Magnetic switching of charge separation lifetimes in artificial photosynthetic reaction centers

Abstract

Excitation of a triad artificial photosynthetic reaction center consisting of a porphyrin (P) convalently linked to a fullerene electron acceptor (C 60 ) and a carotenoid secondary donor (C) leads to the formation of a long-lived C + -P-C 60 − charge-separated state via photoinduced electron transfer. This reaction occurs in a frozen organic glass down to at least 8 K. At 77 K, charge recombination of C* + -P-C 60 − occurs on the μs time scale, and yields solely the carotenoid triplet state. In the presence of a small (20 mT) static magnetic field, the lifetime of the charge-separated state is increased by 50%. This is ascribed to the effect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initially formed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four states have comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. In the presence of the field, the S and T 0 states are still rapidly interconverting, but the T + and T − states are isolated from the other two due to the electronic Zeeman interaction, and are not significantly populated. Under these conditions, recombination to the triplet occurs only from T 0 , and the lifetime of the charge-separated state increases. This effect can be used as the basis for a magnetically controlled optical or optoelectronic switch (e.g. AND gate).

1. A magnetically activated optoelectronic logic gate comprising:
 a. means for receiving and storing ultraviolet or visible wavelength and magnetic field signals wherein the means further comprises a photoreactive molecule capable of forming transient species when activated by said electromagnetic radiation signal, the lifetime of said transient species being modified in the presence of a magnetic field; and  
 b. means for selectively accessing said stored signals to deliver said selected signals for signal processing.  
 
     
     
       2. The optoelectronic logic gate of  claim 1  wherein said photoreactive molecule comprises an electron donor, an electron acceptor and a chromophore. 
     
     
       3. The optoelectronic logic gate of  claim 1  wherein said electron donor is a carotene. 
     
     
       4. The optoelectronic logic gate of  claim 1  wherein said electron acceptor is a fullerene. 
     
     
       5. The optoelectronic logic gate of  claim 1  wherein said chromophore is a porphyrin. 
     
     
       6. The optoelectronic logic gate of  claim 1  wherein said photoreactive molecule comprises a carotene, a fullerene and a porphyrin. 
     
     
       7. The optoelectronic logic gate of  claim 1  wherein said transient species is a long-lived charge-separated molecule capable of decaying by radical pair recombination to yield the triplet state. 
     
     
       8. The optoelectronic logic gate of  claim 3  wherein the lifetime of said transient species is extended by application of a magnetic field. 
     
     
       9. A means for generating magnetic field signals to the transient species of  claim 1  comprises a Helmholtz coil in magnetic contact with said transient species. 
     
     
       10. The optoelectronic logic gate of  claim 1  comprising in addition a means for selectively controlling the time period during which said magnetic signals are generated. 
     
     
       11. The optoelectronic logic gate of  claim 1  wherein said means for selectively accessing and delivering said stored signals for signal processing comprises:
 a. means for activating said transient species;  
 b. means for applying a magnetic field to said transient species for a selected period of time;  
 c. means for transmitting an optoelectronic radiation signal through said transient species;  
 d. means for receiving the transmitted optoelectronic radiation signal in the presence of the magnetic field,  
 
       wherein processing of said transmitted electromagnetic radiation signal comprises comparison of said received signal to a threshold value to provide a Boolean yes/no signal. 
     
     
       12. The optoelectronic logic gate of  claim 7  wherein said electromagnetic radiation signal is light of a known wavelength and the difference between incident and transmitted signal is the absorbance or per cent transmission of said light. 
     
     
       13. The optoelectronic logic gate of  claim 7  wherein said electromagnetic radiation signal is electronic and said difference between said incident and transmitted signal is the conductance or capacitance of said electronic signal. 
     
     
       14. A digital device comprising the magnetically controlled logic gate of  claim 1  wherein processing of said transmitted electromagnetic radiation signal comprises comparison of said signal to a threshold value to provide a Boolean yes/no signal. 
     
