S2 Fluorescence from [26]Hexaphyrin Dianion

Letter pubs.acs.org/JPCL

S2 Fluorescence from [26]Hexaphyrin Dianion Won-Young Cha,† Woojae Kim,† Hirotaka Mori,‡ Tomoki Yoneda,‡ Atsuhiro Osuka,*,‡ and Dongho Kim*,† †

Department of Chemistry and Spectroscopy Laboratory for Functional π-Electronic Systems, Yonsei University, Seoul 03722, Korea Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

‡

S Supporting Information *

ABSTRACT: S2 fluorescence from meso-hexakis(pentafluorophenyl)-substituted [26]hexaphyrin dianion was observed as the first example of expanded porphyrins despite its large molecular size and small HOMO−LUMO gap. The population kinetics among S2, S1, and S0 states have been studied by using femtosecond time-resolved absorption and fluorescence spectroscopies. Broad-band fluorescence upconversion spectroscopy allowed for simultaneous observation of S2 fluorescence decay in the visible region and S1 fluorescence rise in the NIR region, both with a time constant of 0.22 ps. The transient absorption spectroscopy revealed the presence of a direct decay path from the S2 state to the S0 state. The observation of S2 fluorescence from highly conjugated molecular systems is quite rare, and S2 fluorescence beyond 700 nm is also quite rare.

10-fold excess) of tetrabutylammonium fluoride (TBAF) to a solution of 1 in toluene gave dianion 2 as a stable species,22 which was characterized as a more aromatic molecule than 1.23 In fact, 2 exhibited a split but more intensified B-like band and Q-like bands at 831, 872, 910, and 972 nm (Figure 1b). Interestingly, there was negligible absorbance between the Band Q-like bands in the range from 650 to 750 nm, which indicates an increased energy gap between S2 and S1 states as compared with 1. The energy gap between S2 and S1 states was calculated to be ∼4113 cm−1 for 2, which was much larger than that (ca. 2742 cm−1) of 1. Importantly, in the steady-state fluorescence spectrum, S2 fluorescence of 2 was distinctly detected in the vicinity of the B-like band with a definite vibronic feature (Figure 1b). A small peak at 672 nm has been assigned as a solvent Raman signal by blank experiments (Figure S1). When we changed the solution concentrations from 5 × 10−5 to 1 × 10−5 M in order to minimize the effect of self-absorption, the maxima peak of S2 fluorescence was blueshifted from 642 to 630 nm, as shown in Figure 2. The intensities of solvent Raman peaks at 672 nm were almost the same under the same experimental conditions. Also, the intensities of S2 fluorescence from 700 to 740 nm got smaller as the concentration of solutions decreased. When we changed the solution concentrations in order to check the possibility of an aggregation effect, there were no differences in the normalized absorption spectra (Figure S2), which means that there is no aggregation effect in this concentration range. The relative quantum yield of S2 fluorescence of 2 in toluene was

olecules that emit S2 fluorescence against Kasha’s rule1 have still remained rare2−12 but have assumed particular interest recently, owing to their possible involvement in photovoltaic cells,13,14 their potential use in optically operational logic gates,15−17 and the more general demand to understand the fast decay processes of higher excited states. As important examples, S2 fluorescence behaviors of tetraphenylporphyrin (H2TPP) and its ZnII and MgII metal complexes (ZnTPP and MgTPP) have been extensively studied to reveal detailed excited-state dynamics.6−8 In recent years, other pyrrolic pigments such as boron dipyrromethane dyes (BODIPY)4 and meso-aryl-substituted subporphyrinatoboron(III) (subporphyrins)5 have been revealed to display S2 fluorescence. Requirements necessary for the detection of S2 fluorescence may be (1) a large energy gap between the S2 and S1 states that causes relatively slow internal conversion (IC) from S2 to S1 and (2) a large radiative rate of the S2 fluorescence that can rival rapid IC of the S2 state. Meso-hexakis(pentafluorophenyl)-substituted [26]hexaphyrin 1 has been recognized as a representative aromatic expanded porphyrin with regard to a quite planar rectangular structure and 26π-conjugated electronic circuit (Scheme 1).18−21 These properties, which are quite analogous to those of tetraarylporphyrins, suggested some similarity between the electronic natures of H2TPP and 1 and hence prompted us to explore the possibility of detecting the S2 fluorescence of 1. Actually, the steady-state absorption spectrum of 1 in toluene exhibits a sharp and intense B-like band at 567 nm and well-resolved Qlike bands at 712, 770, 893, and 1018 nm (Figure 1a), which are similar to those of H2TPP to some extent. S1 fluorescence of 1 was observed at 1032 nm with a small Stokes shift of ∼136 cm−1, but its S2 fluorescence was not detected under various conditions so far examined. Addition of an excess amount (ca.

