Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic

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Spectroscopy and Photochemistry; General Theory

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin Jun Oh Kim, Yongseok Hong, Taeyeon Kim, Won-Young Cha, Tomoki Yoneda, Takanori Soya, Atsuhiro Osuka, and Dongho Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01829 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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The Journal of Physical Chemistry Letters

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin Jun Oh Kim, † Yongseok Hong, † Taeyeon Kim, † Won-Young Cha, † Tomoki Yoneda, ‡ Takanori Soya, ‡ Atsuhiro Osuka,*,‡ and Dongho Kim *,† †

Spectroscopy Laboratory for Functional π - Electronic Systems and Department of Chemistry, Yonsei University, Seoul 03722, Korea

‡

Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan E-mail: [email protected] (D. Kim); [email protected] (A. Osuka)

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Abstract This study revealed S2 fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin (1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broadband fluorescence upconversion technique. This is the first S2 fluorescence from chromophores with twisted Möbius topology and the observation of the S2 fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand S2 fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the SB state to the SQ state which occurs prior to the relaxation to the optically-dark, lowest electronic state (SD). Therefore, the SQ state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the SB state to the S0 state. In deprotonated [32]heptaphyrin, the internal conversion from the SB state to the SQ state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S2 fluorescent.

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S2 fluorescence which is against one of the key principles in molecular photochemistry, Kasha’s rule, refers to emission from the second excited singlet state.1 There have been limited reports of S2 fluorescence from π-conjugated organic molecules whose absorption spectra cover the visible and nearinfrared regions, due to rapid internal conversion processes from higher excited-states to the lowest excited-state. Such S2 fluorescence from π-conjugated organic molecules has been observed mainly from pyrrolic pigments such as porphyrinoids and boron-dipyrromethane derivatives.2-9 As representative porphyrinoids, free-base porphyrin, metalloporphyrins, B(III)subporphyrins,9 and deprotonated [26]hexaphyrins11 rationalize the detection of their S2-fluorescences on account of energetically well-separated B- and Q-bands in the absorption spectra (~5000 cm-1), which allows for the radiative transition from higher excited-states to the ground-state to compete with the rapid nonradiative internal conversion to the lowest excited-state.10 With this regard, the observation of S2 fluorescence in the near-IR region would be very difficult, since the energy gap between the emitting state and the ground state should be small. Importantly, the detection of S2 fluorescence has been limited to Hückel aromatic porphyrinoids to date, owing to the ease of maintaining their planar conformations.11 In our previous report, meso-pentafluorophenyl-substituted [32]heptaphyrin exhibited a large structural change from a figure-of-eight to a twisted Möbius conformation upon treatment with tetrabutylammonium fluoride (TBAF) in toluene.12 The figure-of-eight conformation is due mainly to effective intramolecular hydrogen bonding interactions, and thus addition of TBAF disrupts such hydrogen bonding interactions, allowing deprotonated [32]heptaphyrin to take a Möbius conformation with related aromatic stabilization. In line with the antiaromatic nature, [32]heptaphyrin displays broadened and ill-defined B- and Q-like bands with a narrow energy difference. In contrast, its deprotonated species exhibits a sharp B-like band at 694 and 724 nm and well-defined Q-like bands at 973 and 1093 nm, indicating its aromatic nature.12 Considering the aromatic character and a large energy difference between S2 and S1 states (4960 cm-1) obtained from TD-DFT (time-dependent density functional theory) calculations, deprotonated [32]heptaphyrin was thought to be a nice candidate of Möbius aromatic molecule that emits detectable S2 fluorescence. ACS Paragon Plus Environment

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Figure 1. Steady-state measurements of (a) neutral and (b) deprotonated [32]heptaphyrin in toluene (top). Their calculated electronic vertical transitions are plotted (bottom).

