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Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

2021; Chinese Chemical Society; Volume: 4; Issue: 7 Linguagem: Inglês

10.31635/ccschem.021.202101193

2096-5745

Shuhai Qiu, Zhiyun Zhang, Yifan Wu, Fei Tong, Kai Chen, Guogang Liu, Lei Zhang, Zhaohui Wang, Da‐Hui Qu, He Tian,

Catalytic Cross-Coupling Reactions

Open AccessCCS ChemistryRESEARCH ARTICLE14 Jul 2022Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings Shuhai Qiu, Zhiyun Zhang, Yifan Wu, Fei Tong, Kai Chen, Guogang Liu, Lei Zhang, Zhaohui Wang, Da-Hui Qu and He Tian Shuhai Qiu Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Zhiyun Zhang Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Yifan Wu Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Fei Tong Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Kai Chen Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Guogang Liu Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Lei Zhang College of Energy, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 , Zhaohui Wang Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 , Da-Hui Qu *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 and He Tian Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.021.202101193 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A series of vibratile π-extended dihydrophenazines ( BPs) and a tetrahydrodiphenazine ( TP) were synthesized via direct C–N coupling reactions. Structural alterations of the fused acene wings lead to diverse intermolecular packing arrangements as well as tunable photophysical properties. These compounds exhibit intriguing features, including large Stokes shift, multiple emissions, and environmental effects. Notably, a dramatic hypsochromic shift in emission is observed when the acene wing is linearly extended from benzene to naphthalene and anthracene. This unusual π-conjugation length-dependent emission is explained by the close correlation between the calculated fluorescence-emitting energy and the π-conjugation length of the acene subunit. In addition, the TP bearing two flexible units exhibits dynamic photophysical properties resembling those of BPs. Our results reveal a balanced control over π-conjugation and luminescence in vibratile π-systems, thereby providing new insight into the molecular design of organic near-infrared fluorophores with large Stokes shifts and dynamic emissions. Download figure Download PowerPoint Introduction Organic fluorescent π-systems with dynamic excited states have played a significant role in the fields of chemistry, biology, and materials science due to their intriguing photochemical and photophysical properties. These molecules usually experience two or more emissive excited states and undergo elaborate photophysical processes, such as excimer formation,1,2 excited-state intramolecular proton transfer (ESIPT),3 and twisted intramolecular charge transfer (TICT).4,5 Besides these intriguing processes, excited-state planarization can occur with dihydrophenazines,6,7N-aryl acridine analogs,8,9 heteropins,10,11 π-expanded cyclooctatetraenes (COT),12,13 and so on. They adopt a butterfly-like bent conformation in the ground state (S0) and relax to a nearly planar structure in the lowest-lying excited state (S1) upon photoexcitation. Sterically hindered dihydrophenazines are a class of dynamic molecules originally discovered by Schuster et al. in 198014 and further developed by Tian and Chou et al.6 The bent-to-planar excited-state dynamics, which we have termed vibration-induced emission (VIE), have introduced new features to the photophysical properties, for example, large Stokes shifts and multicolor emission.15,16 These molecules have been widely applied in ratiometric fluorescent probes, stimulus-responsive materials, and white organic light-emitting diodes.17–20 Studies on these dynamic fluorescent π-systems allow further understanding of the relationship between π-conjugated structures and photophysics and provide possibilities to fabricate new organic functional materials in the future. Theoretical and experimental studies of sterically hindered dihydrophenazines, for example, DHP-16 and DHP-217,21 (Figure 1) composed of phenanthrene or pyrene and benzene fused on a dihydropyrazine ring, have led to a