively charged
phosphepinium structure is particularly suitable for this pₓ⁻π*
conjugation because the energy of the pₓ orbital in the
phosphonium moiety is considerably lower than those of other
phosphorus derivatives such as phosphepine oxides.

To demonstrate the impact of this orbital interaction, we
synthesized dithieno[3,2-b;2',3'-/]phosphepinium dyes **1** and **2**
with different aminoaryl groups, together with the corresponding
phosphepine oxide congeners **3** and **4** for comparison (Figure
1c). The donor-acceptor-donor (D-A-D) fluorophores undergo
a conformational change of the central phosphepine ring upon
excitation, resulting in bright near-infrared (NIR) emission<sup>13-15</sup>
with a large Stokes shift. Theoretical studies on the orbital
interactions and the excitation-induced structure relaxation are
also discussed.

**Results and Discussion**

Dithieno[3,2-b;2',3'-/]phosphepinium-based dyes **1** and **2** with 4-(dibutylamino)phenyl and 4-(diphenylamino)phenyl groups, respectively, were synthesized using dithienophosphepine oxide **5** as the key precursor, whose synthesis was independently

reported by Hissler and coworkers,[10a] and Kivala and
coworkers.[10b] First, the aminoaryl groups were introduced by
the palladium-catalyzed direct arylation of C-H bonds at the
thiophene moieties of **5** to yield **3** and **4**, respectively. Then,
deoxygenation of **3** and **4** with HSiCl₃ followed by methylation
with MeOTf gave dithienophosphepinium ions **1**<sub>OTf</sub> and **2**<sub>OTf</sub> with
TfO as a counter anion, respectively. Compound **1**<sub>OTf</sub> was further
derivatized to **1**<sub>BArF</sub> by counter anion exchange with NaBArF<sub>4</sub> (ArF
= 3,5-ditrifluoromethylphenyl). All these phosphepinium
derivatives were obtained as dark red solids with sufficient
stability to be handled without special precautions in the air. No
noticeable degradation was observed after six months, not even
when **1** and **2** were kept in the solid state at room temperature.
The high thermodynamic stability of these compounds is
reflected in the decomposition temperature at the 5% weight
loss (*T*<sub>ds</sub>) for **1**<sub>OTf</sub>, which was 335 °C (Figure S1). Owing to its
bent structure and the introduction of the large BArF<sub>4</sub> as a
counter anion, **1**<sub>BArF</sub> was soluble even in apolar solvents such as
hexane.

<figure><img src="image_2.png" /><figcaption>Scheme 1. Synthesis of dithienophosphepinium-based dyes **1** and **2**.</figcaption></figure>

Single crystals of phosphepinium ion 1<sub>OTf</sub> and phosphepine oxide 3 suitable for X-ray crystallographic analysis were obtained by vapor diffusion of hexane into dichloromethane solutions of 1<sub>OTf</sub> or 3 at room temperature (Figures 2a and 2b). [16] The central phosphonium-bridged seven-membered ring of 1<sub>OTf</sub> adopts a bent structure with the internal angles summing to 876°, which is similar to the previously reported value for phosphepine oxide congeners including 4. [8d,9,10] The endocyclic P1–C3 bond in 1<sub>OTf</sub> (1.773(3) Å) is slightly shorter than that in 3 (1.794(2) Å), as similarly observed in MeP<sup>+</sup>Ph<sub>3</sub> (P–C<sub>Ph</sub>: 1.78–1.79 Å) and O=PPh<sub>3</sub> (P–C<sub>Ph</sub>: 1.80 Å) (Figures S2 and S3). [17] While the experimentally observed difference of the dihedral angles between the two thiophene mean planes of 1 (133°) and 3 (153°) is relatively high in the crystalline state, those estimated by DFT calculations in the S<sub>0</sub> state are almost identical (see Figure 4b). In the crystal of 3, the formation of a π-stacked structure was confirmed (Figure 2d). On the other hand, such stacking was not observed in crystals of 1<sub>OTf</sub>, wherein the counter anion TfO<sup>−</sup> resides between two π-skeletons of 1<sub>OTf</sub> due to electrostatic interactions with the hydrogen atoms on the methyl group of the P<sup>+</sup> center (Figures 2c and S4). Thus, the
difference of conformers of **1** and **3** observed in the crystalline state is likely due to packing forces. Although the phenyl groups at the phosphorus atoms of **1**<sub>OTf</sub> and **3** are both located at the apical positions, their orientation relative to the seven-membered rings are different (Figures 2a and 2b). This is likely due to the different degree of steric congestion around the phosphorus moieties owing to the bending angles of the seven-membered rings.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** ORTEP diagrams for the molecular structure of (a) **1**<sub>OTf</sub> and (b) **3** as well as the crystal packing arrangement for (c) **1**<sub>OTf</sub> and (d) **3** with thermal ellipsoids at 50% probability.</figcaption></figure>

