Direct Observation of a Photochemical Alkyne–Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor–Acceptor Dyad

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Article

Reference

Direct Observation of a Photochemical Alkyne–Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in

a Donor–Acceptor Dyad

DEREKA, Bogdan, et al .

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 nonequilibrium 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 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 [...]

DEREKA, Bogdan, et al . Direct Observation of a Photochemical Alkyne–Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor–Acceptor Dyad.

Journal of the American Chemical Society , 2017, vol. 139, no. 46, p. 16885-16893

DOI : 10.1021/jacs.7b09591

Available at:

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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*

^,^†

†Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland

‡Department of Pharmaceutical Sciences,^∥Department of Biochemistry and Molecular Biology, and^⊥Fred and Pamela Buﬀett Cancer Center, University of Nebraska Medical Center, Omaha, Nebraska 68198-6858, United States

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

*^S Supporting Information

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 andﬂuorescence 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 nonequilibrium 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 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 involves not only a torsional motion but also a bending of the molecule due to the sp to sp²rehybridization of one of the alkyne carbon atoms. This twisted and rehybridized intramolecular charge transfer (“TRICT”) state decays back to the planar and linear alkyne ground state on a time scale decreasing from a few hundred to ten picoseconds upon going from weakly to highly polar solvents. The diﬀerent solvent dependencies reveal that the dynamics of the allene buildup 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 (D−A) architectures.¹⁻¹⁵These molecules are highly promising from the perspective of materials science for their high two-photon absorption cross sections,^3,4,9,13 long-lived charge-separated states,^10,15 for sensing applications, and as nonlinear 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 com- prehensive photophysical study exists to date addressing the excited-state dynamics of triazine-based donor−acceptor polyads.

Here, we present our investigation of the excited-state dynamics of a triazine-based D−A dyad, D-III (Chart 1), where a

dimethylaniline donor is linked to a dichlorotriazine via an eth- yne bridge. For this, we used a combination of three ultrafast broadband spectroscopic techniques, namely, transient electronic and vibrational absorption and ﬂuorescence up-conversion

Received: September 8, 2017 Published: October 25, 2017

Chart 1. Chemical Structure of the Dyad D-III and of Its Styryl Analogue, D-II

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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.^16,17Although 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

Another importantﬁnding of this study concerns the structural transformation undergone byD-IIIduring 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 anomalousfluorescence of dimethylaminoben- zonitrile (DMABN),²⁰TICT 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.^21,26−36The possibility tofit 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 a large number of model compounds and the application of 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 co-workers for DMABN.^42,43

The results reported here withD-IIIoﬀer a clear-cut illustration of the interplay between charge-transfer dynamics and structural changes. They reveal that both twist and rehybridization take place inD-III, suggesting a“TRICT”state. Moreover, the extent of ICT in this state is close to that of a charge-separated state.

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^RESULTS

The electronic absorption spectrum ofD-III, shown in CHCl₃in Figure 1, consists of an intense charge-transfer band at 440 nm.

On the other hand, only an extremely weakﬂuorescence from D-IIIcan be observed at∼550 nm, pointing to a very eﬃcient nonradiative 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,⁴⁴can be observed (see theSupporting Information for more details on the stationary electronic spectra). All time- resolved spectroscopy measurements were performed upon 400 nm excitation, that is, on the blue side of the S₁←S₀band.

Time-Resolved Infrared Absorption (TRIR) Spectrosco- py.The TRIR spectra measured in the−CC−stretch region with D-III in various solvents, including benzene 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⁻¹and a positive absorption band at 2078 cm⁻¹(2071 cm⁻¹) in THF (benzene) of similar intensity, which can be assigned to the−CC−stretch vibration in the ground and S₁states, 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 low-frequency modes populated via the intermolecular vibrational redistribution of∼3000 cm⁻¹excess excitation energy.⁴⁵⁻⁴⁷

The ∼110 cm⁻¹downshift 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 fre- quencies, 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 time scale (see, e.g., the analysis of the TRIR spectra in benzonitrile in Figure S7). Afterward, all transient features including the ground-state bleach decay entirely to zero, pointing to a full recovery of the ground-state population ofD-III.

Figure 1.Electronic absorption spectrum ofD-IIIin chloroform. The blue arrow represents the excitation wavelength for all ultrafast experiments.

Figure 2.Top: Time evolution of the transient infrared absorption measured upon excitation ofD-IIIin 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: Evolution-associated diﬀerence spectra along with the time constants obtained from a global analysis assuming a scheme with consecutive exponential steps (A→B→C→(D)→).

