Excited-state Intramolecular Proton Transfer Process of Crystalline 6

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C: Physical Processes in Nanomaterials and Nanostructures

Excited-State Intramolecular Proton Transfer Process of Crystalline 6-Cyano-2-(2'-hydroxyphenyl)Imidazo[1,2 a]Pyridine, as Revealed by Femtosecond Pump-Probe Microspectroscopy Yukihide Ishibashi, Mako Murakami, Koji Araki, Toshiki Mutai, and Tsuyoshi Asahi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01044 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

Excited-state Intramolecular Proton Transfer Process of Crystalline

6-Cyano-2-(2'-hydroxyphenyl)imidazo[1,2

a]pyridine, as Revealed by Femtosecond Pump-probe Microspectroscopy Yukihide Ishibashi 1, Mako Murakami, 1 Koji Araki, 2 Toshiki Mutai,2 and Tsuyoshi Asahi* 1

1Department

of Materials Science and Biotechnology, Graduate School of Science

and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan. 2Department

of Materials and Environmental Science, Institute of Industrial Science,

The University of Tokyo, 4-6-1, Komaba, Tokyo 153-8505, Meguro-ku, Japan. Corresponding Author: [email protected]

ACS Paragon Plus Environment

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Abstract We

have

investigated

the

excited-state

dynamics

of

6-cyano-2-(2'-

hydroxyphenyl)imidazo[1,2 a]pyridine (6-CN HPIP) in tetrahydrofuran (THF) solution and crystalline phases by means of femtosecond transient absorption spectroscopy. In the THF solution, the excited-state intramolecular proton transfer (ESIPT) of the hydrogen-bonded enol form (E form) occurred with a time constant of 0.6 ps, followed by the formation of the excited state of a twisted keto form. On the other hand, in the crystalline phase, heteroexcimer formation between the excited keto form with coplanar conformation and the ground-state E form was observed after ESIPT of the E form in a 1-ps time scale. 6-CN HPIP has three crystalline polymorphs: 6-CN(O), 6-CN(Y), and 6-CN(R), which emit fluorescence having different spectral shape and lifetime. We compared the ESIPT and heteroexcimer formation dynamics for three polymorphs. It was found that the time constant of ESIPT process was in the order of 6-CN(Y) (0.30±0.1 ps) < 6-CN(R) (0.50±0.2 ps) < 6-CN(O) (0.80±0.2 ps). The order agrees with the difference in the O-N distance of the ground-state E form in each polymorph; i.e. faster ESIPT takes place in a shorter O-N distance. Furthermore, the heteroexcimer formation, which occurred in a few tens of ps, demonstrated polymorph dependence. We discussed the difference from the viewpoint of the small dihedral angle between the two aromatic rings of the ground-state E form and the longitudinal slippage (the overlap) of the π–π stacked dimeric unit.


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1. Introduction Solid-state emissive organic materials are attracting much attention because of their various applications such as light-emitting diodes, lasers and sensors.1-5 Usually, most of organic chromophores having high fluorescence yield in solution phase are weakly emissive in a rigid medium because of the aggregation-caused quenching process.6 To overcome this problem, extensive studies have addressed rational molecular designs on the basis of aggregationinduced emission mechanism achieved by suppressing molecular motions and/or   planar interaction.4-8 Recently, excited-state intramolecular proton transfer (ESIPT) is a candidate for the solid-state emitter, because ESIPT-capable molecules in rigid medium show highly emissive in the visible region.1,9-17 It is experimentally and theoretically well-known that typical ESIPT process in solution phase comprises a four-level photo-cycle, enol (E) → excited enol (E*) → excited keto (K*)→ keto (K), which involved the enol-keto phototautomerization in a sub-picosecond time scale, and most of the ESIPT chromophores show dual emission, short wavelength emission due to the E* form (“normal” emission) and the longer one due to the K* form (ESIPT emission) through the photo-cycle.18-29, Therefore, by controlling the mode of molecular packings (polymorphic form) and restricting intramolecular rotation and cis-trans isomerization in K* state, the ESIPT emission in solid state shows higher efficiency than that in solution phase. Among highly emissive ESIPT-processing solids,1,9-17 we focused on a three-color polymorph dependent emission of 6-Cyano-2-(2'-hydroxyphenyl)imidazo[1,2 a]pyridine (6CN HPIP) as shown in scheme 1, which was synthesized by Mutai and coworkers.30-34 Three polymorphic crystals of 6-CN HPIP have different modes of molecular packing as shown in scheme 1(b)-(d). 6-CN(O) had a herringbone-like antiparallel dimer stacking mode, and 6CN(Y) and 6-CN(R) showed slip-stacked parallel stacking modes. The molecules of 6-CN(R) were less slipped compared to 6-CN(Y), and the same aromatic parts overlapped one another.


