Combined Picosecond Time-Resolved UVâVis and NMR Techniques

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Combined Picosecond Time-Resolved UV-VIS and NMR Techniques Used for Investigation of the Excited State Intramolecular Triplet-Triplet Energy Transfer Jacek Dobkowski, Alexandr Gorski, Micha# Kijak, Mariusz Pietrzak, Kipras Redeckas, and Mikas Vengris J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03414 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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The Journal of Physical Chemistry

Combined Picosecond Time-Resolved UV-VIS and NMR Techniques Used for Investigation of the Excited State Intramolecular Triplet-Triplet Energy Transfer

Jacek Dobkowski,*,† Alexandr Gorski,† Michał Kijak,† Mariusz Pietrzak,*,† Kipras Redeckas,‡ Mikas Vengris‡

† Institute

of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

‡ Vilnius

University, Faculty of Physics, Laser Research Centre, Sauletekio 10, LT10223 Vilnius, Lithuania

*corresponding authors: Jacek Dobkowski, e-mail: [email protected] Mariusz Pietrzak, e-mail: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The phenomenon of the intramolecular triplet-triplet (T-T) energy transfer observed for spiro[9,10-dihydro-9-oxoanthracene-10,2’-5’,6’-benzindan] (AN) molecule was investigated using stationary and time-resolved techniques in the UV/VIS spectral region. The rate constant for energy transfer from anthrone chromophore to the triplet state localized on the naphthalene subunit of AN molecule is 2.8*1010 s-1. NMR spectroscopy is rarely used for investigation of molecules in the electronically excited states. Here, we propose 1H NMR combined with UV laser irradiation as a useful method of the recognition of an electron spin densities distribution in the excited triplet state that exists for tens of microseconds in the liquid phase. The direct registration of the 1H NMR signals from molecules in the excited triplet state was not possible due to its short lifetime. However, even the short interaction between unpaired electrons and nuclear spins leads to the changes in the NMR spectrum. The analysis of difference NMR spectra delivers information about the electron spin densities distribution over the skeleton of the molecule in the excited triplet state. In order to understand the nature of the excited states involved in the triplet-triplet energy transfer process, the quantum chemical calculations were performed.

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1. INTRODUCTION The interaction between unpaired electrons and nuclear spins is a source of several phenomena in both EPR and NMR spectroscopy. NMR contact and pseudocontact shifts and paramagnetic relaxation enhancement (PRE) are used for gaining information about chemical structure, as well as about electronic structure and delocalization of unpaired electrons.1 Stable radicals are often used as dopants that enable characterization of biological molecules (e.g., proteins),2 or investigation of protein-ligand interactions3 and stable paramagnetic complexes are widely used as contrasts for magnetic resonance imaging (MRI).4 Almost all reported methods deal with paramagnetic substances in their electronic ground states. On the other hand, an example of Fe(II) complex used as an optorelaxer for all proton spins in the solid sample upon lightexcitation to the triplet state was reported recently.5 Nevertheless, the excited electronic triplet state is rarely studied by NMR technique. In order to investigate the electronically excited states, irradiation of samples is necessary. Irradiation of liquid samples with visible or UV light coupled with NMR detection is used for various reasons, and it can be divided into two general classes. The first one concerns light as a technical tool for influencing or controlling the nuclear spins in the whole bulk, e.g., sensitivity enhancement of an NMR spectrum due to optical nuclear polarization (ONP),6 controlling reaction by light,7,8 optical detection of NMR signals.9 The second one focuses on local interactions, and it is used mainly for characterization of photoreactions or molecular structures of photoactive compounds, e.g., detecting of stable or transient products of photoreaction,10,11 enhancement of some signals due to photochemically induced dynamic nuclear polarization (photo-CIDNP).12 The photo-CIDNP relies mainly on the hyperfine coupling between nuclear and electron spins that is responsible for some anomalous signal intensities in the NMR spectrum, and it is widely used for determination of protein structures13 and studies of photosynthetic 3



