despite their usually low stability , organic radical cations1 , 2 have raised substantial interest in a wide range of research areas owing to their critical importance in a multitude of organic and biological reactions , for which they act as reactive intermediates .
furthermore , radical cations that absorb light in the nearinfrared ( nir ) region have attracted much attention in the materials science community , as this feature may potentially enhance the efficiency of solar cells.3 the core structure of radical cations is usually based on fused rings and/or metal complexes .
radical cations with monocyclic rings have received substantially less attention in comparison.4 the core units of these molecules usually contain heteroatoms such as nitrogen , oxygen , and sulfur.1b , 2i , 2j , 5 , 6 the properties of triarylaminederived radical cations have especially been studied in great detail in this context.7 in general , their absorption bands at approximately =700  nm are subject to a bathochromic shift .
n , ndimethylaminophenylsubstituted triphenylamines show absorption bands at approximately =1300  nm . however , most of these compounds are unstable and/or are present only at low temperature .
for example , the radical cation of 1,3,6tris(dipanisylamino)pyrene , which contains fused rings , exhibits a halflife of 3  h.8 in addition , several radical cations generated from sulfurcontaining monocyclic aromatics , such as thiophenes9 and thiazoles,10 have been reported .
some thiazole radical cations , such as 1butyl5methylthiazolium,9d have been reported , but their photophysical properties have not been disclosed .
furthermore , even the radical cations derived from large molecules with heterocyclic rings , such as azathiahelicene , exhibit halflives of only approximately 8  h.2h 
  in this context , we recently developed 5aminothiazoles with various substituents ( scheme  1 ) that could be readily prepared from thioformamides and secondary thioamides .
these 5aminothiazoles represent the first examples of thiazoles that contain diarylamino groups at the 5position,11 and they show a wide range of absorption and fluorescence properties that change upon exposure to external stimuli , such as the addition of acids .
thiazoles contain electrondonating ( d ) and electronaccepting ( a ) moieties that deviate from coplanar alignment .
these da structures usually lead to fluorescence from intramolecular charge transfer ( ict ) states.12 therefore , the electrons of compounds having the da structure are delocalized over the molecule in stages of absorption and emission .
thiazole derivatives moreover exhibit a reversible oneelectron oxidation wave in their cyclic voltammograms ( figure  s1 , supporting information ) .
for example , the halfwave potential of 1 in ch3cn appears at + 0.623  v , whereas that of 3 , which contains a 4methoxyphenyl electron donor at the 5position , appears at + 0.407  v. this result indicates that 3 oxidizes more easily than 1 .
in contrast , the cyclic voltammogram of 4,13 which bears a 4dimethylaminophenyl group at the 5position , exhibits two reversible oxidation waves .
the oxidation potential and waveform of 5aminothiazoles can thus be easily controlled by judicious choice of the substituents at the 5position . on the basis of these results
, we expected that chemical oxidation of 5aminothiazoles would generate stable radicals , for which we anticipated absorption in the uv / vis  nir region .
herein , we report the chemical and electrochemical oxidation of 5aminothiazoles to generate radical cations that show uv / vis  nir absorption bands . in one case , a halflife time of more than 300  h was observed .
to identify appropriate reagents for the chemical oxidation of 5aminothiazole 1 , bf2cf3 
 oet,14 no[sbf6],15 and [ ( 4brc6h4)3n][sbcl6 ] ( magic blue , mb)16 were added to solutions of 1 in ch2cl2 ( scheme  2 ) . upon the addition of bf2cf3 

oet , the color of the solution changed from yellow to orange . however , the thusobtained compounds were very unstable .
the use of no[sbf6 ] did not afford any new species , as evident from quantitative recovery of the starting materials .
notably , a single chemical species was only obtained with mb , and therefore , we subsequently used mb to generate single chemical species of 14 .
reactions between thiazole 1 and bf2cf3 
 oet , no[sbf6 ] , and [ ( 4brc6h4)3n][sbcl6 ] ( magic blue ) .
we then measured the absorption spectra of the radical cations derived from 14 ( figures  1 and s2 ) . whereas 14 showed absorption maxima at approximately =358410  nm ( figure  1  a ) , we expected bathochromic shifts in their absorption bands upon the addition of mb .
the addition of 1  equivalent of mb to 1 afforded new absorption bands at =900 and 650  nm ( table  1 ) .
after the addition of 1  equivalent of mb , the absorption maxima of 2 and 3 , both of which contain methoxy groups , were observed at =918 and 932  nm .
furthermore , the addition of 1  equivalent of mb to thiazole 4 , which contains the strongest electron donor , resulted in a strong absorption in the nir region ( =1180  nm ) .
these results suggest that the wavelengths and strength of the absorptions can be adjusted by the substituents introduced on the thiazole rings . 