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The absorption spectra of three polymorphic crystals were similar spectral shapes, indicating that absorption properties are not sensitive to the mode of molecular packing.30 However, upon 330-nm excitation, the fluorescence peak wavelength and quantum yield were clearly different (6-CN(Y): flmax = 570 nm, fl = 25%; 6-CN(O): flmax = 548 nm, fl = 49%; 6CN(R): flmax = 585 nm, fl = 10%). It is theoretically deduced that these different fluorescence properties are concerned with a combination of intermolecular interactions between neighboring excited- and ground-state molecules and molecular packing modes. In solution phase, Douhal and co-workers experimentally demonstrated that the proto-type of HPIP showed the internal rotational motion involved with phenyl and imidazo[1,2-a]pyridine rings in excited keto state occurred in a few tens of picosecond after rapid ESIPT process.23,24 On the other hand, in the crystalline phase, the mechanism of the ESIPT and the subsequent processes remain unclear. Simply, it is predicted that intermolecular interactions rather than the internal rotational motion will occur because the distance between neighboring molecules become short and large structure changes are restricted. Especially, each of 6-CN HPIP crystals will show different excited-state dynamics dependent on dimer stacking modes as described in Scheme 1. Understanding the mechanism of rapid ESIPT processes in crystalline phase, as well as the extent of intermolecular interactions among the chromophores, is important in order to make significant improvements of the photophysical properties for their application as organic slid-state emissive materials. In this context, we carried out the femtosecond transient absorption microspectroscopy in order to elucidate the ESIPT and the subsequent processes of three 6-CN HPIP single microcrystals. As a result, in three crystals, the ESIPT process took place in 1 ps, and then the intermolecular interaction such as heteroexcimer between K* form and E form was formed on a few tens of picosecond time scale. The rates of the ESIPT and the heteroexcimer formation were dependent on the molecular packing modes. We finally discussed the


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differences of the excited-state dynamics between solution and solid phases, and among three polymorph crystals.

2. Experimental A. Materials 6-CN

HPIP

was

synthesized

by

coupling

5-cyano-2-aminopyridine

and

2-

(bromoacetyl)anisole with subsequent demethylation by tribromoborane as detailed in previous paper.30 Crystallization using diffuse hexane vapor in benzene solution in a sealed tube yielded the prismatic yellow-luminescent crystal, 6-CN(Y). On the other hand, the platelet orange-luminescent crystal, 6-CN(O) was obtained by common heat–cool crystallization of a chloroform solution. Crystallization by the vapor diffusion method from a THF–hexane solution resulted in a mixture of the polymorphic crystals from which the needle-like, red-luminescent crystal, 6-CN(R) was manually separated. The measurements for the solution phase were carried out in THF (Wako, spectrophotometric grade). B. Femtosecond Transient Absorption Measurement Femtosecond transient absorption measurements were carried out using 100 fs pulses (800 nm) from a Ti:Sapphire chirped pulse amplified system operating at 1 kHz repetition rate. This beam was divided into two using a beam splitter (80% - 20%). The stronger beam was frequently doubled (400 nm) using BBO crystals and employed as the pump pulse, which was chopped at a 500 Hz repetition rate with an optical chopper (S2000, Thorlabs). The pump pulses were guided into an inverted optical microscope (IX71, Olympus), and focused at the sample with an objective lens (x60, NA 0.70). The pump pulses were circularly polarized by Berek compensator (Newport), and the intensity was set to be 0.1 mJ/cm2 with a neutral density filter to avoid the photodecomposition. A small portion of the weaker fundamental beam was focused into a CaF2 window (3-mm thickness) to generate the white-