clear understanding of the skeletal planarization process, including the influences of structural elements (e.g., molecular symmetry6 and conformational modulation22,23) on the photophysical properties. On the one hand, the structural changes of these molecules occur mainly on the side substituents at the N atoms, and the simple structures of the fused flapping wings fail to provide sufficient information on the relationship between the size of the flapping wings and the photophysical properties. Compared to the modifications of the side substituents, the π-extension of the flapping wings is expected to facilitate tuning of the electronic structures of the planar S1 state and to more closely correlate with the torque during vibration. On the other hand, the limited active sites on the phenanthrene moiety may impede the construction of dynamic π-systems with additional flexible dihydrophenazine units, which remain undisclosed. Generally, dynamic dihydrophenazine molecules are achieved by one-pot synthesis via copper-catalyzed reactions between aryl diamines and aryl iodides (Figure 1, top). However, the initial intramolecular C–H amination merely produces benzo derivatives, inhibiting π-extension along the long molecular axis.21 In addition, the poor regioselectivity during C–H amination makes linear ring expansion of the molecular skeleton very difficult, even via a two-step approach.24 Thus, the direct coupling reactions between aryl diamines and aryl dihalides may afford a series of laterally π-extended dihydrophenazines with unique photophysical properties. Figure 1 | Concept for the design of π-extended dihydrophenazines (BPs) and tetrahydrodiphenazine (TP). Download figure Download PowerPoint Herein, we report a straightforward synthesis of π-extended 9,14-dihydrobenzo[4,5]triphenyleno[1,12-abc]phenazine derivatives ( BPs) and 5,12,17,20-tetrahydro-triphenyleno[1,12-abc:4,5-a′b′c′]diphenazine ( TP) via multiple C–N coupling reactions (Figure 1, bottom). Compared with the phenanthrene backbone in DHP-1, benzo[ghi]perylene has a larger extent of π-electron delocalization and richer active K-regions, which facilitate the fusion of additional flexible dihydropyrazine units. The direct coupling reactions between diamine or tetraamine precursors and easily prepared acene dibromides afford these molecules with adjustable π-conjugated wings that could be precisely regulated by lateral ring expansion of acene dibromides. Such structural diversity provides more detailed insight into the relationship between the molecular dimension and photophysical properties. Single-crystal X-ray diffraction analyses indicates nonplanar geometries and different intermolecular packing arrangements of these compounds. Notably, an unusual hypsochromic shift in emission in solution occurs when the acene wing is further extended from benzene ( BP-1) to naphthalene ( BP-2) and anthracene ( BP-3). This unusual π-conjugation length-dependent emission is explained by theoretical calculations, which suggest a close correlation between the fluorescence-emitting energy of the planar structures in the S1 state and the π-conjugation length of acene. In addition, the symmetric TP exhibits remarkable near-infrared (NIR) emission at ∼700 nm, with a large Stokes shift of 275 nm. Our detailed studies reveal the straightforward synthesis of π-extended dihydrophenazines and provide insight into the relationship between π-conjugated wings and photophysical properties. Experimental Methods Solvents were purified and dried by standard methods prior to use. All commercially available reagents were used without further purification unless otherwise noted. Column chromatography was generally performed on silica gel (200–300 mesh), and reactions were monitored by thin-layer chromatography (TLC) using silica gel GF254 plates with UV light to visualize the course of reaction. The 1H and 13C NMR data were recorded on a 400 MHz or 600 MHz spectrometer (Bruker switzerland AG, Fällanden, Zurich, Switzerland) using CDCl3 as solvent at room temperature. The high-temperature 1H and 13C NMR data were recorded on a 500 MHz spectrometer (Bruker switzerland AG, Fällanden, Zurich, Switzerland) using 1,1,2,2-tetrachloroethane-d2 as solvent at 373 K. High-resolution mass spectra (HRMS) were obtained from