Ultrafast Dynamics of Tripyrrindiones in Solution Mediated by

Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B 2019, 123, 5524−5535

Ultrafast Dynamics of Tripyrrindiones in Solution Mediated by Hydrogen-Bonding Interactions Published as part of The Journal of Physical Chemistry virtual special issue “Young Scientists”. Alicia Swain,† Byungmoon Cho,† Ritika Gautam,† Clayton J. Curtis,† Elisa Tomat,† and Vanessa Huxter*,†,‡ †

Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States Department of Physics, University of Arizona, Tucson, Arizona 85721, United States

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‡

S Supporting Information *

ABSTRACT: The optical properties and ultrafast dynamics of hexaethyl tripyrrin-1,14dione (H3TD1) are tuned by hydrogen-bonding interactions between the solute and the solvent. In solvents with low hydrogen-bonding affinity, H3TD1 preferentially forms hydrogen-bonded dimers, whereas in solvents that can either donate or accept hydrogen bonds H3TD1 is present as a monomer. The distinction between dimer and monomer determines the dynamics of the system, with faster internal conversion observed in the dimer form. The ultrafast dynamics were characterized using time-correlated single photon counting, fluorescence upconversion, and transient absorption measurements. The time-resolved dynamics of both the monomer and dimer in solution were modeled using a Pauli master equation treatment for a three level system. The solvent-dependent optical properties were measured using steady-state absorption and fluorescence. This data was then used to calculate the quantum yield and extinction coefficients.

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INTRODUCTION Tetrapyrrolic molecules are common in nature. Linear and macrocyclic tetrapyrroles comprise the primary light-harvesting pigments in photosynthetic systems1 from cryptophytes2,3 to higher plants.4 Porphyrins such as heme, the oxygen carrying pigment in red blood cells, or vanadyl porphyrins,5 which are present in crude oils and shales, are macrocyclic tetrapyrroles. Synthetically prepared tetrapyrrolic complexes have been used for applications such as catalysis,6 therapeutics,7 artificial light harvesting,8−10 and optical sensors.11 Compared to tetrapyrroles, tripyrroles are uncommon in nature, appearing primarily as precursors or metabolites of other oligopyrrolic systems. Tripyrrolic compounds possess many of the properties of tetrapyrroles arising from a shared electron-rich conjugated π system and pyrrolic nitrogens. It has been shown that tripyrroles are attractive platforms for redox chemistry.12−14 As ligands, they coordinate readily to metal centers in multiple oxidation states. These systems have optical and material properties that make them of interest for the development of new redox-active materials, for catalytic applications and as optical sensors. While the properties of tetrapyrroles have been extensively investigated, there are comparatively few studies of tripyrroles.13−20 The previously published work on tripyrroles has been primarily concerned with synthesis and material characterization,12,14−18,21−24 demonstrating metal coordination and redox activity13 as well as low-temperature πdimerization.16 Previous work by the Lightner group18 describing the synthesis and characterization of tripyrrindiones, © 2019 American Chemical Society

presented NMR results suggesting tunable hydrogen-bonding behavior. In a nonpolar solvent, the tripyrrindiones preferentially formed hydrogen-bonded dimers, while in a polar solvent the molecule did not aggregate. Building on this work, we present the first study of the ultrafast electronic dynamics of a tripyrrolic molecule, hexaethyl tripyrrin-1,14-dione (H3TD1), and show that solvent interactions dominate the excited-state dynamics. We propose that specific hydrogen-bonding solute−solvent interactions determine the electronic relaxation time scales of H3TD1 in solution. H3TD1 can donate or accept hydrogen bonds, forming hydrogen-bonding interactions with solvents with sufficient donating or accepting ability. In these solvents, H3TD1 preferentially forms hydrogen bonds to the solvent. In solvents with insufficient donating or accepting ability, the molecule can instead form an intermolecularly hydrogenbonded dimer. In general, we observe faster excited-state lifetimes and lower quantum yields in solvents that cannot strongly interact to H3TD1 and where the dimer is expected to be the dominant species in solution. The photophysics of molecules in solution are often determined by solute−solvent interactions.25,26 These interactions can shift energy levels,27 stabilize conformers,28 promote aggregation,29 and drive photochemical reactions30 and energy or charge transfer among many other processes. Received: February 27, 2019 Revised: May 30, 2019 Published: June 9, 2019 5524

DOI: 10.1021/acs.jpcb.9b01916 J. Phys. Chem. B 2019, 123, 5524−5535

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The Journal of Physical Chemistry B Solute−solvent interactions can be broadly divided into nonspecific and specific. Dielectric interactions originating from the averaged polarity of the solvent are nonspecific while hydrogen-bonding is a specific, directional interaction. Both inter- and intramolecular hydrogen-bonding interactions are of fundamental importance to many natural and artificial systems, ranging from protein structure31 to solvent dynamics,32 and excited-state intramolecular proton transfer.33 Hydrogen-bonding interactions are known to have significant effects on excited-state dynamics.34−36 In general, there is a relationship between hydrogen-bonding and vibrational redistribution.37,38 Hydrogen-bonding interactions can facilitate rapid internal conversion and may also be associated with fast electronic relaxation through a conical intersection.39 Conical intersections are thought to be present in the electronic relaxation of a diverse array of molecular systems, driving many excited-state processes.40−42 They are crossings of the adiabatic potential energy surfaces between states of the same spin leading to ultrafast relaxation via internal conversion. Conical intersections correspond to molecular configurations in which multiple electronic states are degenerate.43,44 These degeneracies may be associated with solvent-assisted geometric rearrangement and can be representative of the significant role of solvents in determining relaxation pathways. We propose that electronic relaxation in the H3TD1 dimers may be mediated by a conical intersection. These solute−solvent interactions may provide a mechanism for controlling the relaxation of the excited state and could be used as a basis to design novel tripyrrolic molecules for charge or proton transfer applications.

