Excited state decay of tetrahalomanganese(ii) complexes

CHEMICAL PHYSICS 4 (1974) 295-299.0

NORTH-HOLLAND PUBLISHING COMPANY

EXCITED STATE DECAY OF TETRAHALOMANCANESEJII) COMPLEXES Mark WRIGHTON and David GINLEY LIepartment of Chemirtry, Massachusetts Insriture of Technology Cambridge,

bfassachuserrs

02139.

US.4

Received 18 February 1974

The excitation and emission spectra and decay times of seven1 Mn$ (X = Cl-, Br-. l-) complexes of various terradkylammonium. -phosphonium. and -arsonium salts have been measured for the pure solids at 298°K and 7PK. High luminescence quantum yields (0.3-1.0) reveal tit l&dimes ftily accurately reflect radiative dewy rates. An impressive correlation exists between the lifetime, T, of the ‘Tr(G)-+‘At emission and the ligand, X: for X = Cl’, T = 1.2 3.5 X 10m3set; X = Br-, 7 = 0.35 - 0.43 X lo-’ set; X = I’, 7 = 0.036 - 0.055 x 10e3 sec. We attribute this decreasing lifetime largely to the enhanced spin-orbital coupling associated with the heavier halide. We titd that direct population of high energy charge-transfer (CT) states gives smaller emission yields than excitations in the @and-field (LFI region. 1. Introduction

Perturbation of the electronic structure of atoms and molecules by coupling of the spin and orbital angular momenta of electrons is well founded theoretically. Phenomenologically spin-orbital coupling relaxes the forbiddeness of electronic transitions involving unit changes in the total spin. The incorporation of the heavy atoms Br or 1 into aromatic hydrocarbons is known to yield larger radiative rate constants for singlet-triplet transitions [l] which has been attributed to the increased spin-orbital coupling. Changes in spin-orbital coupling by such chemical modifications may allow identification of reactive excited states and yield changes in reactivity by either more efficient pop ulation or depopulation of the reactive state. And, in cases where substantial spin-orbital coupling exists, one can ascertain the energetic position of spin-forbidden excited states by absorption spectroscopy. Cenerally, factors controlling decay rates of excited states of transition metal complexes are poorly understood, and the possibility that control of the decay pathways in these systems could be achieved by manipulation of the spin-orbital coupling has not been explored. We report herein solid state luminescence studies of pure fiX$ (X = Cl-, Br-, I-) complexes which give systematic changes in decay rates which could be attri-

buted to changes in the degree of spin-orbital coupling. Salts of MnXi- complexes are ideal for the study, exhibiting intense spin-forbidden emission [2]. The T,, high-spin d5 MnXi- complexes have the oneelectron configuration shown in scheme I; 4

4-

4-tz

Scheme 1

All ligand-field (LF) one-electron

transitions are spinforbidden sextet-quartet, and the lowest absorption is associated with the e2t$+e3 tz one-electron demotion. Absorption studies [3] further reveal that the LF strengths of Cl’, Br’, I- for MI&- are similar. reducing complications in interpretation due to differences in the energy of the lowest excited state.

2. Results

2.1. Excitation spectra Typical 298OK and 77OK excitation spectra are shown in fig. 1 and excitation band maxima for all of the powdered complexes studied are set out in table 1. We find that the three band pattern of the excitation

296 Table 1 Excitation

M. Wrighton and D. Cinle_~. Excited

bands for the 4T1*6A~

emission

4p, 4D

6A I-

CK,

km-' x 10~)

(cm” X

IO*)

(Me4N)zMnCls

298

tWN)zMnCLa

17 298

3.73 3.15 3.69 3.71 -

3.55 3.69 3.49 3.52 *-

3.63 3.62 3.65 3.63 3.59 3.59 3.67 3.65 -

3.44 3.44 3.44 3.43 3.40 3.40 3.47 3.46 -

2.84 2.83 2.81 2.81 2.79 2.19 2.78 2.18 2.77 (s) 2.18 2.75 (s) 2.76 2.80 (s) 2.79 2.73 (s) 2.74 2.68 (s) 2.66 2.73 (s) 2.7 1 2.66 (s) 2.68

2.67 214 2.65 2.12 2.64 2.71 2.68 2.68 2.68 2.68 2.64 2.67 2.68 2.68 2.63 2.64 2.55 2.55 2.59 2.57 2.55 2.55

17

WwW2MnCb (Me4N)2MnBr4 WN)lh!nBr4 (nPr4N)aMnBro (nBu4NI2 hlnBr4 (nBuPh3P)zMnBra (hfe4N)2Mn14 (nBucN2hInb (McPh&)2Mnl4

298 77 298 II 298 77 298 17 298 77 798 ll298 II 298 71 298 l-l-

3.41 3.42 (s) 3.50 3.53 -

-

l) Peak maxima are given and shoulders

300 1

Wavelength, 400 i

are indicated

nm 500 .I 1: II II ,I

complexes

in M&complexes*)

Temp. 6A1-4F

Compound

srate decoy ofMnXf

6A ,Pc

(cm-’ X lO*> (s) (s) (s)

2.65 2.64

(s) 2.63 2.61 (s) 2.61 (s) 2.62

2.61 2.62 2.26 2.57

2.29 (s)

2.38 (s)

by (s) after the band

600 I

(s) (s) (s) (s)

2.33 2.32 2.32 2.33 2.30 2.30 2.30 2.31 2.31 2.31 2.28 (s) 2.29 2.31 (s) 2.32 2.20 2.27

2.24 2.24 2.24 2.24 2.23 2.22 2.23 2.23 2.23 2.23 2.20 2.22 2.24 2.22 2.13 2.20 2.23 2.22 2.25 2.25 2.22 2.23

(s)

(s)

(s)

(5)

2.16 2.14 2.15 2.14 2.15 2.14 2.15 2.15 2.15 2.15 2.13 2.14 2.16 2.16 2.01 (s) 2.12 2.15 2.13 2.16 2.16 2.13 2.14

2.10 (s)

2.05 (s)

2.09 (s)

2.04 (s)

2.08 (s) 2.09 2.10 2.09 2.09 2.07 2.08 2.10 2.11

2.08 2.01 2.10 2.10 2.01

(s) (s)

2.04 (s) 2.04 (s)

(s)

2.02 (s) 2.06 (s)

(s)

2.04 2.03 2.06 2.06 2.02 (5)

position; assignments after ref. [ 31.

spectra strongly resembles the absorption spectra [3] energy region, and there is no dependence on the emission wavelength which is monitored. It is noteworthy that in the high energy region of the spectrum there appears to be no luminescence arising from tail absorption of the charge-transfer (CT) excitation which means that the CT state does not decay completely via the LF states. Quantitatively, for [Et,N] 2MnBr4 the emission quantum yields at 298°K are 0.07,0.23, and 0.5 for 254,290, and 460 nm, respectively.

in the low

2.2. Luminescence phenomeno

3.339 Enwgy.

2500 2oOa cm-’ I( IO-’

1.667

Fig. 1. Excitation and emission spectra of (Et4NhMnC& as a pure solid at 29g K ( -) and 77°K (---). Tbc lowest energy Peak is the emission band and atI others are the excitation baiuia monitoring 52Onm emission.

Emission characteristics of the powdered complexes are detailed in table 2. In each case the narrow, structureless emission spectrum, fig. 1, is independent of the excitation wavelength, and the emission overlaps well with the first absorption band. The emission, then, is assigned as the 4T1(4G)“bA1 transition [2]. The temperature effects on the luminescence efficiency are

M. Wrighronand D. Ginley, Excited state decay of MIX:

297

complexes

Table 2 Luminescence characteristics of MI&Compound

(Me4N)z

hln&

(EW’Ozh~Cb

Temp. CK)

Emission max. (cm- x 1o-4,

Emission halfwidth r i 10% (set x 103) (cm’)

298 17 298

1.90 1.89 1.91 1.94 1.94 1.91 1.90 1.88 1.91 1.a9 1.95 1.96 1.94 1.93 1.93 1.91 1.88

1924 1238 1920 1163 1801

17

ftWWinBr4

298 1-l 298 71 298

WW%hlnBr4

298

fnBu4N12MnBr4

298

[(nBuPhs)4P]2hlnBr4

298 77 298 17 298 77 298 71

tPh4As)s MnCL, (Me4N)zMnBr4

11 17 17

(hfe4N)2nIn14 Wu4N)2Mn4

[(MePha)As] 2 Ml4

1.84 1.86 1.88 !.91 1.89

1170

1520 1001 1307 1056 1399 1227 1434 1040 1521 1034 1534 946 1282 1086 1738 911

1.1 3.5 2.0 3.0 2.05 3.1 0.41 0.41 0.35 0.39 0.42 0.38 0.425 0.43 0.32 0.41 0.047 0.053 0.040 0.055 0.0365 0.044

@ t 15%‘)

0.32 0.56

0.27 0.32 0.29 -

l) Excitation wavelength is 460nm; low temperatnre data is for 20°K. small (less than a factor of three) tbougb considerable sharpening of the emission band occurs upon cooling.

3. Discussion

The relative emission quantum yields for all of the complexes are within a factor of three and absolute yields (table 2) reveal that a large fraction of excited states undergo radiative decay. The emission maxima, as well as band shapes, appear to be independent of both X and the cation used. The only remarkable correlation in the decay phenomena are differences in the luminescence lifetimes, T, for the different halides, X. For X = Cl- the lifetime is longest and falls in the range T = 1.2-3.5 X 1W3 set; for X = Br’, T = 0.35-0.43 X 1W3 set; and for X = I-, r = 0.036-0.055 X 1W3 sec. The’emission lifetimes show little dependence on temperature. For a given X the lifetimes are independent of the cation, and in particular for the heavy atom cation Ph,As+ a shorter lifetime was not obtained.

Despite the great difficulty in making quantitative measurements on powdered samples, the large rmmber of MnX~- compounds investigated provides convincing evidence that there is a good correlation between the decay rates and X. The absolute lurninescence yields and the lifetimes allow evaluation of the rate constants for both radiative and nonradiative decay, revealing that the decay rates are in the order Mnl$- > MnBr$ > MnC$-. The absorption spectra [3] also indicate qualitatively that radiative rates follow the same order. The IOO-fold acceleration in radiative decay rates for the iodo complexes compared to the chloro complexes can be attributed to either enhanced spin-orbital coupling due to the heavier halide, or to enhanced covalency in the Mu-X interaction for the heavier halides which would enhance radiative rates for the LF transitions 141. The moIar extinction coefficient [4] for the visible spin-allowed band in CoC$,

298

hf. lt+ighron and D. Ginley. Excired sute decay of MnXr

COBI:‘, and Co@ is 650,790, and 1126, respectively, which is not a large effect, and Mn(lI) is expected to form less covalent complexes than Co(B) based up on their relative position in the nephelauxetic series. Additionally, substantially increased covalency may be expected to manifest itself in other ways. For example, in comparing the luminescence of Mn*+ doped into Td ZnS sites [S] and bis(tetraphenyldithioimidodiphosphinato)manganese(lI) [6] one sees substantially broadened, and red-shifted emission in the latter case, presumably due to geometrical distortion in the excited state caused by changes in the configuration of electrons involved in bonding. For the latter system where a fair degree of covalency apparently exists the luminescence lifetime is > I@ set [7] which is longer than for the Mnl$ complexes. The good overlap of the absorption and emission spectra, the narrow emission band, and the high absolute emission efficiencies in the MnX$ complexes point to relarively small changes in bonding upon electronic excitation. Thus, while differences in the degree of covalency must exist they are not of paramount importance here. Accelerated rates of radiative decay are, then, ascribed to an increased degree of spin-orbital coupling in the heavier halide complexes. The ligand orbitals are in close proximity to the orbitals involved in the spin-forbidden transition, and, indeed, there is some mixing of the metal and ligand orbitals. Apparently, the fact that the halide is in direct coordination to the metal is of importance as there is only a rather small variation in 2E+4A2 decay rates in haloacetylacetonate complexes of Cr(llt): Cr(acac-H)3; 420 psec; Cr(acacCl)j, 250 psec; Cr(acac-Br)3, 230 psec; Cr(acac-1)3, 111 psec [S] . The spin-pairing transition (2E+4A1) in Cr(ll1) which is energetically independent of the ligands and the LF transitions in high-spin Mn(ll) systems are highly metal-localized, consistent with the necessity of direct coordination for a large spin-orbital coupling effect to be exerted by tire ligand. The quantitative information obtained here for MnXi- compounds is paralleled by qualitative observations for the isoelectronic FeXi complexes [9]. The FeCli 4Tt(4GpAt emission lifetime is cr 0.2 X 1W3 set while that for FeBri is estimated to be approximately one-tenth of that value. However, for the tetrabaloiron(lIi) complexes the radiative decay does not account for a large fraction of decay paths since the luminescence yields are about one-hundredth that of the

complexes

Mn(l1) complexes. Unlike the FeCli complex the MI@’ species appear to exhibit luminescence from only one state, but excitation spectra of the MnX$ in the high energy region indicate that internal conversion of the CT excited state to theLF manifold is not totally efficient as was found with FeCli [9]. Demas and Crosby have shown that the changes in radiative lifetimes of even second row cis-dihalo-bis1,lO-phenanthroline-rhodium(ll1) complexes can be interpreted as due to differences in spin-orbital coupling [IO] . For cis-RhCl*(phen);, cis-RhBr*(phen)s, and cis-Rh$(phen)t the radiative lifetimes are 862, 82.5 and 23.7 psec, respectively. The change in radiative decay is substantial, but in this low-spin d6 case it is possible that the variation is not due only to spinorbital coupling since there is an increase in the intensity of spin-allowed absorption in the heavier halide complexes.

4. Experimental 4.1. Preparution of complexes Preparation of the MnX$ complexes are carried out using known procedures or slight modifications thereof [ll]. 4.2. Luminescence phenomena All luminescence and excitation spectra were measured using an Aminco-Bowman spectrophotofluorometer at 298”K, 77’K, or 20’K. The emission spectrometer was equipped with either a lP21 or a R136 photomultiplier tube detector. The dilution of pure solids with KBr was found to yield excitation spectra with the best resolution and give spectra most accurately resembling absorption spectra. Care was taken to insure that the maxima in the excitation spectra are not merely due to the variations in the output of the 150 W Xenon exciting source. Absolute luminescence quantum yields were measured using a modification of a technique [ 121 where photons absorbed by the sample are determined by comparison to a reflectance standard and emitted photons are measured relative to diffuse reflected photons. The technique was reproduced on a different instrument by G.L. Geoffroy at the California Institute of Technology and a value of 0.50 for the room tem-

M. kkighron and D. Ginky. Excited stare deay of MnXr

perature emission yield of [Et4N] ,MnBr4 was found which is in excellent agreement with our values in the range of 0.50-0.57 for several determinations. The error in emission yields is + 15%. Low temperature quantum yields were determined at 20°K by using a Cryogenic Technology, Inc. Spectrim II sample conditioner by comparing emission area with 298°K values. Temperature was varied from 298°K to 20°K then raised to 298’K and back to 20’K with spectra measured at each end point. Emission lifetimes were messrued using a TRW hlode175A Decay Time Fluorometer equipped with a Xenon Corporation Nanopu!ser excitation source and a Kepco Model 2500 ABC powered RCA 93 1 A photomultiplier detector. The output of the PMT was monitored using a Tektronix 454 OScilloscope and recorded with a Polaroid camera. Plots of log (emission intensity) versus time were linear in every case.

Acknowledgement We thank the National Science Foundation for support of this research. David L. Morse of M.I.T. and Gregory L. Geoffroy of the California Institute of Technology are acknowledged for their assistance in the determination of absolute quantum yields.

qwnpfexes

299

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