The electron-accepting character and the bent conformation of the dithienophosphepinium skeleton affect the electronic structure of the resulting π-conjugated system so as to exhibit NIR emission with a characteristic Stokes shift (Tables 1 and S1). The absorption spectrum of **1**<sub>OTf</sub> in CH<sub>3</sub>CN showed an absorption band with a maximum (λ<sub>abs</sub>) at 502 nm, which is blue-shifted by 27 nm compared to that in toluene, due to the strong solvation effect on the positively charged π-skeleton in the ground state (S<sub>0</sub>) (Figure 3a). Given that the λ<sub>abs</sub> values of phosphepine oxide congener **3**

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Absorption (dashed lines) and emission spectra (solid lines) of (a) **1**<sub>OTf</sub> in various solvents and of (b) **1**<sub>OTf</sub>, **2**<sub>OTf</sub>, and **3** in CH<sub>3</sub>CN.</figcaption></figure>

in these solvents are almost identical, such negative solvatochromism in the absorption is specific for charged **1**<sub>OTf</sub>. The fluorescence spectrum of **1**<sub>OTf</sub> in CH<sub>3</sub>CN exhibited an intense NIR emission with a maximum (λ<sub>em</sub>) at 692 nm with a large Stokes shift (Δν) of 5482 cm<sup>-1</sup>. The emission bands were red-shifted upon increasing the solvent polarity from toluene to THF to CH<sub>3</sub>CN (Figures 3a, S5, and S8), reflecting its intramolecular charge transfer (ICT) character induced by the D-A-D structure. Notably, the λ<sub>em</sub> of **1**<sub>OTf</sub> was by 70 nm red-shifted compared to that of **3**, which clearly demonstrates the importance of the highly electron-accepting phosphepinium core for gaining NIR emission properties.

The λ<sub>abs</sub> value of **2**<sub>OTf</sub> was slightly blue-shifted relative to that of **1**<sub>OTf</sub>, which is likely due to the reduced electron-donating ability of the (diphenylamino)phenyl groups. The λ<sub>em</sub> values of **2**<sub>OTf</sub> are similar to those of **1**<sub>OTf</sub> (Tables 1 and S1); however, a large difference was observed for fluorescence quantum yields (Φ<sub>FL</sub>). In particular, high Φ<sub>FL</sub> values were observed for **1**<sub>OTf</sub> even in polar methanol and CH<sub>3</sub>CN (>0.42), whereas a low Φ<sub>FL</sub> (<0.06) were obtained for **2**<sub>OTf</sub> in these solvents. The radiative decay rate constants (k<sub>r</sub>) for **1**<sub>OTf</sub> and **2**<sub>OTf</sub>, which were determined using the values of Φ<sub>FL</sub> and the fluorescence lifetimes (τ), were 0.86-2.1×10<sup>8</sup> s<sup>-1</sup> independent of the solvent polarity (Table S1). In contrast, the nonradiative decay rate constant (k<sub>nr</sub>) of **2**<sub>OTf</sub> increased with increasing solvent polarity, whereas that of **1**<sub>OTf</sub> remained low (Figure S10). Although the reason for the difference in k<sub>nr</sub> remains unclear at this stage, dyes showing such a NIR emission with high Φ<sub>FL</sub> even in polar environments are useful as fluorescent probes for bioimaging.<sup>[13c,18]</sup> Another notable feature is the low k<sub>nr</sub> values of **1**<sub>OTf</sub> compared to those of phosphepine oxide congener **3** in nonpolar solvents. This trend was also observed for **1**<sub>BArF</sub> (Table S1 and Figure S6). A possible rationale for this decrease in k<sub>nr</sub> is due to an increased contribution of a rigid quinoidal resonance structure in the S<sub>1</sub>, which reflects the stronger accepting character of **1** (see Figure 5a).