Figure 2. Absorption spectra of H3TD1 in THF, pyridine, methanol, acetonitrile, chloroform, and toluene solutions. The spectra have been normalized to the maximum of the S0 to S1 feature.

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EXPERIMENTAL METHODS The tripyrrole, hexaethyl tripyrrin-1,14-dione (H3TD1), was synthesized according to previously reported methods.13,14 The structure of the H3TD1 molecule is presented in Figure 1.

Figure 3. Steady-state fluorescence spectra of H3TD1 in THF, pyridine, methanol, acetonitrile, chloroform, and toluene solutions. The spectra have been normalized to the maximum of the fluorescence peak.

Time-correlated single photon counting (TCSPC) and ultrafast fluorescence upconversion measurements were collected using a dual-purpose home-built system. Coherent optical pulses with a 100 fs duration and an 800 nm center wavelength were generated from an 80 MHz Ti:sapphire oscillator (Coherent, Vitara). A type-I BBO crystal (Eksma, 29.2°, 2 mm) was used to frequency double the oscillator output to 400 nm with an efficiency of approximately 20%. A harmonic separator placed after the doubling crystal was used to split the doubled 400 nm pulses from the residual 800 nm light. The 400 nm beam was then focused into the sample to generate fluorescence. The excitation fluence used experimentally was approximately 11 μJ/cm2. Two 90° off-axis parabolic (OAP) mirrors were used to collect the fluorescence and redirect it toward a monochromator and photomultiplier tube detector. In the fluorescence upconversion experiment, the two OAPs collected and focused the fluorescence so that it could be frequency mixed with an 800 nm gate pulse. For TCSPC, the fluorescence was recollimated after the OAPs and focused into the monochromator. Filters and baffles were placed to minimize contributions from scattered light. A Becker Hickl SPC-130 TCSPC module and DCC-100 detector control card were used to detect and bin photon data to generate the TCSPC signal. Solutions of H3TD1 were prepared with an optical density of ∼0.1 in a 2 mm path length cell for the

Figure 1. Structure of the hexaethyl tripyrrin-1,14-dione (H3TD1) molecule.

A Cary 100 UV−vis and a Cary Eclipse spectrometer were used to collect steady-state absorbance and fluorescence spectra, respectively. These spectra are presented in Figures 2 and 3. All measurements were made at room temperature. Linear spectra were collected before and after all time-resolved measurements to monitor sample degradation. 1H NMR measurements were made in deuterated toluene, acetonitrile, and methanol at 23 °C using a Bruker DRX-500 spectrometer at the University of Arizona NMR Spectroscopy Facility. Chemical shifts were referenced relative to those of the residual undeuterated solvent. 5525


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The Journal of Physical Chemistry B

The authors of the π* scale46 developed a method to include hydrogen-bonding interactions as part of their wider theory of solvation. The method includes the Kamlet−Taft parameters50−52 written in terms of an α and a β term, where α represents hydrogen-bond-donating ability and β represents hydrogen-bond-accepting ability. The Kamlet−Taft parameters have been tabulated for reference in the Supporting Information (Table S1). The H3TD1 molecule features both hydrogen-bonding donors and acceptors, meaning that it can engage in multiple hydrogen-bonding interactions with several solvents. In particular, the heterocyclic N−H groups are hydrogen-bonding donors whereas the carbonyl oxygen atoms are strong hydrogen-bonding acceptors. As predicted by the Kamlet−Taft parameters, in the stronger hydrogen-bonding solvents used here (i.e., THF, pyridine, and methanol), H3TD1 likely forms hydrogen bonds with the solvent and therefore is present in the monomeric form. In chloroform and toluene, H3TD1 is expected to form a hydrogen-bonded dimer. In acetonitrile, H3TD1 likely exists as a mix of monomer and dimer forms. The solvent dependence of the absorption spectrum of H3TD1 shown in Figure 2 does not completely follow the Kamlet−Taft parameters; however, it follows a general trend with hydrogen-bond-donating or -accepting solvents blueshifting and non-hydrogen-bonding solvents red-shifting the absorption spectra. The solvent dependence of the absorption spectrum is likely a convolution of the solvent polarity and the hydrogen-bonding affinity. Hydrogen-bonding spectral shifts are normally small compared to dipolar effects. As a result, steady-state absorption and fluorescence spectral shifts are usually dominated by dipolar effects, but the time-resolved dynamics can be strongly influenced by hydrogen-bonding interactions. It is also possible that the red-shifting of the H3TD1 spectra in chloroform and toluene is a direct signature of dimerization due to a potential J-type interaction mediated by the hydrogen-bonding interactions. On the basis of previous work by the Lightner group,18 the dimer is expected to be somewhat coplanar with the orientation of each monomer reversed. This could lead to a J-type coupling that would redshift the absorption spectrum although the electronic coupling might be affected by the specific orientation of the hydrogenbonding interactions. Fluorescence. Figure 3 presents room-temperature fluorescence spectra of H3TD1 in toluene, chloroform, acetonitrile, THF, pyridine, and methanol. The spectra consist of a single, featureless broad peak. There is a lack of mirror symmetry between the absorption and fluorescence spectra suggesting vibronic coupling or a change in the conformation of the molecule in the ground and excited states. The fluorescence spectra show nearly the same solvent dependence as the absorption measurements except that chloroform is red-shifted relative to toluene. The observed Stokes shifts are large, ranging from 55 to 67 nm (∼2100 to 2500 cm−1), which suggests that there is a geometry change from the ground to the excited state. A large Stokes shift is generally associated with a significant solvent reorganization energy indicating a conformational change that affects the potential energy surface of the excited state. Such a geometric rearrangement is often required for deactivation via a conical intersection. Related molecules such as linear tetrapyrrole bilins are known to undergo significant conformational changes upon excitation.53 1 H NMR Spectroscopy. Previous work by the Lightner group using an NMR study and vapor pressure osmometric

