Journal Article•DOI•

Tuning Singlet Fission in π-Bridge-π Chromophores

Q: What are the contributions in "Tuning singlet fission in pi-bridge-pi chromophores" ?

A recent computational study suggests that singlet fission occurs by a direct mechanism in bipentacene, in contrast to the charge transfer mediated ( stepwise ) mechanism widely perceived to be dominant in intermolecular singlet fission of crystalline pentacene. One of the key predictions from this study was that very weak chromophore-chromophore coupling could permit ultrafast singlet fission. In this manuscript, the authors investigate how homoconjugated and non-conjugated bridging units affect the excited state dynamics of pentacene dimers. The chromophores shown in Figure 1 were designed as follows: Finally, the authors use a bicyclooctane spacer ( BCO ) to mimic the distance imposed by the conjugated phenylene spacer in BP1, which has been previously reported. The redshift in the high-energy features in BP1 is potentially due to greater interactions between the chromophores through a conjugated linker.

Q: What is the key prediction from this study?

One of the key predictions from this study was that very weak chromophore-chromophore coupling could permit ultrafast singlet fission.

Q: What is the reason for the redshift in the high-energy features in BP1?

The redshift in the high-energy features in BP1 is potentially due to greater interactions between the chromophores through a conjugated linker.

Q: What is the way to add a singlet pair to a solid-state?

In the solid-state, through-bond singlet fission can be complemented by through-space singlet fission, adding an additional SF channel.

Q: What is the first family of materials with fast rates of iSF?

While it has been observed in twisted dimers that reducing the coupling between pentacene chromophores preferentially extends the triplet pair lifetime,35 this is the first family of materials with fast rates of iSF, and drastically different rates of triplet pair recombination.

Q: What is the recent study suggesting that singlet fission occurs by a direct mechanism?

A recent computational study suggests that singlet fission occurs by a direct mechanism in bipentacene, in contrast to the charge transfer mediated (stepwise) mechanism widely perceived to be dominant in intermolecular singlet fission of crystalline pentacene.

Q: How did Liu et al. design tetracene trimer?

12,33 Recently, Liu et al. designed tetracene trimer through linear oligomerization which resulted in greatly enhanced iSF yield (96%) relative to a similar dimer.

Q: How long does iSF take to recombinate a triplet pair?

Despite the faster time constants for triplet pair generation as compared to conjugated BP1, triplet pair recombination is > 10 times slower in EBD.

Elango Kumarasamy, Samuel N. Sanders, Murad J. Y. Tayebjee¹, Amir Asadpoordarvish, Timothy J. H. Hele¹, Timothy J. H. Hele², Eric G. Fuemmeler², Andrew B. Pun, Lauren M. Yablon, Jonathan Z. Low, Daniel W. Paley, Jacob C. Dean³, Bonnie Choi, Gregory D. Scholes³, Michael L. Steigerwald, Nandini Ananth², Dane R. McCamey, Matthew Y. Sfeir⁴, Luis M. Campos - Show less +15 more•Institutions (4)

University of Cambridge¹, Cornell University², Princeton University³, Center for Functional Nanomaterials⁴

01 Sep 2017-Journal of the American Chemical Society (American Chemical Society)-Vol. 139, Iss: 36, pp 12488-12494

TL;DR: Using transient absorption spectroscopy to investigate photophysics in pentacene dimers, it is found that homoconjugated dimer display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates.

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Abstract: We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.

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Summary (1 min read)

Jump to: [KEYWORDS: Singlet Fission; Homoconjugated Dimers; Non-Conjugated, Organic Electronics, Pentacene Dimers, Electron Spin Resonance.] – [BP1] – [ASSOCIATED CONTENT] and [ACKNOWLEDGMENT]

KEYWORDS: Singlet Fission; Homoconjugated Dimers; Non-Conjugated, Organic Electronics, Pentacene Dimers, Electron Spin Resonance.