Non-fluorescent nature of neutral [32]heptaphyrin is evident in Figure 1, which has been ascribed to its antiaromaticity. In sharp contrast, S1 fluorescence of deprotonated [32]heptaphyrin was clearly observed with a small Stokes shift (196 cm-1), which indicated a structural rigidity of the twisted Möbius conformation. Interestingly, upon excitation at the B-like band, S2 fluorescence of deprotonated [32]heptaphyrin was detected in the vicinity of the B-like band (700-800 nm) with a vibronic structure. As similar to the S1 fluorescence, a small Stokes shift (137 cm-1) of the S2 fluorescence was observed at dilute solutions (0.0035 mM). The observation of the S2 fluorescence in near IR region is remarkable, since S2 fluorescence is rare and near-IR emission is also rare even from S1 state of π-conjugated organic molecule, since non-radiative decays are inversely proportional to their excitation energies, being particularly fast for low-lying excited states.19-22 Importantly, this is the first report of S2 fluorescence in the near-IR region. To confirm the S2 fluorescence from deprotonated [32]heptaphyrin, the concentration-dependent S2 fluorescence spectra were measured (Figure 2). As the concentration decreased, the decrease in fluorescence intensity was observed around 760 nm, where the self-absorption effect was negligible, implying that the detected fluorescence signals originate from the target monomer ACS Paragon Plus Environment

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molecule. Solvent Raman signals at 710 and 790 nm were assigned from a blank measurement upon 640 nm excitation (C-H stretch, ~1500 cm-1 and ~3000 cm-1).23 Furthermore, an aggregation effect was excluded on the basis of the normalized absorbance spectra of the solutions at the measured concentrations (SI).

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Wavelength (nm) Figure 2. Corrected concentration-dependent steady-state S2 fluorescence spectra of deprotonated [32]heptaphyrin in toluene. The excitation wavelength was 640 nm. The spectra were corrected by the PMT response after Raman signals from toluene were subtracted.

The absorption spectra of neutral and deprotonated [32]heptaphyrins match nicely with the TD-DFT calculations. The vertical transitions related to the B- and Q-like bands of deprotonated [32]heptaphyrin are governed by four frontier orbitals (HOMO-1, HOMO, LUMO and LUMO+1), whose electronic transitions are accounted for on the basis of Gouterman’s four-orbital model for aromatic porphyrins.24 The neutral [32]heptaphyrin demonstrates that the major contribution of the vertical transitions involves six frontier orbitals (HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1 and LUMO+2) in line with the optical properties of antiaromatic porphyrinoids.25 The distinct optical properties and aromaticity of the two species were evaluated by their molecular structures, which are the figure-of-eight structures for the neutral species and the Möbius-strip conformation for the deprotonated species sharing the same number of π-electrons.26-29 Removal of the NH proton in [32]heptaphyrin with a base led to the generation of the deprotonayted [32]heptaphyrin with a Möbius aromatic character as revealed by 1H NMR measurements and nucleus-independent chemical shifts (NICSs).12 Anisotropy of the induced current density (ACID) 5 ACS Paragon Plus Environment


plots was calculated for neutral and deprotonated [32]heptaphyrins to visualize delocalized π-electrons and directions of the induced ring currents at the B3LYP/6-31G(d) level (SI). The main π-conjugation pathway of neutral [32]heptaphyrin has π-electron density in the figure-of-eight molecular framework with a moderate counterclockwise ring current suggesting its weak antiaromatic character, while the clear and definite clockwise ring current of the deprotonated [32]heptaphyrin strongly supports its

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Wavelength (nm) Figure 3. (a) Steady-state absorption/fluorescence spectra and (b), (c) fs-transient absorption spectra of deprotonated [32]heptaphyrin in toluene. Excitation wavelengths were selected as 690 and 1100 nm for the B-band and Q-band excitations respectively.

To investigate the higher excited-state dynamics of the deprotonated [32]heptaphyrin, fs-transient absorption measurements were conducted at the excitation at 690 and 1100 nm (Figure 3). As reported previously, the S1 state lifetime of deprotonated [32]heptaphyrin was measured to be 290 ps with a residual component which was assigned as its T1 state. Upon excitation of the B-like band at 690 nm, an ACS Paragon Plus Environment

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additional decay kinetics with a sub 200 fs lifetime was monitored at the red-edge of the ground-state bleaching signals around 720 nm. The amplitude of this fast decay component was monitored to increase when the temporal profiles were plotted near the stimulated emission (S2 fluorescence) region (SI). In addition, a fast rise kinetics was clearly reflected in the temporal profiles at 1100 and 1300 nm where the 0-0 and 0-1 bands of S1 fluorescence appear. Conversely, the fast dynamics was not monitored upon the excitation of the Q-like band (1100 nm) without any spectral evolution. Therefore, the fast time component (sub 200 fs) is assigned as the internal conversion process from the SB state to the SQ state, resulting in the decay dynamics of S2 fluorescence at 720 nm and the simultaneous rise dynamics of S1 fluorescence at 1100 and 1300 nm. Wavelength (nm) 700