TCSPC and fluorescence upconversion measurements. Additional information on the fluorescence upconversion experiment can be found in the Supporting Information. The apparatus used to collect the frequency-resolved transient absorption data has been described previously.45 Briefly, a Spectra-Physics Solstice regenerative amplifier laser generating 100 fs pulses centered at 800 nm with a repetition rate of 1 kHz was used to produce pump and broadband probe pulses. The majority of the Solstice output was used to drive a Light Conversion TOPAS, producing pump pulses with tunable central wavelengths through the visible range. A small portion of the regenerative amplifier output was focused into a sapphire plate, producing a single-filament white-light continuum with a spectral range ∼410−820 nm. This broadband continuum was split, with one part serving as the probe and the other as a reference. The probe pulse was delayed relative to the pump using a mechanical delay stage. The data was collected using dual reference and probe CMOS detectors. A chopper synchronized to 500 Hz was used to generate the difference spectra as a function of time delay between the pump and the probe pulses. The energy density of the pump pulse was set to approximately 10 nJ in a 100 μm spot. The optical density of the sample at the excitation wavelength (600 nm for all measurements presented here) was 0.3 in a 2 mm path length. The instrument response was characterized by nonresonant measurements on a fused silica window. Following chirp correction, cross-correlation measurements performed on solvent showed that the pump-pulselimited time resolution was approximately 60 fs.

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RESULTS AND DISCUSSION Absorption. Room-temperature absorbance spectra of H3TD1 in toluene, chloroform, acetonitrile, tetrahydrofuran (THF), pyridine, and methanol are shown in Figure 2. These solvents were chosen to vary polarity as well as hydrogenbond-donating and -accepting ability. The spectra are characterized by an intense, broad band associated with the S0 to S1 transition. The spectral position of the absorption spectrum is solvent-dependent, and the overall line shape was generally conserved. For many molecules in solution, nonspecific dipolar interactions with solvent molecules shift their absorption spectra. These shifts generally trend with solvent polarity. There are many different measures of polarity such as the π* solvatochromic scale46 that describes nonspecific polarity/ polarizability of solvents, the reaction field factor F(ϵ0, n) model derived from dielectric continuum theory,47,48 or the ET(30) or ETN scale,49 which is empirically determined from the consideration of the free energy change when solvating a photoexcited molecule. While H 3 TD1 appears to be solvatochromic, there is a lack of direct correlation between the solvent polarity and the observed spectral shift of the absorption spectrum. The ordering of the spectra does not correlate with the dipole moment, the dielectric constant, a general solvent polarizability value calculated using R(n) = (n2 − 1)/(n2 + 2) where n is the refractive index, the π* scale, the ETN scale, or the ET(30) model. These values have been tabulated for reference in the Supporting Information (Table S1). As such, these models are not suitable to describe the solvent dependence of H3TD1 in solution; however, the general trend can be understood by considering specific hydrogen-bonding interactions. 5526


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Table 1. Proton NMR Chemical Shifts (δ) for the Pyrrole and Lactam NH Resonances Measured in Deuterated Toluene and Acetonitrile toluene H3TD1 conc (mM)

acetonitrile

pyrrole NH δ (ppm)

lactam NH δ (ppm)

H3TD1 conc (mM)

pyrrole NH δ (ppm)

lactam NH δ (ppm)

10.562 10.576 10.587

10.874 10.914 10.921 10.925

0.45 0.60 0.90 1.80

9.454 9.555 9.560 9.591

10.211 10.231 10.239 10.315

0.22 0.45 0.90 1.80

H3TD1 in toluene, chloroform, acetonitrile, THF, pyridine, and methanol was determined by comparison with a fluorescein standard in 0.1 M NaOH, which has a quantum yield of 0.92554 at room temperature. Fluorescence scans were integrated and plotted with respect to the maximum absorbance. The integrated area was fit to a straight line and compared to fluorescein to determine the quantum yield. The fluorescence plots for all solvents can be found in the Supporting Information (Figure S6). The maximum absorbance values were plotted with respect to concentration and fit to a linear model. The slope of this fit was then used to determine the extinction coefficient for a wavelength at or very near the absorption maximum as listed in Table 2. Table 2 presents the solvent dependence of both the quantum yield and the extinction coefficients of H3TD1. H3TD1 in THF and pyridine has both the largest extinction coefficients and highest quantum yields. H3TD1 has the lowest quantum yield and lowest extinction coefficient in chloroform. With the exception of methanol, the quantum yield is generally higher in solvents that can hydrogen bond to H3TD1 and lower in those that cannot. The extinction coefficient follows the same approximate trend, with the exception of acetonitrile which is closer to toluene and chloroform than to THF, pyridine, and methanol. The Lightner group reported extinction coefficients for H3TD1 in acetonitrile, chloroform, and methanol.18 Our values are comparable but differ by approximately 10−15%. This variation is likely due to the fact that their extinction coefficients were determined at different wavelengths and over different concentration ranges. The low quantum yield of H3TD1 in solution is consistent with fast excited-state relaxation mediated through hydrogenbonding interactions. As we observe in our transient measurements, most of the excitation has relaxed back down to the ground state on the order of tens of picoseconds or less. Only a small amount persists to be detected by steady-state