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© XXXX American Chemical Society

Received: July 12, 2017 Accepted: July 31, 2017 Published: July 31, 2017 3795

DOI: 10.1021/acs.jpclett.7b01799 J. Phys. Chem. Lett. 2017, 8, 3795−3799

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The Journal of Physical Chemistry Letters

Scheme 1. Molecular Structures and Expected π-Conjugation Pathways of Neutral 1 and Dianionic [26]Hexaphyrins 2a

a

Ar = pentafluorophenyl group.

Figure 1. Steady-state absorption and fluorescence spectra of (a) neutral [26]hexaphyrin 1 and (b) deprotonated [26]hexaphyrin 2 in toluene. For the fluorescence spectra in the visible and NIR regions, the excitation wavelengths were 560 and 442 nm, respectively. The asterisk (*) indicates the Raman peak of the toluene.

shift, which is the difference between the band maxima of the B-like absorption band and S2 fluorescence spectrum of 2, was calculated to be 256 cm−1. A small Stokes shift seems to arise from enhancement of the degree of aromaticity and rigid molecular structure of deprotonated 2. Also, the bulky countercations of TBAF (TBA+), which were found just above and below the negatively charged nitrogen atoms of rectangular planar [26]hexaphyrin, as shown in crystal packing structures for charge compensation,23 also help 2 have robust molecular structure. Neutral and deprotonated [26]hexaphyrins were found to display different 1H NMR spectral features with respect to the degree of aromaticity as well as the behaviors of NH protons on the NMR time scale. (Figure S3). The 1H NMR spectrum of neutral [26]hexaphyrin 1 showed two outer β-CH proton resonances at 8.84 and 9.26 ppm, one inner NH proton resonance at −2.55 ppm, and four inner β-CH protons at −3.14 ppm. Importantly, the lower number of signals seen in 1 is consistent with a conformationally mobile system wherein inner NH protons undergo “NH tautomerism” on the NMR time scale. The possible chemical structures of neutral 1 in the ground state are represented in Figure S4. Compared to 2, relatively broad B- and Q-like bands of 1, which is a reason for

Figure 2. Concentration-dependent steady-state fluorescence spectra of deprotonated [26]hexaphyrin 2 in toluene. The asterisk (*) indicates the Raman peak (C−H stretch, ∼3000 cm−1) of the toluene. The excitation wavelength was 560 nm.

obtained to be about 0.093% by using ZnTPP (3.3%) as a reference standard.24 Although an artifact-free S2 fluorescence spectrum was obtained by subtracting the solvent Raman peaks in very dilute solution (less than 10−5 M), we appreciate that the spectral shape and intensity of S2 fluorescence can be affected by self-absorption of intense B-like bands. The Stokes 3796