 
absorption spectra of a )  14 and b )  1+mb to 4+mb in ch2cl2 , [ solute]=110  m 
  uv / vis absorption spectra of thiazoles 14 with mb . 
 
[ a ]  in ch2cl2 , [ solute]=110 
 m. the addition of > 3  equivalents of mb to 4 also induced a shift in the absorption band ( figure  s3 ) .
the absorption of 4 gradually changed from =760 , 1006 , and 1180  nm to 796 , 1474 , and 2726  nm , whereas the color of the solution changed from red to green .
these results corroborate that thiazole 4 generates different radical species , depending on the amount of mb added . to compare the photophysical properties of the chemically oxidized species with those of the electrochemically oxidized species,17 we measured the cyclic voltammograms and absorption spectra of 1 . in ch2cl2 , 1 showed a oneelectron oxidation wave with peaks at e=1.04 and 0.73  v ( figure  s4 ) .
thus , a potential of 1.04  v was applied to a ch2cl2 solution of 1 for the electrochemical oxidation while the timedependent absorption spectra of this solution were measured ( figure  2 ) .
the oxidation potentials and the waveforms of the oxidation waves could be controlled by judicious choice of the substituents of the 5aminothiazoles .
the observed spectra of electrochemically oxidized 1 were found to be similar to those of chemically oxidized 1 ( figure  s5 ) .
hence , the spectral changes should be attributed to the generation of radical species , and the radical species derived from 1 showed absorption bands at 450 and 650  nm . 
 
timedependent absorption spectra of 1 after application of 1.04  v in ch2cl2 ; = 110 
 m , 0.1  m [ nbu4n][clo4 ] ; scan rate=100  mv  s ; reference electrode : ag / agcl , counter electrode : pt , and working electrode : pt gauze . to elucidate the properties of the chemically oxidized species ,
the electron paramagnetic resonance ( epr ) spectra were measured at room temperature for ch2cl2 solutions of 14 that contained 1  equivalent of mb ( 1+mb to 4+mb , figure  3 ) . in all cases ,
the formation of radical species was observed , and the epr parameters are summarized in table  2 .
the g tensor and hyperfine coupling constant ( hfcc , a ) values were determined from spectral simulations , whereby a 
 n and a 
 h refer to interactions with nitrogen and hydrogen atoms , respectively .
the g values of the spectra for 1+mb to 4+mb ( g=2.00302.0034 ) differ substantially from the g value of a pure mb solution ( g=2.0102 ) .
thiazoles 3 and 4 , with electrondonating groups at the 5positions , showed higher g values than 1 and 2 .
moreover , upon adding > 3  equivalents of mb to 4 , the g value of the 4+mb solution changed ( g=2.0044 ) relative to that observed for the addition of 12  equivalents of mb .
this result indicates that the properties of the radical species of 4 drastically change upon adding an excess amount of mb
. 
  observed epr spectra of a )  1 , c )  2 , e )  3 , and g )  4 with 1  equivalent of mb , as well as that of i )  4 with 3  equivalents of mb ; the spectra in panels  b , d , f , h , and j are the corresponding simulated spectra .
signals marked with arrows in panels  a , c , e , g , and i represent the mn marker .
[ a ]  in ch2cl2 ( distilled from p2o5 ) , [ solute]=110 
 m. the g and a 
 n values were determined from spectral simulations .
[ b ]  calculated by integrating the area of the epr signals , normalized to the peak height of the signals from the mn marker as g standard . to elucidate further details from the epr spectra , we simulated the spectra for 1+mb to 4+mb with software for isotropic epr spectra provided by jeol ( figure  3  b ,  d ,  f ,  h ,  j ) . in the spectra of 1+mb to 3+mb , only the nitrogen atom at the 5postition affected the spectral line shape .
interestingly , the epr spectrum of 4+mb , which exhibited two reversible oxidation waves in its cyclic voltammogram , included components of both the nitrogen atoms and hydrogen atoms , which implies that the unpaired electron is more delocalized in 4+mb than in 1+mb to 3+mb , despite the relatively short halflife of 4+mb ( 3  h ) . 5aminothiazoles with electrondonating groups at the 5position and electronaccepting groups at the
the smallest a 
 n value among 13 was observed for 2 ( 0.63  mt ) , which contains an electrondonating group at the 2position . a slightly higher a 
 n value was observed for 3 ( 0.78  mt ) , which contains an electrondonating group at the 5position , and this value is also marginally higher than that of 1 ( 0.72  mt).18 nevertheless , these a 
 n values are much smaller than that of 2,2,6,6tetramethylpiperidin1oxyl ( tempo , 1.59  mt),19 in which an unpaired electron is localized on the nitrogen atom .