reaction. However, it is limited to the cases when radical pairs are generated during photochemical reactions. In our case, the 1H NMR method coupled with laser irradiation will be used for gaining information about the interaction of unpaired electrons with nuclear spins in the excited triplet state. The NMR spectroscopy is commonly used to determine the ground state geometry of molecules. However, this relatively slow technique was successfully used to establish the excited-state charge transfer reaction coordinate.14 In this paper, we apply NMR technique as a useful tool for monitoring the excited-state intramolecular triplet-triplet (T-T) energy transfer. This process was investigated during more than a half-century.15,16 There are two electronic mechanisms, which are responsible for excited-state energy transfer: the coulombic and exchange interaction. The latter one describes inter- or intramolecular triplet-triplet (T-T) energy transfer.17 Several examples of T-T energy transfers have been reported. In most cases, donor subunits were “equipped” with a carbonyl group, and aromatic hydrocarbons were selected as the acceptor subunits.18 Keller has investigated spiro[9,10-dihydro-9-oxoanthracene-10,2’-5’,6’-benzindan] (AN)19 (Scheme 1). In the case of AN molecule, anthrone and naphthalene are connected by spiro bridge. It was shown that for AN the intramolecular T-T energy transfer was the main energy dissipation path.

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O

Scheme 1. The chemical formula of studied compound (AN)

The kinetic data of the T-T energy transfer in the case of AN were obtained by Maki et al.20 They applied transient absorption (TA) technique for monitoring the rise of the T1-Tn absorption of the naphthalene chromophore. The rise time of this band was estimated to be 80  20 ps. Stationary and time-resolved EPR spectra of AN in toluene glassy matrix at 77 K were recorded by Akiyama et al.21 Based on the zero-field splitting parameters, the EPR spectrum was assigned to the T1 state of the naphthalene moiety. Spin polarization was conserved during the T-T energy transfer. Hyperfine splitting in the EPR spectra can provide information about the unpaired electron (spin density) distribution in the molecule. An assignment of measured hyperfine coupling constants to the appropriate nuclei-electron pairs can be challenging, especially, in the case of electrons delocalized over several atoms. The UV-NMR experiment offers an alternative where the hyperfine interaction influences individual NMR signals. The goals of this work are: (i) to monitor the unpaired electron distribution over the molecule skeleton in the triplet excited state via UV-NMR experiment, (ii) to reexamine the kinetic data for

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intramolecular T-T energy transfer, (iii) to perform quantum chemical calculations for AN in order to understand the nature of the states involved in the T-T energy transfer.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Synthesis and Solvents Spiro[9,10-dihydro-9-oxoanthracene-10,2’-5’,6’-benzindan] (AN) was prepared according to the procedure reported previously.19 1H-NMR (benzene-d6, 300 MHz):  8.71-8.64 (2H,m), 7.71 (2H,m), 7.37 (2H,br), 7.32 (2H,m), 7.10-6.99 (6H,m), 3.53 (4H,s).

13C-NMR

(benzene-d6, 75

MHz):  183.6, 151.9, 142.2, 134.4, 134.2, 130.9, 128.4, 127.9, 127.5, 126.7, 126.2, 123.2, 53.6, 48.0. The solvents used for the absorption and emission studies (Aldrich or Merck spectral grade) were used without further purification except for butyronitrile (BuCN) and 2-methyltetrahydrofuran (MTHF). BuCN (Merck, for synthesis) was repeatedly distilled over CaCl2 and P2O5. MTHF (Merck for synthesis) was repeatedly distilled over CaCl2. The NMR solvents CD3OD 99.8% and THF-d8 99.5% were obtained from Cambridge Isotopes Laboratories Inc. or Armar Chemicals. 2.2. UV-VIS Experiments Stationary absorption spectra were recorded with a Shimadzu UV 3100 spectrometer. Stationary fluorescence spectra were measured using the FS900 Edinburgh Instrument equipped with Oxford cryostat. The spectra were corrected for the instrumental response using the fluorescence standards. A home-built instrument was used for recording the transient absorption spectra in the microsecond domain.22 TA spectra in femto/picosecond time domain were recorded using a home-made spectrometer.23,24