ABSTRACT: We have designed a series of pentacene dimers separated by homoconjugated or non-conjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF).the authors.
31,32 Given the sensitivity of SF to structure, it remains unclear how through-bond interactions promote fast and efficient singlet fission, especially focusing on the most basic pentacene dimer model.
In homoconjugated dimers, the two pentacene chromophores are separated by a saturated sp3 carbon, thus the pentacenepentacene coupling and/or electron delocalization is expected to be weaker than in conjugated systems (such as BP1, Figure 1).
The authors postulate that this π-sigma-π bonding scheme will make the excited state dynamics much less sensitive to subtle variations in the geometry of the bridge as compared to analogous conjugated dimers.
11 Complete details of their synthesis and characterization are given in the supporting information.

BP1

11 From inspection, the authors clearly see that the electronic coupling between the chromophores is significantly affected by through-bond interactions; singlet fission in BP1, where pentacenes are connected through four sp2 hybridized carbons, has a time constant similar to homoconjugated dimers, where pentacene chromophores are separated by just one sp3 hybridized carbon.
The authors establish that singlet fission is operative, producing triplet pairs as opposed to free triplet generation by intersystem crossing, by correlating transient absorption and electron spin resonance studies (Figure 5).
Like other iSF dimers, transient absorption studies show that the triplet population decays biexponentially, indicating the presence of triplet pairs (TT) with an enhanced recombination rate (Figure 6) and a minority population of free triplets decay with the expected rate for an individual triplet.
The biexponential dynamics in the TA experiments were further probed by transient electronic spin resonance (tr-ESR) measurements.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.
Experimental methods, including details of the transient absorption spectroscopy and photosensitization experiments, synthetic details, electron spin resonance experiments and characterization of compounds used in this study.
AUTHOR INFORMATION Corresponding Author *dane.mccamey@unsw.edu.au *msfeir@bnl.gov *lcampos@columbia.edu.

ACKNOWLEDGMENT

L.M.C. acknowledges support from the Office of Naval Research Young Investigator Program (Award N00014-15-12532) and Cottrell Scholar Award.
This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven Na- tional Laboratory under Contract No. DE-SC0012704.
This work used the Extreme Science and Engineering Discovery Environment , which is supported by National Science Foundation grant number ACI-1548562.
MJYT acknowledges receipt of an ARENA Postdoctoral Fellowship and a Marie Curie Individual Fellowship.

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Figures (6)

Figure 3. Transient absorption spectroscopy in dilute chloroform solution with 600 nm excitation (~25 µJ/cm2) reveals evolution of the photoexcited singlet into triplets with singlet fission and triplet pair recombination rates that depend strongly dependent on the properties of the bridge. In the structures of the bridges, red color indicates the connectivity of pentacene units, and the color scales have been normalized to facilitate comparison and are therefore reported in arbitrary units (a.u.). Data prior to ~2.7 ns is collected using a mechanical delay, while the same pump pulse and an electronically controlled probe were used to generate data after ~2.7 ns.

Table 1: Time constants for: rate of iSF (τ iSF), triplet pair lifetimes (τ [TT]), and individual triplet decay (τ [T], obtained from slower triplet decay component).

Figure 4. Normalized kinetics monitored at the maximum of the triplet photoinduced absorption of TFM, EBD, Spi, and BCO as dilute solutions in chloroform following 600 nm excitation (~25 µJ/cm2).

Figure 5: Top: Transient absorption kinetics near the triplet absorption maximum, with arrows indicating times selected for ESR spectra. Bottom: Transient ESR spectra of Spi and BCO in toluene at given time delays after laser excitation at 599 nm, ~70 µJ/pulse. Dashed lines mark locations of (TT)0 and T0 transition resonances.

Figure 1. The pentacene-bridge-pentacene model showing the comparison between different bridging units. In the bottom representations, the pentacenes are omitted to highlight the nature of the bridging units.