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light continuum in the wavelength range of 380 to 860 nm, which was used as probe light. After passing through an optical bandpass filter (10-nm bandwidth in fwhm) (Asahi Spectra), the tuned probe pulses were also circularly polarized by another Berek compensator (Newport), guided into the microscope co-linearly with the pump pulses, and then focused into the sample with the same x60 objective lens. The transmitted light of the probe was collected with the x20 objective lens, and then the intensity was detected with a CCD camera (BU-54DUV, Bitran) coupled with a polychrometer (250is, Chromex). The time delay between pump and probe pulses was carefully varied from -20 ps to 350 ps by a computer controlled translation stage (STM-150, SIGMA Koki). The obtained kinetic data were analyzed by using commercial software (Igor 6) and homemade programs. The temporal cross correlation of the pump and probe pulses at the sample position was 350 fs in fwhm, and the diameter of the pump and probe beams was about 770 nm in fwhm at the focusing point.35 We selected smooth and flat area of single microcrystal in the pump-probe measurement. The smooth and flat area of single microcrystal was selected for an excitation spot. For solution phase, we employed a conventional femtosecond spectroscopic system, in which pump pulse set to 266-nm light and probe pulse was the white-light continuum without using the bandpass filter. The home-built rotation cell (2000 rpm) with an optical length of 0.2 cm was used to avoid photodecompositions. The temporal cross correlation of the pump and probe pulses at the sample position was 150 fs in fwhm, and the diameter of the pump beam was about 500 m in diameter at the sample position. The laser fluence was 0.1 mJ/cm2. Magic angle polarization was kept between the pump and probe pulses by using the Berek compensator. C. Fluorescence lifetime measurement


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Fluorescence time profiles for single microcrystal and THF solution were measured using a time-correlated single photon counting (TCSPC) system combined with another inverted microscope (IX71, Olympus). As an excitation laser pulse, a 377-nm picosecond laser pulse (FLP-10, HAMAMATSU) with the pulse duration of 100 ps was employed with the repetition rate of 10 MHz. An avalanche photodiode (PDM, MPD) and a counting board (SPC-130, Becker & Hickl GmbH) were used for signal detection. An optical bandpass filter (10-nm bandwidth in fwhm) (Asahi Spectra) was placed in front of the avalanche photodiode to select monitoring wavelength and avoid the scattering of excitation light. The system response time was determined to be 150 ps in fwhm by a scattered light from a colloidal solution. The fluorescence time profile was measured at different wavelengths with intervals of 20 nm in the range of 500-720 nm. The reconstruction of the time-resolved fluorescence spectrum was based on a method detailed in a previous report.36 It was assumed that the relative integrated intensity of the fluorescence decay at each wavelength corresponds to that of the steady state fluorescence spectrum. The number of photons was counted up to delay of 30 ns with a step size of 12.2 ps and it was averaged with an interval (±250 ps) to obtain a time-resolved fluorescence spectrum with higher S/N ratio. D. Steady-state Measurements A steady-state fluorescence spectra for microcrystals and THF solution were measured by liquid nitrogen cooled CCD camera (Princeton) with monochorometer (SpectraPro 300i, Andor). This system was also combined with the inverted microscopy for the TCSPC measurement. The absorption spectrum of the solution was obtained using a JASCO V-570 absorption spectrophotometer at room temperature.

3. Results and Discussion 3-1. Solution


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Figure 1 shows the steady-state absorption and fluorescence spectra of 6-CN HPIP in THF. * absorption bands around 350 nm appeared, which has quite similar shape as compared to that in another solvents such as acetonitrile, and toluene in Figure S1. This indicates that solvent polarity has less effect on the absorption properties of 6-CN HPIP. On the fluorescence, it was reported that 2-2-phenylimidazo[1,2-a]pyridine derivatives in a polar solution have two species of E form at the ground state; hydrogen-bonded enol and open enol forms.23,24,32 The former shows the rapid ESIPT, whereas the latter does not involve proton transfer but has strong fluorescence with small Stokes-shift and long lifetime in a ns time scale. In polar THF solution, 6-CN HPIP shows a large and broad fluorescence band around 430 nm, which were assigned to the open enol form. In addition, a weak fluorescence with a peak at 680 nm was detected. This band can be attributed to a keto form because the intensity ratio of the open enol form to excited keto form depends on the solvent polarity as shown in Figure S1. Figure 2 shows the fluorescence time profiles of 6-CN HPIP in THF, excited with a ps 377-nm pulse with the pulse duration of 100 ps fwhm. Monitoring wavelengths were set to 450±20nm (black), and >600 nm (red) by using bandpass filter and long-pass filter to select fluorescence species, respectively. The instrumental response function (IRF) is depicted in grey. Solid lines are the results calculated with the IRF and the single-exponential function. The time profile at 450 nm shows the single exponential decay with a lifetime of 2.1 ns, indicative of the lifetime of the open enol form. On the other hand, the time profile at >600 nm due to the excited K (K*) form was in agreement with the curve of the IRF (150 ps fwhm). This implies that the fluorescence lifetime of the K* form will be less than 150 ps. Notes that we accurately determined the K* form lifetime to be 62±5 ps by using another light source with short pulse duration but high repetition rate (100 fs fwhm, 80 MHz) in Figure S2.


