Substituent and Solvent Effects on the Excited State Deactivation Channels in Anils and Boranils

16 Pages·2015·3.46 MB·English

DOI: 10.1002/chem.201404669 Full Paper
&
Photochemistry
Substituent and Solvent Effects on the Excited State Deactivation
Channels in Anils and Boranils
Jacek Dobkowski,[a] Paweł Wnuk,[a,b] Joanna Buczyn´ska,[a] Maria Pszona,[a]
Graz˙yna Orzanowska,[a] Denis Frath,[c] Gilles Ulrich,[c] Julien Massue,[c] Sandra Mosquera-
V(cid:2)zquez,[d] Eric Vauthey,*[d] Czesław Radzewicz,[b] Raymond Ziessel,*[c] and Jacek Waluk*[a,e]
Abstract:Differentlysubstitutedanils(Schiffbases)andtheir trontransfercoupledwithtwistingofthenitrophenylgroup.
boranilcounterpartslackingtheproton-transfer functionality Thesametypeofstructureisresponsiblefortheemissionof
have been studied using stationary and femtosecond time- the corresponding boranil. A general model was proposed
resolved absorption, fluorescence, and IR techniques, com- to explain different photophysical responses to different
bined with quantum mechanical modelling. Dual fluores- substitution patterns in anils and boranils. It is based on the
cenceobservedinanilswasattributedtoexcitedstateintra- analysis of changes in the lengths of CN and CC bonds link-
molecular proton transfer. The rate of this process varies ing the phenyl moieties. The model allows predicting the
upon changing solvent polarity. In the nitro-substituted anil, contributions of different channels that involve torsional dy-
proton translocation is accompanied by intramolecular elec- namicstoexcitedstatedepopulation.
Introduction
excited-state deactivation, either in a competing or coopera-
tive way. The best known examples include proton-coupled
Understanding of the elementary steps of ultrafast photoin- electron transfer[1–8] and excited state electron transfer accom-
ducedphenomena,suchaselectronorproton/hydrogentrans- panied by structural changes.[9–13] It has been recently pro-
fer or conformational changes is the prerequisite for compre- posed that also photoinduced proton transfer can be coupled
hensive description of basic natural processes: photosynthesis to a conformational change, involving mutual twisting of
and vision. Itis also crucialfor applications in numerous areas: protondonorandacceptormoieties.[14]
materials science, optoelectronics, information storage, design Inthiswork,weanalysetheexperimentalandtheoreticalre-
of sensors, biology, medicine, and environmental protection. A sults obtained for a series of anils (aniline-imines) and boranils
particularcase,challengingbothforexperimentalistsandtheo- (Scheme1).ThesecompoundscanformallybederivedfromN-
reticians, arises when more than one channel participates in salicylideneaniline (SA), the most investigated molecule of the
Schiffbase family,famous for extremely complex behaviourin-
[a] Dr.J.Dobkowski,Dr.P.Wnuk,Dr.J.Buczyn´ska,M.Pszona,G.Orzanowska, volving ultrafast excited state intramolecular proton transfer
Prof.J.Waluk
(ESIPT), torsional dynamics, photochromism, thermochromism,
InstituteofPhysicalChemistry,PolishAcademyofSciences
Kasprzaka44,01-224Warsaw(Poland) and solvatochromism.[15–44] Upon photoexcitation, the enol
E-mail:[email protected] form of SA undergoes ultrafast intramolecular proton transfer
[b] Dr.P.Wnuk,Prof.C.Radzewicz from the hydroxy group to the nitrogen atom. This leads to
InstituteofExperimentalPhysics,FacultyofPhysics acis-ketoform,which is subsequentlyconverted into aphoto-
UniversityofWarsaw,Hoz˙a69,00-681Warsaw(Poland)
chromicproduct(trans-ketotautomer).
[c] Dr.D.Frath,Dr.G.Ulrich,Dr.J.Massue,Prof.R.Ziessel
LaboratoiredeChimieOrganiqueetSpectroscopiesAvanc(cid:2)es
UMR7515auCNRS,(cid:3)coledeChimie
Polym(cid:4)resMat(cid:2)riauxdeStrasbourg,25rueBecquerel
67087Strasbourg,Cedex02(France)
E-mail:[email protected]
[d] Dr.S.Mosquera-V(cid:5)zquez,Prof.E.Vauthey
DepartmentofPhysicalChemistry,UniversityofGeneva
30quaiErnest-Ansermet,1211Geneva4(Switzerland)
E-mail:[email protected]
[e] Prof.J.Waluk
FacultyofMathematicsandNaturalSciences
CollegeofScience,CardinalStefanWyszyn´skiUniversity
Dewajtis5,01-815Warsaw(Poland)
SupportinginformationforthisarticleisavailableontheWWWunder
http://dx.doi.org/10.1002/chem.201404669. Scheme1.Theinvestigatedanilsandboranils.