Figure 2. Top: UV-Visible absorption spectra with TIPSpentacene and BP1 included for reference. Bottom: Calcu-

Content maybe subject to copyright Report

BNL-114743-2017-JA

Tuning Singlet Fission in Pi-Bridge-Pi Chromophores

E. Kumarasamy, M. Y. Sfeir

Submitted to the Journal of the American Chemical Society

September 2017

Center for Functional Nanomaterials

Brookhaven National Laboratory

U.S. Department of Energy

USDOE Office of Science (SC),

Basic Energy Sciences (SC-22)

Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under

Contract No. DE- SC0012704 with the U.S. Department of Energy. The publisher by accepting the

manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,

irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to

do so, for United States Government purposes.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the

United States Government. Neither the United States Government nor any

agency thereof, nor any of their employees, nor any of their contractors,

subcontractors, or their employees, makes any warranty, express or implied, or

assumes any legal liability or responsibility for the accuracy, completeness, or any

third party’s use or the results of such use of any information, apparatus, product,

or process disclosed, or represents that its use would not infringe privately owned

rights. Reference herein to any specific commercial product, process, or service

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constitute or imply its endorsement, recommendation, or favoring by the United

States Government or any agency thereof or its contractors or subcontractors.

The views and opinions of authors expressed herein do not necessarily state or

reflect those of the United States Government or any agency thereof.

Tuning&Singlet&Fission&in&Pi-Bridge-Pi&Chromophores&

Elango'Kumarasamy,

1,' ‡

'Samuel'N.' Sanders,

1,' ‡

'Murad'J.' Y.'Tayebjee,

2,3

'Amir'Asadpoordarvish,

Timothy'J.'H.'Hele,

5,6

'Eric'G.'Fuemmeler,

'Andrew'B.'Pun,

'Lauren'M.'Yablon,

'Jonathan'Z.'Low,

Daniel'W.'Paley,

1,7

'Jacob'C.'Dean,

'Bonnie'Choi,

'Gregory'D.'Scholes,

'Michael'L.'Steigerwald,

Nandini'Ananth,

'Dane'R.'McCamey,*

'Matthew'Y.'Sfeir,*

and'Luis'M.'Campos*

Department'of'Chemistry,'Columbia'University,'New'York,'New'York'10027,'USA'

School'of'Photovoltaic'and'Renewable'Energy'Engineering,'University'of'New'South'Wales,'Sydney'NSW'2052,'

Australia'

Cavendish'Laboratory,'University'of'Cambridge,'J.J.'Thom s on 'A v en u e ,'Cambridge'CB3'0HE,'UK'

ARC'Centre'of'Excellence'in'Exciton'Science,'School'of'Physics,'University'of'New'South'Wales,'Sydney,'NSW'

2052,'Australia'

Department'of'Chemistry'and'Chemical'Biology,'Cornell'University,'Ithaca,'NY,'14850,'USA'

On'intermission'from'Jesus'College,'Cambridge'University,'UK'

Columbia

Nano'Initiative,'Columbia'University,'New'York,'NY,'10027,'USA'

Department'of'Chemistry,'Princeton'University,'Princeton,'NJ'08544,'USA'

Center'for'Functional'Nanomaterials,'Brookhaven'National,'Laboratory,'Upton,'New'York'11973,'USA'

KEYWORDS:*Singlet*Fission;*Homoconjugated*Dimers;*Non-Conjugated,*Organic*Electronics,*Pentacene*Dimers,*

Electron*Spin*Resonance.

!"#$%!&$''We'have'designed'a'series'of'pentacene'dimers'separated'by'homoconjugated'or'non-conjugated' bridges'

that'e xhib it'fast'and'efficient'intr amolecular'singlet'exciton'fission'(iSF).'These'materials'are'distinctive' am ong'reported'

iSF'compou nds'b ecause'they'exist'in'the'unexplored'regime'of'close'spatial'proximity'but'weak'electronic'coupling'be-

tween'th e'singlet'exciton'and 'triplet'pair'states.'Using'transient'absorption'spe ctros cop y'to'investigate'p ho toph ysic s'in'

these'm olecu les,'we'fin d 'that'h o moconjuga te d 'dimers'display' d e si ra b le'excited'state'd y n a mics,'with'significantly'reduced'

recombination'rates'as'comp ared'to'conjugated'dimers'with'similar'singlet'fission'rates.'In'addition,'unlike'conjugated'

dimers,'the'time'constants' for'singlet' fission' are'relatively'insensitive'to' the'interplanar'angle'between'chromophores,'

since'rotation'about'σ'bonds'negligibly'affects'the'orbital'overlap'within'the'π-bonding'network.'In'the'non-conjugated'

dimer,'where'the'iSF' occurs'with'a'time'constant'>'10'ns,'comparable'to' the'fluorescenc e'lifetime,'we'used 'electron'spin'

resonance' spectroscopy' to' u neq uivocally' establish' the' formation' of' triplet-triplet' multiexcitons ' and' uncoupled' triplet'

exciton'through'singlet'fission.'Together,'these'studies'enable'us'to'articulate'the'role'of'the'conjugation'motif'in'iSF.'