Journal of the American Chemical Society Article

DOI:10.1021/jacs.7b09591 J. Am. Chem. Soc.2017, 139, 16885−16893 16886

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The spectral region around∼1950−2000 cm⁻¹is very peculiar as only very few vibrations are found there. In general, the 5μm region is a hallmark of the sp carbon. In particular, the 1950− 2000 cm⁻¹range is very characteristic of the absorption of the antisymmetric stretch of the allene group (CCC),^48,49and virtually nothing else appears in this spectral window. As a consequence, the changes observed in the TRIR spectra ofD-III with the appearance of the band at∼1980−2000 cm⁻¹is 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 S₁state of D-IIIconverts 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 subunits, but also a bending at one of the originally alkyne C atoms due to its rehybridization into a sp²atom.

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. Con- sequently, nonequilibrium and, thus, nonexponential dynamics should be expected.⁵¹For 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 time scales. Theﬁrst evolution-associated spectrum (EADS A) corresponds mostly to the optically populated alkyne S₁state ofD-IIIthat 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 of vibrational cooling (VC).⁴⁵⁻⁴⁷The ground-state bleach remains unchanged during the A to C processes, indicating that the whole excited population is either in the alkyne S₁state 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 S₁state, this transient is tentatively assigned to the alkyne T₁state populated with a∼4% eﬃciency.

In addition 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 ofD-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 (seeSupporting Informationfor the TRIR contour plots and EADS, Figures S5−S7).

The TRIR bands due to the ground-state bleach, the nonrelaxed alkyne S₁ state, and the relaxed allene state are compared in all 10 solvents inFigure 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 ofD-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 inte- grated band intensity diminishes.

The time constants extracted from the global target analysis of the TRIR data are listed inTable 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 withη= 2 cP (Figure S9).

Such dependence is expected for structural changes involving large-amplitude motion like those associated with the alkyne− allene photoisomerization of D-III and often follows a η^α relationship, whereα≤1.⁵⁵⁻⁵⁷Theα value of 1 corresponds to the high-friction limit of the Kramers model where the time constant of the process is proportional to viscosity.⁵⁸However, such dependence is rarely observed, and the measured time Figure 3.Comparison of TRIR bands associated with the ground-state bleach (left) and nonrelaxed alkyne S1state (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, dimethylsulfoxide; AC, acetone; ACN, acetonitrile).

Journal of the American Chemical Society

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constant usually exhibits a plateauing behavior at higher visco- sities, which has been explained in terms of local microviscosity,⁵⁹ frequency-dependent solvent friction,^60,61or the involvement of additional intramolecular modes.⁶²

Figure 4indicates that the viscosity dependence of the isomerization time can be relatively well reproduced withα= 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 inFigure 4, which coincide with the best-ﬁt curve, correspond to medium polarity solvents of similar dielectric constant. Therefore, the scattering atη< 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-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).

Broadband Fluorescence Up-conversion Spectrosco- py (FLUPS).The time evolution of theﬂuorescence measured with the FLUPS technique^63,64after excitation ofD-IIIin 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⁻¹red shift 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 ofD-III(seeSupporting Informationfor more details).⁴⁴The FLUPS spectra in water show only the hydrolyzed D-IIIemission, initially at 460 nm and then undergoing a 1000 cm⁻¹red shift due to solvent relaxation.

The low-energy emission band measured in THF and CHCl₃ can be safely attributed to thefluorescence ofD-III. This is the only band measured in hydrophobic solvents such as benzene and di-n-butyl ether. Its large dynamic Stokes shift (2880 and 4780 cm⁻¹in CHCl₃and THF, respectively) points to a substantial change of electric dipole moment upon photoexcitation and confirms the charge-transfer nature of the alkyne S₁state (see also section S4.2 of theSupporting Information). Its very short lifetime explains why D-III fluorescence is hardly detectable by stationary fluorescence spectroscopy. Comparatively, the hydrolysis product has much longer livedfluorescence in organic solvents, with 500 ps and 1.3 ns lifetimes in THF and CHCl₃, respectively, as determined from TCSPC measurements (Figure S11 and 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 nonradiative deactivation (HBIND),⁶⁶ as detailed in the Supporting Information.

Theﬂuorescence decay ofD-IIIin both CHCl₃and THF can be well reproduced with a biexponential function. The major decay component (86% in CHCl₃and 93% in THF) is associated with the alkyne−allene isomerization time constant as extracted from TRIR measurements (1.5 and 1.0 ps respectively). The residual emission decays in 25 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 CHCl₃and THF, as discussed in theSupporting Information(section S4). No additional emission band that could be attributed to the allene state could be detected in the 400−700 nm spectral window.