Chem.Eur.J.2015,21,1312–1327 1312 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
However, both experiment
and theory suggest that several
other channels may be compet-
ing with or following the proton
transfer (Scheme2). These in-
clude twisting about CN or CC
bonds, 1pp*!1np* internal con-
version, and singlet–triplet inter-
system crossing. Using boranils,
compounds devoid of proton-
transfer functionality, allows to
focusontheroleoftorsionaldy-
namics not competing with tau-
tomerization, since the possibili- Scheme3.Syntheticpathwaysforanilsandboranils.
ty of twisting about the C6N1
and C7N1 bonds is retained in
these molecules. Moreover, functionalizing SA with electron- 78and85%.Allthesecompoundhavebeenconsistently char-
donatingand-acceptinggroupsallowsthestudyoftheroleof acterizedby1H,13C,11BNMR,massspectroscopyandelemental
electronic structure on the kinetics and thermodynamics of analysis.
bothprotontransferandrotationalisomerism.Finally,substitu- Figure1 shows room temperature electronic absorption and
tionstabilizesthepp*versus np*states, reducing thepossible fluorescence spectra of 1, 1a, and 1b, recorded in a nonpolar
contribution from theinternalconversion channel. Wedemon- andapolarsolvent,tolueneandacetonitrile,respectively.
strate that the ESIPT rate in anils can be controlled by the
nature of substituent and by the polarity of the environment.
Forbothanilsandboranils,weshowtheexistenceofaneffec-
tive non-radiative excited state depopulation channel which is
activatedinpolarenvironment.
Figure1.Left:absorption;right:fluorescenceof1,1a,and1bin:a)tolu-
ene,andb)acetonitrileat293K.
Scheme2.PossiblechannelsofexcitedstatedeactivationinSA.CI1andCI2
indicatestructurescorrespondingtotheregionofS1/S0conicalintersection. The absorption spectra in two solvents are very similar for
all three compounds. This claim is also true for the emission
profileof1aand1binbothsolvents.Incontrast,fluorescence
Results
ofanil1isstrikingly differentfromtheemission ofboranils1a
The synthetic pathway leading to anils[45] and boranils[46,47] is and1b:itconsistsoftwobands,ofwhichthehigherintensity,
described in Scheme3. Preparation of the anils 1–3 can easily low energy one (F) is located at similar energies in toluene
2
beachievedbytreatmentofsalicylaldehydesAwiththeappro- andacetonitrile.Thehighenergyband(F)isbarelydetectable
1
priate anilineB. This condensation occurs in refluxing ethanol in toluene, but is clearly observed in a polar environment. The
with a catalytic amount of p-TsOH. The desired imines precipi- F/F intensity ratio ismuch higherforacetonitrile thanfortol-
1 2
tate pure out of the reaction mixture and can be purified by uene. The total fluorescence quantum yield is quite low (10(cid:2)3
filtration with yield from 38 to 85%. Subsequent boron(III) orless,seeTable1);itisaboutfour-timeshigherintoluene.
complexation is achieved in 1,2-dichloroethane with BF·EtO The single fluorescence observed for 1a and 1b is much
3 2
underbasicconditionstoformboranils1a–3awithyieldrang- more intense than the emission of 1. It is definitely stronger
ing between 43 and 72%. Boranils 1b and 3b are prepared for toluene than for acetonitrile for 1a and only slightly so in
fromtheBPh precursorinhottoluenewithrespectiveyieldof 1b.Theintensityandlifetimeratiosaresimilar.
3
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1313 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
is similar to that observed for 1,
Table1. Spectralandphotophysicalroomtemperaturecharacteristics.
but in 2 the F band is definitely
1
l [nm] f[a] Decaytime[ps] more intense with respect to F
max fl 2
abs em FU[b] SPC[c] TA[d] in each solvent. As was the case
1 for 1, the dual emission in 2 is
toluene 372 430/553 0.0011 0.25/61 56(550) 0.04/57[e] assigned to the ESIPT process.