()*+,-./*0,):'Understanding'the'fundamental'dy-

namics'of'singlet'fission'(S F)'chromophores'with'idea l-

ized' properties' for' next-generation' optoelectronic' de-

vices' fuels' the ' developmen t' of' families' o f' new' materi-

als.

1-6

'This'includes'the'important'discovery'of'intramo-

lecular' singlet' fission' p o lymers' and' oligomers,' where'

chromophore-chromophore'interactions'occur'prima rily '

through' covalent' bonds.

7-23

' Several' different' bonding'

connectivity' schemes' have' been' demonstrated' to' acti-

vate' singlet' fission,' where' the' triplet' gene ration ' an d '

decay'kinetics'have'been' shown'to'be' highly' sensitive' to'

the' manner' in' which' neighboring' chromophores' are'

linked.' The' interaction' of' chromophores' in' iSF' com-

pounds'differs'significantly'from' the ' through-space' in-

teractions' prim a rily' fou n d' in'molecular' crystals,' w here'

the' through-sp ace' co upling ' betw een' th e' chromophore'

coupling'is'greatly'influenc ed 'by'the'morph o lo gy .

24-26,28-

'For' instan ce,'multiexponential' sing let'fission'rates'are'

observed'in'disordered'systems'when'compared'to'mo-

noexponentional'kinetics'in'iSF'systems.

31,32

Given'the' sensitivity' of' SF' to' structure,' it' remains'

unclear' how' through-bond' interactions' promote' fast'

and' efficient' singlet' fission,' e sp e ci ally ' focusing' on' th e '

most' basic' pentacene' dimer' model.' To' date,' several'

groups' have' reported' that' conn ecting' two' pentacenes'

using' conjugated' bridges' can' b e' used' to' promote' effi-

cient' singlet' fission,' even' when'the' proximity'between'

the' pentacen es' is' significantly' decreased.

9,10,13,14

' For' ex-

ample,'when'the' bridge'shown' in'Figure'1' is' varied'from'

one' to' three' phenylene' units,' we' have' fo u n d' that' the'

rates'of'singlet'fission'and'triplet'pair'recombination'in'

pentacene'dimers'are'drastically'affected.'The'longer'the'

conjugated'bridge,'the'slower'the'rate'of'iSF'and'triplet'

pair'recombination.

'Similarly,'Zirzlmeier'et'al.' reported'

pentacene' dimers' that' wer e' connected' through' 6-

position' by' o-,' m-,' p-diethynylbenzene' spacers,' which'

revealed'that'throu gh-space'and'through-bond'interac-

tions' play' crucial' role' in' singlet' fiss ion ' and' triplet' re-

combination'dynamics.'They'also'found'that'faster'sin-

glet'fission'was'accompanied' by' faster'triplet'recombi-

nation.

'Further,'orthogonally'connected' dimers'report-

ed'by'Lukman'et'al.' resulted' in'ultrafast' singlet' fission'

and'w ere'particularly'sensitive'to'the' p olarity'of'the' m e-

dium.

12,33

' Recently,' Liu' et' al.' designed' tetracene' trimer'

through'linear'oligomeriza tion'which' resulted'in'greatly'

enhanced' iSF' yield ' (96%)' relative' to' a' sim ilar' dimer.'

This'SF' enhancement'was'attributed' to' singlet' exciton'

delocalization.

'These'studies'all'suggest'that'conjug a-

tion'plays'a'sign ifican t'ro le'in 'driving'singlet'fission'and '

triplet' recombination.' Such' con clusion' is' further' sup-

ported'by'the'fact'that'singlet'fission'is'slower'in 'twisted'

dimers' th at' lack' bridging' units,' where' conju g a tio n ' is'

decreased'due'to'reduced'overlap'of'the'pi'orbitals.