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To reveal the rapid ESIPT process in solution, we carried out the conventional femtosecond transient absorption measurement. Figure 3(a) shows the transient absorption spectra and the time profiles of 6-CN HPIP in THF upon a fs 266-nm excitation. Immediately after excitation, a broad positive band with a peak at 380 nm and a weak tail at 480-780 nm appeared. This spectral shape was assigned to Franck-Condon enol (E*) form. The band at 390 nm grew up and the tail at 500 nm rapid decreased, and the negative band around 700 nm appeared in 2 ps. Since the negative band appears in the spectral region of the steady-state weak fluorescence from excited keto form as shown in Figure 1, it is attributable to the stimulated emission of K* form. Therefore, ESIPT process took place in 2 ps. Then, the positive band around 390 nm decreased and the stimulated emission recovered, and after 150 ps a positive band around 380 nm with a weak tail at 480-780 nm gradually decreased. At 500 ps, broad structureless absorption band remained. Therefore, the broad positive band after 500 ps will be assigned to the open enol form. Precisely to understand the dynamics of excited-state 6-CN HPIP in THF, the global analysis for the time profiles were performed in Figure 3(b), and detailed in supporting information. Here, the sum of four exponential functions with 0.6-ps, 9-ps, 70-ps, and 2100ps time constants was required and sufficient to fit the time profiles (Figure S3). The 2100-ps component was the assigned to the open enol form. The fastest component (0.6 ps) was the ESIPT process because of the spectral growth around 390 nm and the appearance of the stimulated emission of K* form in 2 ps. The 0.6-ps time constant was quite longer than that obtained

in

2-(2

′

-hydroxyphenyl)benzoxazole

(HBO)

and

2-(2

’

-

hydroxyphenyl)benzothiazole (HBT). In this study, the excitation at 266 nm attained higher excited state of E form, and vibrational relaxation process should be involved in ESIPT process. Therefore, the time constant of the ESIPT process seemed to be long. The two lifetimes of 9 ps and 70 ps will be attributed to coplanar and twisted conformers in excited


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keto form, respectively. Douhal group discussed that the proto-type of HPIP in solution transitioned from the coplanar to the twisted conformation at the excited keto state with a time constant of 14 ps.23,24 In our case, the 9-ps component showed the negative band due to the stimulated emission around 750 nm, which locates at different peak wavelength of the steady-state fluorescence, and the value was similar to the reported time constant of the internal molecular rotation.23,24 Therefore, the 9-ps component was assigned to the coplanar conformation of the K* form. On the other hand, the 70-ps component was assigned to the lifetime of twisted K* form, because its shape had the stimulated emission band around 680 nm, which located at the same wavelength of the steady-state fluorescence and its lifetime was similar value (62±5 ps) obtained by the TCSPC measurement. Usually, when the twisted form is completely charge-transferred species with large structure change (cis-trans conformation) in excited keto form, its fluorescence is not detected. Vázquez et al proposed the mechanism for the coupled intramolecular proton and charge transfer in the first excited singlet state in HBO, HBT, and 2-(3’-hydroxy-2’-pyridil)benzoxazole, and claimed that the generation of the twisted charge-transferred form (trans keto form) probably led to very smaller energy gap as compared to the coplanar proton transfer state, resulting in the very fast radiationless decay.28 On the other hand, 6-CN HPIP in THF showed low fluorescence quantum yield (0.01),32 and the fluorescence intensity and lifetime of keto form was about 30 times as low as those of enol form. Therefore, the relative fluorescence yield of K* form in 6CN HPIP was also quite low, and the fluorescence was hardly detectable. However, because femtosecond transient absorption spectroscopy permits to observe events on short time scale, even weak fluorescence having short lifetime is possibly detected as stimulated emission (negative transient signal). In the case of 6-CN HPIP in THF, the stimulated emission due to the excited keto form was clearly observed by chance, and slightly blue-shifted from 750 nm to 680 nm in a few tens of picosecond time scale, indicative of the existence of two emissive