a 4.7 Tesla IonSpec Fourier transform mass spectrometer (IonSpec Corporation, Lake Forest, CA, USA). Steady-state absorption and emission spectra were recorded on a Shimadzu RF-6000 spectrophotometer (Shimadzu Corporation, Tokyo, Japan). The fluorescence lifetimes and temperature-dependent fluorescence measurements of the samples in toluene from 293 to 77 K were measured on a FLS1000-stm Photoluminescence spectrometer (Edinburgh Instruments, Livingston, United Kingdom). The geometries of the ground-state and the excited-state structures were calculated by the time-dependent density functional theory (TD-DFT) method with B3LYP hybrid function. The 6-31+G(d,p) basis set was employed for all atoms. More experimental and computational details and characterization of products are available in the Supporting Information. Results and Discussion The synthesis of electron-rich dihydrophenazine derivatives is always challenging. Traditional methods involving aryl lithium,25 unsubstituted dihydrophenazines26 or fluorinated aromatic precursors,27 and oxidizable arylamide28 suffer from harsh reaction conditions, complicated procedures, unstable or limited reacting substrates, and poor regioselectivity. Furthermore, the inherent steric hindrance in bent dihydrophenazines makes the synthesis more challenging. Our previous work reported a one-pot synthesis via a copper-catalyzed C–H amination and Ullmann N-arylation domino reaction,21 which allowed the fusion of a phenyl group to the dihydropyrazine ring. However, this approach would merely afford benzo derivatives ( BP-1) rather than ring-expanded derivatives (e.g., BP-2 and BP-3). Therefore, the direct double C–N coupling reaction between aryl diamines and aryl dihalides is one of the most desirable approaches toward π-extended dihydrophenazine derivatives. However, to the best of our knowledge, the only example of direct double C–N coupling reactions toward electron-rich dihydrophenazines between aryl diamines and aryl dihalides by palladium-catalyzed reactions has a yield as low as 5%.29 Benzo[ghi]perylene has been utilized as a fluorescent probe for sensing and imaging because of its highly emissive property and good solubility.30 Peripheral K-regions and rich active sites make it an ideal building block to construct bent BPs and the double-dihydropyrazine-fused derivative TP. Compared to the phenanthrene block in N,N′-diphenyl dihydrophenazines ( DPAC),6 the larger π-electron delocalization and moiety size in the benzo[ghi]perylene planar unit endows these π-systems with more intriguing properties. The synthesis of the BPs and TP is shown in Scheme 1. Benzo[ghi]perylene-3,4-dione ( 2) and benzo[ghi]perylene-3,4,11,12-tetraone ( 3) were prepared in yields of 53% and 50%, respectively, via oxidation of benzo[ghi]perylene ( 1) with ruthenium(III) chloride (RuCl3) and sodium periodate (NaIO4) at room temperature. The reaction of diketone 2 and tetraketone 3 with aniline in the presence of pyridine and TiCl4, followed by reduction with Pd/C and hydrazine hydrate, afforded diamine 4 and tetraamine 5 in 80% and 76% yields, respectively. To synthesize π-extended BPs, we tested common synthetic methods, namely, Ullmann reactions and Buchwald–Hartwig reactions,31,32 for C–N coupling reactions with 4 and aryl dibromides. Ullmann reactions with copper powder and anhydrous potassium carbonate afforded BP-1, BP-2, and BP-3 in unsatisfactory yields of less than 20%, and no reaction was observed for TP. To our delight, BP-1, BP-2, and BP-3 were obtained as yellow powders in moderate yields of 78%, 52%, and 57%, respectively, by direct double C–N coupling reactions with Pd2(dba)3, tri-tert-butylphosphine tetrafluoroborate and sodium tert-butoxide. In addition, TP was afforded as a yellow powder in 28% yield by quadruple C–N coupling reactions. All these compounds were unambiguously characterized using NMR spectroscopy, HRMS, and single-crystal X-ray diffraction analyses, and they are all soluble in common organic solvents. Scheme 1 | Synthesis of BP-1, BP-2, BP-3, and TP. Reagents and conditions: (a) NaIO4, RuCl3.xH2O, CH2Cl2/CH3CN/H2O, rt; 2: 53%, 3: 50%; (b) (i) aniline, TiCl4/CH2Cl2, pyridine, dry CH2Cl2, 0 °C to rt; (ii) Pd/C, N2H4.H2O, THF, rt; 4: 80%; 5: 76%; (c) copper powder, K2CO3, TCB, 210 °C; BP-1: 