<table><caption>**Table 1:** Photophysical properties and excited-state dynamics data for **1-3**.</caption><thead><tr><th>Compound</th><th>Solvent</th><th>λ<sub>abs</sub><sup>[a]</sup><br>/nm</th><th>λ<sub>em</sub><br>/nm</th><th>Φ<sub>FL</sub><sup>[b]</sup></th><th>τ<sup>[c]</sup>/ns</th><th>k<sub>r</sub><sup>[c]</sup><br>/10<sup>8</sup> s<sup>-1</sup></th><th>k<sub>nr</sub><sup>[c]</sup><br>/10<sup>8</sup> s<sup>-1</sup></th></tr></thead><tbody><tr><td rowspan="3"><b>1<sub>OTf</sub></b></td><td>Toluene</td><td>529</td><td>650</td><td>0.71</td><td>3.4</td><td>2.1</td><td>0.86</td></tr><tr><td>THF</td><td>512</td><td>676</td><td>0.64</td><td>3.2</td><td>2.0</td><td>1.1</td></tr><tr><td>CH<sub>3</sub>CN</td><td>502</td><td>692</td><td>0.48</td><td>3.2</td><td>1.5</td><td>1.7</td></tr><tr><td rowspan="3"><b>2<sub>OTf</sub></b></td><td>Toluene</td><td>506</td><td>642</td><td>0.57</td><td>3.4</td><td>1.7</td><td>1.3</td></tr><tr><td>THF</td><td>486</td><td>677</td><td>0.33</td><td>2.4</td><td>1.4</td><td>2.8</td></tr><tr><td>CH<sub>3</sub>CN</td><td>473</td><td>700</td><td>0.04</td><td>0.50</td><td>0.86</td><td>19</td></tr><tr><td rowspan="3"><b>3</b></td><td>Toluene</td><td>467</td><td>548</td><td>0.20</td><td>0.56</td><td>3.5</td><td>14</td></tr><tr><td>THF</td><td>468</td><td>576</td><td>0.31</td><td>1.1</td><td>2.9</td><td>6.4</td></tr><tr><td>CH<sub>3</sub>CN</td><td>467</td><td>622</td><td>0.51</td><td>2.3</td><td>2.3</td><td>2.1</td></tr></tbody></table>

[a] Only the maximum absorption wavelength are shown. [b] Absolute fluorescence quantum yields were determined by a calibration with integrated sphere system. [c] Calculated taking into account the coefficients of each lifetime.

The incorporation of the phosphonium-bridge also had a considerable impact on the redox properties. The cyclic voltammetry (CV) curves of **1**<sub>OTf</sub> and **3** in THF showed reversible redox waves for two-step one-electron reduction and one-electron oxidation processes (Figure S12). The first cathodic
peak potential (E<sub>pc1</sub>) of 1<sub>OTf</sub> was -1.81 V vs Fc/Fc<sup>+</sup>, which was 0.41 V more positive than that of 3. Meanwhile, the anodic peak potential (E<sub>pa</sub>) of 1<sub>OTf</sub> was +0.46 V vs Fc/Fc<sup>+</sup>, which is 0.12 V more positive than that of 3. This result suggests that the phosphonium-bridge stabilizes the LUMO level more effectively than the HOMO level, resulting in a reduction of the HOMO-LUMO gap, which is responsible for the bathochromically shifted wavelengths of absorption and emission in 1 compared to those of phosphine oxide-bridged 3.

To gain insight into the effects of the positively charged structure of the phosphepinium core on the conformation and electronic properties, DFT calculations were performed at the CAM-B3LYP/6-311+G(d,p) level of theory including THF as a solvent using the polarizable continuum model (PCM). Here, model compounds 1' and 3', wherein the butyl groups in 1 and 3 were replaced with methyl groups, were employed for simplicity. The bent conformers 1'<sub>bent</sub> and 3'<sub>bent</sub>, wherein the phenyl group on the phosphorus atom resides at the apical position, were obtained as energy-minimum structures in the S<sub>0</sub> (Figure 4). In addition, 1'<sub>flip</sub>, 1'<sub>planar</sub>, 3'<sub>flip</sub>, and 3'<sub>planar</sub> were obtained as metastable conformers, in which the energy differences among the bent, flip, and planar conformers are sufficiently small (<2 kcal/mol) to be easily interconverted at room temperature (Figures S14 and S16). Although the counter anion was not included in the calculations, the dihedral angle between the two thiophene mean planes (139°) and the P–C bond length (1.78 Å) in the seven-membered ring in 1'<sub>bent</sub> are comparable to those obtained from the crystallographic analysis (Figure S13).