     
       15. A magnetically controlled optoelectronic logic gate of  claim 1  comprising:
 a. a photoreactive molecule capable of forming transient species when activated by an electromagnetic radiation signal, the lifetime of said transient species being altered in the presence of a magnetic field;  
 b. means for activating said photoreactive molecule to form said transient species;  
 c. means for delivering a magnetic field signal to said transient species for a selected period of time;  
 d. generator means for transmitting an optoelectronic signal through said transient species;  
 e. monitor means for detecting the transmitted optoelectronic signal in the presence and absence of magnetic field signal; and  
 f. output means for delivering signals from said monitor means to a signal processor.  
 
     
     
       16. The logic gate of  claim 11  in a computer processor. 
     
     
       17. A system, comprising:
   a light - producing element;        a magnetic field generator configured to produce a magnetic field;        a photoreceptive molecule comprising a chemical structure excitable to a transient state in response to light, wherein the transient state is maintainable by the chemical structure upon exposure to the magnetic field, and        a detector configured to be responsive to the photoreceptive molecule, and configured to provide a first signal upon exhibition of the transient state by the photoreceptive molecule and to provide a second signal upon exhibition of a ground state by the photoreceptive molecule.     
     
     
       18. The system of  claim 17 , wherein the light has a wavelength between a visible spectrum and an ultraviolet spectrum. 
     
     
       19. The system of  claim 17 , wherein the photoreceptive molecule further includes an electron donor, an electron acceptor, and a chromophore. 
     
     
       20. The system of  claim 19 , wherein the electron donor is a carotene. 
     
     
       21. The system of  claim 19 , wherein the electron acceptor is a fullerene. 
     
     
       22. The system of  claim 19 , wherein the chromophore is a poryphyrin. 
     
     
       23. The system of  claim 17 , wherein the magnetic field generator comprises a coil. 
     
     
       24. The system of  claim 17 , wherein the photoreceptive molecule exhibits increased light absorption in the transient state, and wherein the detector is further configured to detect the increased light absorption to provide the first signal. 
     
     
       25. The system of  claim 24 , wherein the detector is configured to measure the increased light absorption at a wavelength of  980  nm. 
     
     
       26. The system of  claim 17 , wherein the photoreceptive molecule in the transient state has an increase in electrical energy, and wherein the detector is configured to produce the first signal responsive to the increase in electrical energy. 
     
     
       27. A computer processor, comprising:
   a photoreceptive molecule having a chemical structure of the type to exhibit a transient state in response to light, wherein a time period of the transient state is extendable upon exposure to a magnetic field; and        a measurement device configured to detect whether the photoreceptive molecule is in the prolonged transient state, and wherein the measurement device is configured to provide a first signal upon exhibition of the transient state by the photoreceptive molecule and to provide a second signal upon exhibition of a ground state by the photoreceptive molecule.     
     
     
       28. The computer processor of  claim 27 , further comprising a magnetic field generator configured to produce the magnetic field. 
     
     
       29. The computer processor of  claim 27 , further comprising a light- producing element to emit light.   
     
     
       30. The computer processor of  claim 29 , wherein the light- producing element comprises a fiber - optic communication line.   
     
     
       31. The computer processor of  claim 27 , wherein the photoreceptive molecule comprises an electron donor, an electron acceptor, and a chromophore. 
     
     
       32. The computer processor of  claim 31 , wherein the electron donor is a carotene. 
     
     
       33. The computer processor of  claim 31 , wherein the electron acceptor is a fullerene. 
     
     
       34. The computer processor of  claim 31 , wherein the chromophore is a poryphyrin. 
     
     
       35. A solar light energy conversion system, comprising:
   a magnetic field generator configured to provide a magnetic field for a period of time,        a photoreceptive molecule comprising a chemical structure excitable to a transient state, wherein the photoreceptive molecule has a protracted transient state upon exposure to the magnetic field, and        an energy - storing element configured to receive energy produced by the photoreceptive molecule upon a transition of the photoreceptive molecule from the protracted transient state to a ground state.     
     
     
       36. The solar energy conversion system of  claim 35 , wherein the photoreceptive molecule comprises an electron donor, an electron acceptor, and a chromophore. 
     
     
       37. The solar energy conversion system of  claim 36 , wherein the electron donor is a carotene. 
     
     
       38. The solar energy conversion system of  claim 36 , wherein the electron acceptor is a fullerene. 
     
     
       39. The solar energy conversion system of  claim 36 , wherein the chromophore is a poryphyrin. 
     
     
       40. The solar energy conversion system of  claim 35 , wherein the magnetic field generator comprises a coil. 
     
     
       41. A method, comprising:
   generating a magnetic field during a time period;        detecting a transient state of a photoreceptive molecule having a chemical structure excitable to the transient state in response to light, wherein the photoreceptive molecule maintains the transient state of the photoreceptive molecule during the time period of the magnetic field, and        providing a first signal when the photoreceptive molecule exhibits the transient state, and providing a second signal when the photoreceptive molecule exhibits a ground state.     
     
     
       42. The method of  claim 41 , wherein the photoreceptive molecule comprises an electron donor, an electron acceptor, and a chromophore. 
     
     
       43. The method of  claim 42 , wherein the electron donor is a carotene. 
     
     
       44. The method of  claim 42 , wherein the electron acceptor is a fullerene. 
     
     
       45. The method of  claim 42 , wherein the chromophore is a poryphyrin. 
     
     
       46. The method of  claim 41 , wherein the magnetic field is generated by a coil. 
     
     
       47. An electric network, comprising:
   a photoreceptive molecule having a chemical structure that is excitable to a transient state responsive to light, wherein the photoreceptive molecule has a prolonged transient state upon exposure to a magnetic field; and        a measurement device configured to monitor the photoreceptive molecule and to provide a first signal upon exhibition of the prolonged transient state by the photoreceptive molecule and to provide a second signal upon exhibition of a ground state by the photoreceptive molecule.     
     
     
       48. The electric network of  claim 47 , wherein the photoreceptive molecule comprises an electron donor, an electron acceptor, and a chromophore. 
     
     
       49. The electric network of  claim 48 , wherein the electron donor is a carotene. 
     
     
       50. The electric network of  claim 48 , wherein the electron acceptor is a fullerene. 
     
     
       51. The electric network of  claim 48 , wherein the chromophore is a poryphyrin. 
     
     
       52. The electric network of  claim 47 , further comprising a magnetic field generator. 
     
     
       53. The electric network of  claim 47 , further comprising a light- producing element.   
     
     
       54. A switch, comprising:
   a photoreceptive molecule including a chemical structure excitable to a transient state in response to light, wherein a length of time that the photoreceptive molecule is in the transient state depends upon the photoreceptive molecule's exposure to a magnetic field, and        a detector configured to be responsive to the photoreceptive molecule, and configured to provide a first signal upon exhibition of the transient state by the photoreceptive molecule and to provide a second signal upon exhibition of a ground state by the photoreceptive molecule.     
     
     
       55. The switch of  claim 54 , wherein the chemical structure of the photoreceptive molecule is excitable to the transient state in response to either ultraviolet light or visible light. 
     
     
       56. The switch of  claim 54 , wherein the photoreceptive molecule further comprises an electron donor, an electron acceptor, and a chromophore. 
     
     
       57. The switch of  claim 56 , wherein the electron donor is a carotene. 
     
     
       58. The switch of  claim 56 , wherein the electron acceptor is a fullerene. 
     
     
       59. The switch of  claim 56 , wherein the chromophore is a poryphyrin. 
     
     
       60. The switch of  claim 54 , wherein the photoreceptive molecule, exhibits increased light absorption in the transient state, and wherein the detector is further configured to detect the increased light absorption to provide the first signal. 
     
     
       61. The switch of  claim 60 , wherein the detector is configured to measure the increased light absorption at a wavelength of  980  nm. 
     
     
       62. The switch of  claim 54 , wherein the photoreceptive molecule causes an increase in electrical energy in the transient state and wherein the detector is configured to produce the first signal responsive to the increase in electrical energy.