measurements reported that tripyrrindiones could be present in solution as monomers or dimers.18 They found that, in chloroform, a solvent with low hydrogen-bonding affinity, the molecules formed an intramolecularly hydrogen-bonded dimer while in dimethyl sulfoxide, which can engage in hydrogen bonds, the molecules were present as monomers. Consistent with the previous work by the Lightner group, we observed the formation of a H3TD1 dimer in toluene and acetonitrile. 1 H NMR spectra were collected at room temperature for a series of concentrations of H3TD1 in deuterated toluene, acetonitrile, and methanol. In toluene and acetonitrile, we observed a downfield shift of the NH resonances with increasing H3TD1 concentration. This is consistent with the observations of the Lightner group for the formation of a hydrogen-bonded dimer. In methanol, the NH resonances were significantly broadened, as expected in the presence of fast proton exchange with the protic solvent, consistent with solvated monomers in methanol. The results for toluene and acetonitrile are shown in Table 1. The NMR spectra for H3TD1 in deuterated toluene, acetonitrile, and methanol can be found in the Supporting Information (Figures S1−S3). Quantum Yield and Extinction Coefficients. The fluorescence quantum yield and absorption extinction coefficients are presented in Table 2. The quantum yield of Table 2. Quantum Yields and Extinction Coefficients of H3TD1 in Various Solvents solvent THF pyridine methanol acetonitrile toluene chloroform

quantum yield 0.0231 0.0109 0.0011 0.0077 0.0031 0.0008

± ± ± ± ± ±

0.003 0.0006 0.0008 0.004 0.0003 0.0001

extinction coeff (1/M cm) 29 700 33 600 27 700 22 300 21 100 20 900

± ± ± ± ± ±

200 100 200 800 100 400

at at at at at at

470 475 480 485 492 485

nm nm nm nm nm nm

Figure 4. Concentration dependence of normalized steady-state absorption (blue) and fluorescence (red) measurements of H3TD1 in (a) pyridine, (b) acetonitrile, and (c) chloroform. In all panels, the broken line is the low-concentration sample (sub-0.02 mM) and the solid line is the highconcentration sample (0.2−0.3 mM). 5527


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The Journal of Physical Chemistry B fluorescence measurements. As the only strong hydrogen-bond donor studied here, methanol can engage in multiple interactions with H3TD1, which may explain why it differs from the general quantum yield trend. With the exception of methanol, H3TD1 has a higher quantum yield in solutions where it forms hydrogen bonds with the solvent and where we expect it to be a monomer in solution. These solvents are THF and pyridine. The quantum yield decreases as the hydrogenbonding affinity drops, with the lowest quantum yield observed for H3TD1 in chloroform solution. As shown by NMR measurements, H3TD1 forms a hydrogen-bonded dimer in chloroform18 and in toluene. Similarly, in toluene, we observe a relatively low quantum yield. Acetonitrile is only weakly hydrogen-bond-donating, so it is expected to behave as an intermediate between the dimer and the monomer dominated solvents. The dimer is present in acetonitrile, as we have observed using NMR. The quantum yield of H3TD1 in acetonitrile does indeed fall between those of THF/pyridine and toluene/chloroform. Concentration Dependence. The room-temperature concentration dependence of the steady-state absorption and fluorescence of H3TD1 in pyridine, acetonitrile, and chloroform is presented in Figure 4. There is at least a 10-fold difference between the concentrations presented, with the high-concentration plots corresponding to between 0.2 and 0.3 mM and the low-concentration plots to sub-0.02 mM (less than ∼8 ppm). The concentration dependence varies with hydrogen-bonding affinity. In pyridine, which is a reasonably strong hydrogen-bond donor, there is little difference between the high- and low-concentration H3TD1 measurements. In this case, the H3TD1 molecule is expected to form hydrogenbonding interactions with the pyridine solvent. In both acetonitrile and chloroform, which are weak or non-hydrogen-bonding solvents, the fluorescence spectra show a significant concentration dependence. This difference in concentration dependence between hydrogen-bonding (THF, pyridine, methanol) and weak or non-hydrogen-bonding solvents (chloroform, toluene, acetonitrile) can also be seen in the fluorescence data used for quantum yield determination presented in the Supporting Information (Figure S6). Fluorescence is extremely sensitive to changes in solvation or local structure. This concentration dependence suggests that H3TD1 behaves differently in pyridine than it does in acetonitrile. We propose that this difference observed in Figure 4 is due to the formation of a dimer in acetonitrile and chloroform and a lack of similar aggregation in pyridine. Time-Resolved Fluorescence. Room-temperature TCSPC data was collected for H3TD1 in THF, pyridine, methanol, acetonitrile, chloroform, and toluene solutions. These measurements were used to determine the fluorescence lifetime of H3TD1 in solution. Representative data and corresponding fit lines for H3TD1 in methanol and toluene are presented on a semi-log-scale in Figure 5. TCSPC data for H3TD1 in the remaining solvents can be found in the Supporting Information (Figure S4). The small bump at ∼1.2 ns in the TCSPC traces is due to a reflection artifact that also appears in the instrument response. The data was fit in Matlab using two decaying exponentials and an offset, with the exception of H3TD1 in acetonitrile which required three exponentials. Residual analysis was used to determine the lowest number of exponentials required to fit the data, and the error was minimized using a nonlinear least-squares algorithm. All traces contained a time component that was near or below

Figure 5. Representative time-correlated single photon counting data measurements of H3TD1 in methanol (blue) and toluene (red). The data (points) has been normalized and is presented with a fit line on a semi-log-scale.