acetonitrile 372 445/554 0.00025 0.41/45 48(500,550) 0.4/40[e]
The dual emission profile of 2
butyronitrile 0.7/46[e]
has been recently reported and
1a
toluene 396 447 0.046 120(450,500) 0.6/150 interpretedinthesameway.[44]
acetonitrile 395 453 0.0085 0.45/20 20(450) 0.4/25 Figure3 presents the spectra
butyronitrile 0.82/36
of the nitro derivatives 3, 3a,
1b
and 3b. The emission pattern of
toluene 409 488 0.11 760(500)
acetonitrile 409 480 0.07 670(500) the anil 3 is quite different from
2 that exhibited by 1 and 2. A
toluene 398 460/570 0.0019 0.83/236 220(570) 0.9/210[e]
single fluorescence band peak-
cyclohexane 0.49/160
ing at 514nm is observed in tol-
acetonitrile 398 482/567 0.0018 2.2/160 200(480) 2.5/150[e]
butyronitrile 399 476/568 0.0016 3.0/175[e] uene solution. In contrast, dual
valeronitrile 2.8/190[e] emission is detected in acetoni-
THF 400 471/559 0.0017
trile, with the maximum of the
2a 461
dominant, low energy band
toluene 416 468 0.50 1510(460) 2.7/85/1500
acetonitrile 416 466 0.0083 0.77/17 0.7/6/21 peaking at 680nm. A similar be-
butyronitrile 416 466 0.010 0.4/4/33 haviour is observed for butyroni-
THF 411 0.015 0.7/13/90
trile solution of 3, but the maxi-
3 514
mum of F lies at 652nm, about
toluene 428 525/680 0.010 1.2/13 260(525) 8.6/260/>3000 2
acetonitrile 431 514/652 0.0005 0.22/4.2 0.15/0.45/5 30nm blue-shifted compared to
butyronitrile 431 0.0068 35(600) 0.5/1.5/33 acetonitrile. Such behaviour is
3a
different from that of 1 and 2
toluene 426 477 0.53 1610 0.3/4.5/2000
which exhibit F at similar wave-
acetonitrile 429 494/650 0.0006 0.5/2.6 0.3/5.1 2
butyronitrile 429 504/630 0.0045 30(610,660) 1/2/18 lengths in both polar and non-
3b polarsolvents.TheF emissionis
1
toluene 440 509 0.61[f] 2730[f]
barely detectable at room tem-
acetonitrile 444 518/660 0.0007 0.27/80
perature in anhydrous acetoni-
[a]Estimated error:(cid:3)20–30% for quantum yields larger than 0.001 and (cid:3)50% for lower values.[b]Fluores- trile and in anhydrous butyroni-
cenceupconversion.[c]Singlephotoncounting;inparentheses,detectionwavelength[nm].[d]Transientab-
trile. In acetonitrile its intensity
sorption.[e]Long-livedcomponentalsodetected,seeFiguresS4andS6intheSupportingInformation.[f]Ref-
erence[46]. increases with respect to F2 over
a period of several days, which
suggests that it may be due to
Because ofthedifferenceinthefluorescencepatternsinthe
anil and boranils, the origin of the dual emission of 1 can be
readily assigned to the excited state proton transfer, a process
responsible for the lower energy emission band F. The large
2
increaseoftheF/F intensityratioinacetonitrileindicatesthat
1 2
the reaction barrier is higher in a polar environment, which
may be a consequence of a larger excited state dipole
moment in the initially excited form. As discussed below, this
hypothesiswasconfirmedbycalculations.
The emission spectra of 2 and 2a, shown in Figure2, are
qualitatively similar to those of 1 and 1a, but the quantitative
differences are significant. Again, a single fluorescence is ob-
served for the boranil derivative 2a. It is very intense in tolu-
ene(quantumyieldof50%),butdropsdramaticallyinacetoni-
trile,aswellasinlesspolarsolventssuchasTHFandbutyroni-
trile (Table1). Analogous decrease is observed for the fluores-
cence decay time. The anil derivative 2 exhibits a dual emis-
sion profile in both polar and nonpolar solvents. The F/F
1 2
intensity ratio is strongly solvent-dependent: 0.2 in toluene,
Figure2.Left:absorption;right:fluorescenceof2and2ain:a)toluene,
0.6 in THF and butyronitrile, and 0.9 in acetonitrile. This trend andb)acetonitrileat293K.
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1314 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
The temperature dependence of the single fluorescence ob-
served in 2a (FigureS2 in the Supporting Information) is very
different in toluene versus THF and butyronitrile. In toluene,
the emission is intense already at room temperature and does
not change much as the temperature decreases. On the con-
trary, in THF and BuCN the emission intensity strongly increas-
esatlowtemperature.Asaresult,practicallythesamefluores-
cence quantum yield values are determined at 173K: 0.27
(BuCN), 0.24 (THF), 0.21 (toluene). At 77K in toluene, no phos-
phorescenceisobserved,contrarytothebehaviourof2.
Very strong temperature dependence was observed for 3 in
BuCN (Figure4). Upon lowering the temperature from 293 to
123K,theintensityincreasesbymorethantwoordersofmag-
nitude. Drastic changes in intensity are observed in a narrow,
lowtemperaturerangebelow140K,thatis,whenbutyronitrile
becomes glassy (Figure4D). These changes are accompanied
by spectacular blue shifts of the emission: upon lowering the
Figure3.Left:absorption;right:fluorescenceof3,3a,and3bin:a)tolu-
temperature by just five degrees, from 128 to 123K, the fluo-
ene,andb)acetonitrileat293K.Theasterisksmarkartefactsintheemission
spectrainacetonitrileduetonon-perfectremovalofsolventRamanlines. rescence maximum shifts from 592 to 544nm (Figure4C).