102.+3456'The'pentacene-bridge-pentacene'model'showing'

the' comparison' between' differ en t' bridging' units.' In' the'

bottom' representations,' the' pentacenes' are' omitted' to'

highlight'the'nature'of'the'bridging'units.'

Interestingly,'many'of'aforementioned'studies'offer'

hints'that'conjuga tion'may'not'be'strictly'necessary'for'

singlet' fission' in' pen ta ce ne -bridge-pentacene' chrom o-

phores.'A'recent'computational'study'sugg ests'that'sin-

glet' fission' occurs' by' a' direct' mechanism' in' bipenta-

cene,'in'contrast'to'the'charge 'transfer'me d ia te d '(step-

wise)' mechanism' widely' perceived' to' be' dominant' in'

intermole c u lar ' singlet' fission' of'crystalline'pentacene.

One'of'the'key'predictions'from'this'study'was'that'very'

weak'chromophore-chromophore'coupling'could'permit'

ultrafast'singlet'fission.'The'process'is' viable' through' an'

avoided' crossing,' when' resonance' between' the' singlet'

exciton' and' triplet' pair' states' is' rea ched ' through' a' vi-

brational' mode.' However,' the' u ltimate' limits ' o f' th is '

hypothesis'have'not'yet'been'tested,'i.e.,'it'is'unkno w n'

what'happens'in'the'excited' state' when' both'through-

space' and' throu gh-bond' interactions' are' extremely'

weak.'

In'this'manuscript,'we'inves tigate 'how'homoconju-

gated'and' non-conjugated' bridging' units' affect'the' ex-

cited' state' dynamics' of' pentacene' dimers.'We' particu-

larly' foc u s' on' un d er sta tin g ' how' sin gle t' fission' an d ' tri-

plet' pair' recombination' behave' in' the' limit' of' weakly'

coupled'pentacene'dimers'(Figure'1).'In'homoconjugat-

ed' dimers,' the' two' pentacene' chromophores' are' sepa-

rated' by' a' saturated' sp

' carbon,' thus' the' pentacene-

pentacene' coupling' and/or' electron' delocalization' is'

expected' to'be'weaker'than'in' co nju gate d'system s '(such'

as' "75,' Figure' 1).

' We' postulate' that' this' π-sigma-π'

bonding' scheme' will' make' the' excited' state' dynamics'

much'less'sensitive'to'sub tle'variations'in'the'geometry'

of'the'bridge'as' com pared' to' analogous'conjugated'di-

mers.

' Additionally,' th ese' systems' are' su ited' to' in tro-

duce' more' than' one* sp

' carbon' in' the' bridge' to' yield'

non-conjugated' dimers,' thus' allowing' us' to' probe' the'

limits'of'wea k 'co u p lin g 'in te ra ctions.'

%38.9*84:)-4;08/.880,):'

Materials*Design:'The'chromophores'shown'in'Fig-

ure'1'were'designed'as'follows:'In'both'the'ethanoben-

zo[b]decacence' derivative' ( <" ;)

20-22

' and' the' spiro-

bi[cyclopenta[b]pentacene]' derivative' (#=0)' the' penta-

cenes' are' loc k ed ' in' a' rigid' fashion.' The' pentacen e s ' in'

<";4are'm ore'planar'than'in'#=0,'whe re 'th e'tw o 'chro-

mophores' are' nearly' orthogonal' to' each' other.' In' the'

bistrifluoromethyl'derivative'($1>),'the'two'pentacene'

units' are' connected' by' single' satura ted ' carbon,' giving'

the' pen tac ene s' some' freedom ' to' rotate' relative' to' on e'

another.'Finally,'we'use'a'bicyclooctane'spacer'("&? )'to'

mimic' the' distance' imposed' by' the' conju ga ted' phe-

nylene' spacer'in'"75,'which 'has'be e n 'previously'report-

ed.

'Complete'details'of'their'synthesis'and'characteri-

zation'are'given'in'the'supporting'information.'

Steady-State*Optical*Properties.'