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states. Because these stimulated emissions existed in the same wavelength range of the steady-state fluorescence of the excited keto form, at least coplanar and twisted keto forms were weakly emissive. Considering that the trans keto form was non-emissive as described above,28 the coplanar keto form generated through ESIPT process would not be rotated completely by 180 degrees, but twisted to some extent. Therefore, because of this small conformational change, weak fluorescence due to the twisted keto form must be observable even in the solution phase. From above results and discussions, the dynamics of the ESIPT process of 6-CN HPIP in THF is given in Scheme 2. When 6-CN HPIP is excited by a fs 266-nm pulse, the excited state of enol form is generated. The intramolecular proton transfer process from the hydrogen-bonded enol to the keto forms in the excited state takes place in a sub-ps time scale, which competed with the vibrational cooling. After the ESIPT process, the excited keto form having the coplanar conformation underwent the conformational change to twisted forms in a few ps time scale. The twisted excited keto form decayed to the ground state with a lifetime of 70 ps. Finally, the excited state of the open enol form with a long lifetime of 2.1 ns remained.

3-2. Crystals Figure 4(a) shows the normalized fluorescence spectra of polymorph crystals of 6CN(O), 6-CN(Y), and 6-CN(R), excited at 377 nm. Three fluorescence bands appeared around 580 nm shorter than that in THF solution. This large blue shift of ESIPT species has been already explained by the small rearrangement from coplanar to twisted conformations. Araki group reported that the fluorescence energy of the ESIPT species (K* form) decreased with increasing the dihedral angle between phenyl and imidazo[1,2-a]pyridine rings from the theoretical calculations, and indeed the K* form in the rigid environments such as polymer


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and frozen matrices fluoresces around 580 nm because of the suppression of the internal rotational motion to the twisted conformation at the excited state.30-34 That is, the K* form in all crystal will keep more coplanar conformation as compared to that in THF, resulting in the blue-shifted fluorescence. Figure 4(b) shows the time-resolved fluorescence spectra of 6-CN(O), together with their normalized steady-state ones. The obtained fluorescence spectrum at 1 ns showed the peak around 580 nm, which was in good agreement with the steady-state fluorescence spectrum due to the ESIPT emission. This indicated that the ESIPT occurred in 1 ns. Similar behavior was observed in another two crystals, 6-CN(Y) and 6-CN(R) as shown in Figure S4. The fluorescence monotonously decreased without significant spectral change with increasing in delay time. The decay profile of the ESIPT emission can be reproduced by a single-exponential function with a time constant of 5.5 ns in Figure 4(c), and the fitting results of three crystals listed in Table S2 were consistent with the reported values.30 In order precisely to discuss the ESIPT process of three polymorph crystals in 1 ns, we measured the time profiles of the transient absorbance due to the decay of the E* form. Generally, in pump-probe microspectroscopic measurement, when a femtosecond broad white-light continuum is used as a probe light, photodecompositions of a single crystal will easily occur not only by the pump pulse irradiation but also the probe one. To avoid the sample photodecomposition and the light scattering of the pump pulse, we selected the single monitoring wavelength to be 500±5 nm using a band-pass filter, at which the decay of excited-state hydrogen-bonded enol was observed from the results of the THF solution as shown in Figure 3. Figure 5 shows the time profiles of the transient absorbance in three polymorph crystals at 500 nm, excited with a fs 400-nm laser pulse with a fluence of 0.1 mJ/cm2. Excitation wavelength was nearly close to the 0-0 band of the S0-S1 transition. We calculated