Below 123K, the quantitative measurements are difficult, due
tofluctuationsofintensityassociatedwithlong-terminstability
formation of complexes with water (cf. the section below de- of butyronitrile glass. However, it can be safely concluded that
votedtothespectrainproticsolvents). both intensity and shape offluorescence do not change much
The boranils 3a and 3b exhibit single emission in toluene, in the low temperature region. The low temperature emission
but in acetonitrile a second, low intensity band corresponding consists of one band with maximum at 537nm. This band is
toF isobserved.ItslocationissimilartothatofF emissionof barelyvisibleat293K,wherethefluorescenceisdominatedby
2 2
3. abandwithmaximumaround650nm.
In a similar fashion as for 2a, huge differences in quantum The above results demonstrate the influence of viscosity on
yieldsandlifetimesareobservedfor3aand3bintolueneand the photophysics of 3 in a polar solvent, suggesting a confor-
acetonitrile. In the latter fluorescence intensity drops by over mational change involving a large amplitude motion and
twoordersofmagnitude. ahighdipolemomentoftheexcitedspecies.
Theemissionbehaviourof3suggestsadifferentpathwayof The temperature dependence of the emission of boranil de-
S deactivationthanin1and2.Theobservedhugeshiftofthe rivative 3a in butyronitrile (FigureS3 in the Supporting Infor-
1
emission maximum with changing solvent polarity points to mation)issimilartothatobserved for3.Atroomtemperature,
achargetransfercharacteroftheexcitedstate.Moreover,con- weak dual fluorescence is observed, with the maxima located
trarytothecaseoffluoroandcyanoderivatives,dualemission at around 490 and 630nm. The intensity and position of the
isobservedfor3aand3b,analogoustothatof3.Oneshould lower energy band strongly depend on temperature: at 143K,
note, however, very different F/F intensity ratios observed for the intensity increases by a factor of twenty with respect to
1 2
the anil and boranils, suggesting that the photophysical prop- the value measured at 296K. Similarly to 3, the shape of the
erties are sensitive tothepresence of intramolecular hydrogen emission changes dramatically in a narrow temperature range
bonding, which may lead to geometry changes or to different between138 and 133K. (FigureS3C). Further decrease of tem-
relativepositionsoflow-lyingelectronicstates. peraturedoesnotleadtosignificantchanges;thefluorescence
consists of a single band with a maximum at 487nm, very
closetothatofF fluorescenceatroomtemperature.
Temperaturedependenceoftheemission 1
Comparisonoftheemissionpropertiesof3and3aindicates
Stationary fluorescence spectra were recorded as a function of that in both molecules the initially excited structure is trans-
temperatureinanumberofsolvents.Fortoluene,THF,andbu- formed into a strongly polar transient species that undergoes
tyronitrile solutions of 2, similar changes in the F/F intensity aviscosity-dependentrelaxationprocess.
1 2
ratio are observed (FigureS1 in the Supporting Information):
theintensity ofbothbandsincreasesaboutfivetimesatlower
Proticsolventsolutions
temperature.Interestingly,theF/F ratioinitiallydecreasesand
1 2
then increases upon lowering of temperature; at 294 and We have also recorded absorption and fluorescence in protic
180K this ratio is practically the same for all three solvents. At solvents, methanol, and n-propanol. While the absorption
lowertemperaturesinbutyronitriletheF fluorescenceintensi- spectra resemble those obtained in other solvents, the emis-
1
ty significantly increases with respect to F. At 77K, phosphor- sion profile is quite different. Indeed, multiple bands are ob-
2
escence is observed: for toluene, its intensity is similar to that served with relative intensities dependent on the excitation
ofF,whereasinbutyronitrileitisaboutthree-timesweaker. wavelength. This behaviour may have several origins. The first
1
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1315 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
In contrast to the behaviour
of boranils, the F decay in the
2
anils 1 and 2 is insensitive or
only weakly dependent on the
nature of the solvent. This is not
the case for the nitro derivative
3, which exhibits shorter decay
inapolarenvironment.