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2.3. UV-NMR Experiments The setup for combined UV-NMR laser experiments was used as described previously.11 Briefly, the base for it was a Bruker Avance II 300 MHz NMR spectrometer with WB magnet, equipped with a 5 mm BBI probehead for liquids, and a XeCl excimer laser Lambda Physik EMG101 emitting pulses at 308 nm/20ns pulse duration. The energy per pulse was about 30 mJ. A system transferring UV-light from the laser to the sample inside the NMR magnet was described earlier11 except for the aluminum mirrors that were replaced by dielectric mirrors with coatings optimized for 308 nm. The energy reaching the sample was about 8-10 mJ/pulse. The block diagram of the experimental setup is shown in Fig. S1 in Supporting Information. Sample concentrations were in the range of 0.1-0.5 mM. The illumination depth of the solution was checked by irradiation of the samples with the same experimental setup but outside the magnet. The luminescence was observed from the whole volume of liquid. Oxygen-free samples were prepared by bubbling the samples with dry nitrogen gas for at least 15 min inside NMR tubes. Then they were mounted inside the NMR coil in the atmosphere of dry nitrogen. Amount of oxygen in the samples was monitored by measuring T1 relaxation time of solvent 1H signals. A number of scans for UV-NMR experiments was usually 1024, and relaxation delay was set to 5 s. In our UV-NMR experiments, we accumulated two spectra: the “bright” - collected just after the laser pulse and the “dark” – a control spectrum without laser irradiation. The difference spectrum (D-1H NMR) obtained by subtraction of the “dark” spectrum from the “bright” one shows the light influence on the molecule. Pulse program used in the UV-NMR experiment is depicted in Fig. 1. The delay between the laser pulse and NMR pulse was about 1 μs, NMR pulse lasted for 9.8 μs and the dead time after NMR pulse was 6 μs. The temperature was set to 183 K in order to extend the lifetime of the excited triplet state.

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Figure 1. Pulse program used in the UV-NMR experiments (FID – free induction decay). 2.4. Calculations The quantum-chemical modeling of geometries and spectral characteristics of investigated compounds was performed using the density functional theory (DFT) with the B3LYP hybrid functional and the 6-311+G(d,p) basis set. The unrestricted DFT formalism was used to describe triplet states. Frequency calculations, and also wave function stability analysis in the case of triplet states were performed for all stationary points. The vertical excitation energies were obtained by the time-dependent DFT (TD-DFT) approach. All computations were done with the Gaussian 09 suite of programs.25 The global analysis of time-resolved absorption spectra was done utilizing the Optimus software.26 Three components of coherent artifacts and dispersion correction in the second order were taken into account during the global fitting procedure.

3. RESULTS 3.1. UV-NMR Spectra

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Fig. 2a shows the normal 1H NMR spectrum of AN in THF-d8 at 183 K with all eight signals assigned. Difference spectrum of AN in oxygen-free solution was obtained by subtraction of the “dark” spectrum from the “bright” one and is depicted in Fig. 2b.

Figure 2. 1H NMR signals of AN (a), “bright” minus “dark” difference spectra in (b) oxygen-free and (c) oxygen-containing solvents in THF-d8 at 183 K, (d) DFT 1H NMR calculated spectrum. The difference spectra (b) and (c) are ten times magnified with respect to the spectrum (a). Lines labeled with asterisks come from impurities and solvent 13C satellites.

The vertical scale of spectra (b) and (c) was magnified ten times in relation to the spectrum (a) in order to illustrate light influence better. Only some of the NMR signals in the difference spectrum were reduced due to irradiation, namely all naphthalene (N4-N6), methylene (M) and one proton from anthrone part (A4), whereas A1-A3 protons stayed unchanged. The intensity reduction of these lines is about 8-12 % and depends on sample concentration to some extent. It

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is worth to mention that other lines labeled with asterisks, coming from sample impurities, do not appear in the difference spectrum at all. Solvent and water lines in the normal 1H NMR spectrum (a) are 2-3 orders of magnitude higher than those of AN. Therefore, the difference spectrum exhibits some distortions due to imperfections of the subtraction of large solvent and water signals (not shown in Fig. 2, see Supporting Information, Fig. S2). On the other hand, a sample containing dissolved oxygen did not exhibit any change of the 1H spectrum of AN upon irradiation (Fig. 2c). Artifacts coming from solvent and water signals are present in the difference spectrum. In general, the calculated 1H NMR spectrum of AN (Fig. 2d) is in agreement with the experimental spectrum. The largest discrepancy was found for proton A1 (0.55 ppm) that is in the vicinity of carbonyl group which is probably strongly solvated in polar solvents. An analogous experiment was performed in methanol-d4, and the results were similar (see Supporting Information, Fig. S3). Additionally, a control experiment of naphthalene in oxygenfree methanol-d4 was conducted. Both proton signals produced signals in the difference NMR spectrum, and the intensity reduction upon irradiation is about 16-18% (see Supporting Information, Fig. S4).