the resolution of our instrument, which has an instrument response of approximately 200 ps. The traces were dominated by a single exponential term, with the exception of acetonitrile. This dominant time scale generally followed a solvent hydrogen-bonding affinity trend. The solvents that do not form strong hydrogen bonds to the H3TD1 solute, chloroform and toluene, were dominated by the faster, near-identical time scales: 1.33 ± 0.02 and 1.34 ± 0.01 ns, respectively. The primary terms for pyridine, methanol, and THF, which are expected to form hydrogen bonds to the H3TD1 molecule, were 3.6 ± 0.4, 3.10 ± 0.2, and 2.40 ± 0.1 ns, respectively. These time scales are longer than those observed for chloroform and toluene. The relatively rapid decay of H3TD1 in toluene as compared to H3TD1 in methanol can be seen in Figure 5. Fluorescence dynamics are sensitive to many kinds of solvent interactions. The strong solvent dependence of the fluorescence lifetimes observed here indicates that solute− solvent interactions have a significant role in the decay of the S1 state. As can be seen from the TCSPC data, the H3TD1 fluorescence lifetimes are significantly faster in solvents such as chloroform and toluene where H3TD1 preferentially hydrogen bonds with itself to form a dimer. In solvents where H3TD1 is expected to remain a monomer, the fluorescence lifetimes are longer. In acetonitrile, the H3TD1 fluorescence decay was more obviously biexponential, with two significant time scales of 0.52 ± 0.01 and 7.20 ± 0.1 ns. The third exponential of the fit used for acetonitrile was below the resolution of our instrument. Considering that acetonitrile is expected to have only weak hydrogen-bond-donating ability, it is not surprising that it has significant contributions from both fast and slow time scale components. This may be representative of a mixed population of monomers and dimers in solution. The fit parameters for all solvents are presented in Table 3. The time range over which our TCSPC instrument can track the fluorescence decay goes from approximately 200 ps to 10 ns. Fluorescence upconversion measurements can be used to significantly extend this time resolution. Our fluorescence upconversion instrument allows for the time resolution of the fluorescence decay from 200 fs. The time scales obtained from the fluorescence upconversion measurements were consistent with those observed in both the TCSPC and in the transient absorption measurements (see next section), where the measured dynamics could be grouped by hydrogen-bonding 5528


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The Journal of Physical Chemistry B Table 3. Fit Parameters for TCSPC Data solvent THF pyridine methanol toluene chloroform solvent acetonitrile

amplitude 1 47 360 38 030 24 140 30 420 5585 amplitude 1 45 350 ± 320

± ± ± ± ±

80 330 380 200 115

time scale 1 (ns) 0.200 0.210 0.150 0.160 0.115 time scale 1 (ns) 0.150 ± 0.002

± ± ± ± ±

amplitude 2

0.005 0.01 0.002 0.001 0.002 amplitude 2 14 600 ± 400

660 1600 1350 4930 1220

± 20 ± 20 ± 30 ± 50 ± 20 time scale 2 (ns) 0.520 ± 0.01

time scale 2 (ns) 2.40 ± 0.10 3.60 ± 0.40 3.10 ± 0.20 1.34 ± 0.01 1.33 ± 0.02 amplitude 3 1745 ± 20

offset 120 ± 6 200 ± 20 215 ± 20 560 ± 5 260 ± 2 time scale 3 (ns) 7.20 ± 0.10

Figure 6. Representative broadband transient absorption measurements of H3TD1 excited at 480 nm in (a) toluene, (b) acetonitrile, and (c) pyridine. The z-axis scale is presented in mOD.

Figure 7. EADS of H3TD1 excited at 480 nm in (a) toluene, (b) acetonitrile, and (c) pyridine showing the evolution of the signal associated with the three time constants listed in the legend.

dependent. In toluene and acetonitrile, the absorptive feature peaks at approximately 600 nm. In pyridine, the absorptive feature is red-shifted, peaking at approximately 630 nm. The transient absorption traces reveal a near-simultaneous decay of the spectra throughout the whole band. The time scale of this decay appears to depend on the hydrogen-bonding affinity of the solvents, with slower excited-state relaxation dynamics observed for solvents that are expected to hydrogen bond to the H3TD1 molecule. As shown in Figure 6, the decay of the signal is extremely fast in toluene, intermediate in acetonitrile, and relatively slow in pyridine. As shown in the time-resolved fluorescence and NMR data, H3TD1 is present in acetonitrile as both a dimer and a monomer. As such, it represents an intermediate solvent where the observed dynamics lie between the non-hydrogen-bonding solvents (toluene) and the hydrogen-bonding solvents (pyridine). In order to extract time scales and analyze the transient absorption data, global analysis was performed using the Glotaran software package55 and the statistical fitting package

affinity. While all the time scales observed were short, with the exception of methanol, the fastest dynamics were found in the solvents with weak hydrogen-bonding affinity, toluene, chloroform, and acetonitrile. Representative fluorescence upconversion traces, fits, and time scale information can be found in the Supporting Information (Figure S5, Table S2). Transient Absorption. While TCSPC and fluorescence upconversion time-resolve the dynamics of the fluorescent state, transient absorption measurements follow both groundand excited-state population dynamics. In order to further characterize the excited-state dynamics of the H3TD1 molecule, transient absorption measurements were performed in toluene, acetonitrile, and pyridine. Figure 6 presents representative transient absorption data on H3TD1. All of the H3TD1 transient absorption traces have two primary features, a broad bleach (negative signal, indicating increased transmission) centered at approximately 490 nm and a broad absorptive band (positive signal, indicating increased absorption). The exact positions of the absorptive band are solvent5529