Three main conclusions can be drawn from these measurements. First, the disappearance of theﬂuorescence with the rate constant of the alkyne−allene isomerization points to the nonemissive nature of the allene state. Second, the band shape of D-IIIﬂuorescence is close to that expected from the mirror- image symmetry relationship, indicating that the emitting state is structurally similar to the groundstate, 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 ofD-IIIupon excitation, pointing to a substantial ICT character of the S₁state, in agreement with the D−A nature of this dyad.

Transient Electronic Absorption Spectroscopy. Tran- sient absorption (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 Table 1. Time Constants Extracted from the Global Analysis

of the TRIR Data Measured with D-III in Various Solvents^a

solvent

isomerization (ps)^b

vibrational cooling (ps)^c

allene lifetime (ps)^d

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. dimethylsulfoxide 2.4 4.4 14

9. acetone 0.72 16 16

10. acetonitrile 0.34 11 13

aConsidering that population and relaxation dynamics occur on similar time scales, these time constants are assigned to the most dominant process. Limit of error±10%.^bConversion from the alkyne S1state to the hot allene state.^cVibrational cooling of the hot allene state.^dDecay of the relaxed allene state.

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

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ground-state bleach, stimulated emission, and excited-state absorption (ESA). Since the stimulated emission is proportional to the spontaneousﬂuorescence, 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 and 700 nm and no apparent ground-state bleach. The short- wavelength 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 ground-state 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 (Figures 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 S₁state 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.

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 ofN,N-dimethylaniline, DMA^•+.⁶⁷Moreover, 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 (Supporting Information 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)⁶⁷and triazine radical anion (13 000 M⁻¹cm⁻¹).⁶⁸Consequently, this transient spectrum points to a Figure 5.Top: Time dependence of thefluorescence measured after excitation ofD-IIIin 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 bestfits of one (H₂O) or two (CHCl₃and THF) log-normal functions.

Figure 6.Top: Time evolution of the transient electronic absorption measured after excitation ofD-IIITHF (left) and ofD-IIin 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→).

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charge-separated 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 Supporting Information.

In addition, electronic TA measurements were performed withD-II, a styryl analogue ofD-III, where the triple bond is substituted with a double bond, thus eliminating the possibility of an alkyne−allene 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 ofD-IIwill 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 6andS16).

Figure 6demonstrates unambiguously that the excited-state properties of this modelD-IIcompound are completely diﬀerent from those ofD-III. Only minor changes occur within the 100 ps time 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 withD-IIIand assigned to its allene state can be observed withD-II.

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^DISCUSSION

The main results obtained from the above-described 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 S₁state is also polar but planar, similarly to the groundstate, and that the allene state is nonemissive. Additionally, the alkyne−allene isomerization occurs on similar time scales 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 inFigure 7can be proposed to account for the full photocycle ofD-III. Excitation of the dyad at 400 nm populates the S₁ state with ∼0.4 eV vibrational energy. The alkyne S₁state 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 observed⁵⁴ because the conversion to the allene ICT state is faster than vibrational cooling. Consequently, the newly populated allene ICT 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 process 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 allene ICT state rather than via direct intersystem crossing from the alkyne S₁state, which is much shorter-lived.

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 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 sp²rehybridization 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 subunit 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.

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 S₁alkyne 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.

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 subunit. Solvation stabilizes the charge transfer and probably polarizes the electronic distribution further Figure 7.Schematic representation of the full photocycle ofD-III.

Scheme 1. Alkyne to Allene Phototransformation of D-III

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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 time scale. Conse- quently, 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.⁷¹In 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 S₁state population. Its IR band is upshifted by 15−20 cm⁻¹relative 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 S₁state 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 inFigure 7with the varying thickness of the arrows. Further TRIR and FLUPS measurements at diﬀerent excitation wavelengths, that is, 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 theﬁrst 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 readily accessible materials and a 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 charge-separated 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 acceptorπ-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.^42,43 However, the involvement of this state was subsequently disproved by TRIR measurements.³⁷The allene ICT state combines the characteristics 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

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/jacs.7b09591.

Synthesis of D-III and D-II, solvent properties, experimental details, quantum chemical calculations ofD-IIIin the ground state, stationary absorption andﬂuorescence 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 Corresponding Author

*eric.vauthey@unige.ch ORCID

Bogdan Dereka:0000-0003-2895-7915

Denis Svechkarev:0000-0002-4002-6155

Arnulf Rosspeintner:0000-0002-1828-5206

Robert Liska:0000-0001-7865-1936

Eric Vauthey:0000-0002-9580-9683 Journal of the American Chemical Society

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Notes

The authors declare no competingﬁnancial interest.

■

ACKNOWLEDGMENTS

This work was supported by the Fonds National Suisse de la Recherche Scientiﬁque through Project No. 200020-165890, the University of Geneva, the Nebraska Research Initiative, and Vienna University of Technology.

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