The' UV-visible'ab sorption'spectra'of'the'dimers' are'

shown' in' Figure ' 2, ' and'are' plotted' alongside'the' spec-

trum'of'TIP S-pentacene'for'comparison.'In'all'cases'the'

dimer'spectra' are' qualitatively' similar ' to' the' m o n omer'

spectra,' allowin g'us'to' assign 'dimer'transitions'from'the'

known'spectrum'of'pentacene.

'The'absorption'around'

650' nm ' corresponds' to' an' intra-monomer' HOMO' to'

LUMO' excitation' polarized' along' the' short-axis' of' the'

monomer,'and' is'slightly'red-shifted'in'#=0'(by'~12'nm),'

possibly'due' to' a' small'interaction'between' pi' systems'

of'the'monomers.'

The' intense' absorption' in' the' UV' around' 310' nm'

corresponds' to' a'long-axis'polarized' transition'and ' the'

weak'absorption'aroun d'440'nm'to'an' almost-forbid d en '

long-axis'transition.'Both'the'650'nm'and'440'nm'tran-

sitions' are' accompanied' by' vibrational' stretching' pro-

gressions'commonly'seen'in'acene'spectra.'Some'dimers'

exhibit'a'splitting'of' the' 310'nm'absorption,'whereby' th e'

dipole' mom ents' along' the' long-axis' of' each' m onom er'

can' combine' in-phase' or' out-of-phase,' giving' two' dis-

tinct'absorptions.'This'is'clearest'when'the'long-axes'of'

the'mon om e rs' are'app roximately'90°'apart'(as'for'#=0)'

and'absent'if' the'long-axes'are'in' the'same'(as'for' "75).'

The'redshift'in' the'high-energy'features'in'"75'is'poten-

tially' due' to' greater' interactions' between ' the' chromo-

phores'through'a'conjugated'linker.'

Although' changes' in' the' absorption' spectrum'

(where'presen t)'can 'in dica te'th e'e xten t'of'c hro m op ho re'

interaction,'th e'inte r-chromophore'coupling'responsible'

for' SF' is' not' available' fro m ' UV-vis' spectra,' since' the'

relevant'CT'and 'T T'states'(or'adiabatic'states'with'that'

character,' see' SI)' are' generally' dark' and' the' UV-vis'

spectrum'probes' the'adiabatic'electro nic'states,'not'lo-

calized/diabatic'states'from'whose' co upling 'SF'rates'can'

be'determined.

35,38

View along C

(z) axis

View perpendicular to C

(z) axis

EBD

Spi

TFM

BCO

BP1

102.+34 @64 $,='4 UV-Visible' absorption' spectra' with' TIPS-

pentacene'and'"75'inc lu d ed 'for'reference.'",**,A''Calcu-

lated'structures' using' density' functional'theory.'Hydrogens'

and'TIPS'substituents'at'the'6,13-position'of'pentacene'are'

omitted'for'clarity.'

Singlet*Fission*in*Homoconjuga te d*Dimers.'

We'use' broadband'transient' absorption' spectrosco-

py' (TAS)' to' study' the' excited' state' dynamics' of' these'

molecules' in' dilut e' solution .' The' measure m e n ts ' are'

carried' ou t' in' a' standard' nearly' collinear' transmission'

geometry.' A ' 100' fs' pump' pulse' is' tu ned ' to' excite' a' vi-

bronic'feature'associated'with'the'lowest'energy'optical'

transition'(~' 600 'nm)'and'a'supercontinuum 'white'light'

was' used' as' probe' (additional' details' in' the' SI).' Bo t h '

femto-' (mechanical' d elay)' and' nanosec ond ' (electronic'

delay)' broadband' probes' are' employed' in' conjunction'

with'this'same'pump' pulse'to' exte nd 'the'dynamic' r an g e'

of' measurem ent'from'100'fs' –'100'µs.'Fig u r e'3 'sh o ws'the'

resulting'2D'color'plots'produced'by'photoexcitation'of'

the' pe ntac ene ' dimers' with' a' fluence' of' ~25' µJ/cm

' in'

chloroform.