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ally rapid decay of the signal can be seen in both the fitted EADS (Figure 7a) and the raw data, Figure 6a. The transient absorption signal of H3TD1 in pyridine decays much more slowly than for H3TD1 in toluene as shown in Figures 6 and 7. For H3TD1 in pyridine, the first EADS decays in 8.5 ± 0.3 ps. As in toluene, the spectra is dominated by a broad bleach and a broad absorption. The bleach spans from 420 to 585 nm and is peaked at 475 nm. In pyridine solution, SE is not completely overlapped with the ESA as is the case for toluene. The H3TD1 absorption spectrum in pyridine peaks at 475 nm but only extends to 545 nm. The fluorescence spectrum of H3TD1 in pyridine has a maximum at 552 nm and extends past 585 nm. As a result, the shoulder-like component to the red of the main bleach arises from SE. The ESA contribution is more narrow than in toluene, starting at 585 nm and extending to the red with a peak at 630 nm. The second EADS decays in 61.5 ± 3.0 ps and shows a loss of ESA and GSB with the negative-going signal dominated by SE contributions. The ESA blue-shifts slightly between the first and second EADS, with the peak moving from 630 to 620 nm. This 10 nm shift is smaller than the 30 nm shift seen for H3TD1 in toluene. As in the toluene solution data, there is a small third long time scale contribution (≫1 ns). The overall slower decay of the transient absorption signal compared to H3TD1 in toluene can be clearly seen in Figure 6. The time scales observed in the transient absorption data collected for H3TD1 in acetonitrile lie between those for toluene and pyridine as can be seen in Figure 6. As in the other solvents, the transient absorption spectrum is characterized by both broad bleach and absorption features. The overall spectral dependence of the first and second EADS for H3TD1 in acetonitrile (Figure 7b) appears to be intermediate to that of H3TD1 in toluene and pyridine. In acetonitrile, the first EADS decays in 1.3 ± 0.3 ps, which is slightly longer than the initial time scale obtained from toluene and significantly faster than that observed in pyridine. The spectral shape of the first EADS resembles that of the first EADS for H3TD1 in toluene. The second EADS decays in 14.3 ± 1.0 ps, which is intermediate to the time scales observed in toluene and in pyridine. The second EADS more closely resembles the spectral dependence of the EADS generated for H3TD1 in pyridine. This can be explained by the presence of both dimer and monomer forms of H3TD1 in acetonitrile. The dimer complexes are expected to follow the faster dynamics observed in toluene and dominate the transient absorption signal at short delay times. At longer times, the signal would be dominated by the monomer. The observed spectral distribution of the EADS and the time scales obtained from the analysis further support the presence of both monomer and dimer forms of H3TD1 in acetonitrile. The initial excited-state relaxation time scales observed in the transient absorption data are extremely fast, particularly for H3TD1 in toluene and acetonitrile where the first time scale is less than 1.5 ps. These particularly short time scales were also observed in the fluorescence upconversion measurements on H3TD1 in solvents where it forms a hydrogen-bonded dimer. These fast lifetimes indicate that nonradiative decay pathways dominate the observed dynamics. These short time scales may suggest that some of the population proceeds coherently through a conical intersection, with the remaining small amount of population returning to the ground state more slowly. Because the initial population is not in equilibrium with the solvent, it is also reasonable that the populations continue

TIMP.56 Both the decay-associated difference spectra (DADS) and the evolution-associated difference spectra (EADS) were generated from the analysis57 and contain the same overall information.58 The DADS traces are generated by fitting the data using a model where the components decay monoexponentially in parallel, leading to a loss or gain of absorption with a specific lifetime. The EADS analysis describes the transient spectrum evolution of the signal, resolving the spectra into a sequential scheme with increasing lifetimes. Other kinetic fitting approaches including nonsequential schemes were attempted; however, they did not fit the data to an acceptable level of error. For both the DADS and the EADS, global analysis revealed that three time constants were required to describe the timedependent spectral evolution of the transient absorption data. The analysis of the H3TD1 transient absorption data in all three solvents generated similar DADS and EADS results. This is likely due to the simplicity of the kinetic scheme necessary to fit the data and the observation that the components were relatively well-separated spectrally. The EADS are presented here as they provide an initial approximation to the concentration profiles of the transient species and to the evolution of the excited state.59 The EADS for H3TD1 in toluene, acetonitrile, and pyridine are presented in Figure 7. The time scales obtained from the data analysis are listed in Table 4. Table 4. Time Scales Obtained from Transient Absorption Data solvent

time scale 1

time scale 2

time scale 3

pyridine acetonitrile toluene

8.5 ± 0.3 ps 1.3 ± 0.3 ps 1.0 ± 0.2 ps

61.5 ± 3.0 ps 14.3 ± 1.0 ps 8.2 ± 0.2 ps

≫1 ns ≫1 ns ≫1 ns

In all solvents, the bleach and the excited-state absorption (ESA) rise nearly instantaneously with the pump pulse. In toluene, as shown in Figure 6a, the negatively signed groundstate bleach (GSB) spans 415−545 nm with a maximum at 495 nm, which corresponds to its absorption spectrum. Stimulated emission (SE) contributions would be expected to appear between 545 and 660 nm but are likely hidden by strong ESA. The broad ESA contribution begins at approximately 550 nm and extends to at least 725 nm. In the region beyond 725 nm, the signal-to-noise of the transient absorption signal was poor and could not be unambiguously resolved. For H3TD1 in toluene, the first EADS decays in 1.0 ± 0.2 ps, showing fairly even ESA and GSB contributions. In the first EADS, both the ESA and GSB are broad and featureless. The second EADS, which decays in 8.2 ± 0.2 ps, shows a significant loss of both ESA and GSB. In addition, the ESA and GSB bands have a slightly different shape in the second EADS. The GSB has two peaks, one at 495 nm and the other at 525 nm. This corresponds to the absorption spectrum of H3TD1 in toluene, which has a feature at 495 and 525 nm. The peak of the ESA blue-shifts from 595 to 565 nm, and a second peak appears at 660 nm. The peak of the steady-state fluorescence of H3TD1 in toluene is at 573 nm, which does not correspond to a clear feature. Any SE contributions appear to be completely overlapped by the ESA, although a portion of the dip in the ESA band could be due to SE. After the second EADS, there is a long time scale component (≫1 ns), leading to the small amount of steady-state fluorescence observed. The exception5530


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The Journal of Physical Chemistry B to evolve for several picoseconds as seen in the second time scale.60 In all solvents studied here, H3TD1 is expected to form strong hydrogen-bonding interactions either with solvent molecules or with another H3TD1 in a dimer form. Hydrogen bonds can promote rapid internal conversion by acting as radiationless decay accepting modes and can have a significant effect on the electronic spectroscopy of conjugated systems, leading to faster S1 to S0 internal conversion rates.38 Features that are particularly useful for promoting this kind of relaxation have large excited-state displacement, high frequencies, and a significant amount of vibrational anharmonicity. The likely existence of significant excited-state displacement in this system can be seen in the large Stokes shift and deviation from mirror symmetry between the steady-state absorption and fluorescence. In the transient absorption measurements we observe a short time spectral evolution that is consistent with internal conversion through vibrational relaxation.61 In addition, the observed relaxation time scales are extremely fast, especially for the solvents in which H3TD1 is expected to form a dimer. The slight blue-shift of the ESA peak suggests a possible explanation for the extremely rapid decay; it may indicate the participation of a conical intersection. In transient absorption measurements, transitions through conical intersections may be accompanied by an initial redshift of the SE followed by a blue-shift of the ESA.62 We note that the system described in the cited reference differs significantly from H3TD1; however, the general description of the conical intersection is applicable. Following the initial excitation, a wavepacket is generated in the Franck−Condon region. As this wavepacket moves toward the conical intersection, it decreases in energy and the corresponding SE shifts to the red. As the wavepacket approaches the conical intersection, the SE signal decreases as the surfaces come near each other and the transition dipole moment decreases. Once the wavepacket has crossed the potential energy surface, the SE becomes an ESA contribution. This ESA then can blue-shift as the surfaces move away from each other. This process of passing through the conical intersection with the red-shifting of the SE turning into the blue-shifting of the ESA may occur in tens of femtoseconds. The time resolution of our transient absorption measurement is likely too long to catch that initial conversion from SE to ESA and to directly observe the wavepacket; however, we do observe a slight blue-shifting of the ESA accompanied by extremely fast nonradiative relaxation. The analysis of the H3TD1 transient absorption measurements in all solvents returned three time scales. We believe that this corresponds to a three level model, a schematic of which can be found in Figure 8. In this model, excitation from the ground S0 to the initial S1 excited state occurs nearly instantaneously in the Franck−Condon region. Following excitation, there is a transfer of population to a third state. This third state has been tentatively labeled as DS. We believe it may be a dark state or hot ground state. The transfer step corresponding from S1 to DS corresponds to the fastest time scale from our analysis of the transient absorption data. The population in that third state then returns to the thermally relaxed ground state with the second, slower time scale mediated by internal conversion. The small amplitude long time contribution produces the observed steady-state fluorescence and is likely the decay of residual population in the initially excited state from S1 to S0. This interpretation is

Figure 8. Energy level schematic. The fast transfer step is associated with the time scale from the first EADS and the slower internal conversion (IC) with the time scale from the second EADS. The ≫1 ns time scale connecting the S0 and S1 states is associated with the third EADS. DS refers to a likely dark state or hot ground state.

consistent with the time-resolved fluorescence measurements. The TCSPC measurements showed nanosecond fluorescence lifetimes that correspond to the third time scale observed in the transient absorption. The low quantum yields observed for H3TD1 in solution also support the fast decay of the excited state with a small amount of the remaining population available to fluoresce on nanosecond time scales. In addition, the fluorescence upconversion showed dynamics consistent with the shortest transient absorption time scale. While the three state model appears to be sufficient to describe the population dynamics of H3TD1 in solution, the time scales are strongly influenced by the solvent. Pauli Master Equation. To further explore the dynamics of the excited state, we performed a Pauli master equation calculation to track the population densities following optical excitation.63 For this calculation we used a three level model based on the transient absorption data, which was found to be sufficient to describe the transient absorption and timeresolved fluorescence data. In this model, following initial excitation from the ground to excited state, the population in the excited state can transfer to a third state and from there back to the ground state. We label the ground state as a, the initially excited state as b, and the third state as c. In Figure 8, a corresponds to S0, b to S1, and c to DS. The population can return to a either from the initially excited (b) or third (c) state. Using the Pauli master equation/density matrix approach, equations of motion are derived to describe the population densities, where ρaa = |a⟩⟨a|, ρbb = |b⟩⟨b|, and ρcc = | c⟩⟨c|.64,65 Starting from the time evolution of the density operator ρ as described by the Liouville−von Neumann equation ∂ρ(t ) i = − [H(t ), ρ(t )] ∂t ℏ

If we then assume that the evolution of the system can be described as a series of irreversible, energy dissipation processes involving transitions between states, the time evolution of the density operator can then be described using a rate or master equation. By summing over all possible transitions, the general form of the Pauli master equation can be written as follows: ∂ρn (t ) ∂t 5531

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∑ (K nmρm (t ) − K mnρn (t )) m≠n DOI: 10.1021/acs.jpcb.9b01916 J. Phys. Chem. B 2019, 123, 5524−5535

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Figure 9. (a) Population densities calculated using a Pauli master equation treatment. The time scales used here were obtained from the analysis of the transient absorption data of H3TD1 in toluene. In part b, an integrated region of the absorptive feature of the transient absorption data of H3TD1 in toluene (dots) is compared to the calculated response (line). Panel c shows a comparison between an integrated region of the bleach feature of the transient absorption data of H3TD1 in toluene (dots) compared to the calculated response (line).

Here, Knm represents the transition rate coefficient connecting states n and m. Using this expression, the following coupled differential equations can be written to calculate the population densities. Here we assume that μ = 1.

decay of the ESA for the H3TD1 in toluene transient absorption data to the calculated excited-state dynamics. Panel c compares the recovery of the bleach feature in the H3TD1 in toluene transient absorption data to the population density of the ground state (state a). We were able to find similar agreement for the transient absorption data for H3TD1 in acetonitrile and in pyridine using the appropriate time scales. Given that this three level model was able to reproduce the main features of the time-resolved data, we believe it is a reasonable description of H3TD1 in solution.

∂ i ρ = Eρab + Γbρbb + Γcρcc + c·c ∂t aa ℏ

∂ i ρab = (Eρaa + εaρab ) − γbρab ∂t ℏ

■

∂ i ρbb = − Eρab − Γbρbb + k bcρbb + c·c ∂t ℏ

CONCLUSIONS The ultrafast excited-state dynamics of H3TD1 in solution are strongly influenced by the nature of its interactions with the solvent. In solvents that are incapable or only weakly able to form hydrogen bonds with H3TD1, the molecule preferentially dimerizes via intermolecular H3TD1−H3TD1 hydrogen bonds. In this configuration, H3TD1 undergoes extremely fast relaxation, likely assisted by the hydrogen-bonding interactions that drive formation of the dimer. In solvents that can form hydrogen bonds with H3TD1, either as donors or acceptors, the H3TD1 molecule is likely present in a monomeric form. In these solvents, the molecule undergoes fast internal conversion; however, the relaxation is slower than that observed for the dimer. The number of time scales and the overall dynamics are similar in the monomeric and dimer forms. This is likely because fast internal conversion is mediated by hydrogen-bonding interactions in both the dimer and the monomer systems. In both cases, a three state model calculated using a Pauli master equation approach was found to be sufficient to reproduce the observed dynamics. Beyond its dynamics in solution, the H3TD1 molecule readily coordinates with transition metals forming complexes that are of interest for catalytic applications. When coordinated, H3TD1 can act as a stable platform for ligandbased redox chemistry. Future work will investigate the ultrafast dynamics of H3TD1 as a function of metal coordination.

∂ ρ = kbcρbb − Γcρcc ∂t cc

Here, the following apply: E is the incident field of the laser pulses, εa is the transition energy from the ground (state a) to the first excited state (state b), γb is dephasing from state b, kbc is the rate constant of the transfer from state b to c, and Γb and Γc are the population relaxation rates from states b and c, respectively. For the calculation, the dephasing time scale γb was assumed to be 75 fs. The transition energy εa was taken as the maximum of the absorption spectrum in eV, 2.52 eV (492 nm) for toluene and 2.61 eV (475 nm) for pyridine. The Γ factors and kbc term used in the calculations were obtained from the analysis of the transient absorption data. The kbc term was taken as the fastest time scale in the transient absorption data, 1 ps for H3TD1 in toluene, 1.3 ps in acetonitrile and 8.5 ps in pyridine. Γc was the second time scale in the analysis of the transient absorption data, which was 8.2 ps for H3TD1 in toluene, 14.3 ps in acetonitrile, and 61.5 ps H3TD1 in pyridine. Γb corresponded to the greater than 1 ns component. Figure 9a presents the population densities calculated using the time scales obtained from the transient absorption measurements of H3TD1 in toluene. Using these results we were able to reproduce the transient absorption data with good agreement as shown in Figure 9b,c. Panel b compares the 5532



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ASSOCIATED CONTENT

* Supporting Information

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ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge financial support from the UAREN program and the National Science Foundation (CAREER Grant 1454047 to E.T.). V.H. thanks Prof. Scott Saavedra and Prof. Marek Romanowski for providing facility time and support. Funding for the facility was provided in part by NSF Major Research Instrumentation Grant 0958790. We thank Steven Petritis for assistance with sample preparation.

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.9b01916. Concentration-dependent NMR spectra of H3TD1 in deuterated toluene, deuterated acetonitrile, and deuterated methanol (Figures S1−S3); solvent polarity reference table (Table S1); TCPSC measurements of H3TD1 in chloroform, acetonitrile, THF, and pyridine (Figure S4); fluorescence upconversion spectra for H3TD1 in toluene, THF, acetonitrile, chloroform, methanol, and pyridine (Figure S5); experimental description and discussion of fluorescence upconversion; table of fit parameters for fluorescence upconversion spectra (Table S2); and steady-state fluorescence measurements for quantum yield determination (Figure S6) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ritika Gautam: 0000-0001-5826-0719 Elisa Tomat: 0000-0002-7075-9501 Vanessa Huxter: 0000-0001-9692-5312 Notes

The authors declare no competing financial interest. Biography

Vanessa Huxter is an Assistant Professor in the Department of Chemistry and Biochemistry and the Department of Physics at the University of Arizona. She received her Ph.D. from the University of Toronto under the supervision of Prof. Gregory D. Scholes where she worked on the optical and material properties of semiconductor nanocrystals. She then went on to hold an NSERC postdoctoral fellowship at the University of California, Berkeley, in the group of Prof. Graham Fleming. At UC Berkeley, she studied energy dynamics in natural light-harvesting systems and nitrogen vacancy defect centers in diamond. She has authored papers and book chapters on topics ranging from semiconductor spin dynamics to dynamic correlation in molecules and vibrational coherences in solid-state materials. Her current research interests are focused on developing new techniques to understand energy dynamics in synthetic and natural lightharvesting systems, redox-active molecular systems, and quantum materials. 5533


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