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the excitation density to be less than 1%, indicating that the effect of the bimolecular annihilation would be quite small. In the case of 6-CN(O), the appearance of the positive signal with the instrumental response function was followed by the rapid decay (inset of Figure), and then the constant positive signal remained after 150 ps. The profile was reproduced by the triple-exponential function with lifetimes of 0.8±0.3 ps (68%), 25±5 ps (24%), and 5500 ps (8%). Considering the ESIPT emission lifetime in figure 4(c), the third time constant was fixed to be 5500 ps. The 0.8-ps component was assigned to the intramolecular proton transfer from E* to K* forms. Notes that the intermolecular proton transfer process was ruled out because the distance between an oxygen atom and the nearest nitrogen atom of the neighboring molecules was large, and the contribution of the open enol form is negligibly small because the hydrogen-bonded E form was mainly packed in solids.30 The time constant of the intramolecular proton transfer in solid phase was as fast as that in the solution phase (0.6 ps). Comparing the simple excited-state relaxation process (E* form → planar K* form → twisted K* form) in solution phase, the additional lifetime of 25 ps was also observed in the solid phase. Here, we consider the 25-ps component to be a combination of the slight internal rotational motion and the generation of the interaction between K* and E forms such as a heteroexcimer. As mentioned above, the crystalline 6-CN(O) has the emission band around 580 nm, which peak wavelength was by 100 nm as short as that of the THF solution, suggesting that the twisting motion in the excited state will be suppressed as compared to the solution, and the mutual interaction between excited-state and ground-state molecules will be easily formed because of the dense molecular packing in crystalline phase. On the former twisting motion, Douhal indeed reported that the ESIPT emission peak of the proto-type of HPIP encapsulated by -, -, or -cyclodextrin appeared at 500, 550 and 570 nm respectively, indicating that this blue shift originated from the restricted twisting motion around the C2-C1’ bond by the nanocavity23,24. Therefore, in the case of crystalline 6-CN


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HPIP, the internal rotational (twisting) motion to generate the twisted K* form must be inhibited. On the other hand, as is well known in polycyclic aromatic hydrocarbons such as pyrene and pelyrene, molecules in the excited state interact with those in the ground state, leading to the formation of excimers. Furthermore, the excited molecules easily form heteroexcimer by their interaction with electron donating molecules. When heteroexcimers with the weak interaction have similar electronic structures to the excited states of electron donor-acceptor complexes, they are stable also in the ground state.37 In the case of 6-CN(O), because K* form has relatively large dipole,30 the formation of the heteroexcimer between E form and K*form will occur readily and lower the energy level below the monomer K form not only at the excited state but also at the ground state. This means the larger band gap between the ground and excited states than that of the monomer, leading to the blue-shift of the emission. The reported theoretical prediction supports this model of the heteroexcimer formation.30-33 Therefore, we concluded that the heteroexcimer between K* form and neighboring E form was generated with a lifetime of 25 ps instead of the internal rotational motion as observed in the THF solution. From above results and discussion, the dynamics of the excited-state relaxation process of 6-CN(O) is given in Scheme 3. When the 6-CN(O) was excited by a fs 400-nm pulse, the E* form was generated, and then the excited-state intramolecular proton transfer (E* form to K* form) took place with a time constant of 0.8 ps. After that, the heteroexcimer between K* form and neighboring E forms was formed with a time constant of 25 ps, and luminesced in a few ns time scale. Finally, we compared the ESIPT mechanisms of three polymorph crystals. Time profiles of the transient absorbance of 6-CN(Y) and 6-CN(R) at 500 nm were presented in Figure 5(b) and (c), respectively. In both cases, the time profiles were similar behaviors to that of 6-CN(O), while the time constants obtained by the analysis with the triple-exponential


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function were different. The fitting results are summarized in Table 1. The fastest component due to ESIPT process is in the order of 6-CN(Y) < 6-CN(R) < 6-CN(O). This order will be concerned with the distance between the oxygen (O) and nitrogen (N1), d as shown in Figure 6. From X-ray crystallographic analysis,30 the distance was 2.582 Å for 6-CN(Y), 2.622 Å for 6-CN(R), and 2.625 Å for 6-CN(O), respectively. Considering that 6-CN(O) has the longest distance among them, the rate of ESIPT process became slower than those of other crystals. As the O-N1 distance became shorter, the rate of the ESIPT process became shorter. That is, the intramolecular proton transfer process strongly depends on the O-N1 distance in crystals. On the other hand, the heteroexcimer formation dynamics (second time constant) is in the order of 6-CN(R) < 6-CN(O) < 6-CN(Y). We consider that the excimer formation time will be affected by two factors; an interplanar distance, and a longitudinal slippage of dimeric unit. Wasielewski and co-workers reported that the formation rate of the excimer state depend strongly on stacking distance as well as transverse and longitudinal displacement between perylene-3,4:9,10-bis(dicarboximide) molecules.38 In addition, Coppens group reported in xanthone crystal that the interplanar distance become close from 3.39 to 3.14 Å as expected for excimer formation by the time-resolved diffraction experiment.39 These indicate that the closer distance will accelerate the excimer formation. On the distance for three crystals,30 the 6-CN(O) had antiparallel π–π stacked dimeric units with an interplanar distance of 3.35 Å, and 6-CN(Y) had slip-stacked columns, with parallel-stacked molecules with an interplanar distance of 3.38 Å, and 6-CN(R) had parallel slip-stacked columnar packing with an interplanar distance of 3.37 Å, which was less slipped compared to 6-CN(Y) as shown in Scheme 1. Since the interplanar distance of three crystals is similar value, it less affects the heteroexcimer formation dynamics. However, the formation time of the stable excimer will depend strongly on the longitudinal slippage (the overlap) of the π–π stacked dimeric unit as well as the small dihedral angle between the two aromatic rings as displayed in Figure 6. The


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overlap of dimer unit for 6-CN(Y) was the smallest among them, leading to the slowest excimer formation time. On the other hand, both of 6-CN(O) and 6-CN(R) show the larger overlap than 6-CN(Y), while the dihedral angle of 6-CN(O) of the ground-state E form was larger value (2.6°) as compared to those of 6-CN(Y) (1.5°) and 6-CN(R) (1.4°).30 This less planarity of the molecule must obstruct the excimer formation. As a result, 6-CN(O) shows the slower excimer formation time than 6-CN(R). At any rate, the difference of the molecular packings is to some extent capable of affecting the excimer formation.

4. Conclusion In summary, to elucidate the excited-state dynamics of three polymorphs of 6-CN HPIP solids and of the THF solution, we performed the femtosecond transient absorption and fluorescence lifetime measurements. For the solution, rapid ESIPT process took place with a time constant of 0.6 ps, and the excited-state keto form with coplanar conformation was generated. The intramolecular rotational motion (twisting) involved with phenyl and imidazo[1,2-a]pyridine rings in excited keto state occurred in a few tens of picosecond, and the twisted keto form was relaxed to the ground state with a time constant of 70 ps as shown in Scheme 2. For 6-CN HPIP solids, the rapid ESIPT and the subsequent processes were able to be observed in single microcrystal and the whole excited-state dynamics was summarized in Scheme 3. For three polymorphs, excited enol form generated by a femtosecond 400-nm pulse excitation converted to excited keto form with coplanar conformation in 1 ps. After the rapid ESIPT process, the heteroexcimer formation between the coplanar excited keto form and neighboring ground-state enol form rather than the intramolecular twisting motion took place in a few tens of ps time region, and then the heteroexcimer emitted in a few ns time scale. Interestingly, not only the speed of ESIPT process but also the heteroexcimer formation time was different from each other. The time constant of the ESIPT process was in


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the order of 6-CN(Y) < 6-CN(R) < 6-CN(O) as well as the distance between the oxygen and nitrogen (N1) in Figure 6. That is, the speed of the ESIPT process in the crystalline phase was regulated by the O-N distance. On the other hand, the heteroexcimer formation time was in the order of 6-CN(R) < 6-CN(O) < 6-CN(Y). The slower heteroexcimer formation takes place in a smaller overlap (larger longitudinal slippage) of the π–π stacked dimeric unit. Furthermore, as the dihedral angle between the two aromatic rings of the ground-state E form became large, the heteroexcimer formation decelerated. Therefore, the heteroexcimer formation time was affected by the correlation of longitudinal slippage (the overlap) of the π– π stacked dimeric unit and the small dihedral angle between the two aromatic rings. Supporting Information. Supporting Information available: Additional femtosecond transient absorption measurement and fluorescence lifetime data for solution and crystals (PDF) AUTHOR INFORMATION Notes Corresponding Author: Tsuyoshi Asahi, e-mail: [email protected] The authors declare no competing financial interests.