Time-dependent (TD) DFT calculations of 1'<sub>bent</sub> and 3'<sub>bent</sub> at the same level of theory revealed that the longest wavelength absorption bands arise mostly from the HOMO–LUMO transitions (Figure 4a). The LUMO is localized predominantly on the central dithienophosphepinium moiety, while the HOMO is fully delocalized over the entire π-conjugated skeleton. Consequently, a moderate ICT character is induced in the excited state, which is responsible for the experimentally observed solvatochromism in the fluorescence spectra. The stabilization of the LUMO level compared to the HOMO level, which reduces the HOMO–LUMO gap, is larger for 1'<sub>bent</sub> than for 3'<sub>bent</sub>, which is consistent with the results of the CV measurements (Figure S12) and thus most likely responsible for the longer-wavelength absorption of 1.

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

**Figure 4.** (a) Kohn–Sham molecular orbitals for the HOMO (thick black lines) and LUMO (hollow lines) of the energetically most stable conformers of the model compounds 1'<sub>bent</sub> and 3'<sub>bent</sub> (isovalue = 0.03) together with their S<sub>0</sub>–S<sub>1</sub> transition wavelengths and oscillator strengths (f) calculated at the CAM-B3LYP/6-311+G(d,p) level of theory including THF as a solvent using the polarizable continuum model (PCM). (b) Front views of the LUMOs for 1' and 3'.

A closer look at the LUMO distribution confirmed the extension of the π* orbital of the C3 (C3') atom (see Figure 2 for the numbering) toward the phosphorus atom, which is more pronounced in phosphepinium 1' than in phosphepine oxide 3' (Figure 4b). This represents the p<sub>x</sub>–π* conjugation (Figure 1b) between the p<sub>x</sub> orbital on the phosphorous center and the π* orbital of the 1,3,5-hexatriene moiety. Although the corresponding orbital overlap was also confirmed in 3' when decreasing the threshold value for the orbital drawing,<sup>[9,10]</sup> the extent of the p<sub>x</sub>–π* conjugation is significantly lower. The lowering p<sub>x</sub> orbital associated with the positively charged structure of the phosphonium moiety is crucial for the effective interaction with the π* orbital in the LUMO of 1' (Figure S21). It should be also noted here that the bent conformation of the phosphepine ring facilitates the p<sub>x</sub>–π* conjugation (Figure S22). The decreased overlap between the p<sub>x</sub> and π* orbitals in the fully planar conformation of 1' confirms this notion, corroborating the importance of the bent structure of the seven-membered ring. The orbital overlap due to the p<sub>x</sub>–π* conjugation can be confirmed by DFT calculations for other phosphepinium derivatives, including diphenylphosphonium- or dimethylphosphonium-bridged congeners, diphenylamino-substituted 2, and dibenzophosphenium 6 (Figures S15 and S19).

The conformational change in the excited state (S<sub>1</sub>) from the bent seven-membered ring of 1 (Figure 2) is most likely responsible for the large Stokes shift observed in the measurement of the photophysical properties. The DFT study showed that the optimized structure in the S<sub>1</sub>, which was obtained starting from the conformer 1'<sub>bent</sub>, adopts an almost planar geometry with the sum of the internal angles of the seven-membered ring being 881° (Figure 5a). The large conformational change was also corroborated experimentally by decreasing the temperature below the glass-transition temperature in 2-MeTHF (Figure S11), i.e., at 78 K, where the conformational change in the S<sub>1</sub> should be largely restricted, the emission band of 1<sub>OTf</sub>.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (a) Optimized structure of 1*<sup>bent</sup>* together with the S<sub>0</sub>→S<sub>1</sub> transition wavelength and oscillator strengths calculated at the CAM-B3LYP/6-311+G(d,p) level of theory for the S<sub>0</sub> and at the CAM-B3LYP/6-31+G(d,p) level of theory for the S<sub>1</sub>. (b) Optimized structure of the dibenzophosphepinium congener 6 in the S<sub>1</sub>. All calculations included THF as a solvent using the PCM.</figcaption></figure>