DOI: 10.1021/acs.jpclett.7b01799 J. Phys. Chem. Lett. 2017, 8, 3795−3799

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The Journal of Physical Chemistry Letters fast IC processes from S2 to S1 states with a small energy gap, are probably caused by various conformations arising from NH tautomerism. In contrast, deprotonated 2, which does not have inner NH protons, showed the two types of outer β-CH proton resonances at 9.37 and 9.71 ppm and a single peak for inner βCH protons at −3.82 ppm, as shown in Figure S3. In an effort to obtain insights into the different electronic structures of 1 and 2, we performed TDDFT calculations using B3LYP/6-31G(d). The simulated electronic vertical transitions of 1 and 2 coincide with the steady-state absorption spectra, as shown in Figure S5. The energy gap between the second (864 nm) and third (547 nm) lowest electronic transitions corresponding to the S1 and S2 transitions, respectively, was calculated to be ∼6707 cm−1 for 2, which is larger than that (ca. 6217 cm−1) of 1. Also, the highly red-shifted second lowest transition and the two energetically neighboring S1 transitions (the first and second lowest transitions in Figure S5) of 2 were also consistent with the steady-state absorption spectrum. These analyses provide support for a large energy gap between the B- and Q-like bands in the steady-state absorption spectrum. Additionally, the observed split B-like absorption bands of 2 at 601 and 619 nm as shown in Figure 1 are probably caused by the combination of two orthogonal electronic transition dipole moments (red and blue arrows in Figure 1b) in the symmetric and rigid molecular structure formed by deprotonation (Figure S6). The observation of S2 fluorescence of 2 has been accomplished more explicitly through broad-band fluorescence upconversion spectroscopy.25−28 Time-resolved fluorescence spectra were acquired for laser excitation centered at 580 nm so as to prevent spectral interference caused by the excitation pulse in the S2 fluorescence region (Figure S7). The timeresolved fluorescence spectra and temporal profiles of S2 and S1 fluorescence of deprotonated [26]hexaphyrin 2 are shown in Figure 3. It was clearly shown that the decay of the S2 fluorescence in the region from 600 to 750 nm was followed by the rise of the S1 fluorescence at around 970 nm with the same time constant of 0.22 ps. Therefore, this component has been assigned to the IC process from the S2 to the S1 excited state. This is the first example of S2 fluorescence of expanded porphyrins. As noted above, while S2 fluorescence behaviors were reported for some small molecules, S2 fluorescence in highly conjugated molecular systems is rare, and that beyond 700 nm is also quite rare. To better understand the fast relaxation processes of the S2 state of 2, we have examined the excited-state dynamics by femtosecond transient absorption (TA) measurements (Figure 4). Previously, the S1 state lifetimes of 1 and 2 were recorded to be 104 and 870 ps in toluene, respectively (Figure S8).23 A more than 8 times longer S1 state lifetime and significantly intensified NIR fluorescence of 2 suggest that the nonradiative rate in the S1 state of 2 decreases substantially with respect to the radiative rate constant. As such, we just focused on the slow S1 decay of 2 and missed its faster relaxation dynamics at an early time domain. The TA spectra of 2 in toluene with excitation at 590 nm (S2 excitation) are shown in Figure 4a. For the negative bands at around 620 nm (reddish spectra) in early TA spectra of 2, which have been assigned as the ground-state bleaching (GSB) signal, more than 20% GSB intensity decayed within 1 ps without any significant spectral changes. The time constant of this ultrafast decay component was determined to be 0.27 ps. Upon changing the probe wavelength from 605 to 625 nm within the GSB region, interestingly, the magnitude of

Figure 3. (a) Steady-state (top) and time-resolved (bottom) fluorescence spectra. (b) Decay (S2 fluorescence) and rise (S1 fluorescence) profiles of deprotonated [26]hexaphyrin 2. The excitation wavelength was 580 nm.

the short time component of 0.27 ps was significantly increased from 19 to 39% (Figure S9). These results suggest that the red edge of the GSB contains non-negligible stimulated emission (SE) signals despite the fact that the TA spectra do not seem to have a trace of fluorescence. The decay-associated spectrum of 2 at early time delay (0.27 ps) reveals the SE peaks at around 630 and 700 nm, whereas that of 2 at longer time delay (870 ps) has almost the same spectral shape of the ground-state absorption spectrum in the visible region. (Figure S9). On the other hand, when we used a 980 nm excitation source (S1 excitation), similar fast decay dynamics in the visible region was not observed at all (Figure 4b). These results support that the S2 state of 2 returns to the ground state directly, not through any intermediate states or Q-states. The decay profile of 2 probed at 625 nm was well fitted by two exponential functions with time components of 0.27 and 870 ps. As the fast decay component of 0.27 ps obtained by TA and that of 0.22 ps measured by time-resolved fluorescence are almost identical, this time constant can be assigned to the IC process from S2 to the S0 state in the form of fluorescence. The decay component of 0.27 ps is slightly longer than that of 0.22 ps obtained by fluorescence upconversion measurements because TA dynamics may be affected by the interference of the excited-state absorption. In the case of excitation of 2 to the S2 excited state by a 590 nm laser pulse, the temporal profile probed at 975 nm also 3797

DOI: 10.1021/acs.jpclett.7b01799 J. Phys. Chem. Lett. 2017, 8, 3795−3799

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Figure 4. TA spectra in the visible region and temporal profiles (insets) of deprotonated [26]hexaphyrin 2 with photoexcitation at (a) 590 and (b) 980 nm in toluene. Gray dotted lines are the ground-state absorption spectra of 2.