3.2. Stationary and Time-Resolved Spectra

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Naphthalene

Anthrone

AN 15000

20000

-1

cm

25000

Figure 3. Phosphorescence spectra of naphthalene, anthrone and AN recorded in BuCN at 77 K. Low-temperature luminescence spectra of AN in BuCN and MTHF consist of well-structured phosphorescence localized within the spectral range of the phosphorescence of naphthalene (Fig. 3). This emission evidently cannot be assigned to the phosphorescence of anthrone. The phosphorescence excitation spectrum of AN corresponds well with room temperature absorption (see Supporting Information, Fig. S5). It was also proved that low-temperature emission of AN is independent of the excitation wavenumber within the range 31000-37000 cm-1.

15

-1

23450 cm

20 10

10

1 s

0 20000

mOD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25000

30000

5

-1

29500 cm 0.5 ps 2 ps 10 ps 20 ps 30 ps 50 ps 100 ps 1200 ps

0 15000

20000

25000

30000

-1

cm

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Figure 4. Room temperature TA spectra of AN recorded in BuCN as a function of the delay time, exc = 33300 cm-1, insert - TA spectrum recorded for delay time 1 s, exc = 32500 cm-1 (308 nm.). Just after excitation, the TA spectrum of AN consists of the main band with the maximum at 29500 cm-1 and a weak band located in the spectral region . For longer delay times, this spectrum decreases, and the rise of a new band with a maximum at 23450 cm-1 is observed simultaneously (Fig. 4). This band was observed in the microsecond time domain (Fig. 4). The kinetics of the decay of this band and its monoexponential fit is presented in Supporting Information in Fig. S5. The decay time was found equal to 44  6 s. For the spectra recorded in the picosecond time domain, the global and target analysis was performed. Three decay times (Fig. 5) were needed to describe experimental data satisfactorily (see Supporting Information, Fig. S7, S8 and S9 for the comparison of the experimental with the reconstructed map, and some representative kinetic traces, respectively).

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Intensity / A U

10

5

10 ps

1.6 ps

0

35 ps

-10

20000

25000

15

cm

10

A

30000

-1

C

mOD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

5

0

20000

25000

cm

-1

30000

Figure 5. Results of the global analysis of TA spectra of AN in BuCN, T = 294 K. Top – Decay Associated Spectra (DAS) corresponding to 1 = 1.6 ps, 2 = 35 ps, 3 = 105 ps (arbitrary value), estimated error 10%, temporal resolution of the apparatus 0.3 ps. Bottom – Species Associate Spectra (SAS) obtained for the kinetic scheme A(1.6 ps)  B(35 ps)  C(105 ps). Applying the sequential kinetic model with increasing lifetimes A(1.6 ps)  B(35 ps)  C(105 ps) gives reasonable shapes of transient species. On the other hand, the SAS obtained for the reversed kinetic scheme A(35 ps)  B(1.6 ps)  C(105 ps) shows strongly negative amplitude for the intermediate product in the region of absorption bands expected (see Supporting Information, Fig. S10) what indicates that this scheme is not proper. Several attempts to fit

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additional time constant in the range of 0.5-40 ps did not give any distinguishable decay time and/or intermediate spectrum and gave an only minor improvement of residuals. A detailed explanation will be presented in the Discussion.

3.3. Quantum Chemical Calculations To investigate the nature of the excited states involved in intersystem crossing and intramolecular T-T energy transfer in AN molecule DFT and TD-DFT quantum chemical calculations were performed. The calculated stationary absorption spectrum of AN nicely reproduces the experimental one (Fig. 6A). Analysis of the nature of the excited states involved in T-T energy transfer is consistent with the observation of Keller that the sum of the stationary absorption spectra recorded for anthrone and naphthalene agrees with the spectrum of AN molecule, and with the conclusion that spiro-linked chromophores are not electronically conjugated (Fig. 6B).19 Geometry optimization of the ground state of AN gives the expected structure of two planar chromophores perpendicular to each other. However, the energy cross section along the inversion coordinate , depicted in Fig. 7, is characterized by two equivalent minima located at  = +/-14o separated by a small 30 cm-1 barrier. Geometries of triplet states localized at anthrone and naphthalene chromophores of AN were optimized by unrestricted DFT method. According to calculations, the energy of the triplet state localized on anthrone moiety is 2600/3300 cm-1 (vacuum/BuCN environment) higher than that localized on naphthalene. The energy difference of the first maxima of the anthrone and naphthalene phosphorescence is equal to 3900 cm-1, which indicates that the ordering of the low lying triplet states of AN was correctly predicted by modeling. Similarly to the situation for S0 state, modeling of the anthronic triplet gives a tilted structure, two minima located at  = +/-15o and separated by a 40 cm-1 barrier (Fig.

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7). Calculations performed for the lowest naphthalenic triplet state show a single minimum,  = 0o, a perfect perpendicular arrangement of chromophores. The calculated spin densities of two lowest triplet states involved in T-T energy transfer show that, for higher triplet state, spin density is localized mainly on the carbonyl group of anthrone and, for the lower state, it is spread over the skeleton of the naphthalene sub-unit (Fig. 7). Calculated T1Tn TA spectra for the lowest excited triplet states localized on anthrone/naphthalene chromophore show the strongest band around 30500/23000 cm-1 respectively, and they are consistent with spectra of short/long-lived individuals observed experimentally (Fig. 8).

Figure 6. A - Normalized room temperature stationary absorption of AN in BuCN (solid line). Calculated absorption spectrum (dashed line, fwhm of bands is 20 nm). Transitions are marked by bars. The relative height of the bars is proportional to its oscillator strength. B - Calculated spectra of naphthalene (N1) - dotted blue line, anthrone (A) - dash red, the sum of N1 and A 15



solid black, AN – dash-dotted black line. Transitions are marked by bars: naphthalene (blue not filled), anthrone (red). The arrow indicates the calculated n* transition of anthrone.

Figure 7. Top – calculated geometry of the AN in different electronic states with 1H NMR shifts (in ppm, for triplet states shifts are relative to S0 values) and deformation angle marked. Bottom spin densities (isosurface value of 0.05 atomic unit) and partial spin distribution on heavy atoms (Mulliken analysis including hydrogen atoms, values larger than 0.005 e are marked) calculated for the triplet states localized on the anthrone group (T1A) and naphthalene chromophore (T1N).

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1.0

AU

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0.5 ps 5000ps

0.5

0.0 15000

20000

25000

-1

30000

cm

Figure 8. TA spectra of AN recorded in BuCN for delay times 0.5 ps (solid line, black) and 5000 ps (solid line, red). Calculated T-T absorption spectra for the triplet states localized on the anthrone group (dashed line, black) and naphthalene chromophore (dashed line, red). Transitions are indicated by bars: anthrone – black, naphthalene – red not filled.

4. DISCUSSION The comparison of low-temperature phosphorescence of anthrone, naphthalene and AN molecule (Fig. 3) indicates that the emission of AN originates from the naphthalene chromophore, what is in agreement with Keller’s results.19 The starting point of the excited state energy dissipation path is Sn state generated by the S0Sn excitation of AN molecule. It was well established that the first very weak band (  50) observed at () in the absorption spectrum of AN can be assigned to S01n* transition where 1n* is state localized on the anthrone chromophore.19 This experimental result was confirmed by quantum chemical calculations (Fig. 6). The second stationary absorption band at with a maximum at about 36000 cm-1 is

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composed of several S01* transitions localized on the anthrone and naphthalene subgroups (Fig. 6B, S11). In our studies TA spectra were recorded for the excitation at 33330 cm-1, and therefore the anthrone or/and naphthalene subgroup could be excited. Time-resolved spectroscopy is a useful tool for monitoring the steps of the excited state energy dissipation path. For short delay times (

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