Acknowledgment This work was partially supported by JSPS KAKENHI Grant Number JP26107011 to T. A. in Scientific Research on Innovative Areas “Photosynergetics”, and JSPS KAKENHI Grant Number 16K05743 and 17H06367 to T. M.


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Figures

Figure 1. Steady-state absorption and fluorescence spectra of 6-CN HPIP in THF (1x10-5 M). Excitation wavelength was set to 360 nm. Enlargement of the fluorescence spectrum in the wavelength of 550 nm to 780 nm is in inset.


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Figure 2.

Fluorescence time profiles of 6-CN HPIP in THF (10-4 M), monitored at

450±20nm (black), and >600 nm (red). The excitation wavelength was 377 nm, and the pulse duration was 100 ps fwhm. The instrumental response function (IRF) is depicted in grey. Solid lines are the results calculated with the IRF and the single-exponential function. Residual plot for each profile is also shown in upper panel.


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Figure 3. (a)Transient absorption spectra of 6-CN in THF (10-3 M), measured at several delay times after photoexcitation at 266 nm. The pulse duration was 150 fs fwhm and the laser fluence was 0.1 mJ/cm2. (b) Decay-associated spectra obtained from the data analysis of the transient absorption spectra and time profiles in figure S3.


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Figure 4. (a)Steady-state fluorescence spectra of three polymorph 6-CN crystals: 6-CN(Y), 6-CN(O), and 6-CN(R). Excitation wavelength was 377 nm. (b) Time-resolved fluorescence spectra of 6-CN(O), excited with a ps 377-mn pulse (100 ps fwhm). Solid line presents the steady-state fluorescence spectrum. (c) Fluorescence time profile of 6-CN(O), monitored at 580 nm. The instrumental response function (IRF) is depicted in grey. Solid line is the results calculated with the IRF and the single-exponential function with a time constant of 5.5 ns. Residual plot for each profile is also shown in upper panel. Inset presents the optical image of single 6-CN(O) microcrystal.


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Figure 5. Time profiles of transient absorbance of three polymorph crystals, monitored at 500 nm; (a) 6-CN(O), (b) 6-CN(Y), and (c) 6-CN(R). Inset in each figure shows the enlarged transient signal for the short time range from -2 to 10 ps. The solid line is the calculated curve with a triple-exponential function and pulse duration (350 fs fwhm). The excitation wavelength was 400 nm and the laser fluence was 0.1 mJ/cm2.


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.

Figure 6. Molecular structure of the enol form of 6-CN HPIP. d is the distance between oxygen and nitrogen (N1), and  is the dihedral angle between the two aromatic rings (N1C2-C1’-C2’).


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Table

Table 1. Lifetimes (i) and relative amplitudes (in parentheses) obtained from the analysis of the time profiles of the transient absorbance of three polymorph crystals.

1 / ps

2 / ps

3 / ns

6-CN(O)

0.80±0.3 (68%)

25 ±5 (24%)

5.5 (fixed) (8%)

6-CN(Y)

0.30±0.1 (90%)

45 ±5 (6%)

8.0 (fixed) (4%)

6-CN(R)

0.50±0.2 (69%)

11±3 (12%)

2.0 (fixed) (18%)

The decays were fitted to: f(t)=∑iAi exp(-t/i). Relative amplitude was calculated by Ai/(∑iAi).


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Schemes

Scheme 1. (a) ESIPT process of 6-cyano-2-(2'-hydroxyphenyl)imidazo[1,2 a]pyridine (6CN) and top views of crystal structure of 6-CN(O) for (b), 6-CN(Y) for (c), and 6-CN(R) for (d), which was re-drawn on the basis of the X-Ray crystallographic analyses in ref. 30.


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Scheme 2. A simplified schematic diagram of ESIPT and relaxation processes of 6-CN HPIP in THF solution.


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Scheme 3. A simplified schematic diagram of ESIPT and relaxation processes of crystalline 6-CN(O).


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TOC GRAPHIC


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Excited-state Intramolecular Proton Transfer Process of Crystalline 6

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