In order to probe shorter fluo-
rescence decays and to monitor
time-resolved spectra, the tech-
niques of fluorescence upcon-
version (FU) and transient ab-
sorption (TA) were used. Exem-
plary time-wavelength maps ob-
tained using FU are shown in
Figure5, whereas Figure6 pres-
ents the time evolution of fluo-
rescence of 2, 3, and 3a. Table1
summarizes the decay parame-
ters obtained for all compounds
in various solvents. The decays
are biexponential, the shorter
Figure4.A–B)Temperaturedependenceoffluorescenceof3inbutyronitrile.C)Fluorescencespectrarecordedat
296,128,123,andat77K.D)Integratedfluorescenceintensityrelativetothevalueat296K;blackandbluesym- occurs in the subpicosecond
bolsindicatemeasurementsondifferentspectrofluorimeters(JasnyandEdinburgh,respectively). range.Thedecaytimevaluesob-
tained by fluorescence upcon-
version correspond well to the
is trivial for boranils: they can be transformed back to anils by kinetic parameters extracted from transientabsorption studies.
boron decomplexation. Indeed, we have observed that, for in- The components of the dual emission of the anils 1 and 2
stance, 1b is rapidly converted to 1. The other source of com- reveal a characteristic kinetic pattern. The decay of F fluores-
1
plicated spectra may be due to the formation of hydrogen- cence,occurringinafewhundredfemtosecondstoafewpico-
bonded complexes with alcohols. This should, in particular, seconds, is definitely shorter in nonpolar solvents. This finding
affect the spectra and photophysics of anils if the alcohol is in agreement with stationary measurements which show
breaks the intramolecular OH···N hydrogen bond. The complex that the F/F intensity ratio increases in acetonitrile with re-
1 2
behaviourinalcoholsdefinitelyrequiresfurtherstudies. spect to toluene (Figures1 and 2). The F emission decay is
2
much longer and not strongly solvent-dependent, as already
notedonthebasisofTR-SPCexperiments.Time-resolvedemis-
Kineticstudies sion spectra clearly show the ultrafast decay of F. In order to
1
check whether the precursor–successor relationship exists be-
Fluorescencestudies
tween F and F, we looked for the rise time in the latter that
1 2
Thetemporalresolution ofourTC-SPCsetup,estimatedas20– would correspond to the decay of the former. However, no
30ps, allowed the reliable measurement of the longest com- such rise time could be detected. This result does not exclude
ponents of fluorescence decays (Table1). Nanosecond decay the F emitting state as a precursor of F, but it may indicate
1 2
times are observed in nonpolar environments for the boranils a much lower radiative constant in the latter. Additionally, the
2a, 3a, and 3b, whereas for 1b and, in particular, for 1a the risetime maybemaskedbyspectraloverlap between thetwo
fluorescence decays faster (760 and 120ps, respectively). In emissionbands.
polar solvents, fluorescence lifetimes decrease dramatically For 2, we have also studied the isotope effect. In the OD-
with 1b being the only exception. For the fluoro and cyano deuterated 2, the F decay time in acetonitrile increases from
1
derivatives this behaviour correlates well with the correspond- 2.2 to about 9ps. Deuteration does not affect the F lifetime.
2
ing changes in quantum yields, suggesting that the structure This result strongly suggests that the decay of F is associated
1
responsible for the emission is similar in nonpolar and polar with the OH proton translocation. The definitive argument for
solvents. However, in the nitro anil 3a the ratio of quantum photoinduced tautomerization was provided by time-resolved
yields in toluene and acetonitrile is much larger than that of IRstudies,discussedbelow.
thelifetimes.Thisindicatesasmallervalueoftheradiativecon- The kinetic behaviour of 3 is completely different from that
stant in the species emitting in a polar solvent, a hypothesis observed for the other two anils. Now, both decay compo-
that was corroborated by time-resolved emission experiments nents are shorter in acetonitrile. A similar behaviour, a huge
discussedbelow.
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1316 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
signed to the decay of the ini-
tially excited species via proton
transfer. We note that no recov-
ery of the bleaching is observed
on the short time scale. This
means that proton transfer does
not compete with other fast de-
population channels, which was
postulated for SA.[41,42,48] On the
contrary,thesecondcomponent,
about 200ps, assigned to the
decay of the phototautomer, ap-
pears also at wavelengths corre-
sponding to bleaching, indicat-
ing that the rate of ground state
back proton transfer is faster
than5(cid:4)109s(cid:2)1.
The third decay corresponds
to the lifetime which is too long
Figure5.Mapsoftime-resolvedfluorescenceof:a)2,b)2a,c)3,andd)3a.Thespectrawererecordedat293K
usingacetonitrileassolvent. (>15ns) to be measured accu-
rately with the femtosecond
decreaseofthedecayparametersinpolarsolventsisobserved
for3aand3b.
Inspectionoftime-resolvedemissionspectra(Figure6)clear-
lyshowshigherradiativeratesforF thanforF emissions.This
1 2
is true both for F occurring from a proton-transferred species
2
and in the case when the origin of the low energy emission
mustbedifferent(3a).
Transientabsorptionstudies
Representative examples of femtosecond transient absorption
(TA) spectra are shown in Figures7, 8, 9, 10. Table1 contains
the decay parameters extracted from the spectra. Other exam-
ples of TA, together with decay associated spectra are shown
inFiguresS4–S9intheSupportingInformation.