We'find'that'efficient'singlet'fission'occurs'in'all'of'

the' homoconjugated' pentacene' dimers' (<";B4 $1>,'

#=0).'Moreover,'despite'the'significantly'different'geom-

etries,' singlet' fission' rates' are'q uite' similar'amon g' the'

homoconjugated'dimers'as'<";,'$1>'and'#=0'undergo'

iSF' w it h'time 'constan ts'of'10'ps,' 50'ps'and'55'ps,' re sp ec-

tively.' Singlet' fission' is' assigned' following' the' widely'

accepted' criteria.

7,9-12,14,19,23,33-35,39-43

' Briefly,' the' singlet'

decay'is'assigned'by' corre lating'the'tim e'constants' a sso-

ciated' with' the' decay' of' prompt' fluorescence' (using'

photoluminescence' upconversion' techniques,' SI)' to'

features' in' th e ' tra n sie n t' a bs or p tion' spectra.' From' this,'

we' determine' that' the' photoexcited' singlet' exciton' is'

associated' with' photoinduced' absorption' bands' near'

~460'nm ' and' 520' nm,' and' find' that'the' singlet'decays'

on'<100'ps'timescales.'

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Frequently Asked Questions (14)

Q1. What are the contributions in "Tuning singlet fission in pi-bridge-pi chromophores" ?

A recent computational study suggests that singlet fission occurs by a direct mechanism in bipentacene, in contrast to the charge transfer mediated ( stepwise ) mechanism widely perceived to be dominant in intermolecular singlet fission of crystalline pentacene. One of the key predictions from this study was that very weak chromophore-chromophore coupling could permit ultrafast singlet fission. In this manuscript, the authors investigate how homoconjugated and non-conjugated bridging units affect the excited state dynamics of pentacene dimers. The chromophores shown in Figure 1 were designed as follows: Finally, the authors use a bicyclooctane spacer ( BCO ) to mimic the distance imposed by the conjugated phenylene spacer in BP1, which has been previously reported. The redshift in the high-energy features in BP1 is potentially due to greater interactions between the chromophores through a conjugated linker.

Q2. What is the key prediction from this study?

One of the key predictions from this study was that very weak chromophore-chromophore coupling could permit ultrafast singlet fission.

Q3. What is the reason for the redshift in the high-energy features in BP1?

The redshift in the high-energy features in BP1 is potentially due to greater interactions between the chromophores through a conjugated linker.

Q4. What is the way to add a singlet pair to a solid-state?

In the solid-state, through-bond singlet fission can be complemented by through-space singlet fission, adding an additional SF channel.

Q5. What is the first family of materials with fast rates of iSF?

While it has been observed in twisted dimers that reducing the coupling between pentacene chromophores preferentially extends the triplet pair lifetime,35 this is the first family of materials with fast rates of iSF, and drastically different rates of triplet pair recombination.

Q6. What is the recent study suggesting that singlet fission occurs by a direct mechanism?

Q7. How did Liu et al. design tetracene trimer?

12,33 Recently, Liu et al. designed tetracene trimer through linear oligomerization which resulted in greatly enhanced iSF yield (96%) relative to a similar dimer.

Q8. How long does iSF take to recombinate a triplet pair?

Despite the faster time constants for triplet pair generation as compared to conjugated BP1, triplet pair recombination is > 10 times slower in EBD.

Q9. Why are these materials distinctive among reported iSF compounds?

These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states.

Q10. What is the reason for the slow rate of singlet fission in twisted dimers?

Such conclusion is further supported by the fact that singlet fission is slower in twisted dimers that lack bridging units, where conjugation is decreased due to reduced overlap of the pi orbitals.

Q11. What is the effect of the orthogonal connected dimers?

orthogonally connected dimers reported by Lukman et al. resulted in ultrafast singlet fission and were particularly sensitive to the polarity of the medium.

Q12. How do the authors quantify the yields of iSF?

This allows us to quantify the yields using kinetic arguments, since the only significant competing relaxation process is a 12.3 ns radiative decay.

Q13. What are the biexponential decay dynamics in toluene?

These biexponential decay dynamics have also been observed in other systems11,14,39,40 and do not change as a function of concentration or other experimental parameters.

Q14. What is the absorption/emission structure of the triplets?

The absorption/emission structure is indicative of the selective population of T0, as is expected for triplets generated by fission.