16%; BP-2: 8%; BP-3: 9%. (d) Pd2(dba)3, PtBu3.HBF4, NaOtBu, toluene, 110 °C; BP-1: 78%; BP-2: 52%; BP-3: 55%; TP: 28%. THF, tetrahydrofuran; rt, room temperature; TCB, 1,2,4-trichlorobenzene. Download figure Download PowerPoint To determine the nonplanar structures of BP-1, BP-2, BP-3, and TP, single crystals suitable for X-ray crystallographic analyses were grown by slow evaporation of n-hexane or acetonitrile into a chloroform solution at room temperature. As shown in Figures 2a–2d, the crystal structures of BP-1, BP-2, and BP-3 adopt a V-shaped configuration with the N1–N2 line as the rotation axis due to steric hindrance between the benzo[ghi]perylene unit and the phenyl groups at the N atoms. The bending angle (θ), here defined as the dihedral angle between the C1–N1–N2 plane and N1–N2–C2 plane, ranges from 138° to 143°, which is slightly larger than that of DPAC (137°)6 and the optimized structures (∼136°). In addition, the planes of the benzo[ghi]perylene and acene wings are nearly planar, indicating good π-electron delocalization in each moiety. The length of the flapping wing that contains a benzo[ghi]perylene unit is approximately 8.5 Å, whereas that of the acene wing is 3.6 Å for BP-1, 6.0 Å for BP-2, and 8.4 Å for BP-3. The regioregular structures enable us to comprehensively investigate the relationship between the molecular dimension and the photophysical properties in vibratile π-systems. TP can be viewed as a rigid benzo[ghi]perylene block bearing two dihydroquinoxaline wings. Interestingly, only the chair-like configuration is observed, with bending angles of 141° and 142°, similar to that of BP-1. Figure 2 | Front view (top) and bottom view (bottom) of the X-ray crystallographic structures of BP-1 (a), BP-2 (b), BP-3 (c), and TP (d). The bending angle (C1, N1, N2, and C2) is denoted as θ for all title compounds. Hydrogen atoms are omitted for clarity. Download figure Download PowerPoint Figures 3a–3d illustrate the molecular packing arrangements of the four compounds. A pair of enantiomers arising from asymmetrically fused benzo[ghi]perylene blocks and nonplanar configurations are observed in the unit cell for all compounds. In the crystal structures of BP-1, BP-3, and TP, severe π–π overlaps between the benzo[ghi]perylene planes exist in each pair of enantiomers, with average vertical distances ranging from 3.5 to 3.7 Å. However, the benzo[ghi]perylene moieties separate from each other, and only partial overlap between the naphthalene subunits occurs between the paired enantiomers of BP-2. Although BP-1, BP-2, and BP-3 can be viewed as analogs of each other, the subtle structural variation in the acene subunit leads to different intermolecular packing arrangements of the paired enantiomers. In the crystal structure of BP-1, the paired enantiomers form typical herringbone packing with a nearly perpendicular dihedral angle of 87° between the adjacent benzo[ghi]perylene planes in different arrays, together with short C–H…π contacts of ∼2.9 Å. In the crystal structure of BP-2, negligible interactions exist among the benzo[ghi]perylene moieties, and only C–H…π and π–π contacts between the substituted phenyl group and the benzo[ghi]perylene planes are observed. In contrast to BP-1 and BP-2, BP-3 exhibited a one-dimensional (1D) column arrangement, and short π–π contacts of 3.4 Å between the benzo[ghi]perylene planes in the paired enantiomers were observed. The packing of TP is similar to that of BP-1 except loose intermolecular packing occurs due to the more overcrowded structure. Figure 3 | The packing motifs of BP-1 (a), BP-2 (b), BP-3 (c), and TP (d). Hydrogen atoms are omitted for clarify. Download figure Download PowerPoint The absorption and emission spectra of the four compounds in dichloromethane (DCM) are shown in Figure 4, and the key parameters are summarized in Table 1. BP-1 exhibits typical short-wavelength absorption beyond the UV region, with a maximum at 405 nm (ε = 19,250 M−1cm−1), redshifted by 53 nm compared with that of DPAC. Notably, despite one or two additional fused benzo groups, BP-2 and BP-3 show absorptions nearly identical to that of BP-1, indicating the absorptions derive from the common benzo[ghi]perylene wings. The low-energy absorption of TP is redshifted by 20 nm compared with the BPs, implying