The TA spectrum of 2 (Figure7 and FigureS6 in the Sup-
porting Information) consists of several bands. The negative
features at 400 and 500nm correspond to ground-state
bleaching and stimulated F emission, respectively. Due to
1
a much lower radiative constant responsible for F, its contri-
2
bution is not observed in the TA signal. The kinetic profiles
measured at 500nm are in excellent agreement with the fluo-
rescence upconversion results (Table1). In particular, the F
1
fluorescence decay times, longer in acetonitrile than in tolu-
ene,arereproduced.
The transient absorption features a strong peak at 450nm
thatoverlapswithbothbleachingandstimulatedfluorescence,
sothatthekineticanalysisisrathercomplex.Itclearlycontains
the slower decay component associated with F (209 and
2 Figure6.Time-resolvedfluorescencespectraof2,3,and3aobtainedbyFU
156ps in toluene and acetonitrile, respectively), but the faster foracetonitrilesolutionsat293K.
component (0.86/2.5ps) is also detected. Thus, the TA in this
region contains contributions from the species responsible for
bothF andF emissions. spectrometer. The observation that the TA signal persists after
1 2
The time evolution of TA, measured in four solvents may in decay of both emissions indicates the presence of a long-lived
each case be fitted with three components. The shortest is as- species. From the magnitude of the bleaching it can be esti-
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1317 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
an ultrafast proton transfer, the
molecule has no time to reveal
the cis-enol fluorescence shift. A
significant difference between 2
and 2a is the lack of any transi-
ent signal in acetonitrile at
100ps delay in 2a. Thus, contra-
ry to the case of the anil ana-
logue, no long-lived species is
nowformed.Thisresultsuggests
that the long-lived transient in 2
is rather due to the photochro-
micform(trans-keto)thantothe
triplet, since the latter should be
alsopopulatedin2a.
Similar results were obtained
for 1 and 1a (FiguresS4–S5 in
the Supporting Information).
Also in this case, the significant
difference between the anil
1 and boranil 1a is that the
former exhibits a long-lived TA
signal (peaking at 420 and
Figure7.TAof2intoluene(left)andacetonitrile(right),recordedatshortandlongdelays(topandbottom,re-
530nm), whereas the latter does
spectively).Dottedlines:invertedstationaryabsorptionandemission.
not: its decay corresponds to
thatoffluorescence.
mated that about 50% of the initially excited population does The TA behaviour of 3 and 3a is quite complicated (Fig-
notreturntotheinitialgroundstateformonthetimescaleof ures9 and 10, FiguresS8–S9 in the Supporting Information).
at least 15ns. The long-lived species, with the absorption For 3 in toluene, the strong TA band at 459nm evolves into
peaks at 460 and 580nm, may correspond to the photochro- a band with a maximum at 481nm within a few picoseconds.
micproductsand/ortothetripletstate. It should be stressed that this evolution corresponds not to
TheTAspectraof2a(Figure8andFigureS7intheSupport- a band shift, but to the decay of the high energy feature and
ing Information) exhibit a longer
component that can be correlat-
ed with the decay time of fluo-
rescence. Its value drastically de-
creases with solvent polarity, in
parallel with quantum yield
(Table1). In contrast to the be-
haviour of 2, evolution of the
transient absorption and stimu-
lated emission signals is ob-
served on the short time scale.
The kinetic profiles in this range
can be fitted to either mono- or
biexponential decay functions.
Weattribute this evolution to vi-
brational relaxation and to the
stabilization of a large S dipole
1
moment by a polar solvent. The
calculations predict a value of
12.6–15.9D (depending on the
method,seeTableS1intheSup-
porting Information) for the ini-
tially excited 2a. Also for 2, the
S thedipolemomentiscalculat-
1 Figure8.TAof2aintoluene(left)andacetonitrile(right),recordedatshortandlongdelays(topandbottom,re-
ed to be large, but, because of spectively).Dottedlines:invertedstationaryabsorptionandemission.
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1318 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
formation of the lower energy
one. The band at 481nm and
the stimulated emission at
542nmdecayin260ps,whichis
the fluorescence decay time
measured by TC-SPC. Interest-
ingly,theintensityofthebleach-
ing signal does not change
much during this period. It re-
mains practically constant up to
1300ps, the longest delay used,
during which time, a build-up of
TA absorption in the region
above 550nm is observed. An
analogous pattern, interconver-
sion of two TA bands, has been
observed for ethyl ether, THF,
and butyronitrile solutions. The
difference with respect to tolu-
ene is that a significant time
shift of the stimulated emission
maximum is now observed, its
value reaching 551nm in di-
ethylether, 579nm in THF, and
700nm in butyronitrile. Even
a larger shift, to 727nm, is ob-
served for acetonitrile (Figure9).