this result implies that thiazolebased radicals contain more delocalized electrons than , for example , tempo , and hence exhibit higher levels of stability , which is reflected in the relatively long halflives of 1+mb to 4+mb ( e.g. 3+mb :  
 1/2=385  h , 16  days ; table  2 ) .
interestingly , 4+mb exhibited , depending on the amount of mb added , two types of epr spectra . upon the addition of 12  equivalents mb , 4+mb showed multiplet signals , whereas upon the addition of > 3  equivalents of mb , the signals associated with 4+mb disappeared .
the simulated spectrum of 4+mb ( figure  3  h ) was very similar to the experimentally observed spectrum ( figure  3  g ) and reflected interactions with one nitrogen atom ( a 
 n=0.64 ) , two other nitrogen atoms ( a 
 n=0.34 ) , and eight hydrogen atoms ( a 
 h=0.29 ) .
in contrast , the addition of > 3  equivalents of mb to 4 furnished another radical species ( figure  3  i ) .
the simulated spectrum ( figure  3  j ) reflects interactions with two nitrogen atoms ( a 
 n=0.43 ) and is in good agreement with the experimentally observed spectrum . timedependent density functional theory ( tddft ) calculations for neutral 1 showed that the maximum absorption band ( =416  nm ) should be assigned predominantly to the homolumo transition ( 97  % contribution ) .
the homo is mainly localized on the substituent at the 5position and the thiazole ring , whereas the lumo is located predominantly on the phenyl group at the 2position and the thiazole ring .
, we used the longrangecorrected camb3lyp20 functional instead of the b3lyp functional to recalculate the uv / vis absorption spectrum .
the recalculated spectrum revealed an absorption maximum at =349  nm , which is in good agreement with the experimentally observed value ( =364  nm ) .
we also calculated the uv / vis absorption spectrum for the radical cation of 1 . at the b3lyp level ,
an absorption band at =890  nm was obtained , which is in excellent agreement with the experimental value ( =900  nm ) .
this excitation was assigned predominantly to the cationhomosomo transition ( 97  % contribution ) shown in figure  s6 .
the cationhomo is very similar to the homo1 of neutral 1 , whereas the somo is essentially the singly occupied homo of neutral 1 . a detailed comparison between the homo of neutral 1 and the somo showed a slightly higher contribution from the phenyl group at the 2position for the latter , which was probably caused by increased charge transfer from the substituent at the 5position .
as a result , the calculated nc(5position of thiazole ) and c(2position of thiazole)c(2position phenyl group ) bond lengths contract upon removal of one electron from 1.399 ( nc neutral ) to 1.367   ( nc radical cation ) and from 1.472 ( cc neutral ) to 1.455   ( cc radical cation ) . the cationhomo and somo essentially exhibit contributions from the same region , which suggests that the consideration of longrange corrections for the hybrid b3lyp method is not necessary .
the calculated cationhomo  somo energy gap ( 2.10  ev ) is much smaller than the homo 
lumo gap in neutral 1 ( b3lyp : 3.56  ev ; camb3lyp : 5.96  ev ) .
this difference should be responsible for the dramatically redshifted uv / vis absorption after the oneelectron oxidation . for organic radicals ,
the b3lyp method can be used to predict the epr parameters.21 the g tensor shifted from the freeelectron value , and the a 
 n values were predicted by dft calculations for our radicalcation species .
the calculated g tensor and the isotropic fermi a 
 n values for the n atom at the 5position ( 2.0029 , 0.74  mt ) agree very well with the experimental data ( 2.0030 , 0.72  mt ) shown in table  s1 , which verifies the validity of the results obtained from calculations at the b3lyp/631+g(d ) level of theory in this particular case.22 
  the spin density map for the radical cation of 1 ( figure  4 ) shows positive ( blue ) and negative ( green ) spin densities at an isodensity level of 0.004  e  bohr . the spin density is mostly delocalized over the n atom at the 5position and the neighboring conjugated c = cn = c fragment in the thiazole ring . the radical character of the cation is also reflected in the occupancy ( 0.88  e ) of the orbital of the lone pair of electrons for the n atom at the 5position analyzed by nbo  6.0.23 
  mulliken spindensity map for the radical cation of 1 calculated at the b3lyp/631+g(d)/pcm(ch2cl2 ) level of theory .
in conclusion , radical cations 1+mb to 4+mb were generated by adding 1  equivalent of magic blue ( mb ) to thiazoles 14 .