Figure 5. TA spectra in the NIR region and temporal profiles (insets) of deprotonated [26]hexaphyrin 2 with photoexcitation at (a) 590 and (b) 980 nm in toluene.

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shows rise components with time constants of 0.27 and 2 ps (Figure 5a). An ultrafast time component of 0.27 ps originates from the IC process from the higher excited state to the S1 state. According to several literature reports,10,11,29 the components of a few picoseconds likely represent vibrational relaxation accompanied by solvation processes. When we used the 980 nm excitation source (S1 excitation), however, fast decay and rise components were not observed in the early time domain, as shown in Figures 4b and 5b, respectively. In conclusion, the S2 fluorescence was detected not from [26]hexaphyrin 1 but from its dianionic species 2 in toluene by steady-state fluorescence spectroscopy. By broad-band fluorescence upconversion spectroscopy, simultaneous observation of the decay of the S2 fluorescence and the rise of the S1 fluorescence allowed us to reveal the S2 deactivation and S1 population dynamics of deprotonated [26]hexaphyrin 2. Furthermore, femtosecond time-resolved TA spectra revealed the presence of an ultrafast direct decay pathway from the S2 state to the ground state. The observation of S2 fluorescence from highly conjugated molecular systems like hexaphyrins is quite rare, and S2 fluorescence beyond 700 nm is also quite rare. As demonstrated, simple deprotonation made [26]hexaphyrin S2 fluorescence active. Hence, deprotonations of even larger expanded porphyrins including Möbius aromatic species are actively being pursued in our laboratories to explore S2 fluorescence in a more red-shifted region.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01799. Experimental details and spectroscopic results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.K.). *E-mail: [email protected] (A.O.). ORCID

Tomoki Yoneda: 0000-0002-9804-0240 Atsuhiro Osuka: 0000-0001-8697-8488 Dongho Kim: 0000-0001-8668-2644 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at Yonsei University was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2016R1E1A1A01943379). The work at Kyoto was supported by Grants-in-Aid from JSPS (Nos. 25220802 (Scientific Research (S)), 16K13952 (Exploratory Research)). 3798

DOI: 10.1021/acs.jpclett.7b01799 J. Phys. Chem. Lett. 2017, 8, 3795−3799

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Conjugated Networks in meso-Hexakis(pentafluorophenyl)[26]hexaphyrin. Chem. - Eur. J. 2012, 18, 15838−15844. (24) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.; Holten, D.; Bocian, D. F. Effects of Orbital Ordering on Electronic Communication in Multiporphyrin Arrays. J. Am. Chem. Soc. 1997, 119, 11191−11201. (25) Zhang, X.-X.; Würth, C.; Zhao, L.; Resch-Genger, U.; Ernsting, N. P.; Sajadi, M. Femtosecond broadband fluorescence upconversion spectroscopy: improved setup and photometric correction. Rev. Sci. Instrum. 2011, 82, 063108. (26) Sung, J.; Kim, P.; Fimmel, B.; Würthner, F.; Kim, D. Direct observation of ultrafast coherent exciton dynamics in helical π-stacks of self-assembled perylene bisimides. Nat. Commun. 2015, 6, 8646. (27) Gerecke, M.; Bierhance, G.; Gutmann, M.; Ernsting, N. P.; Rosspeintner, A. Femtosecond broadband fluorescence upconversion spectroscopy: Spectral coverage versus efficiency. Rev. Sci. Instrum. 2016, 87, 053115. (28) Kim, W.; Sung, J.; Grzybowski, M.; Gryko, D. T.; Kim, D. Modulation of Symmetry-Breaking Intramolecular Charge-Transfer Dynamics Assisted by Pendant Side Chains in π-Linkers in Quadrupolar Diketopyrrolopyrrole Derivatives. J. Phys. Chem. Lett. 2016, 7, 3060−3066. (29) Turro, N. J. Principles of Molecular Photochemistry: An Introduction; University Science Books, Sausalito, CA, 2009; Vol. 1, p 291.

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