Inthissolvent,onlyoneTAband
is observed, shifting from
487nm, observed immediately
after excitation, to 482nm at
longer delays. A shoulder at
higher energies is detected at
early delays, suggesting that the
initially excited species decays in
acetonitrile faster than in other
solvents. In contrast to anils
Figure9.TAof3intoluene(left)andacetonitrile(right)forselectedtimewindows.Dottedlines:invertedstation-
1 and 2, no long-lived transient
aryabsorptionandemission.
could be observed for both ace-
tonitrile and butyronitrile solu-
tions.
The boranil derivative 3a (Figure10 and FigureS9 in the FemtosecondtransientIRstudies
Supporting Information) exhibits a behaviour similar to 3 re-
garding the temporal evolution of the stimulated emission, Figure11showstheIRtransientspectraof1takenafterexcita-
but some differences are observed in the region of the TA tion at 400nm. At short time delays, the transient spectra are
band. These differences are difficult to quantify because of dominated by four ground-state bleach bands at 1510, 1585,
strong overlap of absorption and fluorescence. Another com- 1607, and 1635cm(cid:2)1, and a positive band at 1545cm(cid:2)1. This
plicationisintroducedbyverydifferentfluorescenceintensities latter band decays completely within about 150ps, whereas
of3and3ainnonpolarsolvents. the bleach bands recover only partially, and new positive
The analysis of the strong negative signal due to stimulated bands at about 1500, 1600 and 1640cm(cid:2)1appear.Fromabout
fluorescence in acetonitrile yielded the kinetic parameters of 150ps onward, the spectra remain essentially constant up to
0.6 and 0.7ps for 3 and 3a, respectively. These values resem- 1.8ns, the upper limit of the experimental time-window.
ble the relaxation time of acetonitrile,[49] suggesting that the Global analysis of the transient IR absorption data was per-
emission initially occurs from a non-relaxed state and that its formed using the sum of three exponential functions, and
decayissolventcontrolled. yielded 2.9ps, 45ps and >10ns time constants and the
decay-associated spectra shown in FigureS10 in the Support-
ing Information. The two longest time constants values are in
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1319 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
1630cm(cid:2)1 and positive transient
bands at 1530, 1550 and
1595cm(cid:2)1,growing within a few
picoseconds. In comparison with
1, where the transient band at
1545cm(cid:2)1 is present within the
time resolution of the experi-
ment, the signal, which is as-
signed to intramolecular proton
transfer, rises more slowly in the
cyano derivative 2. Again, this
finding is in good agreement
with the results of fluorescence
upconversion and transient ab-
sorptionexperiments.
Target analysis using the same
scheme as for 1 was performed
for 2 (FigureS12 in the Support-
ing Information), providing
a time constant of 4ps, compa-
rable to 2.2/2.5ps extracted
from fluorescence upconversion
and transient-absorption studies.
In this case, speciesA and B can
be ascribed to the excited enol
andketoformsof2,respectively.
Quantum chemical calcula-
tions were performed to assign
the transient bands observed in
the time-resolved IR experi-
ments. Figure13 shows the
comparisonofthecalculateddif-
ference of IR spectra of the keto
and enol forms of 1 with the ex-
perimental difference spectra.
After excitation, positive bands
Figure10.TAof3aintoluene(left)andacetonitrile(right)forselectedtimewindows.Dottedlines:invertedsta-
tionaryabsorptionandemission. due to the proton transfer are
expected. The growth of the
good agreement with those extracted from the transient visi-
ble absorption experiments performed for the same molecule.
However, the 2.9ps lifetime is much longer than that of 0.41–
0.45ps obtained from the FU and transient visible absorption
measurements. The transient spectra recorded before 500 fs
havenotbeenincluded intheglobalanalysisoftheIRdatato
avoid the complications related to cross-phase modulation
and, thus, this ultrafast component, associated with the ESIPT
cannot be detected. To better ascribe the 2.9ps time constant
a global target analysis assuming an A!B!C!D reaction
scheme has been carried out. Such analysis yielded the same
time constants, as expected, and the species-associated differ-
ence spectra shown in FigureS11 in the Supporting Informa-
tion. The difference spectra associated with A and B are very Figure11.Upperpanel:transientIRspectrameasuredfor1inacetonitrileat
similarand,therefore,thesespeciescanbeascribedtotheun- differentdelaysafterexcitationat400nm.Thedashedlineistheinverted
andscaledstationaryFTIRspectrum.Lowerpanel:calculatedsteadystateIR
relaxedandrelaxedESIPTproductformsof1,respectively.
spectraofenolandketoforms(redandblue,respectively)andtheirdiffer-
The same measurements were repeated for 2. The spectra
ence(green).Forabettercomparisonthecalculatedspectrawerescaled
(Figure12) show three negative bands at 1510, 1570, and andflipped.