the electron paramagnetic resonance ( epr ) spectra of 1+mb to 4+mb indicated that it was the nitrogen atom at the 5postition that mainly affected the spectral line shape of these thiazolebased radical cations .
radical cations with electrondonating groups at the 5position ( i.e. 3+mb and 4+mb ) showed larger g values than the other radical cations . moreover , upon the addition of 3  equivalents of mb to 4 a different radical species was obtained .
radicals 1+mb to 4+mb showed smaller a 
 n values than 2,2,6,6tetramethylpiperidin1oxyl ( tempo ) , which suggested a correspondingly higher level of delocalization and , consequently , more stable radical cations . among these radical cations
thus , compounds with strong intramolecular chargetransfer character that contain one moiety that is able to react with oxidants may represent a new concept for the design of stable organic radicals .
the absorption wavelengths of 14 showed a bathochromic shift upon the addition of 1  equivalent of mb , and the absorption bands of 1+mb to 4+mb were observed in the nearinfrared region .
the absorption spectra of chemically oxidized 1 showed changes similar to those observed in the spectra of electrochemically oxidized 1 .
therefore , chemical and electrochemical oxidation methods can be chosen for the generation of similar chemical species .
further studies on the applications of 5narylaminothiazoles as stable dyes with a wide absorption and emission range are currently in progress in our laboratory .
electrochemical measurements were performed by using a platinum working electrode , a platinum gauze working electrode , a platinum wire counter electrode , and a ag / ag [ 0.01  m agcl ] reference electrode in ch2cl2 with 0.1  m [ nbu4n][clo4 ] .
measurements were recorded with a bas cs3a cell stand and sec2000 spectra system . prior to use , ch2cl2 was distilled from p2o5 .
thiazoles 14 were prepared according to literature procedures.11a 
  a ch2cl2 solution of the corresponding thiazole ( 110 
 m , 0.1  ml ) was diluted in ch2cl2 ( 0.8  ml ) , and the mixture was stirred for 1  min . a ch2cl2 solution of magic blue ( 110 
 m , 0.1  ml )
was then added , which afforded a ch2cl2 solution of the radical cation ( 110 
 m , 1  ml ) .
then , an aliquot ( 0.3  ml ) of this solution was transferred to an epr sample tube ( diameter=1  mm ) , and the measurements were conducted . to measure the photophysical properties ,
a )  epr spectrum of a solution of tempo and b )  epr simulated spectrum .
electrochemical measurements were performed by using a platinum working electrode , a platinum gauze working electrode , a platinum wire counter electrode , and a ag / ag [ 0.01  m agcl ] reference electrode in ch2cl2 with 0.1  m [ nbu4n][clo4 ] .
measurements were recorded with a bas cs3a cell stand and sec2000 spectra system . prior to use , ch2cl2 was distilled from p2o5 .
a ch2cl2 solution of the corresponding thiazole ( 110 
 m , 0.1  ml ) was diluted in ch2cl2 ( 0.8  ml ) , and the mixture was stirred for 1  min . a ch2cl2 solution of magic blue ( 110 
 m , 0.1  ml )
was then added , which afforded a ch2cl2 solution of the radical cation ( 110 
 m , 1  ml ) .
then , an aliquot ( 0.3  ml ) of this solution was transferred to an epr sample tube ( diameter=1  mm ) , and the measurements were conducted . to measure the photophysical properties ,
a )  epr spectrum of a solution of tempo and b )  epr simulated spectrum .
as a service to our authors and readers , this journal provides supporting information supplied by the authors .
such materials are peer reviewed and may be reorganized for online delivery , but are not copyedited or typeset .
technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors .
 abstractchemical or electrochemical oneelectron oxidation of 5narylaminothiazoles was found to afford stable radical cations . for chemical oxidation , 1  equivalent of [ ( 4brc6h4)3n][sbcl6 ] ( magic blue , mb ) 
 was added to ch2cl2 solutions of the thiazoles , and the thusobtained radicals showed light absorption in the nearinfrared region . 
 electrochemical oxidation also led to bathochromic shifts in the absorption bands , and the obtained spectra were similar to those derived from the chemically oxidized species . 
 these radicals afforded electron paramagnetic resonance ( epr ) spectra that are consistent with the notion of stable nitrogen radicals ( halflife385  h ) . 
 the epr spectrum of a thiazole containing 4dimethylaminophenyl groups on the nitrogen atom at the 5position changed significantly upon adding > 3  equivalents of mb . 
 details of the electronic structures of the experimentally obtained radical cations were generated from theoretical calculations . 
Introduction
Results and Discussion
Conclusions
Experimental Section
General Remarks
General Procedure for the Preparation of the Radical Cations
Conflict of interest
Supporting information