shifted to 603 nm (Δλ = 62 nm). The DFT calculation showed that the endocyclic P1–C3 bond is slightly shorter in the S<sub>1</sub> (1.770 Å) than in the S<sub>0</sub> (1.777 Å). In addition, the bond length alternation between C2–C3 (1.399 Å) and C1–C2 (1.394 Å) is smaller in the S<sub>1</sub>. These structural features suggest a contribution of a quinoidal resonance structure upon conjugating of the phosphepinium center with the aminoaryl groups in the S<sub>1</sub>. This quinoidal contribution, which accompanies the planarization, results in the substantial red-shift of the emission wavelength and an increase in the oscillator strength in the S<sub>1</sub>. While the dimethylphosphonium-bridged congener also showed a similar conformational change (Figure S24), we found that the fused thiophene structure, which is less aromatic than benzene, contributes to take the more planar conformation in the S<sub>1</sub>. Similar excited-state calculations suggested that dibenzo-fused congener 6 only forms a bent conformation with a dihedral angle between the two benzene rings of 147° in the S<sub>1</sub>, where the steric congestion between the methyl group and the benzene ring might also disturb the planarization (Figure 5b). Overall, the fusion of less aromatic thiophene and the introduction of a highly electron-deficient phosphonium bridge is crucial for the bright NIR emission with a large Stokes shift.

Conclusions

A series of dithieno-fused phosphepinium dyes bearing electron-
donating aminophenyl groups were synthesized. The X-ray
crystallographic analysis revealed that the
dithienophosphepinium core in these dyes adopts a bent
conformation in the S₀, and that the π-skeletons are segregated
by counter anions. The resulting D–A–D fluorophores exhibit a
bright far-red to NIR emission with a large Stokes shift.
Compared to an analogous D–A–D dye with a

dithienophosphepine oxide core, the dithienophosphepinium-
based fluorophores with dibutylamino groups exhibited a
bathochromically shifted emission by more than 70 nm while
retaining high fluorescence quantum yields in all solvents tested,
regardless of their polarity. A DFT study revealed p<sub>x</sub>-p<sub>y</sub>* conjugation in the LUMO. Although a similar orbital interaction
appears in principle in other phosphepine oxide derivatives, this
interaction is more pronounced in phosphepinium skeletons due
to the low-lying p<sub>x</sub> orbital of the phosphonium moiety. In addition,
the increase in the contribution of the quinoidal character in the
excited state induces a large conformational change from the
bent seven-membered ring to a more planar structure, resulting
in bathochromically shifted fluorescence with a large Stokes shift.
This characteristic conformational change in the dithieno-fused
structure stands in stark contrast to that of the dibenzo-fused
congener. Overall, the present study demonstrates that the
phosphepinium skeleton provides a useful platform for designing
promising NIR emitters with large Stokes shifts. Further studies
along these lines are currently in progress in our research group.

**Acknowledgements**

This work was financially supported by CREST (JPMJCR21O5)
from the Japan Society and Technology Agency (JST), and by
KAKENHI grants (23H0402 and 22K21346) from the Japan
Society for the Promotion of Science (JSPS). The authors thank
Prof. Masayasu Taki, Mr. Ryusei Aruga, and Dr. Mika Sakai
(Nagoya Univ.) for valuable discussions.

Keywords: Heterocycle • Near-infrared fluorescence •
Phosphepine • Phosphonium • Stokes shift

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Entry for the Table of Contents

<figure><img src="image_9.png" /></figure>

A seven-membered-ring phosphepinium has been developed as the key core skeleton for near-infrared (NIR) dyes. The low-lying p<sub>x</sub> orbital in the phosphonium moiety and the bent conformation of the seven-membered ring lead to a stabilization of the LUMO via a "p<sub>x</sub>-π* conjugation". The observed intense emission with a large Stokes shift was attributed to a large structural relaxation with a contribution from a quinoidal resonance structure in the excited state.