a larger extent of π-electron delocalization. The optical bandgaps of these compounds derived from the lowest energy absorption onset were 2.74 eV for BP-1, 2.73 eV for BP-2, 2.71 eV for BP-3, and 2.64 eV for TP. Cyclic voltammetry (CV) experiments were conducted to study the electrochemical properties of these compounds ( Supporting Information Figure S1). The BPs exhibited two reversible oxidation waves ( BP-1: 0.14 and 0.31 V, BP-2: 0.17 and 0.34 V, and BP-3: 0.16 and 0.35 V) and one irreversible reduction wave ( BP-1: −2.37 V, BP-2: −2.37 V, and BP-3: −2.36 V), while TP had three reversible oxidation waves (0.12, 0.43, and 0.64 V) and one irreversible reduction wave (−2.28 V). The electrochemical energy gaps were calculated as 2.33 eV for BP-1, 2.34 eV for BP-2 and BP-3, and 2.28 eV for TP, which are slightly smaller than their optical bandgaps. Figure 4 | Absorption (dashed line) and emission (solid line) spectra of BP-1, BP-2, BP-3, and TP in DCM (concentration: 1 × 10−5 M) at room temperature. Download figure Download PowerPoint Table 1 | The Photophysical Properties of the Dihydrophenazines and Tetrahydrodiphenazine Compound λmaxabs (nm)a(ε/M−1cm−1) λmaxem (nm) Stokes Shift (nm)a Quantum Yield (%) τ (ns)a PMMA Solid Solutiona Solid Solutiona BP-1 405 (19,250) 472 562 680 275 33 7 (11)b 3.96 (8.42)b BP-2 405 (20,370) 467 471 630 225 36 25 (29)b 13.41 (11.25)b BP-3 403 (14,212) 473 570 608 205 12 4 (24)b 8.86 (6.25)b TP 425 (23,483) 483 580 698 273 16 4 (14)b 3.04 (6.95)b aDetermined in degassed DCM. bDetermined in degassed toluene. BP-1 exhibits a single broad emission with a maximum at 680 nm, redshifted by 70 nm compared with that of DPAC, along with an onset extending to >850 nm. Notably, a large Stokes shift of 275 nm is observed for BP-1, and negligible overlap between the absorption and emission spectra exists, which is rarely reported for other dynamic fluorescent π-systems and NIR-emissive dyes.10,33 The emissions of BP-2 and BP-3 exhibit a similar single peak in the long-wavelength region (500–850 nm). Negligible short-wavelength emission is observed for all compounds, indicating that planarization proceeds smoothly in solution. Surprisingly, although BP-2 and BP-3 bear one or two more fused benzene rings than BP-1, their emission spectra show an unusual hypsochromic shift, with maxima at 630 and 608 nm, respectively, which contradicts general understanding. Theoretical calculations on the optimized planar S1 excited states indicate the emissive S1–S0 transition is dominantly derived from highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transitions (see Supporting Information Figures S9 and S10). TP exhibits redshifted absorption (λmaxabs at 425 nm) and emission (λmaxem at 698 nm) spectra by ∼20 nm compared with those of BP-1. In addition, the spectra of these compounds show distinct bathochromic shifts with increasing dielectric constants ( Supporting Information Figure S2); for example, BP-1 has emission maxima at 622 nm in n-hexane, 645 nm in toluene, 668 nm in tetrahydrofuran and 690 nm in acetonitrile, which arise from inherent intramolecular charge transfer (ICT) (see Supporting Information for the detailed frontier molecular orbital analyses). The fluorescence decay time (τ) of all four compounds in DCM and toluene were determined using the nanosecond time-correlated single photon counting (TCSPC) technique ( Supporting Information Figure S3 and Table 1). The results reveal a single exponential decay lifetime of 3–13 ns corresponding to long-wavelength emission. To further understand the photophysical properties of these compounds, the emission in the poly(methyl methacrylate) (PMMA) matrix and solid state were also investigated (Figure 5 and Supporting Information Figure S4). Deep sky-blue fluorescence was observed in the PMMA matrix containing 0.1% BP-1. The fluorescence spectrum exhibits a single peak with a maximum at 472 nm, and no other peaks are observed at longer wavelengths. The excitation spectrum of the film is nearly identical to the absorption spectrum in DCM ( Supporting Information Figure S5), indicating that the emission originates from the same molecular structures. The small Stokes shift of 67 nm suggests that the planarization process of BP-1 is completely suppressed by the polymer