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1320 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
Full Paper
Calculations
Ground- and excited-state geometry optimizations were car-
ried out in order to determine the structure of the emitting
species and the nature of processes responsible for substitu-
ent- and solvent-dependent photophysics. To simplify and
speed up the calculations, dimethylamino substituent was
used instead of diethylamino group. In addition to optimizing
the push–pull structures of all the molecules under study, we
alsocomputedandanalysedtheparentmolecule,SA,andfour
model compounds, selected to separately assess the role of
electron-donating and -accepting substituents. These included
SA singly substituted with the dimethylamino group on the
Figure12.Upperpanel:transientIRspectrameasuredfor2inacetonitrileat phenolring(SADMA)andSAwithfluorine(SAF),cyano(SACN),
differentdelays[ps]afterexcitationat400nm.Thedashedlineistheinvert-
and nitro (SANO) groups on the aniline moiety (FigureS15 in
edFTIRspectrum.Lowerpanel:calculatedgroundstateFTIRspectraforthe 2
enolandketoforms(redandblue,respectively),thedifference(green),and the Supporting Information). The results are presented in
theexperimentalspectrumrecordedafter6ps(black).Thenumberscorre- TableS1 in the Supporting Information. Three different func-
spondtothemodesdepictedinFigureS13intheSupportingInformation. tionals were tried: 1)B3LYP; 2)PBE0, which is known to yield
accurate electronic transition energies, and 3)CAM-B3LYP, rec-
band at 1600cm(cid:2)1 in the experimental data coincides with ommendedfortreatmentofcharge-transferstates.
that calculated for the keto form. A positive band is also ex- In general, no significant differences were obtained for
pected around 1540cm(cid:2)1. The modes calculated for 2 in the ground state optimizations using different functionals. The en-
region between 1500 and 1700cm(cid:2)1 (FigureS13 in the Sup- ergies of the lowest singlet electronic transitions were repro-
porting Information) involve the OH or NH bending and the duced with similar accuracies by B3LYP and PBE0, whereas
C7H and CH bending in the aromatic rings where an influence using the CAM-B3LYP functional yielded overestimated ener-
canbeexpectedwhentheprotontransferoccurs. gies. For this reason, most calculation were done using B3LYP;
this also ensured compatibility with previous theoretical stud-
ies.[41,42,50] We also checked that double and triple zeta basis
sets yielded very similar results, as can be seen from TableS1
intheSupportingInformation.
Very similar geometries of the dominant, ground state cis-
enol form have been obtained for all the analysed molecules.
Due to the stabilizing effect of the intramolecular hydrogen
bond in anils or boron in boranils, the quasi-six-member ring
assumes a nearly planar geometry. On the other hand, consid-
erable twisting about the N1C6 bond is predicted, with the
values of the anglea (Scheme2) in the range of 30–40 de-
grees. Thesepredictionsare in goodagreement withtheX-ray
structureofSA.[51]Thecalculatedgroundstatedipolemoments
increase from about2D in SA and SAF, to over 3D in SADMA,
and over 5D in SACN and SANO. A spectacular push–pull
2
effect, leading to a considerable increase of the dipole
moment is observed for both anils and boranils. Calculations
systematically yield larger ground state dipole moments for
Figure13.Black:differenceofthecalculated(B3LYP/6-31+G(d,p))steady-
boranilsthanforthecorrespondinganils.
stateIRspectrafortheenolandketoformsof1.Grey:evolutionassociated
differencespectra(EADS)ofspeciesAobtainedfromtargetanalysis. As already reported for some boranils,[52] the calculations
predictthatelectronicexcitationtoS resultsintheincreaseof
1
Time-resolved IR measurements were also performed on the dipole moment. This effect is stronger for anils; no significant
nitro substituted anil 3 (FigureS14 in the Supporting Informa- change is obtained for boranils 1a and 1b. For the cyano de-
tion). All the spectral changes end after a few picoseconds. rivatives,analmosttwofoldincreaseispredictedfortheanil2,
The decay of the excited state is slightly faster than the recov- whereasthechangein2aissmaller(oneshouldnote,though,
ery of the ground state, probably because of the involvement that the S value is larger for the boranil). A spectacular in-
0
of a hot ground state, as testified by the positive feature on creaseisobtained forthenitroderivatives3and3a,forwhich
the low frequency side of the bleach. The intensity of the sig- thevaluesofnearly30DarecalculatedfortheFranck–Condon
nals is lower than for the other compounds and the quality of (FC) geometry corresponding to S. Because the dipole mo-
0
theexperimentaldatadidnotallowcarrying outatargetanal- ments calculated for the optimized excited cis-keto forms are
ysisusingalltheregions. lower, one can expect that the enol form will be stabilized
Chem.Eur.J.2015,21,1312–1327 www.chemeurj.org 1321 (cid:3)2015Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim