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Direct Observation of a Photochemical Alkyne-Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor-Acceptor Dyad Bogdan Dereka, Denis Svechkarev, Arnulf Rosspeintner, Maximilian Tromayer, Robert Liska, Aaron M Mohs, and Eric Vauthey J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09591 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on November 5, 2017

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Journal of the American Chemical Society

Direct Observation of a Photochemical Alkyne-Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor-Acceptor Dyad Bogdan Dereka,a Denis Svechkarev,b Arnulf Rosspeintner,a Maximilian Tromayer,c Robert Liska,c Aaron M. Mohs,b,d,e and Eric Vauthey*,a a)

Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland b)

Department of Pharmaceutical Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198-6858, United States c)

Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163/MC, 1060 Vienna, Austria

d)

Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, Omaha, Nebraska 68198-6858, United States

e)

Fred and Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, Nebraska 68198-6858, United States

ABSTRACT: The excited-state dynamics of an aniline-

triazine electron donor-acceptor dyad with an alkyne spacer has been investigated using a combination of ultrafast broadband mid-IR and visible transient absorption and fluorescence spectroscopies. The transient IR data reveal the occurrence of an efficient alkyne to allene isomerization of the spacer with a time constant increasing from a few hundreds of femtoseconds to a few picoseconds with solvent viscosity. This process is faster than the vibrational cooling of the Franck-Condon excited state, indicative of non-equilibrium dynamics. The transient electronic absorption and fluorescence data evidence that this transformation is accompanied by a charge separation between the donor and the acceptor subunits. The allene character of the spacer implies an orthogonal orientation of the donor and acceptor moieties, similar to that proposed for Twisted Intramolecular Charge Transfer (TICT) states. Such states are often invoked in the excited-state dynamics of donor-acceptor dyads, but their involvement could never be unambiguously evidenced spectroscopically. The alkyne-allene isomerization does not only involve a torsional motion but also a bending of the molecule due to the sp to sp2 rehybridization of one of the alkyne carbon atoms. This twisted and rehybridized ICT (‘TRICT’) state decays back to the planar and linear alkyne ground state on a time scale

decreasing from a few hundreds to ten picoseconds upon going from weakly to highly polar solvents. The different solvent dependencies reveal that the dynamics of the allene build-up are controlled by the structural changes, whereas the decay is limited by the charge-recombination step.

Introduction Triazine is a potent electron acceptor that, owing to its three-fold symmetry, is increasingly found in multibranched and star-shaped donor-acceptor (DA) architectures.1-15 These molecules are highly promising from the perspective of materials science for their high two-photon absorption crosssections,3-4,9,13 long-lived charge-separated states,10,15 for sensing applications and as non-linear near-IR antennae,11 to name a few recent developments. Therefore, much effort is dedicated toward the design of triazine-containing architectures with optimal properties toward the above-mentioned applications. The current approach to synthesize such systems is usually based on the intramolecular coupling of strong electron donors with the triazine cores to impart a charge-transfer character. Surprisingly, no comprehensive photophysical study exists to date addressing the excited-state dynamics of triazinebased donor-acceptor polyads.

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Here we present our investigation of the excitedstate dynamics of a triazine-based D-A dyad, D-III (Chart 1) where a dimethylaniline donor is linked to a dichlorotriazine via an ethyne bridge. For this, we used a combination of three ultrafast broadband spectroscopic techniques, namely transient electronic and vibrational absorption and fluorescence upconversion spectroscopy. Control measurements with D-II, an analogue with a double bond instead of a triple bond, are also presented. We will show that, upon optical excitation, this molecule undergoes an efficient alkyne-allene reaction, a process that has only been demonstrated before with the simplest acetylene and allene hydrocarbons.16-17 Although this reaction is found here to be fully reversible, possible extensions can be foreseen, even for preparative organic chemistry, including structural modifications, trapping and scavenging of the photoproduct and one-pot domino reactions. This reaction is fully photoinduced and does not require elaborate and expensive catalysts, as routinely employed in organic chemistry.18-19

Chart 1. Chemical structure of the dyad D-III and of its styryl analogue, D-II.

Another important finding of this study concerns the structural transformation undergone by D-III during this photochemical process. As one of the sp alkyne carbon atoms transforms into an sp2 carbon, the initially linear molecule adopts a kink at that atom. Moreover, because of the allenic nature of the bridge, the molecular planes of the D and A subunits become orthogonal. Given its strong charge transfer character, revealed by the spectroscopic data, this state is reminiscent of a Twisted Intramolecular Charge Transfer (TICT) state. Proposed in 1973 to account for the anomalous fluorescence of dimethylamino-benzonitrile (DMABN),20 TICT states have since been invoked in the excited-state dynamics of a large number of other D-A systems.21-25 However, the relevance of TICT states is still debated, at least in some cases.21,26-36 The possibility to fit the same data with different models with antagonistic interpretations became a major bone of contention. The main reason for this is that, despite tremendous efforts over the past decades, involving the synthesis of large number of model compounds and the application of

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a wide range of spectroscopic techniques,31-32,37-41 there is still no unambiguous experimental observation of such a TICT state. The structural changes observed here upon population of the ICT state involve not only a twist but also a bending of the molecule as a consequence of the rehybridization of one of the sp carbon atoms. In this respect, this intermediate could be viewed as a R(ehybridized)ICT state, akin to that hypothesized by Domcke and coworkers for DMABN.42-43 The results reported here with D-III offer a clearcut illustration of the interplay between chargetransfer dynamics and structural changes. They reveal that both twist and rehybridization take place in D-III, suggesting a ‘TRICT’ state. Moreover, the extent of ICT in this state is close to that of a chargeseparated state. Results The electronic absorption spectrum of D-III, shown in CHCl3 in Figure 1, consists of an intense charge-transfer band at 440 nm. On the other hand, only an extremely weak fluorescence from D-III can be observed at ~550 nm, pointing to a very efficient non-radiative decay of the optically populated S1 state. In the presence of water traces in the solutions, an additional emission band at shorter wavelength (~460 nm) due to the hydrolysis product of D-III, with one or both Cl atoms substituted by hydroxyl groups,44 can be observed (see the Supporting Information for more details on the stationary electronic spectra). All time-resolved spectroscopy measurements were performed upon 400 nm excitation, i.e., on the blue side of the S1 ← S0 band.

Figure 1. Electronic absorption spectrum of D-III in chloroform. The blue arrow represents the excitation wavelength for all ultrafast experiments.

Time-resolved infrared absorption (TRIR) spectroscopy

The TRIR spectra measured in the -C≡C- stretch region with D-III in various solvents, including ben-


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zene and THF (Figure 2), exhibit spectacular changes during the first few picoseconds after excitation. The early transient spectra consist of a negative bleach around ~2190 cm-1 and a positive absorption band at 2078 cm-1 (2071 cm-1) in THF (benzene) of similar intensity, which can be assigned to the -C≡Cstretch vibration in the ground and S1 states, respectively. The large width of the excited-state absorption (ESA) band on the low-frequency side can be explained by the coupling of this vibration with lowfrequency modes populated via the intermolecular vibrational redistribution of ~3000 cm-1 excess excitation energy.45-47

Figure 2. Top. Time-evolution of the transient infrared absorption measured upon excitation of D-III in benzene and THF (The time axis is linear from 0 to 1 ps and logarithmic from 1 ps to 1 ns. An arcsinh intensity scale was used for the contour plot in benzene to accentuate the weak spectral features). Bottom. Evolutionassociated difference spectra along with the time constants obtained from a global analysis assuming a scheme with consecutive exponential steps (A → B → C → (D) →).

The ~110 cm-1 downshift of the ESA band relative to the ground-state bleach points to substantial weakening of the triple bond in the S1 state. During the first few picoseconds, the ESA band disappears and a new one grows at even lower frequencies, that is ~2000 cm-1 (~1980 cm-1) in THF (benzene). This change is associated with a clear isosbestic point around 2015 cm-1 (2007 cm-1), indicative of a precursor-successor relationship between the two states/species associated with these bands. Immediately after the disappearance of the -C≡C- ESA band, a weak residual band can still be observed at ~2100 cm-1 (2086 cm-1), shifting to ~2070 cm-1 (2071 cm-1) on a sub-10 ps timescale (see e.g. the analysis of the

TRIR spectra in benzonitrile in Figure S7). Afterward, all transient features including the groundstate bleach decay entirely to zero, pointing to a full recovery of the ground-state population of D-III. The spectral region around ~1950-2000 cm-1 is very peculiar as only very few vibrations are found there. In general, the 5 µm region is a hallmark of the spcarbon. In particular, the 1950-2000 cm-1 range is very characteristic of the absorption of the antisymmetric stretch of the allene group (C=C=C),48-49 and virtually nothing else appears in this spectral window. As a consequence, the changes observed in the TRIR spectra of D-III with the appearance of the band at ~1980-2000 cm-1 is a direct evidence of the transformation of the ethynyl fragment of the spacer to an allene. The presence of an allene spacer implies a twisted geometry where the two double bonds are orthogonal to each other. One can thus conclude that the alkyne S1 state of D-III converts to a twisted state. Given the D-A nature of this compound, this state should have a substantial ICT character that is confirmed by the transient electronic absorption data described below. Consequently, this state is reminiscent of a TICT state. However, as discussed in more detail below, the presence of an allene unit in the spacer involves more than a twist of the D and A sub-units, but also a bending at one of the originally alkyne C atoms due to its rehybridization into a sp2 atom. Quantitative insight into the observed dynamics was obtained from a global target analysis of the TRIR data assuming a series of sequential exponential steps (Figure 2, bottom).50 Such an approach is relatively crude considering that solvent/vibrational relaxation and population dynamics are partially entangled. Consequently, non-equilibrium and, thus, non-exponential dynamics should be expected.51 For this reason, the resulting evolution-associated spectra (EADS) cannot be ascribed to a single state/species and the associated time constants should only be considered as representative timescales. The first evolution-associated spectrum (EADS A) corresponds mostly to the optically populated alkyne S1 state of D-III that transforms into the allene state (EADS B) in less than 2 ps in both benzene and THF. EADS B is mainly related to the vibrationally-hot allene state as testified by its evolution in 5-8 ps to EADS C, with a narrower and frequency upshifted allene band, as a consequence



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Figure 3. Comparison of TRIR bands associated with the ground-state bleach (left) and non-relaxed alkyne S1 state (middle, measured 300 fs after excitation) and relaxed allene state absorption (right) in solvents of varying polarity. The spectra are normalized to the minimum of the bleach to allow comparison of the intensity of the other two bands in different solvents (BEN: benzene; DBE: di-n-butyl ether; DEE: diethyl ether; PrAc: propyl acetate; THF: tetrahydrofuran; BZN: benzonitrile; DMSO: dimethyl sulfoxide; AC: acetone; ACN: acetonitrile).

of vibrational cooling (VC).45-47 The ground-state bleach remains unchanged during the A to C processes, indicating that the whole excited population is either in the alkyne S1 state or in the allene state. The lifetime of the equilibrated allene state (EADS C) is strongly solvent dependent and varies from 53 ps in THF to 400 ps in benzene. In the latter solvent, an additional weak band at 2074 cm-1 is visible at late times (EADS D) and remains unchanged within the time window of the experiment. The intensity of the associated ground-state bleach amounts to ~4% of its initial value. Given its long lifetime, and its frequency identical to that of the alkyne-S1 state, this transient is tentatively assigned to the alkyne-T1 state populated with a ~4% efficiency. Additionally to benzene and THF, TRIR measurements were performed in 8 other solvents (Table S1) providing a smooth gradual change of the Onsager polarity function. Unfortunately, it was not possible to use truly apolar solvents, such as saturated hydrocarbons, due to the insolubility of D-III, and benzene is the least polar solvent used here. Qualitatively similar excited-state dynamics to those measured in benzene and THF were found in all solvents (See SI for the TRIR contour plots and EADS, Figures S5-S7). The TRIR bands due to the ground-state bleach, the non-relaxed alkyne S1 state and the relaxed allene state are compared in all 10 solvents in Figure 3. The bleach frequency shows only weak vibrational solvatochromism varying within 5-7 cm-1 (Figure S8).52 The shape of the alkyne ESA band measured at early times after photoexcitation is very similar in all solvents (Figure 3, middle) except in acetone and acetonitrile where it is broader. The latter feature can be explained by the ultrafast isomerization dynamics in these two solvents that already appears at 300 fs.

In contrast, the frequency, shape and oscillator strength of the relaxed allene band exhibits a significant solvent dependence. The increase of solvent polarity is accompanied by a large frequency upshift (up to 50 cm-1), a narrowing and a loss of intensity. Such variation can be explained by considering the electronic density in the bridging unit of D-III. Increasing the dipolar character of the dyad leads to a downshift of the bridge band in the ground state, but to an upshift in the excited state.53-54 Although it might seem counterintuitive, this effect originates from the ICT character of the excited state: the density of the ‘excited’ electron on the acceptor subunit increases with solvent polarity, and, thus, decreases in the bridge. Therefore, the anti-bonding character of the bridge is smaller and the stretch frequency is higher. Similarly, as the density of the excitation on the bridge decreases with increasing solvent polarity, the integrated band intensity diminishes. The time constants extracted from the global target analysis of the TRIR data are listed in Table 1. The isomerization time constant corresponds to that for the conversion of the alkyne S1 state to the hot allene state. It exhibits a clear correlation with the macroscopic solvent viscosity, η, increasing from less than 1 ps in the least viscous solvents like acetonitrile and acetone (η < 0.5 cP) to more than 2.4 ps in DMSO with η = 2 cP (Figure S9). Such dependence is expected for structural changes involving large-amplitude motion like those associated with the alkyne-allene photoisomerization of DIII and often follows a ηα relationship, where α ≤ 1.55-57 The α value of 1 corresponds to the highfriction limit of the Kramers model where the time constant of the process is proportional to viscosity.58 However, such dependence is rarely observed and the measured time constant usually exhibits a plateauing behavior at higher viscosities,


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which has been explained in terms of local microviscosity,59 frequency-dependent solvent friction,6061 or the involvement of additional intramolecular modes.62 Table 1. Time constants extracted from the global analysis of the TRIR data measured with D-III in various solvents. a)

Solvent

isomerization / b ps

vibrational cooling / ps

c

exponentially as the latter increases (Figure S10). In the most polar solvents, the decay of this state is so fast that a positive transient band due to the hot electronic ground state, can be observed on the low frequency-side of the ground-state bleach (Figure S7).

allene lifetime / ps

1. Benzene

1.6

5.2

400

2. Dibutyl ether

1.7

13

510

3. Diethyl ether

0.98

4.1

230

4. Chloroform

1.6

7.1

230

5. Propyl acetate

1.3

6.4

66

6. Tetrahydrofuran

1.2

8.0

53

7. Benzonitrile

1.9

10

34

8. Dimethyl sulfoxide

2.4

4.4

14

9. Acetone

0.72

16

16

10. Acetonitrile

0.34

11

13

d

a)

Considering that population and relaxation dynamics occur on similar timescales, these time constants are assigned to the most dominant process. b) Limit of error ±10%. Conversion from the alkyne S1 c) state to the hot allene state. Vibrational cooling of d) the hot allene state. Decay of the relaxed allene state.

Figure 4 indicates that the viscosity dependence of the isomerization time can be relatively well reproduced with α =0.67, pointing to substantial departure from the Kramers model. However, a clear scattering of the time constants is observed at low viscosity, η < 0.5 cP, corresponding to solvents of very different dielectric constants, namely diethyl ether, acetone and acetonitrile (Table S1). The red points in Figure 4, which coincide with the best-fit curve, correspond to medium polarity solvents of similar dielectric constant. Therefore, the scattering at η < 0.5 cP is a clear indication that, at constant viscosity, the photoisomerization of D-III is favored in more polar solvents. The lifetime of the allene-ICT state exhibits a strong dependence on solvent polarity, dropping quasi-

Figure 4. Viscosity dependence of the time constant of alkyne-allene isomerization and best fit of the function τ iso = C + η α (numbers are according to Table 1). The inset illustrates the large-amplitude motion associated with the process. The magenta, orange and pink markers at below 0.5 cP correspond to diethyl ether, acetone and acetonitrile respectively.

Broadband fluorescence up-conversion spectroscopy (FLUPS)

The time evolution of the fluorescence measured with the FLUPS technique63-64 after excitation of DIII in CHCl3 and THF is shown in Figure 5. At early time, the spectrum consists of a broad emission band peaking around 490 nm. This band undergoes a >2500 cm-1 redshift up to about 550 nm that can be assigned to solvent relaxation.65 In parallel, its intensity decreases and another band centered at ~460 nm becomes apparent. This new band originates from a small amount of the above-mentioned hydrolysis product of D-III (see SI for more details).44 The FLUPS spectra in water show only the hydrolyzed D-III emission, initially at 460 nm and then undergoing a 1000 cm-1 red shift due to solvent relaxation. The low-energy emission band measured in THF and CHCl3 can be safely attributed to the fluorescence of D-III. This is the only band measured in hydrophobic solvents such as benzene and



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Figure 5. Top. Time dependence of the fluorescence measured after excitation of D-III in various solvents. The time axis is linear from 0 to 1 ps and logarithmic from 1 to 75 ps. An arcsinh intensity scale was used for the contour plots to accentuate the weak spectral features. Bottom. Transient fluorescence spectra measured at specific time delays (dots) along with the best-fits of one (H2O) or two (CHCl3 and THF) lognormal functions.

di-n-butyl ether. Its large dynamic Stokes shift (2880 and 4780 cm-1 in CHCl3 and THF, respectively) points to a substantial change of electric dipole moment upon photoexcitation and confirms the charge-transfer nature of the alkyne S1 state (see also Section S4.2 of the SI). Its very short lifetime explains why D-III fluorescence is hardly detectable by stationary fluorescence spectroscopy. Comparatively, the hydrolysis product has much longer lived fluorescence in organic solvents, with 500 ps and 1.3 ns lifetimes in THF and CHCl3, respectively, as determined from TCSPC measurements (Figure S11, Table S2). In water however, the excited-state lifetime of the hydrolyzed species is much shorter, most probably due to the occurrence of H-bond induced non-radiative deactivation (HBIND),66 as detailed in the SI. The fluorescence decay of D-III in both CHCl3 and THF can be well reproduced with a biexponential function. The major decay component (86 % in CHCl3 and 93 % in THF) is associated with the alkyne-allene isomerization time constant as extracted from TRIR measurements (1.5 ps and 1.0 ps respectively). The residual emission decays in 25 ps and 30 ps, respectively. As discussed below, this slower emission has the same origin as the residual alkyne band observed in the TRIR data around 2080 cm-1.

FLUPS measurements have also been carried out in benzene, di-n-butyl ether and acetone and the observed dynamics are qualitatively similar to those in CHCl3 and THF as discussed in the SI (Section S4). No additional emission band that could be attributed to the allene-ICT state could be detected in the 400-700 nm spectral window. Three main conclusions can be drawn from these measurements. First, the disappearance of the fluorescence with the rate constant of the alkyne-allene isomerization points to the non-emissive nature of the allene state. Second, the band shape of D-III fluorescence is close to that expected from the mirror-image symmetry relationship, indicating that the emitting state is structurally similar to the ground-state, namely that it is planar. Third, the large dynamic Stokes shift observed upon solvent relaxation is indicative of a significant change of the permanent electric dipole moment of D-III upon excitation, pointing to a substantial ICT character of the S1 state, in agreement with the D-A nature of this dyad. Transient electronic absorption (TA) spectroscopy

TA spectra measured with D-III in THF, CHCl3 and benzonitrile are presented in Figures 6 and S14. In principle, such TA spectra contain contributions from the ground-state bleach, stimulated


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emission and excited-state absorption (ESA). Since the stimulated emission is proportional to the spontaneous fluorescence, which was discussed in detail above, we focus here on the other contributions to the signal. The early TA spectra feature two prominent ESA bands at 400 nm and 700 nm and no apparent ground-state bleach. The shortwavelength ESA undergoes a spectacular shift (> 3500 cm-1) toward the UV region, out of the observation window, and only the onset of this band around 360-370 nm can be seen at later times. In parallel to this, the long-wavelength ESA band shifts to the blue as well and transforms into a band peaking at ~625 nm with a pronounced asymmetric wing up to ~500 nm. This ESA band is quite intense because it is not masked by the concomitant red shift of the stimulated emission. As these changes take place, the negative band due to the groundstate bleach appears at 440 nm. The time constants obtained from a global target analysis of the TA data are very similar to those extracted from the TRIR and FLUPS data (Figure 6 and S14). EADS A and B are dominated by the alkyne S1 state with absorption bands at 400 and 700 nm superimposed with the stimulated emission. Consequently, the A →B step corresponds mainly to solvent relaxation. EADS C is substantially different and can be associated with the allene state absorbing below 360 nm and between 500 and 625 nm. Therefore, the B → C step can be assigned to the isomerization of the alkyne S1 state to the hot allene-ICT state. The relaxation of the latter state is clearly visible in CHCl3, whereas in THF and benzonitrile this step is not resolved in the TA data.

Figure 6. Top. Time evolution of the transient electronic absorption measured after excitation of D-III THF (left) and of D-II in propyl acetate. The time axis

is linear from 0 to 1 ps and logarithmic from 1 to 100 ps. Bottom. Evolution-associated difference spectra along with the time constants obtained from a global analysis assuming a scheme with consecutive exponential steps (A → B → C →).

The shape of the 500-625 nm ESA band of the allene state suggests the presence of a vibronic structure with a ~1400 cm-1 progression. To obtain a ‘pure’ absorption spectrum of the allene state, the contribution of the ground-state bleach was subtracted from the TA spectrum (Figure S17). Apart from the 500-625 nm band, the resulting spectrum additionally contains a band at ~475 nm. This band coincides well with that found in the absorption spectrum of the radical cation of N,Ndimethylaniline, DMA·+.67 Moreover, the 500-625 nm band bears strong similarity with that of the radical anion of a cyano-substituted triazine,68 apart from a 600 cm-1 blue shift, which can be accounted for by the cyano groups (SI Section S5). The relative intensity of the 425 nm and 500-625 nm bands is in very good agreement with the absorption coefficients of DMA·+ (4700 M-1cm-1)67 and triazine radical anion (13000 M-1cm-1).68 Consequently, this transient spectrum points to a chargeseparated state of the dyad as expected for an ICT state with a structure where the D and A moieties are strongly decoupled. It should be noted that the hydrolyzed product is not observed in either transient absorption experiments (both in IR and UV-Vis), pointing to a low concentration. Additional details on the TA data and analysis are presented in section S5 of the SI. In addition, electronic TA measurements were performed with D-II, a styryl analogue of D-III where the triple bond is substituted with a double bond, thus eliminating the possibility of an alkyneallene photoisomerization. The chlorine atoms were replaced by methyl groups to avoid hydrolysis, whereas butyl groups were substituted on the amino group to ensure solubility in a broader range of solvents. While the detailed excited-state dynamics of D-II will be discussed somewhere else, we present here a few representative TA measurements on this molecule in apolar (hexane), medium polar (propyl acetate) and highly polar (DMSO) environments (Figures 6 and S17). Figure 6 demonstrates unambiguously that the excited-state properties of this model D-II compound are completely different from those of D-III. Only minor changes occur within the 100 ps time



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window. The ground-state bleach around 400 nm is clearly seen in apolar and medium polar solvents but is obscured by a weak ESA band in DMSO. Most importantly, stimulated emission does not disappear and only undergoes a moderate dynamic Stokes shift. There is only one major ESA band present at ~460 nm whose shape remains unchanged. No ESA band can be observed above 500 nm. In summary, none of the spectral features and dynamics found with D-III and assigned to its allene state can be observed with D-II. Discussion The main results obtained from the abovedescribed transient experiments can be summarized as follows: i) the TRIR spectra reveal the occurrence of an efficient alkyne-allene photoisomerization; ii) the electronic TA data point to a high charge-separated character of this allene state; and iii) the FLUPS results show that the alkyne S1 state is also polar but planar, similarly to the groundstate, and that the allene state is non-emissive. Additionally, the alkyne-allene isomerization occurs on similar timescales as vibrational and solvent relaxation and its dynamics depend mostly on the viscosity of the solvent, and, at constant viscosity, is faster in polar solvents. From this, the energy level scheme shown in Figure 7 can be proposed to account for the full photocycle of D-III. Excitation of the dyad at 400 nm populates the S1 state with ~0.4 eV vibrational energy. The alkyne S1 state is thus vibrationally hot, as testified by the shape of the –C≡C– stretching ESA band. Narrowing and possible frequency upshift of this band is not observed,54 because the conversion to the allene-ICT state is faster than vibrational cooling. Consequently, the newly populated alleneICT state is initially vibrationally hot and relaxes within 10-20 ps, before decaying back to the alkyne ground state on a timescale ranging from a few tens to a few hundreds of picoseconds, depending on the solvent polarity. Only in less polar solvents, where this processes is the slowest, the triplet state of D-III in its alkyne form is populated with an efficiency of ≤ ~0.04. This correlation between the triplet yield and the lifetime of the ICT state suggests that this state is populated from the alleneICT state rather than via direct intersystem crossing from the alkyne S1 state, which is much shorterlived. The allene fragment detected by TRIR implies a formal quinoid structure of either of the two aromatic rings of the dyad (Scheme 1). Such structure

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has important implications: i) a charge-separated character of the allene state, as found from the electronic TA measurements; ii) a bending of the molecule as a consequence of the sp to sp2 rehybridization of one of the alkyne atoms; and iii) an orthogonal mutual orientation of the D and A moieties and, thus, a breaking of the conjugation between these two groups (Scheme 1). From the experimental data, it is not possible to specify which of the two alkyne carbons is undergoing this rehybridization, that is which of D or A is in a quinoid form. A quinoid form of the DMA sub-unit is assumed in Figure 7, but the other form is almost equally probable, considering the number of resonant substructures that can be drawn for both geometries. More insight would require quantum-chemical calculations, which, given the complexity of the problem, are beyond the scope of this investigation.

Figure 7. Schematic representation of the full photocycle of D-III.

Although the energy of the allene state relative to the alkyne S1 state is not known, the very fast alkyne-allene conversion suggests that the allene state is below the S1 alkyne state. Moreover, the alkyne-allene energy gap can be expected to increase with the polarity of the solvent, accounting for the faster dynamics measured in more polar solvents at constant viscosity.


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Scheme 1. Alkyne to allene phototransformation of D-III

The photoisomerization results from a parallel weakening of the triple bond of the spacer and a partial charge transfer from the DMA to the triazine sub-unit. Solvation stabilizes the charge transfer and probably polarizes the electronic distribution further to such an extent that the originally linear structure rearranges to the energetically more favorable charge-separated allene form. Therefore, both solvation and large amplitude motion are actively contributing to the reaction coordinate. The importance of solvation is also illustrated by the dependence of the allene stretch band on solvent polarity (Figure 3, right). As discussed above, the frequency downshift observed in the least polar solvents points to a smaller charge-separated character. Moreover, the larger bandwidth most probably arises from a distribution of the geometries and charge-transfer character, in agreements with TRIR spectra of exciplexes in bimolecular photoinduced electron transfer reactions.69-70 The decay of the allene state involves both charge recombination and back-isomerization. The latter is associated with the same large amplitude motion as the isomerization and should therefore not occur on a largely different timescale. Consequently, the lifetime of the allene state is controlled by the charge-recombination step rather than by the isomerization. The strong solvent dependence can be explained by the decrease of the energy gap between the allene charge-separated state and the ground state with increasing solvent polarity, in agreement with electron transfer processes occurring in the Marcus inverted region.71 In this respect, the decay of the allene-state population differs from its build-up, where large amplitude motion and charge separation are strongly entangled. Both the TRIR and FLUPS data reveal the existence of a weak longer-lived component of the alkyne S1 state population. Its IR band is upshifted by 15-20 cm-1 relative to that measured at early time. This behavior, together with the observation of a hot allene population, strongly suggests that the faster decay component of the alkyne S1 state is

related to the isomerization of vibrationally hot molecules. Excitation of the low frequency modes upon redistribution of the vibrational excess excitation energy can be expected to facilitate distortion and the transformation of the molecule into the allene form. The slower reacting population is that with the lowest vibrational energy, characterized by the residual frequency-upshifted band. Because of the small amplitude of this slower component only an approximate ~20-30 ps time constant can be deduced from the data. This dependence of the isomerization dynamics on the vibrational energy is schematized in Figure 7 with the varying thickness of the arrows. Further TRIR and FLUPS measurements at different excitation wavelengths, i.e. with different amount of excess excitation energy, will be carried out to obtain deeper insight into the nonequilibrium nature of the isomerization dynamics. Conclusions and outlook We reported here on a detailed investigation of the excited-state dynamics of a triazine-based donor-acceptor dyad. Thanks to the combination of three different femtosecond broadband spectroscopic methods, an alkyne-allene photoisomerization reaction beyond those reported until now with the simplest representatives of both classes of compounds could be directly observed and resolved for the first time. Our data reveal that this process is associated with a substantial charge transfer and, therefore, is most probably enabled by the strong D-A character of the dyad. A systematic TRIR study with a series of similar dyads with varying push-pull character would be particularly useful for obtaining a deeper insight into the mechanism of this process. This photochemical sigmatropic rearrangement may open many fruitful avenues for the in situ generation, trapping and scavenging of the allene-type intermediates, including such applications as multistep organic synthesis, one-pot domino reactions and multicomponent reactions. The interest in the chemistry of allenes has recently undergone a veritable boom,72-73 because this class of compounds is known to serve as valuable building blocks for the synthesis of complex molecular targets, including natural products and drugs, and for material science. In general, these compounds are prepared using classical reactions, such as addition, elimination, substitution and rearrangement. However, photochemical isomerization has not been considered at all, and alkyne-allene transformations have never been shown before, although alkynes are



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readily accessible materials and multitude of alkyne-allene transformations have been developed.72 However, successful application of this type of isomerization for organic synthesis requires an allenic intermediate with at least an order of magnitude longer lifetime than that found here. Since the decay of this intermediate is essentially a charge recombination process, the same approaches as those developed to lengthen the lifetime of chargeseparated states for applications in solar energy conversion or photocatalysis could be applied. Finally, the photophysics of this triazine-based dyad offers an unambiguous illustration of the major structural changes that can accompany an intramolecular charge transfer process. The formation of an allene bond implies an orthogonal orientation of the donor and acceptor π-conjugated systems. In this respect, the allene-ICT state is strongly related to a TICT state. However, the alkyne-allene transformation involves not only a twisting motion but also a bending from linear geometry due to the sp to sp2 rehybridization of a carbon atom. Rehybridization of the cyano carbon atom, resulting in a R(ehybridized)ICT state was proposed for DMABN. However, the involvement of this state was subsequently disproved by TRIR measurements. The allene-ICT state combines the characteristic of both TICT and RICT states. A large number of push-pull molecules with an alkyl spacer have been developed so far, but only a few of them were investigated using time-resolved vibrational spectroscopy. Here again, systematic TRIR studies would be necessary for a better understanding of the relevance of such ‘TRICT’ states in the photophysics of these molecular systems. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Synthesis of D-III and D-II, solvent properties, experimental details, quantum chemical calculations of D-III in the ground state, stationary absorption and fluorescence spectra, TRIR spectra in all solvents, solvent polarity dependence of the allene lifetime, TCSPC data, additional FLUPS data and analysis, additional electronic transient absorption (PDF). AUTHOR INFORMATION ORCID

Bogdan Dereka: 0000-0003-2895-7915 Denis Svechkarev: 0000-0002-4002-6155 Arnulf Rosspeintner: 0000-0002-1828-5206

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Aaron M. Mohs: 0000-0002-9353-5404 Eric Vauthey: 0000-0002-9580-9683 Corresponding Author

[email protected]

ACKNOWLEDGMENTS This work was supported by the Fonds National Suisse de la Recherche Scientifique through project Nr. 200020-165890, the University of Geneva, the Nebraska Research Initiative, and Vienna University of Technology.

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TOC GRAPHICAL ABSTRACT


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Direct Observation of a Photochemical Alkyne ... - ACS Publications

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