matrix. The powder sample exhibits yellow fluorescence at 562 nm with a quantum yield of 0.33. This redshifted emission arises from the intermolecular π–π interactions of the benzo[ghi]perylene moieties, as supported by the molecular packing in the single crystals. BP-2 and BP-3 exhibit similar emission to that of BP-1, with peaks at 467 and 473 nm in the PMMA matrix, respectively. Notably, the powder sample of BP-2 shows negligibly redshifted emission at 471 nm, similar to that in the PMMA matrix, due to weak intermolecular π–π interactions. This is further supported by the calculated Hirshfeld surfaces ( Supporting Information Figure S11). The emission of TP in the PMMA matrix is redshifted ∼10 nm compared with that of BP-1, and the powder sample gives a yellow emission at 580 nm. Figure 5 | Emission spectra of BP-1 in the PMMA matrix, powder form, and DCM solution (concentration: 1 × 10−5 M). Corresponding photographs of BP-1 under 365 nm UV lamp irradiation are shown in the insets. Download figure Download PowerPoint To understand the multicolor-emission behavior of these compounds in different environments, variable-temperature fluorescence measurements of the four compounds in toluene were conducted ( Supporting Information Figure S6). When the temperature was above the melting point of toluene (178 K) but below room temperature (298 K), only the emission maximum of BP-1 at ∼650 nm, corresponding to the planar S1 state, changed in intensity, and no new peaks appear. As the temperature decreased to 153 K, two new peaks at ∼470 and 580 nm were observed because the conformation vibration was partly restricted by the frozen media. Upon further lowering the temperature to 93 K, the peaks at 650 and 580 nm disappeared, while the peak at 470 nm was greatly enhanced and shifted to a shorter wavelength of 464 nm. These results indicate that the molecular planarization process is completely suppressed and that only emission from vertical excitation occurs. These results resemble the emission behavior of DPAC in n-butyl alcohol with various viscosities.6 BP-2, BP-3, and TP exhibited emission changes similar to those of BP-1 when the temperature decreased, implying that they share a common photoexcitation process. In addition, the emission spectra of all these compounds in frozen toluene at 77 K ( Supporting Information Figure S7) are analogous to those measured in the PMMA matrix, further verifying the complete suppression of planarization in the lowest-lying excited state. Notably, TP bearing two flexible dihydropyrazine rings shows emission changes similar to those of the BPs, which bear one flexible ring, indicating that only partial planarization occurs in the lowest excited state of the π-systems with two flexible units. To illustrate the relationships between the photophysical properties and the excited-state conformational and electronic changes of these compounds, potential energy surface (PES) simulation scanning over a bending angle (θ) range of 130° to 170° was performed by the TD-DFT method at the B3LYP/6-31+G(d,p) level in combination with a polarizable continuum model (PCM). Figures 6a–6c depict the simulated PES of BP-1, BP-2, and BP-3 for both the ground and lowest excited states scanned along θ in the PCM model in toluene. In contrast to the single minimum S1 state of V-shaped oxepins,34 one local minimum S1 state (I*) corresponding to the bent geometry and one global minimum S1 state (P*) corresponding to the planar geometry were observed at bending angles of 135–142° and 160°, respectively. Three emissions at 482, 575, and 635 nm for BP-1 are attributed to the transitions from the initial excited state (R*), an intermediate with the local minimum energy (I*), and the final state with the global minimum energy (P*) to the ground state, respectively, which are in accordance with our previous results.6 This also reveals the origin of the three emission peaks observed in frozen toluene at 153 K. The PESs of BP-2 and BP-3 at the S1 states exhibit energy changes like that of BP-1. Notably, the fluorescence-emitting energy of the global minimum S1 state (P*) (1.95 eV for BP-1, 2.09 eV for BP-2, and 2.14 eV for BP-3) increased distinctively as the π-conjugated skeleton extended. These results show good agreement with the observed hypso