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In addition to the transient absorption measurements, the observation of S2 fluorescence was further accomplished utilizing fs-broadband fluorescence upconversion technique. The time-resolved fluorescence spectra (Figure 4) were corrected based on the S2 fluorescence spectrum obtained from the dilute solution (0.0035 mM). For the excitation pulse, NOPA (Noncollinear Optical Parametric ACS Paragon Plus Environment

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Amplifier) pulses centered at 580 nm were generated to minimize spectral interference caused by the excitation pulse. Time-resolved S2 fluorescence was explicitly observed in the region of 700 - 800 nm with a lifetime of 150 fs which was not precisely assigned by the fs-transient absorption measurements as described above due to its time resolution, limit of 200 fs. Therefore, it is concluded that the S2 state depopulates with a time constant of 150 fs via deactivation processes such as internal conversion processes to the lowest excited-state and a radiative transition to the ground-state. Because the upconversion efficiency drastically decreases from the near-infrared region, it is not easy to observe the rise dynamics of S1 fluorescence from time-resolved fluorescence measurements.

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To understand the origin of S2 fluorescence, the higher excited-state dynamics of the neutral [32]heptaphyrin was investigated as a control molecule (Figure 5). Upon photoexcitation by NOPA pulses centered at 580 nm (excitation of the B-like band), the decay profile was observed with time components of 65, 300 fs and 9.5 ps when monitored at the excited-state absorption bands. Conversely, upon the photoexcitation of the Q-like band (800 nm), the fast component of 65 fs disappeared in the ACS Paragon Plus Environment

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temporal profile. Due to the saturated absorption of white light continuum by the sample in the spectral region where its ground-state absorption is strong, the differential absorption spectrum was probed limited to the spectral regions of its excited-state absorption bands (750-780 nm). Based on the pump energy-dependent temporal profiles, the fast kinetics of the 65 fs time constant was assigned as the internal conversion process from the higher singlet excited-state (SB) to the second singlet excited-state (SQ). Thus, the bright state (the B-like band) of the [32]heptaphyrin is depopulated at a much faster rate compared to that of its deprotonated aromatic counterpart (150 fs), which rationalizes our observation of S2 fluorescence for the deprotonated [32]heptaphyrin. This is further supported by the energy gap law based on the energy spacings between the SB and SQ states of the two [32]hetaphyrin species referring to the SQ state of neutral [32]heptaphyrin as a trap state limiting the S2 fluorescence located between the SB and SD states. In conclusion, for the first time, the S2 fluorescence in the near-infrared region was observed for Möbius aromatic [32]heptaphyrin generated by the deprotonation of the [32]heptaphyrin. Higher excited-state dynamics was examined by fs-transient absorption spectroscopy monitoring the decay and rise kinetics of stimulated emission of S2 and S1 fluorescence simultaneously. Furthermore, the observation of S2 fluorescence was explicitly accomplished utilizing fs-broadband fluorescence upconversion technique monitoring the decay profile of S2 fluorescence with a 150 fs lifetime. Importantly, this is the first examination of the excited-state dynamics of antiaromatic expanded porphyrinoids. Based on ultrafast transient absorption spectroscopy with sub 30 fs resolution, triexponential decay profiles were measured, three time components assigned as two internal conversion processes (SB-SQ and SQ-SD) and the SD state lifetime respectively. In this regard, the SQ state of neutral antiaromatic species acts as a deactivation channel for the bright state (B-like band) demonstrating its non-S2 fluorescent nature.


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ASSOCIATED CONTENT Supporting Information Concentration-dependent absorption and S2 fluorescence spectra, temporal profiles from transient absorption measurements, results from theoretical calculations and experimental details

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work at Yonsei University was supported the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2016R1E1A1A01943379). The quantum calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with supercomputing resources including technical support. The work at Kyoto was supported by JSPS KAKENHI (18H03910, 25220802, and 16K13952).


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Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic

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